MV106 Odd Failure

[Insatman]

Hi...I have had an EDC MV106 operating for about 3 years and recently it failed.  The odd thing is that it appears to be in the first decade switch assembly somewhere.  My logic is as follows.   Power supplies are good.  Voltage reference appears good.  The voltage out is approx 0 when the first switch is set to 0, About 2V when set to 1V, Overload (~13V) at 2V, about 3V when set to 2V.  Every even number gives an overload.  Every odd number gives about +1 volt than it should.  ??!???    One other odd thing.  The power switch is also open circuit and I had to bypass it to get anything to work.   I don't believe in coincidences in troubleshooting...so...    Anyway, I can't find a front panel schematic for the MV106 and was hoping someone out there would have one or at least can clue me in on how they use only 6 resistors for each decade.  I like to avoid removing the rotary switch if possible.

All other decades appear to work as expected.  I'm pretty sure the amplifier is good because of this.

My gut tells me there is enough information to find culprit or at least narrow things down.

[Insatman]

One more thing....a visual inspection of the switch shows no loose wires and the switch has been cleaned with deoxit.

[floobydust]

Rather than troubleshoot to the contact that is failing, I would guess the rotary switch wafers needs cleaning. Silver oxidises really bad due to air (sulphur) pollution. Rarely I see the rotary switch leafs get bent up (due to slop or a gorilla operator) and you have do surgery. Do a visual on the spring contacts, both sides.

DeOxit is convenient but may not deal with a thick black oxide buildup if still there. I use something abrasive to scrub it off: a tiny pencil eraser on the rotating ring and then a piece of paper as sandpaper on each spring contact. Just a gentle rub. It's painful, the kind of thing you do every 40 years. A Q-tip gets tangled and leaves cotton hair snagged on stuff.
Don't forget to lube the switches afterwards.

MV106 service manual/schematic: https://www.eevblog.com/files/MV106 Service Manual.pdf but no detailed switch wiring info I could find.

[dacman]

Set the switch to zero and measure the resistance across the black 19.99 kOhm resistors.  The second one may be open.  There was one resistor company that did not construct the resistors well and they could open up or become unstable.  What are the markings on the resistors?

[Insatman]

Set the switch to zero and measure the resistance across the black 19.99 kOhm resistors.  The second one may be open.  There was one resistor company that did not construct the resistors well and they could open up or become unstable.  What are the markings on the resistors?

there are six resistors.  5 are marked KRL 565C 19.99K +-02.%.  the odd resistor is marked ULTRONIX S207A 19990 ohms.  I can't read the rest of it on that resistor.  The odd resistor is the one nearest the side panel and it looks like perhaps it was a replacement because it's not installed quite like the others.

After measuring the resistors as you suggest all are 19.99K except one which measures 80K.  This is the 5th from the edge or 2nd from the edge depending on which card edge you choose.  I think you have hit the problem on the nose.   Thanks....now where to get a replacement resistor?   Any idea what the ppm rating was on the original parts?

[Insatman]

Update.   I temporarily shunted the bad resistor with a 27K which results in approx 20K net just to check if anything changed for the better.   And yes the values are much closer to setting with this janky temp setup.   I don't know why the power switch failed (open).  The neon lamp inside it works just fine but the contacts are shot.   I'll need to find a replacement for that as well.

I found a datasheet for the Ultronix resistor and it's spec'd at 10ppm/C.   So I'd want something of that order.   Digikey only stocks 20K, not the 19.9K that allows for trimming with 20 ohm pots.  I suppose I could do a painful Ebay search.   My location in the Philippines makes buying difficult for most items, although Digikey ships here for free with $50 min order.   I suppose I could use a 20K resistor and shunt it with a high value to give room for trimming.