VTVM Restoration, Alignment, and why you should own one

[@W3KW]

I completed my first KG-625 resto inspired by this video. I went a bit further. I widened the ground probe hole a bit to accommodate a grommet, I added a shielded supply line from the selector wafer to the new BNC connector, I added a modern and more secure battery holder, added a new polarized two conductor AC supply cord, added an internal fuse holder, and finally, rubber feet for the stand. The battery holder fit between the original battery contacts and is reversible. I used RG-58A/U cable for the probe. It has a braided core and is soft and flexible. I replaced the cap and a few out of tolerance resistors in the bridge circuit. I calibrated the meter and polished the meter lens with car wax. An old aviators trick. It looks amazing and performs perfectly. Now, on to restore another -625. Fun! Thanks for the inspiration and education, Paul.

[@xraytonyb]

Another great video! When I was 12 years old, I saved my money for months and ordered a Heathkit VTVM that is almost identical to the KG-625, even down to the mounting bracket! When I built it, I only had a low-end 20K ohm/volt multimeter to do the alignment (12-year-olds are inherently poor). I remember doing those unsafe alignments on the AC. I didn't even have an isolation transformer! I used that meter for many years. It is now packed away in my storage unit. I think I will take it out and give it a proper restoration after 35 years. It still looked like new when I packed it away many years ago as I worked so hard to be able to buy it and take care of it. Thanks for inspiring me! Keep up the great videos!

[@MrCarlsonsLab]

Hey, That's great! I would love to hear about the resto process! These really are great meters, and fun to restore. Thanks for the comment xraytonyb.

[@kerryp1525]

Mr. C, I have a B&K 177 vtvm on my bench that,s used every day for 50 years.. I repair vintage stereo equipment. The VTVM has been the best transistor tester I have ever used ! Never lies ! Won't find noisy ones, but everything else. Techs I know ask why I still use a VTVM..?. They just don't get it, and they struggle at times when they wouldn't have to..

[@Mork007]

Super video. As an amateur radio operator I have to say you have one of the best channels I have ever seen. Thanks for all your effort in the videos. Jim

[@MrCarlsonsLab]

Thanks for the nice comment Jim!

[@arthuruzi2759]

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[@arthuruzi2759]

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[@erin19030]

I'm a well seasoned professional electronics technician , my education goes back to 1961 at RCA Technical school. While in the Army I studied Radar, and missile guidance systems. I watch your tutorial video's as a refresher course. Your teaching methods and circuit understanding are excellent. During my teen years I did kit building of EICO products. While going to RCA tech I worked as a wireman for GE, a skill that has gone the way of robots. Now in retirement I build Experimental circuitry and revitalize vintage electronics. With all my experience I still learn something new each day. Keep,up the good work Doc!

[@brucejenner4800]

You remind me of my Father. He was in the British Army in the Artillery. At the time the Artillery were developing missile systems. My Father was into electronics, I remember looking at all the trays of components, screws, and most of all, remember the smell of solder! He was a subscriber of the Practical Wireless magazine, and made all sorts of projects from its pages.

[@jimmymac5012]

I have to say I am a big fan of your channel. Excellent presentation and knowledge of your trade. My hats off to you sir.

[@MrCarlsonsLab]

Thanks Jimmy Mac!

[@sonyxperiathree4554]

I love your tenacity and attention to deal. I broke my leg, a watched all your excellent videos as I just sit idle waiting for some healing to occur. You are one of the last true omni - knowledgeable electronic engineers. Thanks for the experience and knowledge you convey on KZbin.

[@mikemarshall1394]

A trip down memory lane. I built the Heathkit version of that vtvm when I was about 12. I'm 72 now and wish I still had it.

[@johnwest7993]

Clear and complete description, and all the knowledge of an old-timer. I get the impression you started learning electronics at a very early age, and were taught by a master.

[@vernonbosshard9317]

You sparked a renewed interest in tubes for me, and I'm old enough to have taken tubes out of the family TV and carry them down to the local drug store to test and buy new tubes. But they did not teach us a darn thing about tubes in tech school, all I know is they are voltage amplifiers.

[@iamhe999]

It doesn't get any better than this folks... Kudos to this great electronics teacher....

[@dancamp1515]

just watched the Forest Service Radio video, and then this one popped up in my suggestion box. "Hey, I have that VTVM!" Thanks for the great overview, I now have a better understanding of one of my pieces of test equipment.

[@tashpaug]

Mr. Carlson, I am once again impressed by your ability to repair equipment. In this throw away society it is so refreshing to see someone who rebuilds equipment and thinks through how to find and fix problems. Excellent video, keep them coming. Tony

[@MrCarlsonsLab]

Hi Tony. Thanks for the comment. I'm working on the next video right now.

[@marklowe7431]

KZbin should conduct awards for channels this good.

[@TJSurma-se6dl]

Thank you for your efforts in putting together these videos. The quick 'aside' tips offered up throughout are gold. I often reach for a larger 'gun'-type soldering tool when working with a component that will sink/pull the needed heat away from the solder joint. I would have never thought of replacing the tools standard/long tip element with a short piece of 10 gauge copper wire. So simple, yet effective. Brilliant!

[@MrCarlsonsLab]

+T.J. Surma Glad your enjoying the video's T.J. Thanks for the kind words!

[@paulmichaelson7203]

This reminds me of my old CONAR VTVM that I built when I was a student at NRI. As I remember that meter only had one tube. I remember when I visited NRI they had a bench with about 50 tubes being de-gassed in preparation for use in their VTVMs. They had them running with the filament energized and no voltage on the grid or plate. I used that old CONAR meter for many years until I finally got an FETVOM that had an analog meter. That VTVM was a great instrument. I think the Knight meter is probably a better design. I have always loved analog meters. Very useful. Another very enjoyable video, Thanks for the memories Mr Carlson.

[@geraldwellborn5047]

I have a RCA solid state VTVM (although no vacuum tubes in it) I bought in 1973. It has been stored away for 40 years or more. After watching this, I believe I will dig it out and change its battery and verify its accuracy. Very enjoyable video Mr Carlson. Thank you very much.

[@boulder89984]

The man can smell an electron that's gone the wrong way. Amazing troubleshoot.

[@williamstarklauf6085]

No a electron that has gone wrong way is identified buy the rainbow flag

[@greggaieck4808]

Mr Carlson your cabinet of your VTM is in good shape the cabinet looks like new that's cool

[@DucatiMTS1200]

You have what it takes mate to be deservedly called a great electrical engineer.

[@robbo6460]

Another excellent video. Thank you Mr. Carlson. I've done several calibrations on Heathkit V-7A's and this video has taught me I was screwing up the red dot adjustment by following the manual. Now I know. Thank you

[@MrCarlsonsLab]

Glad to help Rob!

[@robbo6460]

@@MrCarlsonsLab what is that voltage adjustment dial please?

[@edvaioli7340]

Thanks so much for this. Using your video as a guide I was able to restore the flea market RCA Volt Ohmist I scored for $5.00. The RCA is essentially the same as your Knight and your explanation of the bridge circuit was the key. I too had a 12AU7 that was way out of balance along with a couple caps that had drifted badly. Thanks again and keep up the excellent work!

[@MrCarlsonsLab]

You're welcome Ed!

[@AB1Vampire]

@@MrCarlsonsLab I was about to write the same as Ed (above) so I'll put it under his thread. I'm also renewing an RCA VOM. It's the Voltohmyst Junior model. It has the same 2-tube bridge circuit. Thanks Mr. Carlson!

[@barquisimetido1]

I have a vintage Heathkit VTVM model V7-A and have noted that it, the Heath IM-18, Heath IM-28, EICO VTVM's, this one and others of about the same era uses the same tube numbers and schematics in general with a few different values and cosmetic variations, but in essence is the same instrument design (electronically speaking).

[@bassman7115]

Thanks Mr. C. I have two RCA VTVM's; WV-87B-Master and a WV-98c Senior. Both have been restored to their former glory thanks to your video's and Patreaon channel.

[@MrCarlsonsLab]

You're very welcome!

[@TachiTekmo]

Oh, boy, does this bring back memories! My dad had one of these in his "shack," and was constantly fiddling with it. I remember being - oh, six or seven years old - and "helping" him by handing him resistors and what-nots as he'd work on his ham radios. I love these "old-fashioned" VTVMs with the big sweep indicators and solid "ka-chunk" switches! Thanks for bringing this one back to life, and may she serve you long and well!

[@MrCarlsonsLab]

+Tachi Tekmo Thanks Tachi!

[@cristrigotti9933]

I just got one of these at our club auction for $7! I followed your video and changed out all the resistors and followed very closely you cal procedures. It is all cleaned up and working very well. Thanks for doing your videos.

[@bugdrvr]

A few days after I watched this video I found a sad Heathkit V-6 VTVM in the "junk" pile of an estate sale. After some cleaning, polishing and replacement of a few bits it's now a proud member of my workbench. Thanks for the inspiration!

[@MrCarlsonsLab]

Glad to be that inspiration Jim!

[@vasilification]

" A problem well stated is a problem half solved" Charles Kettering Excellent video, a key learning point is the set up Mr. Carlson does before starting the repair. Careful analysis of the problem, from how to remove the meter from the case, the cord the rubber grommet, looking at the condition of the unit, studying the schematic, understanding how the unit works and determining the approach for repair. The net for me is to slow down and familiarizing your self with the problem before beginning the work, which will may make the restoration more enjoyable. I watched the video before starting my restoration of a vintage Hickok VTVM, definitely helped.

[@restoredoutboards]

Hey Mr. Carlson! Thanks for this video. I just finished bring one back to life thanks to you. Replaced a couple of resistors and cleaned the pots, etc. works like a dream now. Pretty good investment for 15 bucks. Thanks so much for your videos and sharing your knowledge. Inspiring.

[@MrCarlsonsLab]

Glad to be there Greg!

[@chrismckenna4689]

Really nice! I'm new to electronics, but with the help of this video, I just restored a DeVry 1S14 VTVM. Some caps, some fixed solder joints, and it works great! I just wish you lived next door so I could pop over and learn more stuff!

[@MrCarlsonsLab]

Glad this helped Chris!

[@chrismckenna4689]

Do you have a video that explains what test equipment one should acquire, and in what order, to work on older ham radios?

[@tuckermoreland147]

at one time i had a complete set of sencore tube testers, an eico oscilloscope. signal generator, isolation transformer, capacitance tester, vom meters, variac, flyback transformer probe, and other things. got divorced and lost it all. those two oscopes you got for 5 bucks apiece reminded me of the eico. didn't surprise me they still worked. your ability at troubleshooting is amazing. i can watch you at work for hours on end. i wasn't anywhere near as good at it as you. but with a schematic i fixed a lot of stuff. mostly audio stuff. a problem i used to run into was some instrument amplifier manufacturers would use components ordered with their own number printed on them. wasn't difficult to find the problem but made it difficult to cross reference a replacement and a component might be worth a few cents but ordered from the manufacturer would cost many dollars. with your ability you can probably figure it out for yourself. i wasn't that gifted. you are an excellent teacher. i have learned more watching you than i ever did in electronics school. i was certified for radar and communications back then but never worked in the field so i wasn't a complete dummy. i am retired now and during this pandemic i found you on the internet. it's true u use it or lose it. but audio is pretty simple and i would like to get back to that. keep the vids coming and i'll keep watching.

[@greggaieck4808]

Mr Carlson your VTM Restoration and Alignment is cool

[@CraigHollabaugh]

Hey Mr. Carlson, I now have 3 VTVM's, EICO, Senior VoltOhmyst and Conar (through estate sale buys). This video is just what I needed to kick my butt in gear and get at least one of them running. Thanks. Craig PS. killer tips here: bounce dryer sheet for meter face de-static and Weller 10AWG tip. Thanks for these little gems

[@MrCarlsonsLab]

+Craig Hollabaugh Glad your enjoying the video's Craig! They all sound great, choosing the candidate would be a hard choice. I have an EICO with a huge meter that I would like to have working as well. Good Luck!

[@oldschool-68]

Thank you for being on KZbin. You have taught me so much in a short time. I've tried reading books and QST magazines to learn more stuff but get confused more than anything. You on the other hand are my "guru" so to speak. I've been a ham since '93 and I love to tinker. I just love your style of presentation the most. Thank you and 73s. KB5WUF

[@MrCarlsonsLab]

Thanks! Glad your enjoying the video's.

[@absurdengineering]

Oldschool68 Unfortunately, most authors have some gray area in their knowledge so they can’t but gloss stuff over, and/or they simply have no knack for effective communication to the reader. There’s a lot of cargo cultism even among people who do engineering, so a lot of written text seems impenetrable: the problem usually is not with the reader, but with the writer.

[@BrandonHall916]

I never thought I would be so excited to see someone calibrate something, GREAT JOB!

[@MrCarlsonsLab]

+Brandon Hall Thanks Brandon!

[@Andre-on5kr]

I found this video very helpful for restoring my DeVry VTVM. Your description of the processes will be invaluable to me. Thanks, mr. C!

[@ThePapa41]

What a fine tutorial on the restoration of the Knight VTVM meter, you have included so much education along with the restoration! Your work is outstanding! Louis

[@TubeRadioRepairRestoration]

I love your videos. Your so intelligent and I'm learning alot from you ( happy patron). One of the things I love most about your videos is they are very comprehensive and just one part. No BS just trouble shooting / repairing in one video, not stringing along with multiple videos

[@RestoreOldRadios]

Another excellent video! Your presentation skills are excellent and your knowledge is superior. Glad to see you promoting safety too. Best. Don

[@MrCarlsonsLab]

Thanks for the nice comment Don! Glad you are enjoying the videos. You have great videos yourself! That Silvertone 4569 is beautiful!!!

[@CraigHollabaugh]

Mr Carlson, I spent about 3 hours fixing another dead VTVM today B&K 375. Finally found that the 'ohms' pot wiper was internally shorted to the shaft, hence also to the frame. This threw off the PS voltage, the bridge bias point, and just about everything else. I pulled the pot's cover, and reseated the wiper on the phenolic, put it back together and bingo, works like a champ. I suspect this VTVM was never used, yay, brand new meter! Thanks again for this video, I wouldn't have spent the time today otherwise. BTW, this B&K VTVM has great meter with a geared changing scale. Replaced my Voltohmyst. Have a great day!

[@MrCarlsonsLab]

Great to read Craig!

[@benthere8051]

Very well done. I enjoyed following your thought process. Finding the steel wool in the pot is a great example of why you can't make assumptions when troubleshooting. It is sometimes the most outrageous circumstances that are hardest to find. That is also a VERY good reason to be very careful with steel wool around electronics.

[@icyrax314]

Found out about you a few days ago and I must say I love the depth that you go into what your talking about.

[@MrCarlsonsLab]

icyrax314 Thanks, Glad your enjoying!

[@jlucasound]

I Loved it. The techniques you have demonstrated can possibly be used on other devices. (Barring any specific requirements, i.e. aligning a graticule on a scope vs. the needle on your meter). The trouble shooting and calibration and safety tips you teach are very clear. Thank You, from (almost) 5 years later!

[@KG5IF]

I never knew the significance of the last digit in a vacuum designation till now. Been working in electronics for 50 years. Thank you sir. Like your B+ warning too for the new players with tube gear.

[@magic1bean]

Back in the 80's I was a bench tech for HP. One of the other techs had an HP VTVM that he could not get to pass specs at high end frequency. The fix was drilling the rivets out, sanding the mating surfaces and riveting the pieces back together. FYI, in case it helps anyone else. Worth trying the antisieze first tho. I really have enjoyed your videos. I learned on tubes in 1968 and have had a great trip down memory lane while learning.

[@jimthannum7151]

Wish I never gave away my VTVM! This was an excellent tutorial, so clear and understandable, and a great learning experience, not just the restoration, but your superb troubleshooting technique.

[@MutethatBozo]

Watching this as I am working on some minor repairs to an old DeVry vtvm from 1955. Thanks again for the videos, and peace :)

[@MrUbiquitousTech]

Great job on restoring that VM! I'm seeing the trend here, when you're done with a piece of equipment it looks as good as new and works even better! I like your attention to detail. Nice work on diagnosing and repairing that VR; it's not as easy as it looks when you're doing it, it takes knowledge and experience to accomplish that.

[@TheLawrenceWade]

Yeah, was that a tin whisker forming in the VR? Something like that? It seems more likely than steel wool, especially in old high voltage equipment.

[@vancemyers8286]

@@TheLawrenceWade Would a razor help???

[@BellTechnical]

I enjoy every second of your videos. Thank you for the vacuum tube lesson and theory of operation. I grew up with late 1970's heathkit projects in place of sports. While discussing vacuum tubes I had childhood memories of tube testors familiar at hardware or drug stores. We've come along way with solid state components. Thank you for your channel I appreciate your patience and great narratives. Keep up the Great work!

[@MrCarlsonsLab]

Thanks for your kind comment!

[@paulk.bowden7134]

Back in about 1952 I built a Heathkit VTVM that is still working fine on my bench. I did switch out the tubes for solid state equivalents. They plug into the tube sockets. This was the first of about a dozen Heathkits I’ve built.

[@mikesteffes9999]

Same here, except I didn’t build mine until about 9 years later.

[@roygarcia7835]

i loved this so much i grabbed an Eico model 249 that i got in a bunch of equipment that i knew wasnt working. i purchased all new Capacitors and replaced the 1 electrolytic. Once i was done i fired it up just to see if anything worked. Well then i tried a few other things and came to realize the actual meter was faulty. Although it didn't go as planed i still had fun and now will be searching for another Eico and keeping my eye for a good deal to end up with a nice VTVM. I just want to thank you for motivating us to bring life back to these awesome machines. Also for showing us how to do it and teaching us in a manner an amateur like myself can get a grasp. Being a visual learner these youtube videos make my life easier lol.

[@MrCarlsonsLab]

+Roy Garcia Glad your getting into some of this old gear Roy! Thanks for the comment as well!

[@davidwhelchel8774]

Just to let you know that your video on this VTVM is still o great use and very interesting. Thanks for the content

[@MrCarlsonsLab]

Hi Alan Janover. This response may not be in order to your post as you do not have a reply tab under it. You may need to go into your G+ settings and edit your circles to "Anyone" Then the reply tab should appear under your future posts. The reason I didn't use the battery for calibration is, the scale on the Knight only goes to 1.5 Volts. A new battery is "1.6 Volts plus" and would create an error factor, even if aligned to the red dot. The .5 and 1.5 Volt supplies are taken from the Voltage divider that I did show on the schematic a number of times. "The accuracy of any meter is only as good as the alignment." LOL, I will try harder next time to get my camera in there to get the glow of the tubes. Thanks for the comments!

[@MrCarlsonsLab]

+Dewey Rayburn Your welcome Dewey, glad you enjoyed!

[@gchalljr]

Thank you for creating the video on VTVMs in such detail. I am getting ready to restore a couple of VTVMs and now I feel I understand the circuit balancing. The way you explain things is awesome.

[@MrCarlsonsLab]

Thanks for the comment Gary! Many more video's like this coming.

[@erin19030]

I've been an electronics tech Ian since 1961 and like to consider Mr. Carlson' videos to be great refresher and continuing education courses. I have the HealthKit version of this meter.

[@cranegantry868]

At 1:42, sometimes you want a center zero for tuning of RF circuits etc., so the zero is enabled to both zero at left of scale and at to center the needle at center. Another convenient and beautiful feature of VTVM's. I have 3 of them. Love them.

[@waynecarlson3736]

Once again Paul outstanding as always. I recentl bought this meter and will bring back to life AT MY OWN RISK Thank You

[@davidmaddison2628]

I love your videos and repair and restoration techniques Mr Carlson. You also show that old equipment can be very good.

[@johncunningham5435]

Very nice demonstration of what you can accomplish when it comes to old vintage equipment that in my view does not get enough praise. I believe as you do; they have a place on the bench. I recently scooped up a Sencore FE20 Field Effect Multimeter in mint condition. The only major concern was the meter was pegging, which required the removal of the front plate and surgically correcting the issue. Then I used a 1% 100 mega ohm resistor and bingo it was right on the mark. There is nothing more satisfying when you bring back a vintage treasure to life and show it can still stand up, congratulations on a superb video.

[@MrCarlsonsLab]

+John Cunningham Thanks for the kind words John! Sounds like a great job on the Sencore.

[@Screamingtut]

i had the heathkit VTVM plus someone gave my dad an Eico VTVM which I repaired I was 15 at the time I am 62 now LOL enjoyed the video

[@The1jonnyz]

I ALWAYS learn something new from your videos Paul. Another great presentation full of tips, tricks, and knowledge!

[@MrCarlsonsLab]

Glad to help!

[@bobkins270]

With out this video I would have never have calibrated my Knight VTVM. It was the 12au7. I just replaced both tubes, and it calibrated beautifully. Thank you for going though the steps and explaining the problems that could and did arise. I appreciate the videos. Thanks again Brad

[@MrCarlsonsLab]

Glad the video helped Brad!

[@keithstudly6071]

AUH, A-jil-lent? I have been familiar, for years with units that looked like the one he was pointing to and some were H-P and some were A-gi-lent but this is the first time I ever heard of A-jil-lent. Mr. Carlson you teach this old dog new tricks everyday! Thank You!

[@sincerelyyours7538]

This makes a really good first time tube project. I probably paid too much for mine but I got one in nearly pristine condition. I think the previous owner had trouble aligning it so he stopped using it. Almost new inside and out, the few very light scratches disappeared easily with a bit of plastic rubbing compound. That creates a lot of static but Staticide sheets (from Amazon) gets rid of the static buildup without leaving a soapy film on the plastic. All resistors were more than 20% off their markings so they all got replaced. The bias pots were 20% out as well causing a lack of range in the alignment. Had to rebuild the 1.5V 10K pot with parts from a Fender amp bias pot which are the only ones I could find still available online. A final bit of work repairing the probe tip and she's good as new. Thank you, Paul for your excellent video! Thanks to you, antique electronics restoration has become my new retirement hobby.

[@MrCarlsonsLab]

Thanks for your kind comment! I'm glad you found a great hobby!

[@stevedonahue2517]

Really thorough video- I like how Mr Carlson takes you through his troubleshooting steps, and how he located the poor mechanical ground connection. It was a real treat to find any video on Knight Kit equipment. Bravo!

[@MrCarlsonsLab]

Thanks Stephen! I do have some "rare" projects coming soon as well. Glad your enjoying the video's.

[@davidprice2861]

Fancy some stray bits of steel wool messing it all up. Excellent video as usual, very thorough and informative.

[@DuhBiggestDog]

Mr. Carlson you continue to amaze me. Every video you do is a work of art. I'm a gray beard from the vacuum tube days and your knowledge of and the clarity with which you describe these circuits are exceptional. Would love to have an eyeball QSO with you someday .73,DonK2PMC

[@MrCarlsonsLab]

+Don K2PMC Thanks for the kind words Don! Glad your enjoying the video's.

[@rlamb51]

Very interesting part number breakdown of the vacuum tubes. I worked with them for years but didn't know how the numbering worked. I knew the first numbers were filament voltages but the rest was just a designation. Goes to show you that you are never too old or experienced to learn something new.

[@MrCarlsonsLab]

Thanks for the comment Roger! Glad you found this interesting.

[@frankcollins5109]

Just wanted to comment on the clear and excellent presentation of all your videos. This one takes me back 50 years from when I left school and started work as a trainee in a radio and TV repair shop. It was all valve (vacuum tube) technology then so I can really understand and follow your projects. I remember building my own valve receivers and AM transmitters. Those were the days! 73's de G0RXO

[@MrCarlsonsLab]

Thanks for your comment Frank!

[@erichgugle5500]

Another great troubleshooting and alignment demonstration. Very nice find in the chassis mounted Variable Resistor with the steel wool hairs inside causing the plates of the 12AU7 to be incorrect. I am extremely impressed with your skills.

[@jenniferjohnson708]

Just got one of these two weeks ago. I got it up and running. I installed a three prong grounded plug and cleaned it up. The parts looked to be in good shape and it read voltage and resistance with good accuracy. Your video showed me that I have a lot left to do though. Thanks a million.

[@MrCarlsonsLab]

Glad the video helped Jennifer. I have to warn you though, the case of the VTVM was designed to float, and not be grounded. The negative lead from the VTVM is directly connected to the case. This may pose some issues with reading bias and Negative Voltages in some situations. Having the case earthed could make some measurements dangerous. This is the reason they used a 2 wire plug.

[@jenniferjohnson708]

Thanks for the advice. I will change out to a two prong plug. In what cases could it be dangerous?

[@jenniferjohnson708]

How is the case designed to float when the grid via pin 2 is grounded to the chassis and a red lead from the PT as well as the cathodes from the 12au7 both connect to the chassis via resistor R-23? Is it because the PT has no center tap to ground? I am concerned because of the obvious safety implications.

[@MrCarlsonsLab]

Hi Jennifer. When I say float, I mean the AC side. What you mention above is the circuitry inside the VTVM which is isolated from the mains by an internal transformer. The unit was issued with a 2 prong plug for a scenario like this: If you plug in a guitar amp with a 3 prong plug, and plug in your VTVM that now has a 3 prong plug as well, the common test cable of your VTVM is already attached to the chassis of the amplifier through the earth connection of both units. Both chassis will be connected together with that 3rd pin on the plug. Then if you attach the common of your VTVM somewhere in circuit, what ever (possibly isolated supply) you attach the common lead too will automatically be shorted to the chassis. Whether you choose to keep the 3 prong plug, or change it back to the original design (2 prong plug) is completely up to you, and **this is your responsibility and decision.** I strongly suggest you do some study on this, as this sort of situation could be dangerous. You need to realize that the chassis of the VTVM will be connected to what ever you have the common lead of the VTVM connect too. The case or chassis of the VTVM is metal and conductive so be very careful and understand what you are working with. These units are not all plastic like the newer DMM's and you can put yourself in a dangerous predicament. **If you use this advice, or choose not too, and you choose to use that VTVM (or any other piece of test gear), you are doing this all at your own risk!** Until you are fully familiar with the way this VTVM (or any other piece of test gear) works, I strongly suggest that you don't use it. Please be careful, and do some study first. There are also other dangerous scenarios mentioned in the manual, please see these as well.

[@jenniferjohnson708]

Thank you very much. I will avail myself of the manual and return it to a two prong configuration. Thank you for taking time to explain this to me. It is really appreciated.

[@davideppert6486]

Thanks for taking the time to go into so much detail in your videos. It really does help us beginners in the learning and after a while many of the points you discuss just start falling into perspective.

[@MrCarlsonsLab]

+David Eppert Glad to help Dave!

[@MrMoggyman]

Beautiful and useful piece of kit for RF sections. Had one in the radio workshop. Ours was Heathkit.

[@ppdan]

I am amazed how little editing there is on your videos. Takes some great skills to narrate like that. Very good job, keep on the good work!

[@MrCarlsonsLab]

ppdan Thanks for the nice comment!

[@82abn34]

Thanks, I didn't understand the bridge circuit function on my Heathkit V-6 meter. This meter restoration is my first journey into the world of vacuum tube technology. When you build something, you really find out where your knowledge gaps are. I've had this thing for a couple of years and it keeps getting pushed to the bottom of the pile but I should have it completed soon.

[@jamesmdeluca]

Greetings: My Heathkit V7A VTVW also uses the same tubes (6AL5/12AU7). The 6AL5 is wired as an AC voltage-doubler. The view at 15:20 of the 6AL5 in your schematic shows it as a similar voltage-doubler. I have never heard the doubler called a full-wave rectifier before. You are correct as usual.

[@dhansel4835]

WOW I built one of those decades ago and still have it. I don't use it but it brings back memories. I guess one day when I can get into that closet I will open the door and see what falls out. Thanks....

[@satellite_guy3768]

A really super duper video!! Presented this way and to this much detail is the way they should be done. A very excellent tutorial. Thanks so very much for the education and insight to an almost lost and forgotten art!

[@MrCarlsonsLab]

Thanks satellite_guy for the nice comment!

[@CH_Pechiar]

Jee, I now have to restore a VTVM I have in order to continue the alignment of the tube radio I mentioned on another of your videos! On thing leads to another. Thank you for your really helpful and clear videos. Charlie.

[@MrCarlsonsLab]

Charlie Pechiar LOL, I'm creating work for you Charlie. Glad your enjoying the videos!

[@kellingc]

I can't believe I watched that who;e thing. Great video. I want a VTVM, now.

[@laser31415]

twice

[@ele4853]

This one really made me learn a lot about these meters. Thank you so much for your great job at teaching! I tried to find one to buy with no luck...

[@MrCarlsonsLab]

+Really? Your welcome..... Really! :^)

[@harrystevens3885]

I wish I had discovered your videos a few years ago, for me the best KZbin channel ever, the wire wool strands found in the VR was a weird one, another enjoyable watch just brilliant in fact ....thank you!!

[@MrCarlsonsLab]

Thanks for the kind comment Harry!

[@PurityVendetta]

Great explaination of how the bridge in the meter. I have normally used AVO 8's as my analogue meter as they've served me well over the years. I can't actually remember how many I have but one, ex Royal Navy has the kit with the high voltage shunts. After watching this video I think I need to add something like this to my bench. Thanks again.

[@johntriplett848]

Your ability to blueprint electronic gear and show the process on video is motivating to the extreme. For example, you not only showed how to locate the zero adjust problem area, you showed how to fix the problem. You took the potentiometer apart, observed the steel whiskers that were causing the short, cleaned them out, and put the pot back together - problem solved. Thank you.

[@MrShobar]

I also find an analog meter easier to use, as opposed to a DVM, for the reasons you state. I find that VTVMs were just perfect for their intended purpose. They are also quite inexpensively had at hamfests, etc. Thanks again for an enjoyable video.

[@MrCarlsonsLab]

+MrShobar Glad you enjoyed!

[@brig.4398]

what a neat clean shop...this is someone who takes pride in their work.

[@MrCarlsonsLab]

Thanks Bri G!

[@podxtv]

Really enjoy your videos - anyone studying electronics repair should watch these

[@fredclark4033]

Great to see how accurate the old vom meters really are even today.

[@cmbower100]

All of your videos are great, I've learned more in a week from these than in some textbooks that took months to get through! However, this one is my favorite. I'm just starting out restoring "hollow state" radios and test gear. I just finished my first working piece, an Eico 249 VTVM. After seeing your video, I'll be going back over my work to correct several minor problems that I've had with it. Thanks so much for taking the time to share your expertise!

[@MrCarlsonsLab]

Chris Bower Glad to help Chris! That Eico 249 has a huge meter, and looks great. Keep'em glowing! Thanks for your comment!

[@jackiefarmer4892]

What a pleasure to hear the word "SOLDER" pronounced without the silent "L" !

[@danmackintosh6325]

Indeed, it's nice to hear someone so knowledgable and whom doesn't partake of "soldermy" haha

[@budboggy4699]

I think it's a Canadian thing, eh? It causes him to mispronounce Farad as FAYrad, too. The guy's name was Faraday: Fare-aday, not Fay-raday...wonder how he pronounces "soldier"? Not a bit of that is an issue here though. His amazing teaching skills, along with an extraordinarily extensive understanding of broad and diverse technologies trump his localized pronunciation idiosyncrasies.

[@petenikolic5244]

@Ken Mason soLder :-) ..

[@DaleDix]

Amen

[@oldbatwit5102]

Solder doesn't have a silent L.

[@xenia5101]

Great stuff. 55 years ago I was using a similar VTVM (RCA) as a high school student to troubleshoot radio and tv gremlins.

[@chrismosley409]

being just a beginner, I'm way out of my league watching these videos but I'm still amazed at the tiny little things that can go wrong like that fine strand of wire in the v.r. and the static on the plastic cover or the glue going conductive on that old radio. nice job.

[@MrCarlsonsLab]

+Chris Mosley Glad your enjoying Chris!

[@clytle374]

Another great video, with great advice. I found one a while back in an antique store and bought it. Just got done getting it working, building a probe for it now. I think it had the original dry cell in it. Corrosion was half an inch deep, but still had 1.49 volts. Only question i had was finding coax for a probe lead. Hard to find anything with a rating that really gave me a good feeling about holding it in my hand and probing HV with it. Thanks again.

[@wa4aos]

I always enjoy your well done and detailed videos. My parents bought me an RCA Voltohmist for my 8th Christmas present. I still use it 52 years later. I also have a nice hP 410C VTVM and would not consider doing alignments without an analog meter. My best friend's father repaired TV's and radios from his home workshop. This is when my interest in electronics started. I asked him what would be the best piece of test gear to start with and he suggested the VTVM.. That led to my interest in Ham radio and later engineering courses in college.. vy 73, Glenn WA4AOS

[@MrCarlsonsLab]

+wa4aos Thanks for the interesting write Glenn!

[@saturn5tony]

Even though I have had the radio shack VTvm for years, this review and callibration of the knight was very close and helped me understand it's operation beter. Thanks

[@MrCarlsonsLab]

saturn5tony No problem. Thanks for the comment!

[@tomwoehle3519]

Much appreciated your educating style. Will continue my schooling during retirement.

[@steeljan]

Very well done. I know how hard it is to fiddle with the camera while making a "teaching" video like this. Thank you for taking the time and effort. Great work on the Knight too, good job.

[@MrCarlsonsLab]

steeljan Thanks for the nice comment steeljan!

[@Daveyk021]

Picked a KG-625 VTVM up on Ebay last week. No issues balancing it at all. When I took it apart, I found the negative lead of C6 was making contact, but not at all soldered! Its ESR was 3.0 Ohms, or slightly greater. I replaced it with a 20uF 200v (Sprague 500D206F250FJ7 ) from my old stock that still checked ~24uf with a ESR of about 1.5 ohms and it is working fine. I am really happy with this old meter and will make a great addition to my antique work bench that I am building. I'll stick with my Fluke 87 for precision resistance measurement though 8-)

[@Joejoe-eo6yz]

I really enjoyed this video. You did an excellent job describing how the circuit works. I really learn a lot more when someone explains how the circuit works then goes into the circuit and shows how theory is applied in that circuit. I am looking forward to more videos like this one. Thanks for the fine video.

[@MrCarlsonsLab]

Glad you enjoyed the video Joe! I'm sure there will be many more video's like this. Thanks for the comment too!

[@bobkins270]

I recently have purchased a Knight VTVM. The upright model. It has the PCB in it. And basically I,m having the same problems that you where having. right down to the funky tube. So I've got a new tube ordered for it. For the most part it is spot on,after I changed all caps and several resistors.The tubes will be coming from a supplier the will make sure that the tube coming to me has balanced triode. Appreciate the videos Brad

[@MrCarlsonsLab]

Great Brad! Would be great to hear how that turns out. Keep me posted.

[@bobkins270]

Yeah, It was the 12au7. It was shorted on one side. So I ordered two new tubes for the Knight.

[@MrCarlsonsLab]

Great! Now you get to calibrate it :^)

[@mitchlee3008]

Your information about the ground on the input jack was dead on for the Knight KG-625 meter I just received. A simple ohms check revealed no connection between the ground and the tube support bracket. Gonna give this meter the once over and try to find the missing bracket and probe, or make them. Thank you for what you do Sir, from a fellow Canadian born man who loves in Florida now. Cheers!