HP E3631A triple output power supply

[tribble222]

This is my first time trying to repair anything like this 

When powering on, the display lights up fully, then shuts off. Nothing I do illuminates anything on the display after that. Voltage setting is correct on the back of the device.

I hooked up my oscilloscope to the serial interface where the power supply's top board should be communicating to the display, and I don't see any activity. The clock goes from 0 to 5V and stays there, and so do the DI and DO lines. So, unless I'm wrong, the board isn't even trying to communicate to the display.

The power supply also doesn't beep. According to the manual it's supposed to beep twice if the display communication fails, but it acts exactly the same whether or not I even have the display cable plugged in.

As a test, I kept the reset pin of U17 high while powering on. This time the power supply made a single beep. So the speaker works.

I checked the bias supply voltages as indicated in table 5-1 on page 99 of the service manual and they were within spec.

I checked the crystal for U17 and it appears fine. I checked U16's counter output, which is functional.

I'm not sure what else to test since the problem seems to be it's not even trying to communicate to the display at all.

Here's a link to the service manual & schematics http://www.evernote.com/l/AIYJimveyXtBlrgRLf5X79Jrqq5ZUT4cgXY/

Thanks for any help!

[vtp]

I just this week repaired one with similar symptoms as yours and in this case the SRAM (U15) was faulty.

The MCU was resetting itself continuously - so obviously there was something in the MCU subsystem that it did not like.

So, is the MCU (U17) resetting itself continuously in your supply?

Measure U17 reset behaviour at R42-C33 junction at the via there. R42 acts as a pull up after U1 has provided reset for the MCU. U17 pin 16 is open drain bidirectional and there seem to be weak pull-up somewhere, possibly in U16. If necessary, to disable U1 linear regulator provided reset just lift its pin 5 up.

After SRAM replacement the reset remained high and the MCU started working.

Edit: Before going after the SRAM I resoldered the program EPROM socket and tried another EPROM to have fairly good confidence that the MCU was executing valid code.

[tribble222]

Oh yeah, I forgot to mention it is resetting itself approx every 200ms or so. I wasn't sure if that's normal behavior. Thanks for the hint, I'll look into it.

[bitseeker]

I had not encountered this before. Interested to hear if yours turns out to be the same fault as vtp.

[permantier]

I have a E3631A with the same problem, did you identify the problem?, is SRAM faulty,?

[Kwakerman]

It was on mine when I fixed it, pretty common issue and not too expensive to replace

[pubo]

Just wondering where did you guys manage to source these SRAMs from? It seems like they've been long obsoleted and unavailable everywhere.

[z01z]

What do you mean? Digikey has a lot of different 5V capable 32kx8 SRAMs in SO package, I'm pretty sure you can find a replacement among them.

[pubo]

My mistake, I've got it mixed up between the SRAM (U15) and gate array SCX6206AK0(U16). Lack of sleep and I'm somewhat fixated on a busted U16/U17. Don't mean to hijack this thread but I've attached my captured waveform on the turn_on_reset points. Will try the SRAM and report back.

[pubo]

Just to update, replacing the SRAM fixed it for me, turn_on_reset was stable 5V after the swap.  Cheers for the help