Wandel Goltermann NFA - self test fails after readout of Eproms

[Apfelmus]

Hi all,

as the subject says... I had the "good" idea to make a readout of all the Eproms in the unit. When I finished I was rewarded with an error 61 on startup. Selftest showed errors in the digital part and threw up all sorts of errors in the analogue part, too. Error 61 points to the "send processor", it turned out a DIL switch was in the wrong position. Then error 61 disappeared, digital part all OK, but the analogue errors remained (all fail except Mixer2). Ensuing calibration fails, of course...

I did not do this kind of work for the first time... i have several SNAs where I did that without any problem. I only opended the NFA from the backside to gain access to the cards nest, and pulled out the digital cards one by one. So how could I possibly get analogue errors now?

The audio generator seems to work. Output is correct, level and frequency can be set. But the input seems like disconnected - it constantly reads some 42dB, whether there is a signal or not. I checked the synthesizer, and got the 10MHz signal but no output on the 4....6 MHz socket at the back. There is a link of the synthesizer to the co-processor card that drives the dividers through the "parallel interface card". If that link is broken somehow that may explain all the errors in the analogue part. So my main suspect right not is the output of the co-processor. There are ribbon cables connected from the co-processor going down somewhere to the interface card. I checked the cables visually, they look fine, the connectors look fine, too.

I tried the test-routines in the service manual, they are all OK. Now I'm trying out my HP5004 signature analyzer, but up to now I even cannot make the first test as I'm unsure where to connect the analyzer. The manual is a bit short on that. I tried various locations, but the "frame signature" is far from connect.

Any idea of what else could have gone wrong during Eprom reading are welcome!
If anyone has made a copy himself we could cross-check. My firmware is 6.1, the unit has an E.... serial.

And of course any help in signature analysis is welcome, too!!

cheers
Martin

[Apfelmus]

Update...

I found the list of nominal DIL-positions, and there was one on the send-processor that I had to correct. Strange, because its one in the middle, but OK - now I have no more self-test-errors on the digital part, the send-processor seems to work normally. But still all sorts of errors in the analogue part (all lines are filled with "1"s except Mixer2). There must be a common cause somewhere...

The generator seems to work (frequency and level can be set, correct sine at the output), but the receiver is stuck at some 42dB. The co-processor is supposed to read in these values, so I'm having a closer look into that. All self-test and internal-monitor routines are OK. W&G uses signature analysis to trace faults in the digital part. Having recently acquired an HP 5004, this is my first try with that type of tool...

I did a hundred measurements or so (there are many more in the manual), all correct except at one chip where the "0000" was correct but the probe did not blink as required. Its the output of a latch 74HC259 going to a control line on the Eproms. I reckon that means there should be short pulses outside the signature window but are absent. Maybe a short to GND?

Conspicous, since I fiddled with the Eproms...

cheers, so far
Martin

[YetAnotherTechie]

your fiddling is more likely to have caused an open than a short...

[Apfelmus]

yeah, and guess what: common cause turned out to be a shorted small tantalum in the power supply, dragging down one of the two -15V rails.

I wonder if these little chaps have a sadistic mind of their own to blow up the instant when I'm touching the machine somewhere else, misleading me to search anywhere but not where you usually start when something is wrong: in the power supply.

Thinking positive, I learned something about signature analysis! Never did it before.

cheers
Martin