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Electronics => Repair => Topic started by: WikusVanDeMerwe on June 22, 2017, 05:48:13 pm

Title: Barn Find Oscilloscopes
Post by: WikusVanDeMerwe on June 22, 2017, 05:48:13 pm
Greetings all!

Recently I scored two 'barn find' scopes which I'm a bit terrified to try powering up - the technology they use is entirely alien to me although I did once inherit a valve based scope which eventually exploded on me so I threw it out (something if I'm honest I regret doing). 

Can anyone tell me what sort of pitfalls I should watch out for in attempting to service these old beasts? Can you tell if a Valve has gone bad by visual inspection? The larger of the two (I think it is a 'Cossor Instruments LTD Cossorscope Model 2000') had a date on a component of 1959 and the second unit which is a 'Telequipment Serviscope Type D33' has a date on a component of 1961, both of which are full of various valves and giant capacitors of strange design. I have not tried powering them on as I value my life but I did use an air brush and a soft bristled brush to give them a bit of a dusting off.

If Dave smelled them he'd definitely make a face. 

I have taken a few pictures for you all to look at, of the two the Telequipment unit seems to be in better condition, I can see that the other one has what seems to be a leaky cap block... Maybe these devices would be better left alone and if that's the advice then I'll do just that as they were free and I have modern equipment for doing actual work but you know how it is when you get something for free.

Behold that retro design!

I suspect they both came from this department:

Inside the Telequipment #1

Inside the Telequipment #2

Inside the Cossorscope 2000 from the top - here we can see the leaky capacitor block thing:

Inside the Cossorscope 2000 #1

Inside the Cossorscope 2000 #2

As Dave would say, pretty crusty:

A vintage probe:

Thanks for any advice or taking the time to read!

Title: Re: Barn Find Oscilloscopes
Post by: CJay on June 22, 2017, 06:02:33 pm
Oh wow, I think that Cossor is the same as my very first scope, I was 14 and had *no* idea what to do with it and cannot for the life of me remember where it went.
Title: Re: Barn Find Oscilloscopes
Post by: DaJMasta on June 22, 2017, 06:17:04 pm
Those look remarkably clean for being in a barn, you may have less of an issue getting them started than you expect.  As I've seen mentioned around, a lot of people like to power up old beasts on a variac, gradually increasing the voltage seems to be much gentler than just plugging the thing in and bringing it back for the first time in 30 years or whatever.

Other than that, I'd probably dust a bit in the messier parts and just keep inspecting things to see if there's an obvious fault.  There can be clear valve failures in the form of hazy discoloration if, for example, air has gotten in, but I don't know many of the specifics.
Title: Re: Barn Find Oscilloscopes
Post by: Gyro on June 22, 2017, 06:54:50 pm
A very dry barn, clearly [Edit: Ah, South Africa]. No signs of rust or corrosion that I can see.

As long as all the valves have a dark shiny getter flash at the top then they should have reasonable vacuum still - that's not to say that they're completely gas free so they (together with the electrolytics) will need to be brought up slowly to minimise cathode damage. Replace any valves where the getter flash has gone white, they're dead (I can't see any).

The Telequipment looks in very clean condition, the Cossor less so, but not too bad. You'll want to spend some time cleaning it up before trying to power it up.

In Your Telequipment #1 picture I can see a couple of high voltage Selenium rectifier stacks (marked K8/50). They may emit very unpleasant smells if the capacitors that they feed have gone high leakage! If necessary, you can rebuild them using Silicon rectifiers and a series resistor fitted in the original tubes.

Yes, a variac is the way to go. Bring them up very slowly (over several minutes) while looking, listening and smelling (from a safe distance) for problems. They're bound to smell a bit dusty at the start. Oh, and report back on progress!

Edit: That leaky Nitrogol capacitor in the Cossor obviously needs replacing, be careful of the leaking oil, it might be nasty. I see two Selenium rectifiers next to that too.
Title: Re: Barn Find Oscilloscopes
Post by: WastelandTek on June 22, 2017, 07:06:25 pm
I would NOT power these before replacing capacitors.  I think you have the right instincts there.

Then a dim bulb tester and variac to bring them up.

watch a few episodes of Mr. Carlson's lab ( and heed his wisdom regarding 50 year old capacitors please.
Title: Re: Barn Find Oscilloscopes
Post by: Gyro on June 22, 2017, 07:43:16 pm
Hmmm, that's always a difficult one. It depends on the OP's plans for the scopes. Scattergun capacitor replacement can involve significant expenditure in something like a scope - I notice that several of them are multi-section too, so will require some ingenuity to safely mount the replacements. The majority will probably respond fine to gentle re-forming.

Only the OP can judge how much they are worth to him in terms of up front expenditure. He may decide that gently attempting to power (at least the Telequipment) very slowly with a variac is worth the risk, or alternatively donate them to a collector or whatever (the Cossor is probably more in that territory).

One good thing with valve gear is that they don't suffer from cascade semiconductor failures (aside from the aforementioned Selenium rectifiers) that a more modern scope would under similar circumstances.
Title: Re: Barn Find Oscilloscopes
Post by: Tomorokoshi on June 22, 2017, 08:12:01 pm
Be careful with the white silkscreen printing on the valves (vacuum tubes) when cleaning them. It comes off very easily.
Title: Re: Barn Find Oscilloscopes
Post by: EHT on June 22, 2017, 08:59:16 pm
So you have scopes from Newcastle Uni in an SA barn!? Might be just as interesting to find out their history than fire them up. I like the look of the Cossor. I used to have a telequipment one which looked similar to that one except was smaller and single ch.

I'd echo many of the previous comments but also consider what you want to do with them first. For instance if you end up wanting to pass them on then a buyer who is going to restore them would prefer them to be untouched rather than fiddled with (and potentially damaged). I don't consider them to be of any practical use.

If you proceed with restoring them:
- look for the service manual to help getting valve model #s and capacitor values in case any are not clear
- clean up the valves
- replace high voltage and high capacitance caps, i.e. the power supply. New ones will be way smaller. I know sometimes people would hollow out the case of the ones to keep it looking the same but I'm sure there are horrible chemicals inside.. can you be bothered...
- not sure about the selenium rectifiers - see if the consensus advice is to replace before powering up

Here is a pic of one I've just replaced in a 70's piece of equipment (Tektronix). It was getting really hot - (presumably high ESR?) so out it went. New jap cap is higher capacitance and way smaller.

Best luck!
Title: Re: Barn Find Oscilloscopes
Post by: james_s on June 23, 2017, 12:04:45 am
Those look like they're in remarkably good shape, I see nothing particularly unusual about the design or construction, it's very typical of the era. The same advice applies as when restoring antique radios, check electrolytic capacitors first, then cautiously power up with a series lightbulb or variac while monitoring the current as you slowly turn up the line voltage. Watch closely for smoke, sparks, excessive current draw or other signs of trouble. It would not surprise me at all if one or both scopes powers up and more or less works as found.
Title: Re: Barn Find Oscilloscopes
Post by: Nemo1956 on July 20, 2017, 04:53:35 am
I had two of the right hand side scops back in the days. They are proper duel beam scops so the tube has two sets of guns in the. But look out for the EHT stick they often go up in smoke. These can be replaced with high voltage diodes with leak resistors across them.
I had my for many years. I don't know what happen to them. I most probably gave them away to a student.
But have fun with them and it's nice to see one again.
Good luck with it.
Title: Re: Barn Find Oscilloscopes
Post by: voltz on July 20, 2017, 09:45:21 am
Just to echo whats already been mentioned, dont be tempted to just plug it in and switch on. Either use a variac to slowly bring up the mains maybe staying at 50% for a while before increasing further. Check for excessive current/heat. Or use a light bulb in series with the mains which serves two purposes: gradually charging capacitors back to life and giving a visual indication of over current. A good trick for all vintage gear.

Try to clean out the dust with gentle compressed air and a dry paint brush, but be careful of causing any damage to components. I see neon lamps standing up for example, they could break. Remove dust from the valves with a cloth. Clean valve bases with a contact cleaner.

I would not just go in and change all capacitors. Too much work only to find out the CRT or the transformer is broken.. Try to get the thing powered up first, then determine what capacitors to change etc. Mind you, the Dubilier Nitrogol most certainly needs replacing.

If you're lucky, they both come back to life and you can start cleaning switches and rotaries which will most certainly be dirty.

good luck with them.
PS. Based in Sunderland here, not far from Newcastle Uni!.. how strange for them to end up in SA.
Title: Re: Barn Find Oscilloscopes
Post by: Kjelt on July 20, 2017, 10:35:32 am
Our universities and colleges donated many out of date T&A to other world countries over the decades, could be what happened here?
Title: Re: Barn Find Oscilloscopes
Post by: WikusVanDeMerwe on September 02, 2017, 05:24:27 pm

Hi all, sorry I have not updated this post for a while (like you) I have many projects...

After exhaustive checking, measuring and re-checking I finally powered it up - things were in better condition than I thought and I only had two minor fixes to do.

Here we go fingers crossed...

It's alive!

Both channels seem to function well for their age:

After running it for a while to ensure no magic smoke escaped I removed the side of the casing to inspect the components again - nothing failed:

The valves seem to be holding up:

As a bonus the guy who tipped me off in the first place about the scopes had one more freebie up his sleeve so I also got this small 'Telequipment Serviscope Minor'. The case is in rough shape as the mounting bracket plastic tabs went brittle and cracked long ago but this unit had been stored inside an old filing cabinet and was relatively undamaged. I found a date code on several of the components indicating that it is from around 1974. I gave it a clean and check over and it powered up first time. It had an ancient microphone hooked up to the inputs so I think it was only set up to demonstrate this effect to the students. Here are some badly taken photos of it in action:




So even though I won't use either of them in anger very much I'm still glad to be able to get them going again, there's nothing quite like the high-voltage hum of a vintage scope!  Apologies for the quality (or lack thereof) my pictures, the light in the workshop today was against me at all angles and I was using my kick around ipad  :o

Title: Re: Barn Find Oscilloscopes
Post by: b_force on September 02, 2017, 05:48:43 pm
Greetings all!

Recently I scored two 'barn find' scopes which I'm a bit terrified to try powering up

That's why people use a variac (or variable transformer) :)
Title: Re: Barn Find Oscilloscopes
Post by: VK5RC on September 03, 2017, 05:59:48 am
Good to see 'most'  has gone well, use one scope to look for ripple on the power rails (not the high voltage), or an in circuit capacitor esr tester. I restored an old tek 545, its description is here (eevblog forum) somewhere. One of the main power rail filter capacitors was bad, a '1k resistor', but found before switch on.
Keep up the good work.
Title: Re: Barn Find Oscilloscopes
Post by: IanMacdonald on September 03, 2017, 07:11:25 am
Interesting that they've done some really neat point-to-point wiring, but then then that middle PCB is like chaos on a plate. You can see that the guy who did the layout was still thinking in handwiring terms.
Title: Re: Barn Find Oscilloscopes
Post by: Ian.M on September 03, 2017, 09:38:53 am
Restored, with attention to meeting current safety standards, the Telequipment Serviscope Minor would still be a useful first scope to lend to a novice doing audio and *slow* digital stuff till they can get something better.  Its got broadly comparable capabilities to a sound-card scope with the advantage of DC coupling and a reasonable input attenuator, but the disadvantage of only being single channel, with no X-Y mode.
Title: Re: Barn Find Oscilloscopes
Post by: WastelandTek on September 28, 2017, 03:55:28 pm
chaos on a plate

lol, I rather like that, not heard it before
Title: Re: Barn Find Oscilloscopes
Post by: Gyro on September 28, 2017, 05:27:28 pm
We used those at senior school, I always wondered what they looked like inside. Iirc, the Neon used to flicker in time with the sweep - it made me wonder if it was a neon sawtooth oscillator driving the X plates.
Title: Re: Barn Find Oscilloscopes
Post by: james_s on November 17, 2017, 07:46:43 pm
Hey that's cool! It gives me a warm fuzzy feeling to see ancient equipment like that brought back to life.