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-----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.

----- Original Message -----

From: "w.m.d.koch via groups.io" <w.m.d.koch@...>
To: "tekscopes" <[email protected]>
Sent: Friday, January 3, 2025 11:02:34 AM
Subject: [TekScopes] Adapter PCBs for 148-003x-00 relay replacement

Hello all,

after quite a few years of absence from this group and from my trusty
7000-series equipment, I am back.
The reason is quite obvious. I have a special measurement job which needs my
7A13.
As expected, after almost 10 years of sitting around my 7A13 has quite a few
issues, but this is a story on its own.

As the subject already tells all the relays within the 7A13 have gone bad. Some
seemed to work fine, but on closer inspection every relay has its own problems.
Mostly contact resistance of the relays is highly unstable. Sometime you can get
the relay down to 100mOhm, sometimes the same contact has up to 10 Ohm.
Of course, I tried all the recommended reviving techniques like repeated
switching at high frequency under load, etc. but nothing really helped.
I remember that I replaced 2 of the relays already 10 years ago by a self-made
free-floating solution to fit a new relay into the old socket.
This was quite tedious and I don't want to repeat the same thing again. Leave
alone the fact that this free-floating solution might not be capable of
providing the right HF and Isolation performance.

By searching for a better solution in this group I saw the solution of Tom Ford
back in 2019. (/g/TekScopes/album?id=91736)
Unfortunately, he didn't provide any design data and he seems to be offline
since then. Therefore, I went to action and designed my own little adapter PCB.
And I will also try to document my work as good as possible, to give anyone
else enough information to do this on his own. Related pictures can be found
here and maybe on my GibHub. /g/TekScopes/album?id=299851

My first Idea was to replace the internals of the original TEK relays with a
modern relay with the original TEK Housing still in place. Therefore, the PCB
had to be so small to fit inside the original housing and connect to the relay
and the header pins.
In addition to the relay there also should be a small bridge rectifier to get
rid of the polarity issue most modern relays have (most modern relays are
polarized) and optional a little SMD LED to see if the relay is activated.
As the space within the relay is really tiny, I came up with the idea of using a
2layer Flex PCB which would be bent and hold the relay, a bridge rectifier and
an LED + resistor.
This concept (concept 1) looked really good but I had quite a few problems:
1. Crosstalk between channels and isolation between open contacts was not that
good because of relatively long tracks in the flat flex running in parallel.
2. Flux residue between the flat flex and the header caused leakage which cause
"enormous" leakage currents. Not the required +/-0.2nA more the +/-15nA or even
more.
3. Due to the tight space within the relay the bending radius of the flat flex
was quite tight and only in the range of 1-1.5mm. This caused on some trials
problems with broken tracks.
All in all, the ideas sounded great but the realization was not the great at
all.

For my next concept (concept2) I reverted back to the idea of Tom Ford and
created my own little adapter PCB for both needed relay types used in the 7A13.
At the moment this is an unproven design as the relays are still on back order
and I'm waiting for the distributor to dispatch my order.
For the relay I did a longer market study to find the best fitting relay. In the
end I decided to use the AXICOM IM06TS which is really tiny and still provides
relatively good switching and HF performance.
The footprint of the relay is a little issue as the row spacing between the pins
is only 5.08mm. This makes the pads slightly interfere with the through hole
pads of the socket header. To still accommodate the socket header and the relay
on the same PCB without interfering I decided that the pins of the relay need
to be bent outwards a little (~.7mm per side). This is not a really nice
solution but I assume it will work fine. If the socket header would have been
available in a SMT version the use of a SMT relay would have made everything a
lot easier.

I presume Tom Ford used the BAS3007 as bridge rectifier. Unfortunately, this
part was not available at my favorite distributor. Thus, I selected the
BAS40DW-04 which is not exactly a bridge rectifier, but it can be connected as
one while still being really tiny.
As I already wrote I also added a small surface mount LED together with a
resistor to the PCB because I experienced on my unit some issues with the
Cam-Switches which could have been easily debugged if there had been a little
indicator in place.

All in all, the BOM for a 148-0034-00 relay replacement looks like this, for the
148-0035-00 it is basically the same, you only need to remove some pins from
the pin header rows.
1x Relay (IM06TS)
0.1x Pin Header (W+P 952-09-080-2 cut into 10 pieces)
1x SMD Resistor 0603 1.8kOhm
1x SMD LED 0603 yellow

The resulting PCB is measuring 8 x 14.5mm and is still smaller than the original
relay (8.5 x 16mm). My favorite Chinese PCB manufacturer charges me the same no
matter if the PCB is 8x14.5mm or 100x100mm. Therefore, I decided to make my own
custom panel which utilizes the 100x100mm as good as possible. Now I have 5
panels of 66PCBs each lying next to me waiting for assembly. Each PCB holds 36
148-0034-00 compatible PCBs and 30 148-0035-00 compatible PCBs.

As already stated above I was not able to do any tests, but I will definitely do
tests as soon as I have some relay. As soon as my concept of the PCB is tested
and proven good, I will give you all a follow up on the status and will also
publish the design data in case you want to replicate or improve my design.

I will not need all 330 PCBs, maybe 15 to 20. Therefore, I am willing to sell
these to anyone interested in getting some relay replacements. As I also
ordered a solder paste stencil I will wait how big the interesst in these PCB
is before i start populating a whole panel. I think it would be the easiest if
I sell these PCBs with the SMT parts already soldered on.

Best regards,

Wolfgang