Picotest M3510A - R201 and R203 in Hi Input line

[trobbins]

My unit is working on most ranges but not the 0.1 and 1V or resistance or capacitance ranges.

The Hi Input has a trace that goes over to a relay K201, and the trace goes via R201, and a parallel LR comprising L201 and R203.  R201 and R203 are damaged such that the resistance markings can't be read, and I don't have access to any service docs or details.  A photo of the pcb region is in the link.  If anyone has a unit with cal sticker off then I'd appreciate a quick look-up of those two resistor values. 

There may be more damage, but this is the visible damage so far.  The K201 relay NC goes through to a large 220nF ACin coupling cap C617, but likely the contact was energised and that would then go through to a FET for ohm reference switching.

The Keithley 2000 schematic on xdevs (https://xdevs.com/doc/Keithley/2000/K2000.pdf) is a close representation on Switch sheet 3 for L109, R324, R115, but I guess there is some doubt without a visual confirmation.

https://www.dalmura.com.au/static/Picotest%20M3510A.jpg

Ciao, Tim

[trobbins]

xani has helped out with a photo of his board - with R201 as 2k, and R203 as 100 ohm.  The board rev/layout is a bit different, but I'll go with those values and see how much functionality is recovered 

Thanks again xani 

[trobbins]

I also tracked down two failed protection diodes D303 and D304 that were on the HI signal line, and replaced them with BAS16 as the Keysight 34401A schematic has certain similarities and it typically uses BAV99.  That restored the 0.1 and 1V ranges.

I then identified a bad logic FET Q1401 that grounded the hi signal line through a 1k, but couldn't specifically identify the FET model, so swapped in a BSS138 that had the same footprint and likely nominal ratings.  But still had bad resistance capacitance and diode ranges. 

The only other part I could identify as possibly suspect was a MAX4635 dual spdt switch, but I couldn't easily confirm it, and didn't want to replace a 10-pin soic just for the fun of it.

So if anyone comes across a service manual or schematics then I'd appreciate a pm.  Unfortunately the 34401A schematic is significantly different in many sections so I can't even start probing for correct signals.

[Kleinstein]

The BAV99 is similar to a dual 1N4148, so not really low leakage. The common low leakage diode is BAV199.

The 34401 schematics may look different,  there should be more similarity to the Keithley 2000, 2015 and the like. AFAIK the picotest 3510 is similar to the Keithley2100 (but no service info there, more like using the little info on the 3510 to help with a 2100).

A simple MOSFET grounding the input via only 1 K sounds odd - there should be at least some high voltage switch to separate this FET and a way to handle the other polarity. So chances are there would be 2 FETs in series. Still 1 K sounds more like the current range or maybe be part of the ohms current source.

For the ohms more a usual test is to measure the test current with a 2nd meter ( hand held is sufficient).

[trobbins]

Thanx Kleinstein.  Yes, the BAS16 was what I had to hand and showed up the fault location.  I will wait till I do a calibration cross comparison to see if the leakage of those diodes shows up as a concern.

The Keithley 2000 schematic was my first on-line schematic for comparison, but it has little in common and the 34401A was closer, but still not a good enough fit unfortunately.

The hi input uses spdt dual latching relays to switch between voltage and ohm functions, so I can track through the relays to generally identify the high current fault path that caused the damaged parts.  And yes there is no current being sourced at the connection terminals - that is also indicated by the effectively 'zero' reading for all the ohm ranges.