Electronics > Repair
HP 34401a DMM with leaking segments
Cyclotron:
Any chance this modification can be used on a HP 34410a. I've tried finding a replacement VFD but no luck.
If it hasn't been done already, maybe I can get some time to do the work for 33410a. I can start by modifying one of my 33401a units with what has been done I guess and learn from that.
coromonadalix:
cut the vfd filament power is possible, or a display off command if there is
the 34410a vfd clone(s) are available and work good, they became more expensive since covid, but a good investment
i had a 34410a and changed the lcd, worked very well, it is not the same as the 34401a
34410 34411a share the same designs for the lcd, just acquisition speed are different
just need to be very very careful, they come with long pins, once put in place, they may kick on the front panel when you slide the pcb in place
an original 34401a lcd came with the front pcb and was sold at almost 200USD from Keysight when they had it ...
robert.rozee:
--- Quote from: trobbins on March 05, 2024, 05:52:46 am ---iMo, did you progress with a simple and practical way to de-energise the display filament?
I want to operate my meter for logging, and the display is showing some intensity variation, so I have some incentive to turn the display off as well as de-energise the filament - for the purpose of lowering internal temp rise, and prolonging useful service life of the display. The assumption would be that de-energising the display filament has some correlation with display related failure/degradation.
--- End quote ---
this is something i've been thinking about for a while too. is there concern that just removing filament power is not sufficient to prevent wear?
i see that the 34970A has software control over the filament power, see the below abridged schematic from the manual. this suggests that a simple 2-pole relay (possibly with a pair of diodes added to match the substrate diodes in the MOSFETs that the 34970A uses) may suffice. does anyone have a 34970A on which they can check if drive is (or is not) also removed from the anodes and grids?
the question then just remains as to where would be an appropriate place to fit a switch? on the front, or hidden on the rear panel perhaps?
cheers,
rob :-)
coromonadalix:
well for a 34401a if you never use the back plugs, make yourself a new switch rod and a filament pcb interface ??
that way you are original, and use the hidden front back as a filament switch loll
iMo:
--- Quote from: trobbins on March 05, 2024, 05:52:46 am ---iMo, did you progress with a simple and practical way to de-energise the display filament?
--- End quote ---
Nope, I did not, also thinking where to put the switch. I run my 34401 always with the display off (as I get the DMM data and 2 temperatures from serial off a stm32 via a Bluetooth into my logging app in my old smartphone), but the filaments are still powered on. I've been planning to put the stm32 (or whatever MCU) inside the meter and switch on/off the filaments via a relay upon a command, but no progress so far.. ::)
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