Gould DSO400 failing Amp Shift calibration, no trace

[philpem]

Hey guys.

A colleague fished this machine out of the scrap a few years ago, and now it's mine (ho hum). It's a Gould DSO 400 digital storage scope, 20MHz, 100MSa/s. Nothing even remotely fancy, but perhaps a little interesting by virtue of not being made by one of the "big three".

It blew out the capacitors in the Schaffner mains inlet filter shortly after power up (having replaced those on Solartron 7150s, I can honestly say, I bloody hate those failure-prone little sods and the smell they make when they go BANG!). Besides that, no trace on the screen. Ran auto-calibrate, and it ran to completion, but no change in symptoms.

Switched to Service Mode (Control/9 - 777) then checked the Cal Status, all the Cal values are pegged. I'm getting gain 21.5%, offset 49.5mV and CCD -6, "V" errors for all ranges, "S" for a few, "TL" error at the bottom, a "CAL INCOMPLETE" annunciator... and for bonus points, a "Failed Amp Shift calibration" error. That last one is probably the smoking gun.

Before I go clearing my bench enough to open this thing up, has anyone done a repair on one of these before with a similar fault? Was it repairable? Simple, cheap etc, or one of the custom parts?

I'm trying to decide if it's worth my time... a friend wants a scope, but I'm fifty-fifty whether to fix this one or let the old girl rest in peace and give him my old Tek analog scope...

Cheers,
Phil.

[philpem]

I spent a few hours on the beast this evening... For anyone who wants to follow along at home, the service manual is in KO4BB's archive:

  http://www.ko4bb.com/manuals/download.php?file=06_Misc_Test_Equipment/Gould/Gould_DSO400.pdf
  http://www.ko4bb.com/manuals/index.php?dir=06_Misc_Test_Equipment/Gould

Fault symptoms:
  - CAL fails CCD offset cal
  - Calibration Hardware Check function reveals that none of the cal hardware is functioning. Gain slope is horizontal (should be a ~45 degree slope) and offset slope is also horizontal (permissible but not really ideal).
  - 1V calibrator output on the panel was as dead as a doornail, flat zero output
  - Traces can't be moved up/down the screen

First visual check revealed that the CMOS battery (a Varta Mempac NiCd) on the CPU board had leaked and muckied up the connectors and eaten a few traces on the CPU board. Those traces were repaired. Interestingly there are a few traces under the battery which seem to be routed through, then end abruptly! I thought I had three broken traces, when in fact I only had one... (I hope?)

On to Rule Zero, always check the rails -- this revealed that the 4051 which serves as an offset DAC for the preamp chip had no +5V supply. Checking further back, the +12V supply was gone too, along with the input to the +12V regulator in the Preamp/Trigger section.  This was traced back to a smoked 2R2 resistor, R12. Replaced that, rails came back up. Traces are still stuck doggedly in the middle.

Further checking revealed there was no VREF+ input to U307 and U314 (two DAC08HPs). These are fed with +5.2V via a 3k9 (one per DAC), and I'm seeing 0V at the VREF+ pin of both. Surely that's not right?
Or do these DACs load down the VREF input?
I must confess I'm not that clear on the how multiplying DACs work... most of my experience is with the more common voltage-output type.   

There's also another fault - the power rails seem to have a voltage spike (more accurately a short-duty square-wave, about 200ms high every second) which is causing the screen and FP LEDs to change in brightness. To try and rule out a problem on the AC side of the power supply, I hooked up a 12V supply to the input on the back. A few seconds later, during the self-calibration phase, the sodding 2R2 resistor blew again!    
This after an hour or two of running fine (ish) off the mains!

I swear this machine is possessed...     

[le captain]

I Recently repaired a Gould DSO400.

Being puzzled by finding so little online I decided I would comment here to share what problems I encountered during my repair
And hopefully help anyone else who is willing to give one of these scopes a new life 

Most of the problems in the scope where power supply (AND sub-power supply) related
Even more specific tantalum capacitor related .

I was recently reminded that "If tantalum caps go bad ,they plainly short out"
With that knowledge in mind I started measuring out all the tantalum capacitors with a multi meter in diode function.(this helped me so much)

Lo and behold i found 4 shorting tantalums, all in sub-power supply's (i remember C75)
Which in turn blew their 2r2 "fuse"resistors.(R12,R25)

After replacing these with some plain electrolytics the scope fired up beautifully
(the machine has some age, so components turn bad sometimes, especially capacitors)

Another thing is that the CPU battery was spilling battery goop on the CPU pcb.
I had to do some minor repairs to some broken traces,sprayed some contact cleaner on the big connector between the CPU board and the main board and put a fresh battery in it.

I also had to replace the IEC power socket (with internal line filter, which blew up while testing the machine, it does not smell nice!)

I'm quite enjoying my repaired DSO now