Electronics > Repair

HP 34401a DMM with leaking segments

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Hi all,

I recently became a happy owner of this used but in good condition DMM (see first pic) which seems to have only one issue that I could find so far: display shows dots where there shouldn't be any and some symbols (4w, buzzer) show up as well.

It is old (cal string says "feb 1996", revision 04-01-01) but perfectly clean on the inside and other than the display everything seems to be in good shape.

So I started searching the forum for similar symptoms and found https://www.eevblog.com/forum/buysellwanted/ebay-many-faulty-hpagilent-34401a-multimers/msg811170/#msg811170

It talks about leaky drivers with NEC part that are used in newer front panels but mine is the old one with unobtanium UPD7527. Is this issue relevant to old style panels too?

Either way I decided to take it apart and look at what the driver is sending to display. Like I imagined the dots and symbols are segment 14 and 15 and they are somehow linked together. See the scope screenshots. Both have the following setup: ch1 - chr11, ch2 - seg14, ch3 - seg15.
Whenever segment 14 is high (+18v) segment 15 goes to around ground, although when 14 is low (-18v), 15 can be driven both high and low successfully.
Btw I did not figure out which segment is 14, it seems to be the lower horizontal one (think underscore).

So here is what I am thinking.
1. Somehow "help" the driver with some outside circuit that will pull pin to -18v when driver outputs anything less than say 5v.
2. Much more involved but also should be great help to others: reverse engineer the protocol between main board and front panel, reimplement it on another MCU and output the display on a nice big OLED screen like this guy did for his 34970a:

With multi layer board doing 1 may not be possible without splicing pins of the display itself. There may be other issues as well, I have not thought about this too much. Your ideas are welcome.

For option 2 looking at the schematic suggests that likely SPI is used in 34401 instead of usart as in 34970a so it can be completely different in terms of packet format, acks, etc. But should be still doable if it's SPI, would be much harder if it's something else. Protocol is referred to as "4 wire serial" in the service manual but the pins on UPD7527 are marked SCK, SO, SI, what else could it be? I'm not sure what is the purpose of INT(errupt?) pin though. Will have to take a scope to it under different modes and see whats going on there.

If I succeed than the display can be swapped for a 3.2" OLED with a blue pill on a side. Cheaper and more importantly much more accessible than replacement VFD display with UPD7527.


UPD: segment 14 must be upper-left vertical segment. Deduced from both "V" and "4" having the dot next to it, that's the only segment they have in common.

I've never had the opportunity to have such a nice instrument  :'(, but if the display driver exhibits this behavior I think that a quad op amp, put in the most basic comparator mode, should be able to do the trick as long as there is still a way to analogically differentiate between signal levels of the driver output.
That is put an LM614 or similar (it has to accept >= than +/- 20V as supply, and be quad or more)  to the most simple comparator mode, that is the [- Input] to ground or some reference voltage  ( an 10k potentiometer also connected to the voltage supplies) and the [+ Input] to the driver output, the op amp [Output], of course to the display segment. Basically this is it, the not so rich man adjustable signal signal rejuvenator :).

I have no idea what is the logical interface to the display board (parallel input, SPI-like sync or actual async serial) but if you can hook a cheap LA to it and post some traces, I promise to have a look and try to decode the protocol, maybe the extra karma and sympathy bonus will allow me to get a similar device sometime.


I will definitely hook it up to a logic analyzer but first I need to find any dip comparator  |O Everything I got is smt. Not ideal for experiments.

Interestingly the old front panel works with +13 to +18v digital logic levels while new one is on the low side -18v to -13v. The schematic shows that data out from front panel goes straight to an ASIC pin, with a pull up to +5v. It forms a voltage divider with a series resistor that is present in new panel so that logical 0 is around 0v on the ASIC instead of -18v. But on old panel there is no series resistor, are they sending +18v straight into logic board?  :o I'll have to measure that tomorrow, 3:30am right now, too tired to think :)

I'll come back with some experiment results soon.


@Samogon: The OP already referred to that link near the beginning of his post. :-//

@qu1ck: Interpreting the front panel communication with the motherboard in order to completely replace the old display would be interesting and useful for keeping alive the many old 34401A devices out there, if you're inclined to dive into that endeavor.

I had heard that some of the old driver ICs were out in the world, but I had never located any.


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