| Electronics > Repair |
| HP 34401a DMM with leaking segments |
| << < (38/45) > >> |
| floobydust:
I have one 34401a, new VFD fixed the polka dots problem - only for a while, when the DMM is hot they faintly appear. It is intermittent now. The dots are a combination problem of aged VFD having low transconductance, needing high drive voltage for blanking, as well as the NEC VFD driver PMOS aging getting old and lazy. |
| dreamcat4:
got here the same problem, dots between the characters. and am going to leave it like this for the time being while considering options. you see maybe the oled would be lasting for a longer time than a new vfd, given the way the driver is already leaking and degraded as explained above here ^^. if it really cannot be replaced? just am still a bit unsure. for example wanted to know if by installing the oled it might affects negatively the precision of measurements. but maybe there was already some testing with that in mind? and can see existing results for that? look up somewhere? :-+ |
| floobydust:
The polka dot problem is caused mainly by both- an aging NEC MCU and VFD. If you replace or rejuvenate the VFD display, it can work for a while longer. But I have one 34401a that polka dots sometimes show up as it runs warmed up. So I started working on a replacement front panel board, to keep the stock VFD and just use a VFD driver+MCU compatible with the old main board firmware. I forgot Qu1ck's schematic but plenty of power available from the unused filament winding on the tranny. Adding noise or load to +/-17.4V rails would affect the analog sections of the multimeter, I would not run the OLED power from that. |
| dreamcat4:
thanks for these insights. so then i shall see what comes of this... BTW checking the schematic on github here.... https://raw.githubusercontent.com/openscopeproject/HP34401a-OLED-HW/master/schematic.png it seems qu1ck gets the power in from a +12v J5 marked there as DISPLAYPWR ? not sure where that is supposed to hook up to. but yes that was my main concern with the oled mod. in the meantime will also be patiently waiting to see if you can eventually replace the vfd driver in yours flooby. not in any particular hurry |
| floobydust:
3.12" OLED need 12-15VDC at 25-50mA typ, depending on how much is lit up. About 2/3W. If this is generated by an on-board boost-converter, it will draw a lot from a 3.3V rail, up to 250mA. So how this is powered is important not to pollute or overload the 34401a power supply. |
| Navigation |
| Message Index |
| Next page |
| Previous page |