Restoring an HP-120B Oscilloscope from 1961!

I have a soft spot for old test equipment, particularly old HP test equipment. My HP-150A is my all-time favorite oscilloscope. It’s rock solid, looks gorgeous, and is stunningly good. It also weighs about nine million pounds (that’s roughly four million kilograms). I was lamenting that I needed a front-end loader to pick my 150A up to a good buddy of mine, Chris Fala, and he said “I’ve got an HP tube scope that won’t send you to the doctor for back pain.” Chris is an absolute legend and hooked me up with said scope, which is the HP-120B you see here today. It’s not currently working though, so let’s dive into it. Should be easy, right? Right?
If you want to support the channel please hop over to Patreon:
/ usagielectric
Also, we now have some epic shirts for sale!
my-store-11554688.creator-spr...
Come join us on Discord!
Discord: / discord
Intro Music adapted from:
Artist: The Runaway Five
Title: The Shinra Shuffle
ocremix.org/remix/OCR01847
Thanks for watching!
Chapters
0:00 A bit of reorganization
2:34 Scopes!
4:43 Testing the test equipment
5:40 Throwing electrons at the 120B
7:25 Deep cleaning time
12:43 6U8 vs. 6GH8
14:05 Testing after cleaning
15:21 So close, let’s dig in deeper and find out what’s wrong
17:38 Thor: “Is it though?”
19:35 I fought the HP and the HP won…
21:56 Battling someone else’s modifications
24:11 Please work correctly, for my sanity’s sake
25:48 Nailed it!
26:48 What am I going to do with another oscilloscope?
28:46 Derpy cat is derpy

[@chrisjpf33]

So cool to see this on your channel already! I am thrilled that it now has a much better home that I was able to provide. ;-) I had this since the mid 1990s. I used it maybe twice in all that time. It was given to me by a friend and coworker, Brad Forrester (sadly I lost touch with him since then), when I worked at VDO Yazaki in Winchester, VA. Before that, I don’t know the history. Brad, if you catch this, please tell us what you know.

[@UsagiElectric]

Thanks for hooking me up with the scope! I'm really curious how Brad had it set up with the modifications. They were clearly meant for a specific purpose, and may have been totally fine for that particular job!

[@rfburns5601]

@@UsagiElectric I had one of these when I was a student in trade school in the late 70s. It was an H41-120B Cardiac monitor that came from the charity hospital. It had a bad 12AX7 and a 5 milliamp fuse. 5 ma fuses were used for patient protection into the vertical input. It also had the orange long persistence P7 phosphor. You can connect your chest to it and view your heart waveform only so many times before the novelty wears off. I had thought about building a slow-scan receiver out of it but never did. In the 80s I sold it to a man who had heart trouble so he could make sure his heart was OK.

[@MrCHrisfj]

Maybe the long lost label that was just to the side of the screen was telling the operator to be careful of the modification?

[@chrisjpf33]

@@UsagiElectricjust guessing, but I would bet that the modifications pre-dated Brad's ownership of this scope.

[@KeritechElectronics]

Remember, Dave: There's NEVER such a thing as "too many scopes". Period. It's a thing of beauty and a joy for ever - and your kitty fully agrees.

[@UsagiElectric]

Haha, I mean, I agree, but they sure do take up a lot of space. Maybe, never such a thing as "too many HP scopes"!

[@stamasd8500]

Agreed. I only have 3 of them and feel naked. I need more. Especially since my oldest one is from the 1970s and transistor-only (Hitachi V660). :)

[@KeritechElectronics]

@@stamasd8500 ha! I need a tube Tek on a scopemobile. And that won't be the end of it... if I Keri on, I'll end up like Paul Carlson!

[@JamesHalfHorse]

@@UsagiElectric Collecting test gear can get addictive. You have seen Curious Marc's place...

[@WagonLoads]

@@KeritechElectronics I have a scope mobile for a 585 and one for a 475 scope.. The wheels on the 585 cart went flat on one side from sitting too long in one place. There was a metal piece near the base of the 475 cart that snapped.. Tektronix made some wonderful items... I think the 475 also had a power supply problem. I thought it might have been the transformer, but I never really tested it for sure. My latest scope is a Tek 7904 mainframe scope.. I love scopes that have plugin modules to add extra features.. I really hope to get a spectrum analyzer for it..

[@prairiedawg792]

10:43 Use some soapy solution with the razor! Then the blade slides over the paint and it softens the residue too making it much more effective.

[@georgegonzalez2476]

They may have had to cut some corners on the sweep and sync circuitry. Maybe Tektronix had some patents on really good sync circuits? In any case the 120 has, IMHO, a barely passable sync ability. You may notice that at some input frequencies there is some sync jittering going on. That's probably because the next cycle arrives just before the sync circuitry has fully settled down after the retrace. Also realize that all the neon bulbs used as level-shifters, their Krypton-85 gas has long worn out. So the bulbs are going have an unstable trigger voltage. You may want to replace the bulbs with fresh new ones. There are two in the CAL square-wave generator, those are probably jittery too.

[@dell177]

When i was in high school I built an EICo 4.5mhz scope with a 3" CRT, that worked well for about 8 years but it really ran to hot and eventually the transformer cooked itself. After getting out of the army in 1970 I was working for Analog Devices as an engineering tech. when they were on Binney St near Kendal Sq. Sometimes on our lunch breaks a fiw of us tesks would trek down to Eli Hefferons in Summerville, ELi's was an old style surplus dealer who sold everything and anythisng from parts, electronic assemblies. and old test equipment. I ended up buying a HP120 and a HP 200 oscillator, both were rack mount inits and i hot them cheap because they didn't quite work right. The scope just needed a good cleaning and a couple of leaky caps replaced. The 200cd had a pair of tunes that were substitutes and looking at the tube manual the pinouts were the same but Rp (plate resistance) was off by 4:1. I bought a pair of the right tubes (6F6) and that fixed that. I also added a attenuation switch to the generator out put so I could attenuate the output in decade steps (copied from a PACO generator schemamatic) so now the generator worked fine for audio work. I still have the HP generator but sold the HP 120B to a coworker a few years later when I bought a Tek 545a scope which did a pretty good job of heating my shop. Twelve years later the Tek was getting iffy so bought a Hitachi 35 mhz scope which I still have along with a Teck 2230 analog storage scope. Both HP and Tek made some impressive instruments back in the '60's solid as a rock.

[@ctbcubed]

Way back in 1974, a friend and I came across some used HP120B scopes at a local surplus store. We each bought one and upon firing them up at home, neither one worked. These must have been used in a factory where there was oil mist as the internals were coated with oil. I took mine outside and used a spray bottle filled with denatured alcohol to thoroughly clean the insides. It looked almost brand new when I finished, but it still didn't work. I purchased the excellent service manual and after some head scratching discovered that the neon bulb in the sweep generator was bad. I replaced it and the scope came to life. I told my friend and he found the exact same problem with his unit! I calibrated it and it was a great for audio work. I recently gave it away and it was still working perfectly with all of the original components (except the neon bulb) from the day it was built. As far as having too many scopes, the benchmark for that is KZbinr Mr. Carlson's Lab!

[@Choober65]

That large scope is simply BEAUTIFUL.

[@jordanzish]

There's a video of him cleaning/restoring it. I think 2 or 3 years old. It really is a thing of beauty.

[@simonkormendy849]

7:15 That's actually a very good sign, the fact that you're able to get a stable trace on the CRT proves that pretty much all the internal circuitry does indeed appear to be working.

[@rsbohn]

The 120B is a beautiful device. You did a great job cleaning and restoring it.

[@atkelar]

When you showed the "pin compatible layouts", I'd like to point out that some pins in one of them are shown as "not connected" whereas the ohter one has them connected. I've seen devices that use "not connected" pins as solder points on the sockets, so be careful when replacing one with the other, it might lead to magic smoke incidents; Of course that also depends on the device and soldering behind it.

[@UsagiElectric]

That is definitely something to keep an eye out for! Though, in this case, I think the 6GH8 is just drawn in a confusing way. Near as I can tell, the 6GH8 and 6U8 are actually 100% pin compatible. But for something like the 6CB6 and the 6AU6 there are subtle differences that don't make the swappable in all cases. Either way, I'm happy to have an original 6U8 back in place!

[@atkelar]

@@UsagiElectric sorry if it came across wrong; I was not implying that you in your case had an issue with going back to the original one; The comment was intented as a bit of suplement for interested watchers who might swap out tubes based on that information. I'm far from being a tube expert myself, but after a few successful restorations and troubleshootings, this is one of the more surprising things I learned and it struck me as unusual enough to point out. I've seen both cases: unused pins for solder points and internal connections that save external wires; a quick check at the socket should be done when changing them from away from factory specs at least. And given the mods that were in your scope, I'd also double check there 😊

[@Runco990]

My Jr. High School Electric shop teacher gave me an HP 122A scope in 1978. That was the most monumental gift I ever received, and it truly kicked me off into a lifetime electronic career. ( He knew that, I think ) I always love that graceful bezel on those scopes. You should look up an inside pic of the 122A, it's point to point wiring and an absolute work of ART!

[@ttyR265]

Similar story for me. High school electronics teacher sold me a Tektronix 514AD scope. I still have it, thought it didn't fair well sitting in storage for a few decades. I had some ham radio guys go through it for me and it works perfectly again. I never got into electronics heavily, but I've been an enthusiast ever since.

[@kennethdegruchy5503]

My High school math teacher sensed my keen interest in electronics from speaking with my electronics shop teacher and one day summoned me to the teachers room where he bequeathed me with a Waterman Pocket Scope (s10A) which would actually display video signals acceptably. My first serious piece of test equipment and truly compact for it's time. A life time electronics career did follow.

[@herbertsusmann986]

That 100 Ohm grid resistor is known as a "grid stopper". It prevents high frequency oscillations. Same trick is used on solid state circuits on either the base or collector terminal of bipolar transistors.

[@trevorvanbremen4718]

And on the gates of power MOSFETs too????

[@jdolecki100]

It’s simple you need to make that room bigger.

[@MonkeyUnit]

I'm a collector of vintage HP gear. I have many of the items you are looking for to include an HP 200CDR and HP 3440a. HMU! BTW, this was my favorite episode so far.

[@MonkeyUnit]

Fudge, my 200CDR (rack mount version of your bench top oscillator) does not have the integrated rack handles. But the 3440A does.

[@MonkeyUnit]

It appears my comment has gotten lost in the stack. I didn't want to leave my email dangling in the wind on my channel for too long so I've removed it. If you're still interested in the HP 3440A, as a donation to the channel, let me know here.

[@roboticus3647]

The mod you removed looks like an external sync mod, as many people find external sync to be more useful than the z channel.

[@gcewing]

There's already an EXT SYNC socket on the front, though...

[@roboticus3647]

Ah, I didn't see that. Even so, that's what the circuit seems to look like. ;^) @@gcewing

[@gcewing]

@@roboticus3647 Yeah, that was my first thought as well. It's very strange. Maybe the built-in ext trig function didn't do quite what they wanted?

[@sebastian19745]

When I was young and scopes were way too expensives, I wanted so badly to have one. My projects were mostly in AF range or CMOS up to 1MHz so any scope would fit the need. Now, I have around 7 working scopes, from digital to analog with transistors, from handheld to bench and I found that I use each and every one. I recently found a pair of old 20MHz dual channel analog scopes that were broken and I just fixed one so I share your excitement. The other one needs some more troubleshooting but Christmas hollydays are coming and I plan to fix it then.

[@jeremiefaucher-goulet3365]

Dave: Complains his oscilloscope is too heavy Dave: Gets and painstakingly repairs a smaller oscilloscope Also Dave: Plans to put the new oscilloscope in a rack to make it even heavier than the first. ;) (yes yes, i know there will be wheels)

[@UsagiElectric]

I mean, you're not wrong!

[@SeekingTheLoveThatGodMeans7648]

Well, I was wondering why not put the "heavy scope" on a cart? But again that might not help when you want the scope to be on the benchtop. Maybe a cart that has a sufficiently sturdy projecting top shelf that will slip and/or pivot right over the benchtop?

[@WagonLoads]

hmmm, moving heavy lab equipment around effortlessly always sends me back to my happy place when I saw a movie featuring a hand powered micro forklift made from a movie set camera dolly.. It was used to lift a miniaturized submarine and precisely lower it into an oversized hypodermic, which they also shrunk... I rewatched that movie a year ago, and as cool as it was.. There were so many technical errors(design flaws)... Well... 10000 points to anyone who can guess what the movie was...

[@chrisg6597]

@@WagonLoads Fantastic Voyage (1966). I remember Raquel Welch and Donald Pleasence as part of the cast, but I can't remember the others.

[@WagonLoads]

@@chrisg6597 You get 10000 points. That movie is on KZbin..

[@ashraf9472]

Thanks for more power!!

[@bitteroldskunk]

If you hadn't noticed, some of the pins on the tube sockets looked a little cracked around a few of those sockets. You may have a few bad solder joints on those boards. Other than that, that's a sharp looking scope

[@UsagiElectric]

Sometimes the camera makes things look like cracks when they're really not. It definitely does look weird on a lot of the joints, but I went through and inspected them and it's really just blobs of solder, but they're all still connected nice and strong. I also went through and rocked the tubes back and forth while it was on an running to see if flexing the tube socket or PCB would cause changes in the pattern, and it didn't, so I think we're all good!

[@bitteroldskunk]

@@UsagiElectric awesome ;] an yeah, I can agree the camera can fool the eye.

[@robertlinder6414]

Flashback to 1976 High School electronics shop

[@UsagiElectric]

I think high school electronics classes should still use old analog scopes, they're wonderful little tools!

[@mamulcahy]

Thanks for the video. I love seeing you breathe new life into old equipment.

[@jamesdecross1035]

Now you've fixed it your challenge, should you choose to accept it, is to recreate those beautiful on-screen patterns. Go for it!

[@pac1085]

I'd say the HP 651A Test Oscillator meets the rack mount requirement and the aesthetic, but it's a bit newer (1965) and transistorized. They can be had super cheap though. I bought one in the hopes of using the case for it to fit my HP Barney Oliver amplifier, but it wasn't quite right... so it just sits here on a shelf now. lol

[@qzorn4440]

So many sweet devices from the past. 🥰 Thank you.

[@shalffon]

Awesome! Thank you so much for sharing!

[@Zadster]

This is why I always add a note inside equipment that I have modified or repaired - especially my own! Not just stating what was changed, but why. You *might* find that tube you replaced was a factory upgrade or mod. Looks like you are setting up a museum for HP test gear from the generation immediately before CuriousMarc's collection. And that is a really good thing!

[@MsFireboy2]

Thanks for posting very informative.

[@macgvrs]

Good job repairing that scope. That was a tricky one. When repairing vintage radios I have encountered bad Sprague caps. It is possible there are commercial higher quality caps being used in test equipment. Just wanted you to know you can't always rely on the fact they are Sprague caps.

[@jeromethiel4323]

The college i attended in the mid 80's was still using a gigantic vacuum tube o-scope, as it was the only scope they had that could do two very important things. One, was to generate a transistor family of curves. Two was that you could hook a land camera to it, and take a picture of said transistor curves, and develop them yourself darn near instantly. Bonus, was that you could actually draw on the photo to pick your bias point for that particular transistor. That o-scope had to weigh at least 200 pounds. It was 3 feet deep and 2 foot x 2 foot across the front. You could only move it in it's wheeled rack. Amazing how well that o-scope worked. We had regular old analog, transistor based tektronix scopes at each workbench, but you could not do transistor curves on them. At least not with the equipment the school had on hand.

[@skfalpink123]

My phone line reconnected, I can FINALLY watch the second half of this video!

[@restingsmirkface]

Nice work!

[@jamesdecross1035]

What… loose an old hand-made embossed Demo label. I love to see those things!

[@chrisjpf33]

I said the name “Dymo” to David and he didn’t know what I was saying. ;-)

[@FJL4215]

11:28 Removing that big black 450KHZ sticker made it 10x more beautiful. Satisfying. The zener + transistor was a nice simple regulator circuit. When the output voltage reaches the zener voltage minus the Vbe of the transistor, the zener diode starts to conduct and starts turning off the transistor. The resistor from the -150V makes essentially a current source that provides the base current to turn on the transistor.

[@Ihamfp]

I have an HP 8405A "Vector Voltmeter" (well actually, just the case... wanted to use it for an homemade amplifier), with a very similar style, handles and all, and with the stackable plastic feet this 120B seems to be lacking. It fits perfectly on my other 19" HP things ! If you need I could model them in CAD and send the files for 3D printing copies.

[@borisj4054]

Have a full working hp vector volt meter with all the accessories. Very useful instrument used with directional couplers to measure insertion losses or used to measure electrical length of cables.

[@AndersNielsenAA]

Wow that 120B is beautiful! The CRT bezel really makes it shine!

[@phildem414]

What a hunk of a scope 😍 Digital scopes are cool for they one shot capture, protocol interpretation and digital modes, BUT; an analog scope is a special connection to the signal. I love both!

[@jurjenbos228]

I just love that the litton looks like a washing machine. This form factor is pretty unique.

[@drrenard1277]

I worked for Litton industries before acquired by Northrop. I worked in Aero department but I've seen so many of Litton old stock in storage

[@rickhuang4493]

The PNP transistor and the Zener diode forms a basic current follower. The zener is the voltage reference and the PNP transistor create a voltage that's close (600mV delta) to Zener's voltage with higher current capability. This is essentially the same as a current follower circuit that uses a NPN transistor and a Zener diode that is more commonly used today except with the use of PNP transistor, the whole circuit is a mirror of the one that uses NPN transistor. Hint, the placement of the ground is confusing. Redraw the circuit by putting the ground at the +19V rail will make the analysis easier.

[@simonkormendy849]

Cool, as it just so happens, I have an old Oscilloscope stored away in one of my cupboards, it's an old Telequipment D33R Tube Oscilloscope, it's currently not working and needs a new, or second-hand CRT, plus some restoration work to get it going again, it's a great restoration project.

[@matthewspence3251]

Beautifly simple scope - I used one of these for my phd in 1992, triggered by the laser pulses … measurements of molecular dynamics photon counting in a pitch black university basement except for the glow of the tubes and an ibm 5150 for data logging.

[@Charlottesville798]

I've got a 1974 mini portable oscilloscope in a brown leather case im trying to resurrect, great video!

[@johngalt7382]

As the zener conducts it positive biases the base of the pnp (off), held on with the 15k resistor to the negative supply, establishing a nfb loop, increasing the v drop to the ground.

[@paulmoir4452]

It also looks weird to us who are more used to power NPN devices. It's just the familiar pass transistor linear regulator topology upside-down. The pass transistor is on the bottom 0V rail rather than the top positive rail.

[@johngalt7382]

@@paulmoir4452 I'm guessing that it dates to the pnp germanium era, before npn silicon power devices were common, but it is exactly that.

[@georgegonzalez2476]

Nooooo! I've interfaced a Selectric to a computer. Around 1980. It took a large amount of hardware and software. The Selectric basic mechanism wasn't designed with computer control in mind, so it is difficult to interface. It's possible if you're not careful in sending it pulses, to lock up the shift mechanism. Have a month or two and a full service manual. Also it's in no way an ASCII machine, so you need a bit of coding to map ASCII to tilt and rotate signals for the golf ball. And it won't accept a full-speed serial stream, you have to wait after each character for a handshake signal. And there is no handshake signal if you're shifting to upper-case when it's already in upper case, so yo have to special-case that situation or the whole thing locks up. Not a lot of fun. Good print quality though.

[@UsagiElectric]

The good news with the Selectric Composer is it already has the six solenoids and switches set up to type and interpret what is being typed, but as you said there's still a ton of serious hurdles to overcome. The marriage of mechanical and digital is a finicky one, and getting that thing working reliably as a data terminal is going to be a proper challenge! But, I have to get it working reliably as a Selectric first. We have a serious mountain to climb for sure, but I think it's going to be a fun one!

[@mm-hl7gh]

using this old hp gear to get the old bendix running would be aweseome !! :D cant wait !

[@radarmusen]

That bendix teletype carrier looks like it are able to knock things off the desk from a good distance.

[@TheHylianBatman]

Don't forget the power supply for the mobile test rack! I wonder what that would be. Also speaking of not forgetting, what about the other old HP equipment that's been waiting for work for a couple of years? You could restore those, or, if they're not getting touched any time soon, you could move them out of the room to free up space? I love that you're just collecting old equipment. Someday, you're gonna need a bigger space! I can see it now, in 10 year's time: "Now opening, the David Lovett Texas Museum of Technology"! Featuring wings on cars, Japanese tech, Centurion, and vacuum tubes, of course!

[@50shadesofbeige88]

Reminds me of Curious Marc. Very cool video. I love old HP gear.

[@fernandonovoa3261]

Muy buen trabajo !!, saludos.

[@Renville80]

When removing old adhesive, I find that soaking it with isopropyl (sometimes with a rag or towel laid on top) will go a long ways towards softening it to where it will come more easily.

[@SeekingTheLoveThatGodMeans7648]

Yes; another possibility is naptha (cigarette lighter fluid). "Keep away from fire, flame, or spark."

[@milanzizka4636]

The cat 😼is great after all! 👍

[@LeonDerczynski]

hey, that cut at 6:58 was subtle! not bad

[@annieworroll4373]

One of these days I want a classic unit like this. My Hantek handheld does the job I need it to do, but there's something about the look to these beasts.

[@vincentguttmann2231]

I'd go ahead and ask CuriousMarc if they have something for you, they're like THE masters of HP equipment.

[@JCWise-sf9ww]

I seen a similar thing happen with a solid state Telequipment Scope, spent days trying to TS the problem, replacing some caps, in the end the bias pot for the schmitt trigger need calibrated by using another Scope. Then the triggering stayed locked in when changing frequencies on my signal generator.

[@neilthomas6042]

Interesting video, glad you got it working. Cute cat having a lick at the end; adorable.

[@StefanWolfrum]

Wow, such cool equipment from HP! Love ❤ it!! Makes me think about _my_ lab equipment … 🤔 😂 💸

[@UsagiElectric]

Thanks! I have a soft spot for old test equipment, especially HP stuff, and this one was right up my alley!

[@justicelut]

Good work. You can never have enough scopes! IMO!

[@RobsonWilliam82]

"Wavy sine wave". That's a true sine wave! 😂

[@mikegLXIVMM]

I used to have an HP 130C scope. Same crazy tube/ Transistor mix.

[@clyde3013]

ooh more big hp brick stuff!

[@UsagiElectric]

Every once in a while I need my vintage HP test equipment fix!

[@barryyoung6159]

What A lovely thing 🙂

[@JohnDlugosz]

re the thumbnail: Wow! I had that exact model 'scope, too! I bought it on-line in the mid or late '90's -- not sure if Ebay was around yet, but something like that. Around $80 IIRC. The rack-mount ears had been cut off. In the early '00's, after getting married, I gave it to a friend who collected oscilloscopes (he also had *every* Star Trek novel paperback). I suppose there were quite a few of these still kicking around, since they would just keep on working. Update: seeing it in the video, it's not the identical model.

[@nickk6109]

That regulator and zener could be a clamp on the regulator to stop it from spiking over, or, it could be an early shunt regulator providing a low noise power supply by having a constant current bypassing through the zener (would need to see what’s elsewhere on the schematic.

[@alanpecherer5705]

The 6U8 turned out to be somewhat of a problematic tube in many applications. They have a propensity to heater to cathode leakage. This may or may not show up on a tube tester, but is exactly the kind of thing that will throw noise on your trace and contribute to instability in your H-sweep. Exactly the issues you have. That is why, I'll bet, that they were substituted out in your scope as-found with allegedly superior types. Bottom line, it's kind of a defective design tube. They were reasonably popular tubes in old TV sets and if that is the source of your bag of 6U8's, all of them could be hinky on H-K leakage side. Before going nuts searching for a defective component in your scope, I would invest

[@simonkormendy849]

The level of serviceability shown with this Scope is something that seriously needs to be brought back into consumer electronics, electronic equipment manufacturers these days try to build stuff as cheaply as possible to maintain their healthy profit-margins, which is a bit unfortunate, there's deliberate "planned-obsolescence" as well, most modern electronic devices end up on e-waste piles in India when they become obsolete, and, or, defunct.

[@PCFixer]

Oh, man, this comment is ALL MY FRUSTRATION with modern technology, arrrrgh!!! I miss the days when everything was serviceable and repairable, not held together with locking tabs!!!

[@WagonLoads]

The Tektronix TM506 is a rack mount system for solid state plug in modules.. I don't know if there is a vacuum tube equivalent to it though.. But that would be a starting point in form factor search possibilities.. By the way.. I have a Tektronix 585 laying around somewhere... I think it had a problem with the power supply...

[@colinstu]

I hope you can source the rest of that HP gear, that would be a sweet setup

[@jrhalabamacustoms5673]

Yay, an oscope that's basically the same age as me, its prettier too! Nice find!

[@computeraidedworld1148]

I adore your channel, and was just curious, where did you learn your way around schematics? Reading them and finding how to pin point possible places to check.

[@KameraShy]

This vintage electronic equipment was designed when nominal voltage was 110. Now it ranges from 124 to 129, at least in my area. If a vintage device is randomly blowing its fuse for no apparent reason, I am wondering if voltage fluctuations and spikes could be a factor. Maybe monitor the power line or use a variac. This may be particularly important for precision test equipment - to get correct readings - versus something like audio equipment.

[@TheAtomstrike]

current through r363 opens the transistor. When sufficient voltage appears on the red wire, current begins to flow through the zener and this current closes the transistor. In this way, a constant voltage is maintained on the red wire

[@maskddingo1779]

7 scopes seems like a pretty ideal amount😂

[@trevorvanbremen4718]

Makes me feel soooooo unworthy... I only have three!

[@pfitz4881]

I see you have a Marantz receiver!!! I wish I had the one I owned in the late 70s.

[@mathewmcgill6266]

I use to use that scope.

[@JamesHalfHorse]

There is a reason that big equipment had optional (but not really) rolling stands. There are some Tektronix ones I hope come into my possession.

[@joshj88]

I really think you should use Hllrl as your hello. I know it’s silly but I love your modified hello world

[@byterock]

A one point you had a classic Fuzzy Worm, trace. It is usually caused by one wire crossing a wire or pressing close against it. That mysterious white wire could cause that.

[@gd2329j]

Looks like a old external sync mod . Used with rf sweep generators or possibly part of a diy curve tracer .

[@Renville80]

Just putting this out there, since you mention the Model 15 teletype... see if you can find one that was modified for use by deaf people as the wiring harness was greatly simplified to have just the power and two pairs of cables (one would produce a Baudot serial stream from the keyboard and the other would be a similar Baudot stream to drive the printer). Both data cables would go into a box that would have an acoustic coupler for a telephone handset.

[@ebayscopeman]

OMG a HP-150 Oscilloscope. HP's first plug-in oscilloscope close to disaster, they recalled and repaired a ton of these for flaky PCB's.! Bumblebee capacitors and all. I bought one of these at a hamfest when I was in engineering school. I had a couple of plugins for it. If anyone finds one of these that has all of the front panel painted with baby blue Rustoleum paint and painstakingly redone lettering it was the one I had. I have not seen it since or one like it) I sold it decades ago. If you have it I would like to know! W3OHM

[@RPrice_OG]

I'd love to have a rack full of vintage test equipment like I had back in the 70s/80s but I don't have the space for such a thing. I know my Siglent does more in much less space but it does not look nearly as cool.

[@emilschw8924]

Old school beauty trumps modern electronics 😂

[@tony359]

Amazing repair and restoration! So we don't know why that modification was done in the first place? Weird!

[@nickolaswilliams935]

6U8 and 6GH8 are interchangeable in nearly all applications, but in my experience the GH doesn’t last as long. Remember that they came out near the end of the vacuum tube television era, so quality wasn’t as good and most of the circuits they were used in pushed them to their design limit.

[@HimmelsDaemon]

I think Mr. Carlson spoke of them at least once, or something similar with the same triode/pentode difference. That comment about towards the end of an era sound familiar, but might be combining things, later part of what you said not included. Would have to scrub projects from the last year to three, I think it was of relatively recent memory.

[@colonelbarker]

28:52 This is what we are here for.

[@ozzie_goat]

I need to show my scope some love now... I have a Philips PM3240 from God knows how long ago

[@dan3a]

Nice! Got its little brother, the PM3055, wonder how long it'll last before I'll have to repair it though. (Maybe that's why you never have enough osciloscopes!)

[@ScottGrammer]

How the PNP and the Zener "play off each other": CR 304 and C328 form a half-wave rectifier producing an unregulated and probably ripple-laden 30 or so volts. The bottom of the transformer secondary is not grounded directly, but rather it is grounded through the PNP power transistor (Q301). Q301 is biased on by about 10mA of base current provided by the -150V supply through the 15k resistor (R363). This allows current to flow from the filter capacitor's (C328) negative lead to ground, powering the filament circuit. As soon as the filament circuit voltage reaches about 18.7 volts, the zener conducts, passing positive voltage to the base of Q301, reverse-biasing it so that it begins to turn off. An equilibrium will be reached where the voltage at the cathode of the zener sits right at the zener voltage, regulating the filament supply. If the voltage dips, the zener conducts less, and the -150V supply biases Q301 positively, allowing the filament voltage to rise. If the filament voltage goes too high, the zener will conduct more heavily, turning Q301 off until the voltage drops to normal. It's a weird circuit, but in the days of Germanium transistors, NPN power transistors were uncommon, so this circuit was devised so as to make use of the more common PNP power transistors.

[@miroslavstevic2036]

Nice analysis. There are some obvious problems, like temperature drift and unpredictable Zener characteristics. Regulation loop -> Higher voltage on output, Zener conducts more -> less current to base -> bigger Vce, lowering the output voltage. I don't think that Q301 is turning off completely unless C328 is damaged - its base current is "eaten" more or less by Zener, and Vce changes accordingly to that. One interesting detail is that filtering cap C328 is not grounded but connected to Q301's collector. I suppose they saved on the capacitor voltage rating since most of noise is coming from the transformer and CR304.

[@ScottGrammer]

@@miroslavstevic2036 There was no choice but to connect C328 as they did, because the secondary + CR304 +paralleled resistors + cap = the complete unregulated supply. And it's the grounded side that's getting regulated, not the positive side. They did that because NPN Germanium power transistors were rare, and also they had really bad temperature characteristics that were inferior to the PNP versions. N-doped Germanium does not like heat. The paralleled resistors are for inrush current limiting. When the three 12AU7's this circuit powers are up and running, the total current would be about 300mA. But when they are first turned on, they'll try to draw about 4 times that much for about 1 second. The resistors keep this current down until the tubes warm up, to protect the power supply. And you're right, the pass transistor should never turn off completely.

[@miroslavstevic2036]

@@ScottGrammer Yes, good point about parallel resistors, filaments draw large current until they warm up. Also 1000uF is quite a punch. Since they are in parallel I suppose they've used two cheaper packages to dissipate excessive heat. NTC resistor would probably save that power, but also bit more expensive. My observation and question about C328 is what if its minus lead was grounded. I don't see why it couldn't do filtering if connected from +19V rail to ground? Harmonic current would go through Q301 in that case though, heating it up a bit, and the voltage on C328 would be a bit higher. I believe an additional cap from +19V to ground would be beneficial for loop stability, but they probably didn't care about regulation that much as thermal inertia of filaments is effectively doing the similar job.

[@EIs4Excellence]

Hey, it was cool to meet you in Maryland. As far as your question about what to do with another oscilloscope... You could always give one to me! I just have a cheapo $10 handheld jobbie :(

[@justinpakarno4346]

The Litton computer looks like a laundry machine.

[@UsagiElectric]

Isn't it fantastic!

[@CATech1138]

i though 60s roll around dishwasher

[@AttemptingAstro]

All that VC troubleshooting and it was a resistor....well honestly, yay. At least that's what it was and you found it haha

[@janwzalewski]

yes we did

[@lenovovo]

You are so CUTE!!!! -Melvin - Sunday, November 12, 2023 - Colorado Springs, Colorado - 12:33 PM

[@antoniodias2776]

Excelente.

[@emmettturner9452]

I have a AN/USM-338 “portable” scope that is the U.S. Marine Corps version of an old HP 1707B scope from the mid-‘70s. I have no idea how to use it! I was hoping to get an old analog CRT scope so I could get the eye pattern when adjusting CD lasers but I’m told I need at least 100MHz and this is 75MHz.

[@emmettturner9452]

It is, of course, able to be battery-powered and supports AC or 12v DC operation. I guess that’s what makes it a portable military scope. ;)

[@MaxWithTheSax]

I believe the modification might have been to create a marker dot on the external trigger pulse.

[@anvz6]

I think the mod was for adding external trigger. Probably the previous owner needed it and thought that he never will need z axis input then used it's jack.

[@SeekingTheLoveThatGodMeans7648]

Another possibility is going back one stage from CRT blanking signal to get a positive=more intense/negative=less intense beam response (and amplified, and without a high voltage pulse coming from the input) to Z axis. I did this on an old Conar 250.

[@timothystockman7533]

I had an HP 120B at one time!

[@VorpalGun]

Have the wang writer in the main house area, so you can use it to write video scripts!

[@coldfinger459sub0]

I love old test EQ but it takes so much bench top & shelf when you start getting too much of it.