Electronics > Repair
HP 34401a DMM with leaking segments
bitseeker:
--- Quote from: qu1ck on July 09, 2018, 07:41:54 pm ---
--- Quote from: zucca on July 09, 2018, 07:12:00 am ---Probably you will sell a truck of them. Please consider a group order.
--- End quote ---
Doing logistics of group order and delivery is not worth it to me. Board designs as well as everything else will be open so anyone can do it.
--- End quote ---
Yep. I might look into organizing a group buy for this. It may be OK in the form of kits (i.e., DIY soldering). Getting it completely manufactured might be too much of a project. We can revisit after qu1ck is done.
qu1ck:
Heh. I thought of completely replacing the front panel PCB too. It is doable but will need even more firmware development (I have button codes but I don't currently emit anything onto data lines). Also manufacturing that pcb will be quite costly.
As floobydust mentioned it has lots of cutouts and would need precision machining, as well as tight tolerance on thickness to fit snugly and gold plating for button pads and it's pretty big and... yeah it's gonna cost ya. Nowhere near the $2 for 10 bargain that I can get for tiny 2 layer mcu board.
bitseeker:
Yeah, it's another level for sure.
floobydust:
--- Quote from: bitseeker on July 09, 2018, 11:38:56 pm ---... handling of the buttons would still need to be added, right? I assume that's what's meant by "finish the replica PCB."
--- End quote ---
At first I thought of making a small daughter board to replace only the NEC uPD7527 but not enough space.
Then I figured just replace the entire front panel PCB, keeping the VFD. New tubes are plentiful from China.
I started with the mechanical, including the keypad etc. Then the H/W and F/W design is how I usually go. Getting a PCB made to verify how it fits, it's on the list.
I don't want to compromise the instrument by having a noisy, high current MCU running off the +/-18VDC rails which run to the A/D converter. Designing for conducted EMI and keeping it quiet to keep life rosy at 6-1/2 digits is important.
I think an underclocked Bluepill would be OK, or Mega328PB- something with two SPI ports and under 12mA. The original uPD7527 is 3mA, and 12mA for 87C51.
To bring the multimeter up to date, adding wireless is tempting. WiFi needs a lot of TX current, so power would have to come off the 5VAC filament winding. ESP8266 is max. 170mA chirps but the second SPI port would have to be bit-banged I think. ESP32 seems like overkill? But only $5.
I haven't really nailed down what I'm doing.
This is the keypad drawing, one key needs vias moved but the slots and VFD and bracket crap I have also figured out.
I can try add qu1ck's OLED and see how that fits. The huge 5.5" OLED was too big.
qu1ck:
You have to bit bang SPI either way. In 34401a protocol FP notifies the master of readiness to receive next char or button press event by pulling MISO line low and waiting for the clock. I don't think that kind of shenanigans are normally supported in hardware SPI on most MCUs, but I may be wrong.
Thanks for the drawing, it will come handy.
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