Okay, I think I give up.
I have now replaced all 63V or less rated the caps inside the PSU and readjusted the Reference-Voltage.
The 14V-Rail is now about 0,5V below spec, the 5V-Rail sits at 5,08 to 5,19V, depending on the measured test-point.
The Scope is a bit more stable, but it still locks up after some time.
Measuring the Status-Signal of the ADC and the DataAvailable-Testpoint on the DPU, I noticed that the DAVA-Signal stops when the trace-updates stop, and so does the ADC-Status Signal.
If increase the timebase, the ADC-Status Signal returns at the switch from 200µs to 500µs, but there is still no DAVA-Signal and the traces do not update.
I suspect the reason to be either clocks (I can't measure those, as the PM3350 only goes to 50Mhz and shows only a faint amplitude at the 100Mhz Crystal and almost nothing at the 125Mhz crystal), or the DPU itself, with a tendency towards suspecting the DPU.
@guido:
Unfortunately, I don't know exactly what the PLS is supposed to do. The Service-Manual only states that, together with the SRAMs, it generates control signals and a part of the instruction-addresses for the Data Processing Unit that is entirely made with 74-Logic chips.
Because the chip seems to work properly for at least a time, I think it could be possible to read it one or two times before it misbehaves.
I am currently trying to get another PM3320A (about half a year older than mine and with the FFT-Option), but it looks dark: Current bid: 215€ and 5minutes to go. - Didn't get it -.-
AAARRRRGH!
Why can't it decide whether to work or not?
?
I pulled the PLS-Chip and a 74LS125 out of their sockets (the only socketed ICs on the DPU-Control board), and carefully sprayed the contacts with Kontakt 60 Cleaning agent, cleaned it with a metal brush, sprayed Kontakt 2000Gold on it (Oil for connectors) and also botched a 10µF/16V SMD-Cap between the legs of a 100nF ceramic cap.
The scope ran for about 20minutes with the PM3350 and the DS203 connected to the Testpoints on the DPU-Control board, showing the various status signals.
The signals that disappeared when the trace-updates stopped were: Interrupt-Level 2, DataAvailable and TransferReady. Clock Register and EndConversion remained active.
I think this pretty much eliminates the possibility that there is something wrong with the clock and isolates the problem to the DPU-Control board, a.k.a. Module A8