RAM replacement on GOULD scope (4ch) - distorted traces

[Alex Eisenhut]

" I pulled the ASIC (U44, Acquisition Controller) from its PLCC socket and re-seated it. After that A9 looked proper on the scope and the problem was SOLVED! "



Well, looks like the scope is in good hands.

But I'll sleep better tonight if you can adjust those voltages a bit.

[Bashstreet]

" I pulled the ASIC (U44, Acquisition Controller) from its PLCC socket and re-seated it. After that A9 looked proper on the scope and the problem was SOLVED! "



Well, looks like the scope is in good hands.

But I'll sleep better tonight if you can adjust those voltages a bit.

I do not think those voltages are that bad to be fair although it highly depends on components in use as some are more sensitive than others.

Reminds me of Commodore 64 if the 5 volt line go to 5.3 it will highly likely soon destroy CIA chips among few others ... so might be good idea adjust the voltages if one suspects that sensitive chips are used.

[rg58]

Wow, what a DAY!

Took the whole day to investigate all the issues. Soldered 7 wires to the tube base PCB and monitored various voltages with DMMs and both X deflection coils.
On startup in the cold room again, the heater voltage came up as 6.3VAC right away. The X deflection looked quite weird I thought, and seemed a bit instable too. The 75V ACC voltage coming directly from the 75V rail and going to the CRT however started at 63V and moved up slowly, so I connected the other DMM to the 180V rail which showed just 160V increasing slowly as well. No trace to be seen. I was onto something here. The trace started to show up at around 67V on ACC (162V on the 180V rail). Both voltages stopped increasing at around 71V and 169V (target 75V and 180V) with bright trace showing. I previously tested both rails in the 'warm' state as 72V and 171V and didn't bother. I then changed the line voltage selector from 240V to 220V, even though I was a little concerned as we use 230V here (measured this as well). This way I got very close to the target voltages - now being 73V and 178V. But this still didn't solve the issue(s), although it seemed to have improved.

Next I took out the PSU and decided to go through all resistors with an Ohmmeter first. Three showed odd but turned out to be good when unsoldered, so I put them back. As the issue was heat related, I now took the time to take a good look at all electrolytic caps. As I didn't see much ripple on any of the power rails, I believed the big caps would be OK. So I started to suspect the small caps close to the voltage regulators. If they are leaky caps connected to the reference inputs, that could in fact shift the reference voltage (or if Vref has ripple). So I decided to replace all 5x 10uF and 1x 2.2uF (increasing 3 from 10uF to 22uF) - the blue axial ones that you can see in the picture. The old 10uFs measured 6.8-11.3uF out of the board, the 2.2 as 2.3uF. Not all too bad, but I did have bad experiences with leaky caps before. I consider caps as possibly leaky when the capacity shows up HIGHER than the nominal value, as if there was a resistor in parallel to the cap and the cap becoming conductive for DC.

So I attached an 8 Ohm load to the output of the L296 switching regulator on the +5V rail, giving me roughly 0.6A of load current. The 5V rail now was 4.93V and the -5.2V rail was -5.22V! Something had changed! So I put the PSU back in and checked the voltages. Still -5.2V which is good, but the 5V suddenly was back to 5.3V. I have no idea if this is a ground issue are whatever - outside the case it was 4.9V under 0.6A load, if load is higher (2.5A?) the voltage increases!? Possibly something to do with the ground connection of my DMM on the case rather than the PSU itself.

After I pushed the ON button, the traces appeared after just 3-4 seconds and the image was sharp and crisp. What a fantastic sight!!! All issues were suddenly gone. No long heatup time, no more fuzzy stuff, no more too low screen top on startup, no more blurry readout and traces.

So for the most part it comes down to the 75V & 180V being much too low when cold, little too low when warm, and the -5.2V rail which was 0.2V too low as well (-5.0V before). It was fixed by changing line input from 240 to 220V and the rest by changing 6 small(!) caps (10uF and 2.2uF). What a day!

I also fitted a brand new fan and 2 new light bulbs for the grid illumination that I got delivered today. The 3 SRAMs that I also received today I will keep as souveniers as I never needed them. They were just 2.15 EUR each.

Last issue to fix is a contact problem on the input of CH4, maybe a relay. The trace is moving up and down and is never on the centerline, knocking helps to move the trace randomly. Very last thing to do before closing the hood would be the calibration.

@Alex Eisenhut: You gave me the right clue already in the beginning with the PLCC socket, I just looked at the wrong one! And also with the voltages it seems. You're the man! Hope you didn't have nightmares about my 5.3V last night. Coz, it'll stay that way forever now - until it breaks.

@james_s: You were right. Thanks for taking the time explaining what a bad CRT looks like.

[james_s]

Nicely done, that thing looks great now.

I remember back when everyone still had CRT TVs the CRT was almost always improperly diagnosed. When a TV was dead, people always assumed "the tube had blown" even though that was *never* the case with that symptom. When the tube actually was shot "it just needs an adjustment!"

[stj]

yes,
and just as the public realised being told the tube was gone was a scam, we got widescreen.
and then all the tubes started suffering gunshorts!