Electronics > Repair

HP 34401a DMM with leaking segments

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floobydust:
Did you measure the filament bias voltage, or try changing it? VFD cathode (filament) -12V bias, from zener CR556/C559 at the Xfmer on the PSU.
I keep thinking this problem is due to the VFD aging and cutoff voltage shifted, making the driver look like the culprit- but it's not.

For substituting the VFD with an LCD or OLED display, I realize radiated EMI is a big concern. It would actually be the hardest part I think.
You have to keep things as quiet as possible. There are no buck converters here, and the metal shield around the VFD is there for good reason.

The display and main cpu do talk back and forth as a self-check, so that might be the occasional garbage you see.

qu1ck:
Filament bias looked fine on the scope, waveform was completely within -18 and 0v, which is all that is needed, I think. Did not do any more precise measurements.
Problem is that for ghost segments there appears fuzzy 0v to +2v potential on the display pin from some internal short (as I deduced from that potential appearing even on unconnected pin). Can't fix that with bias unless you shift it much higher than it is intended to be which will reduce contrast.

LCD I'm currently using does not have any converters, just a linear reg. Also it has a shield.
For final version with OLED I will have to rip out the 12v boost converter and supply it directly with linear regulator as well. It should be even better EMI-wise because much smaller voltages are involved here instead of +/-18v swings of VFD.

Regarding garbage I'm sure it's my code, I just don't have the interrupts and timings right (I'm doing software SPI to read both input and output at the same time).
I already reverse engineered main portions of the protocol and have a good idea of packet format, there is no meta info exchanged between front panel and main cpu, only "display this text", "display these symbols" and "button/combination pressed".

I will post everything in detail as I do the rest of the work. I don't have as much time as I would like to tinker with this, so updates may be slow :)

floobydust:
The 3.12" OLED displays I looked at are about 0.6W-0.7W; 100% pixels lit is max. 50mA at 12-15VDC, and 33mA with 50% pixels on. I don't know the 34401a power budget for how much current is drawn for the VFD and its MCU.

Because the +18V rail goes right to the A/D and analog sections (after 3.3V zener to make +15V) I would not power a display from that rail, in order to keep it super quiet.
It may be the multimeter's limiting factor- noise on the analog +ve rail from the display. Gen2 of the display board also added RFI filtering.
Gen2 display board, if the main MCU does not get a response from the display MCU, the main MCU will reset the display MCU by asserting its reset, as I recall.

I hope the project bears fruit, the multimeters are commanding very high prices now on eBay.

qu1ck:
We are only displaying some text on the OLED, it will never light up more than 30% of pixels. Power requirement is quite low, but I agree on the noise. I will look for a good place to tap power from.

I fixed the garbage glitch (turns out micros() in stm32duino is just not reliable) and reversed 99% of the protocol. I am not certain about purpose of couple control frames and characters but for all practical purposes I can show exact same information on the display and decode button presses. Unless there are some more special cases in the modes that are not accessible to me at the moment (remote control mainly) I think I am done with this part.
Now on to improving code to display all the annunciators properly.

coromonadalix:
would be nice to have another type of display adapted  ...

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