Repairing the aquisition-system of the Philips PM3323 Oscilloscope

[SaabFAN]

Alright, second try!

I've got a new PM3323 with some goodies in the box. Among them a Keithley 6147 Triax to BNC-Adaptor (at least that's what the sticker on the bag says) and two 300 MHz probes.

Unfortunately, the scope was badly packaged: The only shock absorbing quality of the package was the fact that it was in an old cardboard box that was inside another old cardboard box. See pictures for the results 

Good thing I've held on to the other PM3323 - It's Frankenstein-time! 

The "new" scope exhibits the same noise on both channels that's expected from the PM3323.
I've ordered caps (I'm going to use electrolytics this time as I suspect that using ceramics was the reason it didn't work last time) and will replace damaged parts with good parts from the other unit where necessary.
The tube looks good. Sharp picture and very bright. And inside the scope there's no visible buildup of dust. Like the unit sat in the attic for most of its life.
Once I have replaced all the SMD-Caps, I'll post if it works again.

btw. The Triax-Adaptor is for sale.
https://www.eevblog.com/forum/buysellwanted/keithley-6147-male-triax-to-female-coax-bnc-adaptor/

The last picture isn't showing the damage to the shaft of the rotary encoder properly - That thing is bent about 10 degrees and unusable.
The knob beside it bites into the plastic of the front panel and is stuck. The remaining elements of the front panel seem to be ok.

[SaabFAN]

I've replaced all the caps, which included having to repair two traces that had been eaten away by the electrolyte. One pad was in particularly bad condition and I had to resort to "unconventional" methods. Including using some WD-40 to break through the corrosion - I do NOT recommend this method to anyone unless there's no other way to get through a 1mm thick crust of corroded metal (usually there is) - and using tons of flux with the soldering iron turned up all the way. In the end, I managed to remove most of the corrosion and the new capacitor has contact now.
The traces I had to repair were the -7V Supply-Trace to a LM337 that is surrounded by capacitors and is responsible for creating the CCD-Gain reference voltage, as well as a GND-Connection of a cap close to one of the LM317 that create the 9V-Supply for Channel B.

After this treatment, Channel B works pretty well. It's out of calibration, obviously, but I get a clean signal with only very little amount of noise.
Channel A on the other hand still doesn't work. Down to 500µs/div I was able to get a trace on the screen by playing around with the potentiometers (which means that the CCD should work), but there's absolutely no sign of any input-signal being converted.
At faster timebases, which use the "time conversion"-method, the trace jumps up high and leaves the screen at the top.

2 possibilities: A problem with the clock-drivers of the CCD, or the circuits responsible for the leakage correction aren't working.

[SaabFAN]

Well, channel A gives me a trace on screen now.
The problem was that the trace from a 2V Zener-Diode, which is responsible for providing a -2V Rail for the Substrate of the P2CCD-Chip via R8089, was interrupted by corrosion right next to the solder-pad of R8089 (see pictures).

I bet destroyed traces were also the problem with the PM3323 I tried to repair earlier, which then failed rather spectacularly

The symptoms were pretty clearly centered around the P2CCD-Chip of Channel A (with the added problem that my Scope-Probes sometimes had problems breaking through the oxide-layer of solder joints), with Channel B working sort of (Noise and jumps of the trace at timebase-settings slower than 200µs/div).
Using Channel B as a reference, I walked through the entire signal-chain and discovered that the outputs from the P2CCD-Chip of Channel A were at the same DC-Level, but their amplitude was 10x higher.
With everything set exactly the same on the input, I started focusing on the output-side and then the supply of the Chip - I know: Supply should be checked first, but because everything else around the chip worked, I assumed supply-voltages to be ok.

After repairing the trace, Amplitude of the outputs was basically the same on both channels and by following the procedure for the pre-adjustment of the P2CCD-Chips I managed to get the picture on my measuring scope that is specified in the Service-Manual.

Searching in Forums, it seems a lot of people have the same problem (Channel A or B being outside the screen), but nobody wrote about the cause, so hopefully this will help others fix their scopes: Channel A / B being outside the screen = Check Substrate-Voltage - Mine is at -1.409V and pretty much the same on both channels.

Next I'll check why the 14V-Rail is only at 13,15V and if there are other rails that are out of spec. I haven't seen much noise on the supply-rails so far. At least on the P2CCD-Unit the rails seem pretty clean.

What can I do btw. to prevent any further corrosion? I've cleaned all areas with a brush and Alcohol and during soldering I used large amounts of no-clean Flux. Is that enough or are there additional methods I could try?

[tautech]

............
What can I do btw. to prevent any further corrosion? I've cleaned all areas with a brush and Alcohol and during soldering I used large amounts of no-clean Flux. Is that enough or are there additional methods I could try?

Aerosol based PCB lacquer.
Not all rework jobs call for it but you'l find uses for it in many things that are in adverse environments.

I use it for self etched PCBs to protect the copper from tarnishing until you get around to populating the PCB. No worry about removing it as most of them are solder through.
This is what we commonly use in NZ:
https://www.jaycar.co.nz/circuit-board-lacquer-spray-can/p/NA1002

[SaabFAN]

I've managed to get through most of the calibration-steps and get the majority of the adjustments done (I stopped because it is too hot here now to work on this scope).
Unfortunately, the scope still has a 14-Rail that sits at 13,5 V at the PSU, which becomes 13,15 V on the P2CCD-Board.
The 9V-Rails are a bit high (9,30 V), but considering the fact that everything is working at about 50°C ambient temperature inside the case, I think a little bit of drift can be expected by LM317-Regulators and their feedback-networks.

The 0,5 V too low voltage on the 14 V-Rail is probably caused by weak capacitors. Or one of the many 1 Ohm-Resistors in the power-rails has drifted up. The rail isn't watched and  only the 5V-Rail is regulated, and one other voltage monitored.

During calibration I stumbled across what seemed to be a problem with one of the potentiometers: I had it turned all the way in one direction and just barely managed to get the specified 6 div deflection.
But apart from that, I think I can call this repair a success for now
Front-Panel replacement was done in under 5 minutes btw. Much easier than anticipated.