E3631 HP Power Supply wont start

[nz_brett]

I'm an enthusiastic amateur attempting the following repair -

I bought a non-functional E3631 HP triple output DC power supply which on startup flashes all screen elements and beeps then nothing - dark screen and no output voltages. I tested all the bias supply voltages with a Fluke multimeter +5V, -5.1V, +/-15V floating voltages for both the +6 and +/-25V outputs, and +5V ground reference and all are within specification.

The unit has a top and bottom board and I found another apparently functional top board (mostly MCU+digital and +/-25V supply) and swapped it in with no change.

I checked the reference voltages coming out of the DAC buffers - all are close to zero. The multiplexer and shift register and other chips in the area have the correct supply voltages (these are all on the replaced top board).

I would appreciate thoughts on where to look next - presumably on the bottom board. This includes the 6V circuit where there is 12V post rectifier/regulator, but no voltage past the series power MOSFET, possibly because the 6V REF voltage is zero? There is also the GPIB circuitry which I assume is unlikely to cause this problem, and a number of opto-isolators which I wondered about as a cause.

Thanks

[DanielDlouhy]

If outputs from all DAC buffers are zero and the screen is "dead", then it is probably a problem in the digital section. It kinda looks like a memory problem, but you said, that you replaced the digital board with no change... Are you sure, that the replaced board is working? Otherwise it looks like that the MCU "doesn't know what to do". The EPROM might be to blame... If EPROM has power, then check if the MCU is active. You can take a look at the signal coming to the DAC from the MCU, if there is any... If not, then it probably is the memory.

I don't think there is a battery in this instrument, but that would also be a valid suspect...

but no voltage past the series power MOSFET, possibly because the 6V REF voltage is zero?

Yes

and a number of opto-isolators which I wondered about as a cause.

Those are in the 6V PS isolation, if the outputs of DAC buffers are zero, then those opto-isolators don't see any voltage.

Anyway, definitely keep me updated! I gladly help you more, if needed .

-Daniel

[nz_brett]

Thanks for the help. I am unable to definitively confirm that the new digital (upper) board is functioning correctly. It was sold to me as a functioning board, and given that the fault is unchanged, I am assuming that the fault lies elsewhere. This means that the MCU, ROM, RAM and ADC digital circuitry has been replaced, but I agree that it feels like this is where the problem is.

The front screen is a separate board and also contains a MCU (inconveniently located under the display), connected as a slave over a serial bus to the main MCU on the upper board. Given that the screen is not functioning and there seems to be a digital fault, I am wondering if a failure in this board would lead to a lack of input to the main MCU? I have checked the ribbon cable between the two and confirmed that all the voltage supplies are correct, but will now take a look at the serial bus with a scope to see if this is helpful.