#218

In this video we restore a HP 200AB audio oscillator from 1952.
Virtual museum:
www.hp.com/hpinfo/abouthp/hist...
Support us on Patreon;
/ theradioshop
Contact Information;
gokarters.com/smf/index.php?to...
Discuss this on my website;
www.gokarters.com/smf

[@hectorpascal]

That's REAL HP equipment! I'm not at all surprised that even at 67 years old, after just a little bit of TLC, it will still work exactly the way it was designed to. Their innovative use of a "pilot light" as a thermistor in a negative feedback circuit stabilises the oscillator amplitude, and minimises distortion - and this almost certainly helped the 200 series to become a great financial success for the company.

[@TheRadioShop]

You are correct Hector. I only clanked and thought fuse lamp. Was not until I dug deeper to see what was really happening. Thing works pretty well. I can tell the lamp is there because you can see the signal float as you turn the dial. Once stops it gets stable quickly. Thanks for the information.

[@mikehuedepohl7927]

Another great video Buddy I recently bought one for doing a recalibration and alignment on my on my Swan 700CX. Unfortunately I bought it off of eBay the add read previously owned what it didn’t' say was it wasn't working at all after replacing the caps and checking all resistors and 3 of the 6 tubes there was still no tones. I have watched the video 218 about 10 times from start to finish and was about to put it on the shelf as a not working piece. then I was looking at this video and checking out the the C-8 cap that is shown in your video Bigger than life right in front of my face the wires were in the wrong position that was view 11 and low and behold the audio oscillator came to life. Thanks for everything you do to bring quality videos to KZbin.

[@curtvincent3728]

Well, I am sorry you didn't get the job. I have an electrical engineering degree, but I don't have the expertise, talent or skills you have demonstrated in these videos. Thank you for sharing them! I am learning a great deal from you!

[@lylewatts6370]

I like the old test equipment that's easy to work on as long as it's accurate. Please keep up the good work and glad to hear things at work are slowing down for you.

[@TheRadioShop]

I agree Lyle, HP made some pretty good old test gear back then. This is a old 50's unit and is in pretty good shape. Thanks for watching my friend.

[@33cdh]

Thanks, Buddy, for another great video. I have had this same generator for about 20 years and it is probably about time that I did the same restoration. I appreciate that you went thru it first. It looks to be a very straight forward job. Thanks again for your excellent work! 73 & all the best. Chris, N9WHH.

[@TheRadioShop]

It is a pretty simple design which makes it easy to repair. Lots of room and no hard to get to stuff. The manual is not bad either and shows how to set it up for different impedance when needed. Thank you for your nice comment Chris.

[@IrishvintageTVRadio]

I just tidied up a 202C last week. Came out of a TV repair shop in Athlone. They are a lovely bit of kit, that variable capacitor is a work of art!

[@ianbutler1983]

That method of installing a tube socket and mounting the caps on a plug is an elegant solution that I never thought of using. Thanks.

[@radio-ged4626]

What a great job. Loved the idea of using a tube base for the filter capacitors, that looks really neat.

[@TheRadioShop]

Glad you enjoyed it

[@voltagefreak6350]

I love vintage radios and the test equipment to compliment them. Good show. Would like to see more of these.

[@scottlee1211]

Another fine restoration by Buddy! Great job!

[@TheRadioShop]

Thank you very much Scott

[@alanpecherer5705]

Nice work! I've probably repaired 2 dozen older pieces of hp test gear in my career, and 98% of the time, you just replace the power supply caps and all the bumblebee and black beauty caps and the thing will fire right up.

[@mr1enrollment]

Looks like it works correctly. Note: when changing the frequency you see a transient change in the amplitude, not a distortion. The gain of the system is being changed, as frequency is altered by the action of the Lamp. The lamp is not a protection circuit, but rather a clever way of causing the oscillator to have a loop gain of 1 exactly. The tungsten filament of the lamp is in a feed back loop with a bridge circuit. The time it takes the filament to manage the circuit gain when frequency is changed is seen as that slow amplitude change of the envelope as it eventually gets back to the amplitude set point. This was an HP innovation.

[@James_Bowie]

Glad your working situation still suits you fine. Looking forward to many more videos as you get the chance.

[@TheRadioShop]

Thanks, I am happy to get back to normal. Many more videos to come now.

[@majordisappointment8692]

Thanks nice job love the brown wrinkle finish and that tube plug cap idea is fantastic and clean and neat.

[@TheRadioShop]

Thank you. Yep paint turned out real nice. Got that tip a while back from D-lab on the caps.

[@dcdortho]

Beautiful restoration of that quality equipment. I enjoy your videos even though I basically know nothing about electronics.

[@TheRadioShop]

Thank you, very glad you watch them. Electronic repair is very soothing to me.

[@randallissimo]

Great in depth video. I restored my 200 CD some years ago, tho I didn't go to the lengths you did. My fine adjustment assy was frozen. Once I got it freed up, I was amazed that the spring loaded bakelite knob was machined perfectly to engage and fine tune the back of the smooth metal dial with no other gripping material, rubber, cork etc. That's when things were built to last! My current issue is the sealed 2 gang output pot is crappy and intermittant. I'd like to get that fixed someday if I can find a suitable replacement. Probably should re-cap it as well, but it is still working great.

[@bblod4896]

Beautiful restore work Buddy. I've learned much from you and I thank you. HP quality shows through the entire generator. Glad the job situation is working out. Hopefully, you will have more time to "play" in The Radio Shop.

[@TheRadioShop]

Thanks B. All well and ends well.

[@old64goat]

That is a great audio generator, you did a great job on it Buddy

[@PeopleAlreadyDidThis]

Enjoyed it, Buddy. Glad you’re happy at work. I hope your “old” job remains secure as the new people come in. I’d hate to see you shortchanged there. Looking forward to more frequent videos. I like your practical approach. I just bought an Apache/Mohawk pair that I think are unmolested originals. I’ve already reviewed your related videos. Thanks for all you do.

[@TheRadioShop]

It is pretty secure, but with the ways things are today you never know. This company has been in business since 1974 and is now global. They also have a mega warehouse in China making similar product. Yes more time for videos coming soon. Great to hear you picked up those gems. Should make a nice pair. Thanks for sharing.

[@dl7majstefan753]

Glad to see that your situation at business went into the right direction - yes the world is crazy today in many respects. And I´m happy to see your new videos!

[@TheRadioShop]

I think it was for the best. More time for me. Thanks for watching.

[@sparksw9dkb187]

Well done, looks and works great. I had one for years. But, too large for my workbench so went with a smaller solid state unit. 73.

[@TheRadioShop]

Yep works a treat now. And it is big, bit will be fine in the other bench. Thanks for watching

[@cruxinterfaces]

Nice work, we will be at Knowledge Fest Thank you for the content

[@SoundsLegit71]

The HP signal generators are great devices. I have both the new and old chassis. I really enjoy showing young people the radio waves are very high frequency sound you can't hear. Except for my dad he can't hear above 5,500hz which is apparently the frequency my mom talks at.

[@TheRadioShop]

Cool deal. I don't think I can hear that high myself. Deaf in one ear.

[@SoundsLegit71]

@@TheRadioShop I actually have the hp signal generators that go too 600,000 Hertz they look nearly the same. The RF really starts picking up at about 28,000hz.

[@pierrelataillade7564]

Well Buddy ....a very nice vid. as usual we learned a lot watching you. thanks for the show.

[@TheRadioShop]

Thanks for stopping by Pierre.

[@billpowell5931]

A new video from the Radio Shop. Cool! As usual, your approach to getting this unit back on-line is super. I am wrestling with an EICO audio generator and can't seem to get good looking signals. Next stop is to replace a few resistors in my unit. I also noticed an HP 200AB on eBay right now and have placed a bid. Here's hoping. Thank you for sharing your systematic process for restoration.

[@TheRadioShop]

Thanks Bill, which model generator you have? The 377 was a decent one. I would take a look at those resistors. Good luck on your ebay bid

[@billpowell5931]

@@TheRadioShop It is a 377. After the capacitor replacements some of the signals look a little weird on the oscilloscope. Thank you for taking the time to respond. You surely get a lot of posts with requests. Looking forward to your next video.

[@billpowell5931]

Update: I won the auction for the HP200AB and look forward to closely following your process for getting it operating the best I can. I really appreciate your taking the time to provide guidance for us noobies. Thanks again.

[@johnbellas490]

Hello Buddy ! That project turned out to be a nice older HP audio oscillator!! The older HP stuff was built like a tank physically, and they were designed electrically really well too!!

[@TheRadioShop]

Hi John. Yes HP gear from this era was built pretty well. This one should run for many more years now. Thanks

[@geckoproductions4128]

Very clever cap substitution, nice video

[@TheRadioShop]

Thanks!

[@chopper680]

Ahh the 200ab brings back memories. I used to work with the 200cd mostly. Both are good work horses and quick and easy to calibrate or repair

[@TheRadioShop]

Thanks for sharing that

[@VintageElectronicsGeek]

I'm with you, don't like top handles and for that very same reason. You do an amazing job and are a true craftsman! This device turned out real nice. Bummer about the job but also understand about more free personal time! Will contact you sometime in Sept in regards in shipping my next item. 73's my friend! ~Jack, VEG

[@aaronson2540]

Nice work Buddy wish I was that talented, thanks for the warning about the volume in the headphones!! 😀

[@TheRadioShop]

Not sure if it is a talent or curse lol. Thanks for watching Aaron.

[@daneguitarist1]

This answered so many questions for me, i just purchased my first test equipment ascillator, i assume its going to be relatively dirty and need a little bit of work

[@James_Bowie]

Nice job. 👍 HP quality shines through again. I believe the 200AB is dates from 1952. I have an HP 202C (from c1959) in my queue. I have already replaced the leather handle. I take your point about stack-ability of instruments but I don't like to stack anything on top of tube equipment because of the heat.

[@TheRadioShop]

Thanks, I understand what you mean about the handle. You do not want to stop air flow that is for sure. Good luck on the 202C

[@cuban9splat]

Another excellent video Buddy. Good job in the old HP osc. I like your tip with the "sawed-off toothbrush" to scrub the really bad smudges. Thanks for sharing it with us. 73 DE K7RMJ Frank

[@TheRadioShop]

Good to see you Frank. Hope all is well. Thanks for stopping in

[@jamessutherland2011]

From Corning NY, “you’re amazing!”

[@TheRadioShop]

Thanks for the very kind words my friend.

[@N5KDA]

Great job Buddy. When you replace those banana jacks it will look almost new. You train the new boss and all will go well.

[@TheRadioShop]

Thanks, going to order up some high quality jacks for it.

[@1903A3shooter]

GREAT JOB, looks brand new. thanks. W4GSM

[@mikehuedepohl7927]

another great video Buddy

[@mikehuedepohl7927]

Just bought one and will be restoring it for my bench

[@jake2213b]

I feel for you because the same thing happen to me at work. After the new guy got the job I was told to do his work for him to make him look good. I think you know my answer. I made his boss do his work. Any way glad to see you doing what you like to do.

[@TheRadioShop]

Thanks for sharing that. Really getting crazy these days.

[@larrysherwood9]

Sorry you didn't get the promotion Buddy, Glad we'll be seeing more of you on here though! Great restoration video!

[@TheRadioShop]

All good with it :) Yes more time to make videos for you guys. Thanks Larry

[@dayleedwards3521]

The lamp in the cathode of the first vacuum tube is to regulate the amplitude of the oscillator. As the amplitude increases, the filament resistance increases as it begins to heat, reducing tube gain, and keeping the amplitude reasonably constant.

[@mcanduiken2224]

Pretty amazing to think that negative feedback was still a new concept and under research in the 1930s, and Bill Hewlett had the brilliant (pun intended) idea to use a light bulb to stabilize the amplitude.

[@chessapk5034]

Nice work Buddy. It was my impression that the last 20uf you replaced was meant to be a non-polar cap. A few HP 200cd that I repaired in the past I used a non-polar cap in that part of the circuit. I guess I could have gotten away with a regular electrolytic.

[@TheRadioShop]

Yes the way the circuit is you could use either. Since the original was an electrolytic that is what I went for. Thanks for the comment/

[@alpcns]

Good to see you back, Buddy! Happy to hear you're happy with the work situation, and looking very much forward to new episodes and electronic adventures. I was getting withdrawal symptoms already! Tried to email you (both addresses) but I think your email is swamped, or I inadvertently ended up in the bit bucket. I had some questions about acquiring equipment. Have a great day and take care!

[@TheRadioShop]

Thanks, Let me dig through about 4 thousand emails and see if I can find you.

[@alpcns]

@@TheRadioShop : sorry to hear that, Buddy. You can narrow it down perhaps by looking at emails starting July 22 and August 22. The (my) first name in the email header is "Andre". That makes it hopefully a lot easier. Thank you again for your excellent videos and work.

[@paulbione5267]

Nice one👍

[@mikeadams3977]

Sorry you didn’t get the job Buddy but it might be for the best. I had a similar situation happen toward the end of my working career (I’m retired now) but in my case I got the job. It turned out to be the worst experience of my entire 29 years with that particular company. Looking back now I wish I hadn’t gotten the damn job. But enough about that. I want you to know I really enjoy your vids and admire your skill and pride of work. I’m looking forward to seeing what’s ahead now that you’ll have more time to entertain and educate us. Keep ‘em coming!

[@TheRadioShop]

Yep probably better I did not get the job. Way too much head aches anyway. Sorry to hear you went through that. Thank you very much for the kind words Mike.

[@Cavemannspace]

Sounds like the job situation worked out for the best for you. Glad you are happy with it. I just hope you don't wind up doing your new bosses job for him, but I don't think that will happen.

[@TheRadioShop]

It is what it is, their problem now. Just hate it I had to spend 8 months holding things together. I am glad it is about done with so I can get back to normal life. Thanks

[@douro20]

The layout is certainly cleaner than in my Eico 377. The 377 uses a 6SJ7/6SK7 as the oscillator. Interesting that it has a push-pull output.

[@TheRadioShop]

Bit of difference in the two that is for sure. Although the 377 is a decent unit. Had one and gave it away years ago. The older 200's had a 6SN7 and 6K6 for the oscillator but was later changed to 6AU6 and 6Aq5. Agreed, was surprised to see that it had a push pull output. Thanks for stopping in.

[@PapasDino]

Great job Buddy! That HP dial "sound" as it turns is to typical, sounds just like the dial on the HP distortion analyzer I picked up lately. 73 - Dino KL0S

[@TheRadioShop]

Thanks Dino, Yes have to love the sound of the old HP dials. I think they use the same tuning system in all their gear back then. 73

[@dougmcartin3881]

Good for another 65 years Buddy. Well done and still a very useful piece of gear today. I wonder if you put the old can caps on the leakage tester. I would be curious as to just how good they were. 73 Doug.

[@TheRadioShop]

Thanks Doug, The caps are still on the bench so will do that in a future video. Well built piece of old gear.

[@Masterkill45588]

I have the same oscillator great video

[@OC35]

If you look at en.m.wikipedia.org/wiki/Wien_bridge_oscillator you'll see that the bulb is used to stabilise the amplitude. It takes a bit of time to work, hence the amplitude variations when changing frequency but it soon stabilises.

[@TheRadioShop]

Thanks for the link. Good old 1950's technology.

[@Goforitmanager]

Hi Sir, thank you for all your great videos really enjoy watching them. Couldn’t help but notice the low frequencies amplitude modulation at around 34m 7s mark. I recently restored an Eico 377 tube audio generator and suffers from the same amplitude modulation up until 180Hz and after that the amplitude is nice and stable. It is worst at just above and below 60Hz but very stable right at 60Hz. Do you happen to know if this is normal or what causes it? Also, glad to hear your job situation will get better and hope you don’t stay up until 2am finishing up a video. My hat off to you, you’re a hard working man. Sorry for the long post.

[@TheRadioShop]

Normal in this unit. It is because of the Lamp that is used as it stabilizes the amplitude. It does seem to get better after she has been on for a spell. As far as the job, well closer to retirement now, then more time in the shop :) Thank you for the comment my friend

[@shartne]

I guess you would need a frequency counter to use this signal generator to set it to 27.185 for example. To see where your at exactly. I have a frequency counter that I need to fix its a BK I have never opened it. Was wondering what it needs capacitors probably. Thanks for this interesting teaching video.

[@TheRadioShop]

This unit is an audio generator. It is below 1 megahertz. Will not cover HF (cb) frequencies. The B&K counters are fairly easy to repair. Thanks for watching.

[@1903A3shooter]

Hey Buddy, as always very nice, W4GSM Dave.

[@TheRadioShop]

Much appreciated Dave

[@OrbiterElectronics]

Great job Buddy.. Hey.. Looks like you could do with a Sonic Scrubber like mine 👍 LOL 😃 Cheers friend John

[@TheRadioShop]

Thanks John. Yes I would like to have one of those, sure beats the old way.

[@docholliday3150]

Great repair and restoration Buddy. How flat is the signal amplitude across the ranges?

[@TheRadioShop]

It is pretty flat, surprised me. However the amplitude does drop a bit at higher frequencies. Thanks

[@TheDefpom]

@24:00 nice solution for the cap.

[@TheRadioShop]

Thank you Scott. That is sort of a D-Lab feature.

[@peterharband326]

Nice old unit. lots of room under the hood.

[@TheRadioShop]

Lots of room for mods to add. Thanks for the comment Peter

[@andrewbarnett84]

Nice repair there Buddy, do you recommend spraying the dial with any coating to preserve it or will the ali dial be OK as it is?

[@TheRadioShop]

Thank you. It depends on the environment it is used in. Since it is aluminum it is subject to a surface corrosion. Mag wheel polish with keep it shiny for many years to come.

[@garybalanesi610]

I've got one of those, from my father in law.

[@johnwelcher2305]

Great job on the restoration The old HP audio generator looks Great !! Keep the video comming Tks KM4KYA

[@TheRadioShop]

Thanks John

[@moodyga40]

good unit i owned one

[@IamScarhead]

Outsourcing - I can tell you some stories. 5.5 years in college and university, only to schlep bananas and cabbage at Walmart. Who says life is fair.

[@TheRadioShop]

My son did the same. He studied for IT tech and cuts meat in a meat market.

[@GrahamNye]

You mention at 6:58 that the lamp is there to protect the oscillator. As Hector Pascal mentioned, the lamp acts as a thermistor to stablize the amplitude of the signal. It adjusts the gain of the oscillator so it operates linearly without clipping. This gives a very low distortion signal (much lower than you'll get from the function generator visible in the background). There's more information about this design on Wikipedia: en.wikipedia.org/wiki/Wien_bridge_oscillator

[@TheRadioShop]

Yes that is correct. When I first looked at it I though lamp fuse. But after getting more into it I saw the feedback circuit.

[@Joe-KN4IFI]

Nice Resto Buddy. Great Video as always. 73 KN4IFI

[@TheRadioShop]

Thanks Joseph

[@Lee-NN7X]

Hope that's on a future give away.....hint...hint...

[@TheRadioShop]

He HE, well we will have to see :)

[@deadlinkofficialartst5592]

i wood make that into music

[@laceradewilde4056]

Great video. Would like to have watched you do a calibration to round out the restoration. I have one with three of the rotating plates on the main tuning capacitor slightly bent, throwing off the outputs. Looks like someone reached in and squeezed the end plates together on two sections. Is there a way to accurately readjust the plate seperation? After 50+ years in the work force with no college, you tend to see the good, bad and the ugly side of management. Glad to see you survived the ordeal and happy with the outcome.

[@TheRadioShop]

Since there is no tuning control for some of the band settings it is common practice to bend the plates on the capacitor. That could have been some ones attempt to get the dial scale to line up with the frequency. Before you try and straighten then go ahead with the restoration if you have not already. Then when aligning tweak the plates so the dial aligns with the frequency at both ends. I did not align mine since it was so close after the rebuild.

[@laceradewilde4056]

@@TheRadioShop Now I would not have thought of someone purposely bending the plates to achieve alignment. It does make since and could explain why they are. I can align one end correctly but when I set the other end the first end becomes off. I've lost linearity. I have been able to adjust the plates somewhat to reduce the differences compared to what I had originally. The plates were rubbing so hard they broke the flexable ceramic coupling between the turning capacitor and dial assembly. I repaired all that so all's left is to replace the caps and bad resistors then we will see