HP54501A fails A/D and D/A converter self test. Thoughts on repairing?

[mr_coffee]

I have a HP54501A 4-channel digitized scope and it just doesn't show anything except baselines.

The HP54501A manual online just says to send the mainboard back to HP if it fails the A/D and D/A tests. Everything else seems fine. I suppose I'm lucky the power supply and monitor all seem to be working OK, as are all the buttons.

Does anyone know of this is a minor or major repair scenario? I am a fair troubleshooter. Are parts likely to be unobtanium?

Thanks in advance for any experience you can share.

[Runco990]

I had one of those.  I believe I had to replace the battery or Dallas chip in it and do a self calibration.  That should fix it.

It's been many years, but I'd check that first on any HP scope of that era. 

[mr_coffee]

So the Dallas chip and it's battery could cause the A/D and DAC to fail the self test?

[Runco990]

Yes.  In fact: 

I had the same problem.  Followed this video.

[Stray Electron]

So the Dallas chip and it's battery could cause the A/D and DAC to fail the self test?

   Probably not IMO, but I could be wrong. I have used two of these old HP scopes in the past week and both worked fine and both pass the self test but I suspect that the batteries in them were long dead. Bad A/Ds are pretty common in these because of people applying too high input voltage.

  If you do swap the Dallas chip, post your process here and your results. The 54501s are only 100 MHz but are still good and useful scopes, IMO and they have a nice large display.

[Kleinstein]

It seems that a faulty cal data set can cause the ADC/DAC fault error message.  So definitely have a look at the self test and see if the battery buffered RAM is bad. If bad, maybe also try the load default values step as there is a good chance the data would stay there till the next power down. So one could check if the ADC is actually working, no to spend money on the RAM only to find out that the ADC is also bad.

[mr_coffee]

When I run the full self-test, the protected non-volatile ram also fails self test along with the A/D and DAC.

On startup, the screen also shows

"cal-ram checksum error
re-cal instrument"

So I should try to do the re-cal FIRST which could work OK even if the Dallas module battery is dead, and it might PASS the self test even if the calibrate is off with the default values?

Also,

Other than trying to find a real, recently-manufactured Dallas replacement, which I see from the videos on youtube is soldered in, (I'm with the guy who said "Why no socket?"), but anyway...

Is it possible to just remove the battery-backed ram module and do some "surgery" on it to remove and replace the battery inside the module? OR...

Maybe I'm being naive or ignorant about the battery backed circuitry, but it seems that an ordinary RAM IC with a diy backup-battery add-on would make as good or better sense.

Thanks for ALL the help, guys. You are great!

[Kleinstein]

So I should try to do the re-cal FIRST which could work OK even if the Dallas module battery is dead, and it might PASS the self test even if the calibrate is off with the default values?

I think this is worth a try, as the CAL data are bad anyway. So not much to loose and a chance to confirm that the ADC is still OK. The re CAL is mainly just recall defaults.

AFAIR there are some reports on removing the original battery and adding something on to it. I remember having seen X-ray images of the RAMs. So one can get a pretty good idea on where to expect the battery. There were also RAM versions with an exchangable battery clip, but these were rare.
One could give it a try and if it fails one could still buy a new complete RAM module with battery.

[mr_coffee]

Hooray!

I have done the "restore default calibration" and run the recalibration routine, as suggested by Kleinstein, which now results in PASSING ALL  the self test routines.

THANK YOU for all the help!!!!!

I have searched for x-ray images or other DIY solutions to restore storage of non-volatile SRAM, but haven't been able to find anything about it so far.

Does anyone have recollections about where that x-ray view might have been posted, or other ideas about replacing battery (or other) backup?

[mr_coffee]

NEVERMIND - Found what I needed.

For posterity



Thanks everyone.

[Kleinstein]

An xray may be shown in the "signal path" site/blog when changing a RAM with an 3458 or similar.  I know he did some 3458 repair and has a x-ray maching to make pictures.

[Runco990]

Well, I am glad that worked out for you.  I have done some grinding, sever the battery and then piggy back a button cell on top the NVRAM before.  It's not too difficult.   

[mr_coffee]

Came across this other NVRAM chip from ST, the

STMicroelectronics M48Z35Y-70PC1,

in a 28-pin Dip package with a "cap hat" that appears to hold the battery.

Anyone had any experience with these?

[Runco990]

I have not..... but you COULD try a FRAM replacement... put a socket under it regardless.

[mr_coffee]

Hi Runco 990 and all,

I asked the youtube video guy linked above, and he said he just connected the positive of the new battery to the positive of one of the old batteries, which I wonder about since that would put the new coil cell in parallel with the old depleted one. I don't know much about the business of leaving the old one in circuit, but my instinct is to be a bit leary of doing so. It looks like he cut into the side of the coin cell in the process of reaching the positive side of the depleted cell, so maybe opening the cell may make the old battery less of an issue. That's beyond my knowledge level.

You said you "severed" the connection to the old battery; how did you figure out how to disconnect one of the old cells and attach to the positive connection, leaving the old coil cell out of the circuit? I have no idea where to "dig" in the old DS module.

Any recollection you can share on that would be really helpful.

Thanks soo much for sharing.