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Old 21st Feb 2020, 2:50 am   #1
Uncle Bulgaria
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Default Gould OS255 Oscilloscope Repair Advice Needed...

Ahoy,

My oscilloscope is possibly in need of some adjustment or repair.
  • It's started to get a funky smell as it gets warm
  • The trace doesn't appear until the intensity knob is turned up over halfway
  • The trace cannot be made sharp at maximum intensity, and is of varying width. I have to knock the intensity down in order to get a reasonably sharp trace.
There are other useability issues I've put down to age and lack of use, but I don't think they're representative of major faults.

I've got the case off and there are no obviously problematic components. I have the manual, where chapter five explains some points to be measured. See attached.

The requirements for equipment it demands are of course beyond my hobby possessions. Is it worth my attempting to work through the instructions?

I am not well versed in high voltage procedures. What should I be careful of and how should I proceed, bearing in mind the test procedures in chapter 5?
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Old 21st Feb 2020, 9:47 am   #2
ahankinson
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Default Re: Gould OS255 Oscilloscope Repair Advice Needed...

Check the manual for voltage test points, and see what they measure on yours. Keeping in mind, of course, the voltage limits of your multimeter (i.e., don’t probe 1.5kV with a 1kV meter!)
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Old 21st Feb 2020, 4:52 pm   #3
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Default Re: Gould OS255 Oscilloscope Repair Advice Needed...

Sounds like the usual drift of the resistor chain than gives the CRT voltages, or problems with the high voltage smoothing capacitors.
Check all the voltages around the CRT. Expect them within 10% or so.
Make yourself a high voltage probe, to 2.5kv will be alright for the OC255, as the tube (Mullard D14-250 or 56803) operates below that.
Notes on making a probe previously posted here (perhaps under Telequipment D75/D83). Otherwise, make up a string of a couple of 10M or other high value resistors with a 1M at the bottom. Put your DVM across the 1M and select the ones above to give you the approximate ratio you want -say 5:1. Put the string in a plastic pen tube or a length of PVC tube, cement a tip into the point. Get the ratio about right. The secret is to calibate the probe each time you use it against some other point on the scope - the 200v rail will do. Measure with and without the probe and calculate the ratio.
Any resistor type will do, as its value will not change enough under voltage stress to affect your results.
Now measure the voltages down the EHT chain, R706, R720 network and across the zener D706. I have posted the actual values of the resistor networks in the past, though they are in most copies of the manual. The voltages you can check with the CRT data which is on Frank Philipse's invaluable Tube Data site.
If stuck, I can provide all this data again.
It is not as difficult or dangerous as you might think, just be careful and don't rush or poke wildly at things.
Study the EHT regulator and Bright -up/Blanking circuit theory described in the manual carefully, as Advance used their own method which works very well.
When measuring voltage around the CRT, make sure the bright-up is not operating. Keep the TB off, or on ExtX or X-Y.
Let us know how you get on.
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Old 26th Feb 2020, 12:51 am   #4
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Default Re: Gould OS255 Oscilloscope Repair Advice Needed...

Thank you. The chapter I mentioned does have the voltages I will endeavour to measure. You've made it sound less daunting than the manual does!

I've found your previous notes on making a probe, and some on other websites where they discuss stringing 80 resistors together...
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Old 26th Feb 2020, 7:32 am   #5
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Default Re: Gould OS255 Oscilloscope Repair Advice Needed...

Most scopes use a resistor potential divider to get the various voltages for various electrodes of the CRT as Bill says, a very easy bit of circuit to fault find. You can start by measuring the resistances of the various R's with the scope not powered, some R's will be pots.

You can measure the intensity control by instead of measuring from ground to grid, measure from the cathode to grid, remember the cathode may be at -1kV or more, clip meter leads on, hands off. The meter on DC volts should give a varying voltage of about 40v when intens knob is turned.

Andy.
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Old 27th Feb 2020, 6:17 am   #6
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Default Re: Gould OS255 Oscilloscope Repair Advice Needed...

Hi!

OH DEAR!

Before you get involved in the power supply and CRT e.h.t. circuits, these scopes do suffer from the dreaded Gould malarkey of short–circuited turns in the 950V e.h.t. winding of the mains transformer!

Prepare yourself a 6 way piece of 5A chocolate block connector strip, disconnect the 210V leads from pins 721 and 722, the 25V leads from 723 & 724 and the 950V leads from 727 & 728 on the PSU board and terminate them in the ways of the chocolate block, one lead to each way, making sure the block isn't touching the metalwork, then run the scope for 15 minutes – the CRT heater 6.3V winding can be left as it is – and afterwards switch off and feel the mains transformer – if it's still warm or smells of varnish, etc., I'm afraid it's time to chat to Ed Dinning, get another one for parts or scrap it, and yes, I have personally come across a case of this with another Member!

If you have an AVO 8 or Selectest Super 50 you can directly measure the 950V winding on the 2.5kVAC range – it should be about 1050V a.c. unloaded!

Please check the above first as these transformers are very fragile and do fail!

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Old 27th Feb 2020, 11:51 pm   #7
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Default Re: Gould OS255 Oscilloscope Repair Advice Needed...

Thanks Chris55000. It looks like I now have a few tests I can perform on the resistor chain while off, then at the transformer. I have an AVO 8.

Diabolical Artificer - is measuring the grid-cathode rather than grid-ground to avoid destroying the tube? I'm sure I've read somewhere about that being a pitfall to be wary of, but I've never done anything with CRTs before.

I will report back...
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Old 28th Feb 2020, 12:19 am   #8
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Default Re: Gould OS255 Oscilloscope Repair Advice Needed...

Hi!

Yes, that is the reason for Andy's precautions!

However, I always advocate, for the same reason, that it is far safer to test Oscilloscope CRT grid and cathode components with a good quality reliable DVM after the scope has been switched off than trying to measure CRT grid voltages, etc., direct!

Many modern Oscilloscopes with externally accessible Z mod/unblanking often rely on high–voltage zener diodes and transistors in their unblanking circuits that can leak or short, sometimes partially or intermittently, and in such cases working thro' the unblanking circuits methodically against the appropriate circuit–diagram is well worth the time spent!

Another reason for the use of complex unblanking circuits is to enable intensity controls to operate at low voltages relative to chassis–earth (not forgetting microprocessor–controlled push–button Oscilloscopes that must use low–voltage intensity controls of course!) – whilst the large carbon metal–cased potentiometers sold in the valve days would tolerate many hundreds of volts above or below earth, the modern small ones don't!

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Old 28th Feb 2020, 2:12 am   #9
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Default Re: Gould OS255 Oscilloscope Repair Advice Needed...

It looks as though you were right, Chris!

I've just finished the test you described as my first foray. The AVO measured 850V and the transformer was almost too hot to touch after switching off.

I have a spare which I can do the same test with in case the transformer is transferable. Luckily it's quite accessible. Why does this happen, do you know?
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Old 28th Feb 2020, 9:25 am   #10
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Default Re: Gould OS255 Oscilloscope Repair Advice Needed...

Hi!

The reason was stated by C.A. Quarrington in the Fault Diagnosis Chapters of Vol. 3 of "Radio & Television" (1963 ed) where he states "All too often the necessary wattage output has been obtained by using too much iron and too little copper", in other words, the transformer is run at a very high magnetic flux density in order to get the minimum possible "turns per volt" figure, and this, as well as Gould's transformer maker cutting the winding wire size to the barest minimum they can get away with, means that the enamel insulating the turns from each other gets very hot and starts to flake off the wire of the 950V winding, causing one or more turns to short together, causing a large circulatory current in the winding, which rapidly increases the heat dissipation in the rest of the winding, and so on until the transformer rapidly becomes totally unusable!

Ed Dinning's transformers are expensive to make but he does a first–class job of them that are cool running and reliable when run within the design ratings!

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Old 28th Feb 2020, 1:22 pm   #11
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Default Re: Gould OS255 Oscilloscope Repair Advice Needed...

I suppose it has been around for some years, in its defence. I don't know how many of those years it was used for, however, as I was given it by an old physics teacher when it became surplus from school.

The other transformer checks out, with about 1050V on the same winding so I can swap them over. I've heard good things about Ed Dinning's transformers, but I'd rather try to fix it myself as it isn't critical. I would have thought unwinding and re-winding would be a tedious but plausible task for my ability level.

Would using modern spec. wire be effective, or improving the insulation between layers be a better option?

I will also check that resistor chain while I have the case open.
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Old 28th Feb 2020, 3:02 pm   #12
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Default Re: Gould OS255 Oscilloscope Repair Advice Needed...

Hi!

It sounds like the spare transformer you have should be ok!

Removing and rewinding a 950V winding which is likely to be the order of 40 SWG or finer isn't a task for the faint–hearted unless you're experienced at it and have a good quality machine with an accurate and stable wire tension control! However if you can get "double–thickness" enamel for the 950V wire this will help, and modern plastic insulating sheet impregnated with shellac or high–temperature varnish afterwards will be adequate enough – it's the 6.3V CRT heater winding that had to be the most well insulated from any others!

Chris Williams

PS!

It's NOT necessary to worry about counting the turns on the 950V winding – the CRT or LT 24V winding will give you the "turns/volt" figure, which applies identically across all windings, so if you find 42 turns on the 6.3V winding, then the 950V winding requires 7 × 950 or 6650T.
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Last edited by Chris55000; 28th Feb 2020 at 3:08 pm. Reason: 6650
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Old 8th Mar 2020, 9:01 pm   #13
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Default Re: Gould OS255 Oscilloscope Repair Advice Needed...

I'm working through the test procedures having put the spare transformer in. See attached for figures.

I have 240VAC in. 30VDC across C704 as expected. However, I have 222V instead of the 210V expected from the secondary in the next entry. This means the 268VDC is actually 288VDC. I feel I should adjust R742, as 268V is quite a specific requirement!

However, this is the unregulated side, so why the specificity when the regulator can be adjusted, and (as can be seen) has +-21V for the 210V supply? I haven't yet tested the regulated side of things.
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Old 9th Mar 2020, 3:41 pm   #14
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Default Re: Gould OS255 Oscilloscope Repair Advice Needed...

A PM has alerted me to the fact that R742 is just a bleed for C705. Now I've had a fresh look I've gone through the regulated section and end up with:

+213V (210V +-21V) All Right.
+11.1V (11V +-0.5V) Hunky Dory.
+7.4V (7V +0.4V, -0.6V) Seems Legit.
-11.36V (-11V +-0.5V) Top Banana.

So the only one on the edge is the +7V.

I've adjusted the EHT regulator R725 for the required +405V on the collector of TR707.
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Old 9th Mar 2020, 7:06 pm   #15
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Default Re: Gould OS255 Oscilloscope Repair Advice Needed...

I've got some strange readings around the Y Amplifier (mid right bottom of the diagram). Perhaps someone can tell me what could be going on here.

The nominal voltages should be +3.7V wrtg on the collectors of TR401 and TR402. I get that on TR402, but I get 4.7V on TR401. The base voltages are the same. The 11V rail is providing 10.8V, and resistors 401, 402, 403, 404, 406, 409, 412 and 414 all measure correctly out of circuit.

Doesn't the 'Preset Y Gain' circuit balance between the collectors of TR401/402? So there shouldn't be a discrepancy there. However, I get 5.9V on the collector of TR402 and 9V on TR404... Should I look further afield?
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Old 9th Mar 2020, 10:02 pm   #16
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Default Re: Gould OS255 Oscilloscope Repair Advice Needed...

OS255. Y amplifier voltages appear out of balance.
Have you considered the shift controls. By their very nature, they will make the voltages different on each side. Otherwise the spot will never be deflected.
The gain control should not alter the voltages much in themselves.
Your power supply readings look good enough. You do not say what the -5v rail is. If that is slightly more positive, then that will lift the +7v rail. For IC701 is a 12 v regulator, sitting between -5 and +7volts.
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Old 10th Mar 2020, 2:37 am   #17
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Default Re: Gould OS255 Oscilloscope Repair Advice Needed...

Ah. That could be the trouble! I thought that the controls took effect later, and this section was merely an amplifier to make the signal useful.

I see the instructions say that the trace should be centred. However, I have no trace! On switching on and off, there's a flash of a beam off to one side, but the screen is blank at any position of the intensity knob, and the other controls have no effect.

Oh, and there's -4.95V on the 719 test point.

Operating the Y 'trace rotate' knob changes the voltage on the collector of TR402 between 4V and 5V, while the equivalent measurement on TR401 only varies between 3V and 4V.

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Old 10th Mar 2020, 3:32 am   #18
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Default Re: Gould OS255 Oscilloscope Repair Advice Needed...

TR513 and TR514 are getting hot. They should both have 135V on their collectors, but TR513 has 215V and TR514 has 65V. The latter varies by about 10V as the X knob is rotated, but the other doesn't.
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Old 14th Mar 2020, 1:18 am   #19
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Default Re: Gould OS255 Oscilloscope Repair Advice Needed...

Hi!

I'd replace TR513 and TR514 as a pair to begin with – TR513 should be (theoretically!) almost cold if it's collector is up at or very near to the h.t. line voltage, since that implies TR513 is passing virtually no current, which would cause TR514 to conduct more heavily as it's not being biassed back by TR513's collector current, hence TR514's low collector voltage!
.
However if TR513/TR514 are mounted close to each other on a common heatsink, then conducted heat from TR514 will make TR513 feel hot as well!

Don't forget the thick–film resistor unit R569 a/b/c (3 X 3k6) as well whilst you have the o/p transistors out!

Also don't forget that e.f. TR511 gets base current from the 210V h.t. line via R560 – if R560 wanders up too high as high–value resistors bleeding tiny currents from the h.t. supply are wont to do, TR511 won't operate and TR513 will turn off and TR514 conduct too heavily as mentioned above!

Chris Williams

PS!

Gould's infuriating habit of not putting any component–values, test voltages or waveforms on their circuits together with their endearing habit of cramming everything close together on a small page size makes fault–finding their tackle most infuriating!
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Old 18th Mar 2020, 2:16 am   #20
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Default Re: Gould OS255 Oscilloscope Repair Advice Needed...

I have a trace back! TR513/514 don't have any heatsinks beyond their own tags. They are still hot, but perhaps they're running within their dissipation.

What do you propose for the thick-film R569? Is it likely to need replacement with a DIY network? Should I remove it to check the resistances?

Thank you for the clarity, Chris. It's amazing how the wood is invisible for the trees until someone more knowledgeable gives some guidance. The Zener diode is dropping 6V from TR511 and R560 appears fine. I get the required -3V on the base of TR514.

I'm now following the setting up chapter, and trying to set the EHT as described: 'set the incoming a.c. supply via the autotransformer to the nominal centre voltage of the selected range. Set to mid-brilliance on the CRT. Monitor the collector voltage on TR707. Adjust R725 for this voltage to be +405V."

I take this to mean 230V for me, as the selectors on the back have "100/220V" and "0/+20V" options. We have 240V, so I have 220V+20V selected. With the autotransformer on 230V it is impossible to achieve 405V on R725, with the most I can get being approx. 360V.
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