HP 3478A SELF TEST looping

[Barrelshifter]

Hi everyone.

I have a HP 3478A. It was functional a few weeks ago. I stored it for some time in a cabinet. Today I get it out  to do measurements.

When I power it up SELF TEST is shown on the display and the buzzer(?) is sounding rapidly. 
The text in the display flickers. It seems to me that the processor is restarting all the time.
I checked all voltages with a scope. All are fine and within the limits.
Even there are no spikes on them.

I checked the RESET-Line at the main processor: Nothing special to see. Still at High-Level after power on.

The SRAM battery was changed 8 month ago and the voltage is still at 3,2V. After the change I used the meter for several weeks without any problem.

Anyone an idea?

Thanks.

[Ordinaryman1971]

check if all the buttons on the front are... depressed. Or just take the front off, clean it and put it back together.

[Specmaster]

Hm, the fact that you have a buzzer sounding is worrying in own right. I have HP3478A and it doesn't have a buzzer, to double check I have just done a word search of it looking for "buzzer" and it drew a complete blank on that word also searched for "sounder" and zero came up so I'd suspect things like the power transferor which might buzz if excessive loading is placed on it. 

[Barrelshifter]

That's why I wrote "Buzzer". I sounds not very healthy.  Thanks.

[Barrelshifter]

I found the source of the "buzz" and the source of failure.
It is the display unit!
I do not know why this happens but it is definitely the LCD Unit.

Does anyone have the schematic / protocol description for that?

That's not my day

[Specmaster]

You can download the service manual on the web FOC

[Miti]

There's nothing to buzz on the display pcb, not even ceramic capacitors, unless they are covered by the epoxy blobs. Are you sure it's coming from the display? Very interesting.

[Barrelshifter]

After I had time to go on with my 3478A, I found the real source of the buzzing. It is the relay K101 clicking during the Self Test, that never ends.
The source of the problems seems to be the U502, the ROM.
Before I remove U502 from the board I would ask if anyone can provide me with the content of that. It is a 4 x 8 kBit ROM (2732). Also I am searching for a service manual for that version of the 3478A.
Its serial number is 2301A03741, PCB is rev. C and the ROM is labeld with 1818-1752B.
It would be very nice if someone can provide me with this.
Thx.

[iJoseph2]

I've just been trying (and failing) to fix an HP 3478A with similar symptoms... cycling relay and flickering display.
One of the things I did was replace the rom and use the image from the ko4bb site dc118 version... which made no difference to mine.
So it's possible my rom is fine. It reads fine and the programmer has verified it and the data looks reasonable.
You could try the image on the ko4bb website.
In case you want it, I've attached the image from my rom. It might be fine. It might not.
HP 3478A
Mostek MK36769P
1818-1752B
Checksum 00143AB2h
SHA256sum b3b6a8238a3d0ad6327bef3b4471835540aaeb3a8e522e6590ac2c08b65e2cf5