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Old 21st Sep 2019, 6:43 pm   #1
Tim
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Default Gould/ Advance OS3000A/ PO Oscilloscope 16A

Hi all.


The day after I picked up a Philips PM3240 to cure my working 'scope deficiency, I was given one of these today, discovered while tidying a friends loft. One minute one is scopeless, and then suddenly they're knee deep!
He said it didn't work, so I got it.
It didn't power on, but that was due to a bad connection in the mains fuse holder.
When powered up it ran( actually very well) for about an hour before the trace disappeared.
(This was down to a blown fuse for the 20 volt supply feeding the EHT module, in turn caused by a shorted oscillator transistor(BFY51). Transistor replaced and now trace again, and stable current on the 20 volt feed of about 260mA. )
Before it did so, I noticed some strange behaviour. First the traces were pulsing in brightness alternately, which got worse until the solid trace became a series of dashes(but perhaps blanks or gaps)?)
The period of the blanks is variable with the timebase setting, but is not triggerable upon, and affects both traces.
Initially I thought that this might have been related to the loss of trace, but I don't think it is, and merely occurred at about the same time.
I did manage to get some pictures.
No funny noises or smells.


Any one out there who is familiar with this beastie, have any idea where I should start looking please.
I'm inclined to investigate the Bright up/ z mod circuits with a can of freezer spray as it seems to be temperature related, but after that it's the long way round.
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Old 21st Sep 2019, 6:51 pm   #2
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Default Re: Gould/ Advance OS3000A/ PO Oscilloscope 16A

Some pictures of the trace going funny. Happens in all positions of the beam switch( ie, ch 1 , alt, chop, ch2, both)

Thanks in Advance.
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Old 21st Sep 2019, 7:21 pm   #3
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Default Re: Gould/ Advance OS3000A/ PO Oscilloscope 16A

Just thought of something. these "dashes" start off small and get longer over time
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Old 21st Sep 2019, 8:34 pm   #4
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Default Re: Gould/ Advance OS3000A/ PO Oscilloscope 16A

Try adjusting the mains voltage selector by +/- 10V if the switches allow it. That resolved a similar display problem on a Gould scope from around the same era.
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Old 21st Sep 2019, 9:30 pm   #5
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Default Re: Gould/ Advance OS3000A/ PO Oscilloscope 16A

It’s set to maximum at the moment. I’ll try it though. All supplies are regulated though aren’t they. With the exception of the heater voltage. Mains was 242 volts yesterday.
Hopefully it’s not the EHT transformer failing as it looks so regular. I might be able to see how the EHT oscillator is running by ‘scoping a coil of a few turns( it’s about 30 kHz) held near the transformer.
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Old 1st Oct 2019, 6:43 pm   #6
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Default Re: Gould/ Advance OS3000A/ PO Oscilloscope 16A

Actually the fault is more or less as described in this excellent publication:

http://www.smcelectronics.com/DOWNLO...LESHOOTING.PDF

Apparently it’s called intensity’ modulation.

More later. SWMBO wants to go out
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Old 1st Oct 2019, 8:02 pm   #7
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Default Re: Gould/ Advance OS3000A/ PO Oscilloscope 16A

Apparently common causes include heater-cathode leakage in the oscillator( circuitry/transformer?), bad neons or bad coupling caps in the z mod/unblanking CRT circuits.
Appropriate circuit Diagrams etc. to follow.
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Old 2nd Oct 2019, 12:01 am   #8
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Default Re: Gould/ Advance OS3000A/ PO Oscilloscope 16A

Component list and Diagram of Z mod/CRT circuit.

Hopefully it's not the first thing on the list! Given that an oscillator transistor went, and focus isn't great(but might only be resistors) double fingers crossed.
To check the tube, could I simply disconnect everything from the tube except heater. run until nice and hot, then megger between heater and grid?

The other two I can cope with. 2KV caps from Rapid are available, but have some 0.022 1000 volt ones I can put in series for testing..

Presumably the neons are just neons?

Hopefully it's just the capacitors. I did notice some ceramic ones on the EHT board as well, but I think they are all LV.

According to the manual there should be about 25 volts on the collector(metal cased transistor) of TR904. I have 4 volts. I think I'll start round there somewhere!

I have an AVO8. Is that OK for measuring the 1.6KV, if I stand well back?

I haven't got this deeply into 'scopes before so sorry about all the questions.
Thanks
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Old 2nd Oct 2019, 7:11 am   #9
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Default Re: Gould/ Advance OS3000A/ PO Oscilloscope 16A

Sorry, those are really small. I’ll try and post some better ones.
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Old 16th Oct 2019, 11:31 pm   #10
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Default Re: Gould/ Advance OS3000A/ PO Oscilloscope 16A

Hi

Same "dotted" trace, but starts very bright. Brightness control inoperative, but can be turned down slightly after a few minutes.Focusing not very good either.

Started with a few voltage measurements:

Voltage on collector of TR904 with x-y and brightness up full 31 volts(should be around 25. This voltage DOES change with adjustment of the brightness control.
Grid and cathode voltage of CRT are low(or is it high?) at about -650 to -700 volts, as opposed to the quoted -1.6KV. measured on the 2500 volts range of an AVO8. The manual actually specifies " a 20,000 ohms / volt meter capable of measuring up to 2500 volts". Sounds like an AVO 8 to me!
Grid Voltage remains unchanged with CRT base unplugged. The trouble is I don't know whether that's a fault, or whether the voltage is actually OK, being loaded down by the meter and the fault is elsewhere.
I do have another meter(A GEC Select test) which has the same resistance as the AVO8 on 2500 volts(50 meg ohms),
I suppose I could try both meters in series to get 100 meg, and then they would each display half the voltage. If the total is different then my first readings must have been loaded by the meter.
The trace is very bright, and calibration seems fairly good( in vertical direction anyway) so I think the anode(9KV) is OK

I haven't had a chance to get a 'scope on it yet, but I'll have a poke about and see what waveforms are present.
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Old 17th Oct 2019, 7:35 am   #11
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Default Re: Gould/ Advance OS3000A/ PO Oscilloscope 16A

Given that it's been in a loft for years I'd start by putting it somewhere warm to dry it out.

Andy.
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Old 17th Oct 2019, 9:37 am   #12
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Default Re: Gould/ Advance OS3000A/ PO Oscilloscope 16A

The loft seemed fairly dry, but it’s not heated so subject to greater variation of temperature than the house so I guess that’s a fair point. I did bring it up on the variance over a few minutes, and it seemed to work quite happily.
I’d be glad of some advice re CRT grid voltages etc. As I understand it the cathode should follow the input signal to a certain degree. I wasn’t able to check whether the brightness control varies the grid voltage, but will do next time I am able to work on it.
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Old 17th Oct 2019, 5:00 pm   #13
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Default Re: Gould/ Advance OS3000A/ PO Oscilloscope 16A

I was up a pole today when it occurred to me that this might be ripple on the power supply.
The regulated supplies are spot on dc wise, but I haven’t yet checked for ripple going in. Actually the EHT oscillator is powered from an un-regulated DC feed. Hmmmmm.
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Old 23rd Oct 2019, 8:50 pm   #14
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Default Re: Gould/ Advance OS3000A/ PO Oscilloscope 16A

Well. I was nearly right. In my first post I mentioned the strange effect looked like gaps in the trace. And that's exactly what they were.
All voltages and signals on the Z-mod/Bright up board looked normal. Although this was disappointing in one respect it did eliminate a section, and I found all the 2,2 meg resistors in the "low" grid resistor chain had risen significantly in value. Luckily I had some replacements so those went in.
As the cathode and grid volts were low my next stop was going to be the EHT module. I was given a clue completely by accident. While clearing some stuff off my temporary work table , the BNC plug of my scope probe was dangling and hit the EHT box. When it did so I noticed the trace flicker. A few more "scientific" taps verified the phenomenon.

Inside the EHT box, the voltages around the regulator transistor for the EHT oscillator were about right( so hopefully nothing dragging down the EHT) so I decided to probe the collectors of the oscillator transistors with my spare, spare scope. The moment I touched TR815 the trace disappeared. this was down to a broken track on the board. A thin wire looped round the transistor's ,now padless collector leg and taken to the next nearest solder pad in lieu of the missing/broken track cured the problem. This was the transistor I replaced earlier to fix the original “no trace fault. It must have been suffering as the new plastic spacer I fitted had started to melt. While I was in there I checked the 3 electrolytic capacitors ( all good) and fitted some decent heatsinks to the oscillator transistors.
It's been on for about 4 hours now, with rock solid trace and good brightness.
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Old 23rd Oct 2019, 10:10 pm   #15
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Default Re: Gould/ Advance OS3000A/ PO Oscilloscope 16A

it reminds me of the saying"dont fix it if it ain't broke,or keep fixing it until its broke"! lol
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Old 23rd Oct 2019, 10:24 pm   #16
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Default Re: Gould/ Advance OS3000A/ PO Oscilloscope 16A

Quote:
it reminds me of the saying"dont fix it if it ain't broke,or keep fixing it until its broke"! lol
Yes. This fault did actually manifest BEFORE the no trace one, so I wasn't entirely to blame. I have reached the age of 52 without having to wear glasses, but not for much longer I fear. I'm going long sighted, which is rather inconvenient for close up work, as I have difficulty focussing in poor light. Arms length and distance, such as driving is fine, but I'll have to get it sorted or soon my arms won't be long enough.


I have been fiddling some more with the OS3000A, and it seems to be working fine now, although the calibration is slightly out.
The post repair cathode voltage was about 1320 volts, but I had been tweeking the set EHT pot to see if the oscillator feedback loop was working. A quick twiddle brought it up to it's recommended value of 1500 volts.
It's all back together and actually I quite like it. Although its only 40 Mhz( fast for someone who is used to skip find/college throwout scopes) that's going to be perfectly adequate for vintage radio/audio and mucking about with electronics.
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Old 30th Oct 2019, 9:18 pm   #17
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Default Re: Gould/ Advance OS3000A/ PO Oscilloscope 16A

Had it running for 12 hours the other day, and once warmed up the timebase is spot on(well within accuracy possible due to thicknesses of lines and guesstimating scales, inherent parallax etc.) with various frequencies checked with a frequency counter. Y volts div is about two small divisions out on every range on both channels. Both channels are the same though, and the same wave applied to both Y channels simultaneously fit over each other perfectly.
So just some tweeking needed really.
Unfortunately I do have severe distortion on one part of the screen, which I think may perhaps damage or something inside the CRT. if you arrange the trace to miss it the rest is fine.
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Old 30th Oct 2019, 11:38 pm   #18
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Default Re: Gould/ Advance OS3000A/ PO Oscilloscope 16A

Here are some traces. It's possible to arrange the trace to miss the "anomaly", but I expect I will just block it out mentally and work around it.

40 Mhz? Yes but only with all the stops pulled out. Only just with x10 enabled on the timebase.

Other wise the trace isn't bad.
For a free scope it's definitely worth the money!
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Old 31st Oct 2019, 12:17 am   #19
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Default Re: Gould/ Advance OS3000A/ PO Oscilloscope 16A

Hi Tim,
It's good to see that somebody else is playing with one of these! They don't seem to crop up very often, but I've had one since I was a teenager. I remember driving down to 'Stewart of Reading' to buy it second hand in my parents car (Austin 1300 of course!) when I still had my provisional licence!

It served me well through my days of repairing Tv's, vcr's etc and then into timelapse security machines. I've always liked it to use and it's been pretty reliable until a year ago when it suffered insulation breakdown on it's heater winding.

Since getting round that problem with a separate heater transformer, there has been a cascade of failures, the latest being a very non linear trace on some timebase settings.

I was kindly given an OS3000 for spares ( sadly with a soft tube) so have used the timebase module from that for the time being.

I've pressed an old Hameg HM203 into service to repair the OS3000A, You definitely need a scope to repair a scope!

Good luck with yours
Cheers
Nick
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Old 31st Oct 2019, 12:29 am   #20
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Default Re: Gould/ Advance OS3000A/ PO Oscilloscope 16A

Hi Nick

Yes, until recently I was nearly scopeless. My former scope, a Gould 2035 DSO let the magic smoke out of it's transformer, and since then I have had to make do with a Tektronix T912. Nothing wrong wit that, only mine has a knob missing on Y1, so is effectively only single trace. Y1 does work, but it's stuck on the 200mV/Div range.
I recently acquired a Philips PM3240, and had a few anxious moments when the triggering wouldn't.
Everything checked out and the fault magically cleared when I unplugged and re-connected the trace chop multivibrator feed back to the timebase. I think the logic had got stuck in the wrong state, but anyway it's OK now.
Interestingly the Philips has a max V/div of 2 volts(x1) which means only 16 volts p-p(or 160 with a x10 probe) can be displayed. It's probably more suited to working on solid state stuff, whereas the 3000A has 50v/div on x1, ( so in theory 500v /div with a x10 probe) so great for valve work.
I'm getting a soft spot for it, so we'll see how it goes.
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