ADC hybrid for 3456A (U500) I want to try to blow 98k19 resistor inside

[velik_kazakov]

Hi guys,
I continue to work on one of my HP 3456A. It have a problem with one of the resistors inside U500 (98k19 from the ADC part of the chip). It is more 102K and I want to ask - can I try to blow resistor inside the chip. I have good foil resistor 98k2 and if I blow inside resistor I can to solder externel one. All other resistors inside the chip are good.

[coromonadalix]

Never heard of this one,   you know if you try to blow it you can make more damadge ?? be prepared to find this part ...

The problem is with the resistance value, no low voltages will be able to burn it,  unless you are able to grind the part casing and try to cut a trace in it ??

I dont know this u500 ic,  can you try to adjust it with an resitor in parallel, if you are able to measure it it should be doable with a low tempco precision value 

[Kleinstein]

Blowing a 100 K resistor is difficult and it may do more damage than good. One may be able to increase it's resistance by hard to be sure it would be reliably in the GOhms range. The residue would be still in parallel to where it should be.

As far as I see U500, the 98.19 K resistor is at pin 15, toward the 12 V reference. So one could just interrupt the link outside of the chip, e.g. bend away pin 15 or cut a trace on the board. That would be much easier and lower risk than blowing the actual resistor.

[velik_kazakov]

yes, but it have a connection inside from pin 15 to pin 16 via 100k. First I think to disconcet pin 15 and to add 2 externel resistors from 12v to pin 16 and pin 9 , but internal will stay conected ~ 200k betwin pin 9 and pin 16. Thats is the reason I ask can I blow internal one. will try with external parrale ~ 6Mohm to solder in parallel to 98.19, but can't find good resistor with this nominal.

[Kleinstein]

With another 100 K connected to pin 15, it would be nearly impossible to electrically burn the 98.2 K and keep the 100 K intact. So some 6 M in parallel is probably the only option. With the extra resistor much larger than the 102 K inside it does not have to be as stable - the TC and drift may be up to 60 times higher. So no need for 1 ppm/K performance, but 50 ppm/K would be good enough to still be at the 1ppm/K level for the combined resistance. Because of the odd value I would expect some 2 - 4 resistors in series.

[velik_kazakov]

Thanks!
Now it work this way with external resistor, but it need to be very strange nominal . Will kepp this way and to solder some series resistor to adjust the exact value.