HP 54502A repair once again

[Smith]

Looks like the nvram battery is dead. It' s quite common after over 20 years. You can replace the module or connect an external battery.  I did replace mine with a new one. Google for hp 54502a nvram and there are lots of photos and info to be found.

[vanhouten]

Hmm, I replaced the NVRAM around 3 years ago when I got the scope, so I assume it might still be alright. After a self-calibration it has been working ever since until the PSU gave up on me.
I recall that one can even do a successful self-cal with a flat NVRAM-battery, it just looses its calibration data when the power is turned off.
In calibration mode the display tells me that the connection between the DC-Calibration-terminal at the back and the channel input might not be properly established (which I can rule out).
There is a static notice at the top of the screen "failed interpolator cal"... if that provides any clue to anyone more familiar with this scope than me.
Attached are screenshots of the display in normal operation after power-on and in calibration mode.

Is there a way to check if the input attenuators are still working at all?

Thanks again for any feedback!

[vanhouten]

Update:
I double-checked every connection and found merely by accident, that the 3.5V-rail of my PSU-assembly wasn't stable at all times. Turned out that the ribbon-cable socket had loosened up just enough so that the 3.5V-line was slightly wiggling.
After properly reseating the cable everything looks much better  .

So despite not being mentioned in the service manual at all, on a 54502A all these symptoms are directly related to the 3,5V-rail:
-> failed interpolator cal
-> static faulty offset of scope-trace
-> AD/DA-sections fail in self-test
-> Analog Trigger fail in self-test
-> Acquisition-RAM fail in self-test
-> Time-Base fail in self-test

Just ran calibration and self-test once again and all turned out well!
So for the time being I consider this case closed...

Thanks a lot wn1fju & Smith for your support and sticking around!

Best regards,
vanhouten

[Smith]

I'm glad you got it working. I really like these scopes. Technically they aren't that great compared to modern equipment, but I love the interface.

[wn1fju]

For vanhouten:

Glad it works.  I would be interested in knowing if you see any noise problems attributed to the replacement supplies (especially if you used the same two Meanwell supplies I did in my 53310A). 

If you could cap off the inputs, set the timebase for something reasonable for a 50 kHz to 100 kHz signal, and turn the Volts/div settings to their absolute minimum, do you see any transient spikes?  You probably won't be able to trigger on any of them.

In my 53310A, which is a modulation analyzer, not a straight oscilloscope, I swear I am seeing some artifacts in certain low frequency scenarios.  But since I didn't have the unit operational with the original Boschert supply for very long, perhaps they were also there and I just didn't notice them at the time.

[manneke]

Hello,

If there is still aneed for 5450x diagrams then a free set can be found at  www.radiocollection.be

search thru the repair reports ot the author

good luck
Gilbert