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-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of James via groups.io
Sent: Sunday, January 5, 2025 1:13 PM
To: [email protected]
Subject: Re: [TekScopes] 7904 pre-260000 PSU crackling

Hi all and thanks for the replies and help. I'm pretty sure it's in the PSU but I will reconnect it all (again), fire it up, and try the trick with the microphone in a plastic tube. Trouble is, I'm 90% certain it comes from inside the PSU and when I had the scope turned on in a darkened room I couldn't see any corona. My sense of smell isn't the best but I detected no ozone. But I will try again and report back.

I did quite thoroughly clean the HV lead and the connector originally, and cleaned the cap multiplier lead when I had the PSU apart. They were totally blackened with this oily grime, as were all the wiring looms. I imagined that this could lead to arcing and other unwanted stuff, so it seemed a good way to start. It's quite astonishing that the thing worked at all!

What is the dog house hole? Is it the slot in the middle of the PSU panel where the HV lead and other wiring comes out? Once I have that confirmed I might try getting another plastic tube and seeing if I can sniff ozone through that?

Re bumping the HV lead, yes, I have tried it. No change, sad to report.

Currently waiting on a supply of tantalums and other bits to replace those present on the other boards, also a tool for inserting the connectors into new molex connector plugs before the connectors totally fall apart (the blue ones are already partially absent, I'm concerned for shorts).

On 1/5/2025 10:56 AM, James wrote:

Thanks Zen, much appreciated.

I checked the main caps fairly cursorily for capacitance, which was textbook value, but didn't check them at proper operating voltage for issues. I will certainly do so when I pull the PSU apart. I forgot to mention that the crackling continues (decaying) for a minute or two after switch off, maybe that helps narrow the culprit down?

Thanks for the info about the resistances, understood!

Re the neon, my poor description, it flickers dimly (like most neons do) when the psu is switched on and very quickly stops after it is switched off. It doesn't blink though if that matters. I note that the reservoir caps don't hold charge for any time after switch off. The switch, like all Tek switches, looks built to last forever. It functions fine thankfully.

My understanding is that the PSU in the 7904A is rather different but have no experience to back that up.

-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of James via groups.io
Sent: Sunday, January 5, 2025 9:34 AM
To: [email protected]
Subject: [TekScopes] 7904 pre-260000 PSU crackling

Happy New Year everyone!

I have a 7904 s/n 215841. I got it at an auction in the UK where I live, where there don't seem to be many of this series available, and it was FILTHY when I picked it up. Knowing the tantalum issue I immediately replaced as many of the PSU tants as I could get at without dismantling the unit completely, and removed as much of the grime as I could which was like oily black soot and covered all the circuit boards - got a surprise when I discovered that the HV line was bright pink! Now it works, the PSU comes on and the voltages are within spec, screen is bright (albeit not a patch on my old 545 - is this normal?).

The issue is that almost immediately there is a slight ticking noise inside the PSU. It starts as a fairly regular tick (every second or two) but after ten or twenty minutes has become more of a quiet crackling noise. This is far lower in volume than eg. a start up tick. There was no visible light from arcing when looking in a darkened room but I didn't want to stick my face too close, also there is no discernable burning smell. After the crackling develops the voltages remain steady, the screen doesn't change. I dug into the PSU, lord what a job, to look for anything I could find but apart from plenty of grime didn't find any tell-tale carbon arcs or blackened components. I had a look inside the cap multiplier but, again, no obvious arcs. The neon that is on the inverter board had stopped working so I replaced that, changed out the 470 Ohm sense resistors R1429 and 1439 which were burnt but still near-ish to spec, reassembled, and tried again, more in hope than expectation. No change. My questions are:

1. Can anyone point me at the likely source of the crackling? I am fearing it might be the transformer futzing out, but have no idea, after pulling the PSU apart, how I could test it reliably (and safely). If you think it is that, can it be rewound? I can't do this sadly, but I do know a couple of professional trafo winders. If the symptoms scream "cap multiplier" at you I do have a used spare that is labelled good, bought from Sphere's old stocks but, before replacing it haphazardly, I wanted to ask those with more experience than me how to proceed. That's all of you I suspect...

2. The service manual suggests testing impedances on the LV regulator board test points with the power wiring looms connected and disconnected , however their advice is to investigate if the measurements are "significantly lower". But these are for s/n 260000 and up. My PSU is not much different to the circuits in the Tabalabs .pdf. However, my measurements differ from the specs given in table 4-3 as follows:
+50V TP with wiring loom connected - spec 3.4k Ohm, measured 2.95 kOhm. Is this significantly lower? Disconnected, the regulator resistance rises to 23.6 kOhm.
-50V TP with wiring loom connected - spec 270 Ohm, measured 3.74kOhm. Disconnected, the resistance is 4.34 kOhm. This appears significantly higher but is it an issue. Thoughts?

3. The manual suggests testing the Pre-reg power supply across R1287. The waveform is more or less like that illustrated in 4-4 (A) showing correct operation. Looking at the burst voltages at the test points given in table 4-4 there are stable voltages with little evidence of bursts, and they come at 25 kHz rather than 100 Hz intervals, I assume this is normal?

4. The new neon bulb on the inverter board is supposed to blink to indicate charge on the reservoir caps. Mine flickers dimly and continuously. Is this normal or does it indicate something wrong?

Any and all help, advice, and pointers gratefully received.