HP 8594E: FREQ and AMPL CAL Fail

[Bicurico]

Hello,

One of my HP 8594E has developed a problem. It went suddenly out of calibration and shows corresponding error when booting.

The device works as it is, but all frequencies are about 34MHz wrong: for example, the local DVB-T transpodner at 754 MHz shows up at 720 MHz.

I was in some case able to carry out the FREQ CAL routine using some different input signal (actually my DVB-C band from my TV provider). This allowed to pass the first two steps of the frequency calibration routine and after that (a few seconds), I would plugin in the CAL OUT again. After running FREQ CAL this way once, I was then able to run FREQ CAL properly with the CAL OUT connected from start. I know, sounds strange, but worked for a few weeks.

The AMPL CAL always failed and I did try follow the service manual to tune the chrystals. While the intermediate steps with measurements showed improvement, I never got the device to pass the AMPL CAL.

At the current state, I am no longer able to get the FREQ CAL.

Since I own another two HP8594E (one working, one not - no PSU and motherboard fault), I took all boards from the IF section case and one by one swapped them over: no effect, I still have the same problem.

So now I am stuck without ideas. Swapping fillters, YIG, etc. required to dissamble two devices completely and I am not in the mood to do so, without being sure that I will have some chance of success.

Any idea, why the frequency is off by 34MHz? Can I adjust this somewhere?

Any suggestion on what could be broken?

I don't suspect the YIG at this moment, as the device does work and shows the full spectrum...

Thanks for any help.

Regards,
Vitor

PS: If you have suggestions, please break them down, so that I can understand them. I am not an RF engineer and my knowledge is limited when it comes to such kind of repair.

[Bicurico]

So this is what I did today (without any success):

I have one HP8594E with broken PSU. If I fit a working PSU, it will turn on, but the screen remains black - even if I connect an external screen through Composite Video Out on the back. The CRT is working. I am pretty sure that the main CPU board is broken.

I have the other HP8594E which shows random errors (ADC GND FAIL mostly, but not always). It fails to run the calibration and hence the LOCK OFF is displayed asking for CAL to be carried out.

This time I transplanted the whole motherboard to the other unit. It will turn on, but show the same ADC GND FAIL. Plus I got no trace at all.

I put everything back and the units behave as before. One does not turn on/show anything on screen, as if the CPU is dead. The other has a frequency offset of around 45MHz and this fails the FREQ CAL routine. The confidence test does fail, as the AMPL shape fails. This might be due to misaligned crystalsm, though.

Anyway, now I know that I do have an issue with the mainboard (CPU board). It is at least causing the ADC GND FAIL. The description of this error is:

ADC-GND FAIL
During an instrument preset, the analog-to-digital converter reading for ground on the A16 Processor/Video assembly is outside the test limit. An instrument preset routine is also performed during the self-calibration routines.

Any ideas how to fix this and if this has to do with the frequency offset (i.e. a signal at 754 MHz is shown on the device at 708 MHz). Any way to go to a secret menu and enter a compensation frequency offset? Is there any screw to tune this?

Thanks,
Vitor