HP34401A

[lucian]

Solid Beep on boot, I have a hp34401A and it won't stop beeping on boot the display switches ons but nothing works, the beeping does stop if I change to use the rear jacks but it still doesn't go past the boot screen
https://youtu.be/GEM9esejeUU

[lucian]

More photos

[iMo]

fyi
PS: do start with the checking of all supply voltages..
Download the Service manual, there is the schematics with all the components on the pcbs..

[lucian]

5v was fine and 18v looked fine, but I will look to order a replacement for that part any ideas about the buzzer

[trobbins]

Suggest start with all the voltage rails - there are more than two rails, and they include regulator circuitry.  Afaik there is/are beep(s) as mains voltage increases (eg. when using a variac), and likely related to under-voltage lockout style activation of sections of circuitry.

[lucian]

Just noticed the 3amp fuse in the rear holder is burst just an fyi

[lucian]

Any further steps for diagnosis or fixes

[trobbins]

The fuse has blown, so do you have a Variac, or can power from lower than mains, or have a bulb limiter?  Start by disconnecting the 8-pin connector from the transformer. But you need to have the service manual, and need to appreciate the power supply section schematic first ??

[factory]

The 3A fuse is for the current measurement function, I can't imagine it would stop the DMM from working on other functions?
Mains fuses are much lower values.

David

[lucian]

Sadly I do not have either of those, Ive been looking at the schematics for the power supply and I really doubt its an issue with that

[lucian]

Just noticed that this part is getting very hot but its still supplying a pretty clean 5v so Im a bit stumped

[lucian]

Still Stumped on why when the front selector is pressed to use the rear inputs and the multimeter is rebooted the beeping stops

[iMo]

Just noticed that this part is getting very hot but its still supplying a pretty clean 5v so Im a bit stumped

That 5V voltage regulator powers the outguard logic only. There are i/o interfaces (HPIB, serial) driven by an MCU where the chips might be damaged. Also the tantalum capacitors need to be checked.
The +/-18Volts in the inguard logic (with the ADC, input stuff, main CPU etc) go through various serial zeners (3.2V) thus creating +/-15V for all important analog circuitry there ( ). Doublecheck them as well, as they get damaged easily.

[lucian]

Ok thanks, could that be the root cause?

[lucian]

Is this meant to be unpopulated

[iMo]

Yes, unpopulated, that goes to the RS232 connector, there are 2 free pins without a signal on them.

[lucian]

I am assuming by serial zeners you mean this i checked them at it ends at 14.8v

[tggzzz]

I am assuming by serial zeners you mean this i checked them at it ends at 14.8v

To understand the input protection circuits, there's a good explanation in "The Art of Electronics x-Chapters". https://x.artofelectronics.net/

[lucian]

Thanks will take a look at this tmr morning but Im assuming what I did was incorrect?

[iMo]

Those are not the power supply zeners I mentioned above (those are around the ADC part, look at the schematics and pcb view in the Service manual).
CR304,305,306,307
CR400, 401

[lucian]

Oh you are referring to the ones here correct?

[alm]

Did you get it like this or did it suddenly break? If so, did anything particular happen to it before it broke?

Did you measure both voltage and ripple on all power rails (+/- 18V floating, +5V floating, +5V earthed, 6VAC for front panel)? Ideally you use a scope to measure.

The blown fuse and damaged C109 cap (in parallel with the MOV) both suggest excessively high input signals, though on different inputs. But otherwise after the power rails I'd start looking at the floating or front panel logic circuits. Study what the service manuals says about start up steps (does all segments lighting up indicate correct operation of the front panel?), and based on that identify a suspect. Then look for clock, activity on address bus, reset line, etc.

Can you connect to it using RS-232 or GPIB? Maybe it's just the front panel that locks up.

[J-R]

I say focus on testing all the capacitors right away. You might be able to do some in-circuit, but if you can't tell for sure, remove for proper testing.  If you search for 34401A repairs, you'll see a lot of folks just replace all of them, especially on the older units.  Maybe you will get lucky and quickly find one that is shorted.

[lucian]

Ok all the caps are fine anything I shoud check next

[lucian]

all the big ones