General Valve Oscilloscope Restoration Advice

[MrStokie]

I bought a cheap, old 'scope from eBay with the intention of using the case for a bench power supply I'm going to build. For less than £25 it seemed like a steal. Then I opened it up. This being my first valve device I was unsure what to expect, but it's a thing of beauty in there.

So, Plan B, see if I can make it work again. I have a number of problems, not least my lack of any knowledge of this stuff. I do know not to touch power caps and CRTs though.

My initial thoughts are to clean it all up with some compressed air, replace the capacitors, check and do the same for the resistors (especially where the board has clearly gotten hot.

I have no intention of putting any power into it for a couple of reasons. The main one being I haven't got a Variac. It set for 110V but I would want to creep up on that for the first switch on.

This may take some time.

What I am really asking is for general pitfalls, advice, and things a new gamer should watch out for. Any information about the 'scope itself. Also, do you think that my current plan sounds like the right sort of track - or should I just gut it for a bench power supply (not sure that I could bring myself to do that now actually)?

The oscilloscope is an Advance Instruments OS-15a, of which the mighty Google seems not to have heard

Cheers, Stokie

[RobinBirch]

I found this

http://www.collection.archivist.info...dvance+OS15+IS

The inference is that they have a manual.

Hope this helps

Robin

[ms660]

Brief release article here for the OS15:

http://www.americanradiohistory.com/...rch=%22advance os15%22

Lawrence.

[Chris55000]

Hi!

I've got the paperwork all ready but it's too big to post in one lump so I'll install a pdf chopper-upper and post it for you!

Please bear with me a little while!

Chris Williams

[Chris55000]

Hi!

Manual parts 1, 2, 3 & 4. . .!

Chris Williams

[Chris55000]

Hi!

This is an excellent and relatively straight-forward oscilloscope to learn about Oscilloscope circuitry, it's operation and servicing and I'm sure many other Members who are better equipped than me will be able to assist with suitable spare parts!

Remaining parts of Manual. . .!

Chris Williams

[MrStokie]

Wow! Thanks so much for these really useful responses already. And that manual will be (as you know) totally invaluable. I bought a digital variable temp soldering station today. My first oscilloscope accessory

[Phil G4SPZ]

I have restored a few Cossor valved oscilloscopes, and the most obvious sequence of repair would be (1) power supplies, LT, HT then EHT and focus potentials; (2) timebase; and finally (3) Y-amplifiers, trigger circuits etc.

You say the device is set for 110 volt mains. Does it have a 240 volt tapping? You could try making a simple lamp limiter in lieu of a variac to apply power in a gentler manner.

[Chris55000]

Hi!

The first thing to beware of is that this model uses 4uF electrolytic smoothing capacitors in two series-parallel chains across the -1200V negative e.h.t. supply which is very unusual in oscilloscope practice, and these can give a potentially fatal shock if contacted accidentally!

Also take note all the valve-heaters are referenced to approximately half the main h.t. supply voltage to reduce the voltage-stress of the heater cathode insulation of the valves, so please be careful not to accidentally short valve-holder tags, etc., if you make measurements!

Begin by feeding the 'scope with a lamp-limiter (use an ordinary filament lamp) of about 100W rating in series with the live mains lead to your 'scope - if you can't get one in your local supermarket, you can still find "rough-service" GLS filament lamps in DIY stores.

First of all, disconnect one end of each ot the h.t. rectifiers MR1 and MR2 by unsoldering them from the PCB, then disconnect the high-voltage lead from the mains transformer e.h.t. winding from tag 31 of the PCB, insulate the end very, very carefully with a piece of chocolate-block connector then place the end with the chocolate-block in something like an old supermarket plastic salt container, making sure it's supported clear of the metalwork!

This precaution will allow you to power up the instrument without h.t. or e.h.t voltage whilst you check the mains-transformer runs ok without any overheating. It's quite safe to leave the valves plugged in, although they will not be lighting at full brilliance while you're using the lamp limiter!

Have a check to see all eight of the valves and the CRT heater are glowing, and note also each valve is a double-section ECF82 type with two separate cathodes in each, so you should see 16 heaters alight in total.

If one or more of the heaters appears to be very bright or out, that valve should be treated as defective and you will need to obtain replacements!

If you hear any untoward noises or "hot smells" from the transformer with MR1/MR2 disconnected and the 1200V lead from the PCB tag 31 safely insulated out of the way, then this indicates the e.h.t. winding may have short-circuited turns, which would need rewinding!

(Member Ed Dinning will advise on this - he does a first-class job!)

Let us know how far you get with the psu and we'll go on from there!

Chris Williams

PS!

I missed off the first three pages - here they are!

[WME_bill]

Oscilloscope restoration. Advance OS15.
I cannot unload the files of the manual posted by Chris55000.
So as to make it easier for us to consider the points raised, I attach just the circuit diagram of the OS15. I have manuals for both OS15 and OS15A, each totalling about 2.6Mb, quite readable, which can be sent by email, if anyone is also having difficulty with those posted.
Mains EHT power supplies, as used here, were common in the 1960/70 period. They were used for instance in the Telequipment S32 and D43 and Solartron CD1400, very big sellers in their day.
They can be nasty to work upon unless you are careful. I often clip a short circuit across the capacitor chain, for the charge will creep back after a time as the voltage stress held in insulators and capacitors relaxes.
wme_bill

[karesz*]

Hello Bill,

I cannot unload the files of the manual

Maybe you must use "Foxit reader" _I have it on my "quasi modern" XP & dont have such problem.
Regards, Karl

[MrStokie]

Thank you all for the help so far, what an incredible resource this forum is. My confidence has risen ten fold; I am now confident that I cannot really break a broken 'scope much more than it already is - I just have to make sure its EHT doesn't break me! I have been swatting up by watching Mr Carlson's Lab, he seems to know his onions (but his hair looks constantly charged with static!).

Also thanks to the manual that's been posted I have instructions on how to configure the transformer for domestic mains. Actually that's my next point... looking at the chart it seems best to set the voltage for the 210v-230v range which is the 220V tap. Does that sound right?

[M0FYA Andy]

No problem here, the files just downloaded and opened!
Andy

[karesz*]

I can problemless open it, even with Adobe Reader too...
Karl