Keithley 2001 Fails ONLY 407.1

[J-R]

From the service manual:

"Test 407.1 – Front end; 2V range
Cannot measure 7V ±0.15V at A/D IN

The signal at BUFF is routed through R233, K503, K500, the input protection circuit and
Q513 to the input buffer (Q512 and U520). The buffered signal is tied to AMP IN.

The signal at AMP IN is then routed to the variable gain amplifier (VGA) and, being
positive, follows the non-inverting x1 path that consists of R530, Q509, Q507, U519,
R531, U509, Q501, and U516 to pin 12 of multiplexer U511.

The output (OUT) of the multiplexer is routed through buffer U342 to ACV/A. The signal at ACV/A is routed through U320 (/AC pulled low) and applied to the A/D buffer
(U322), which is configured for x1 gain."

This makes no sense to me since the meter works perfectly in the 2V range, and also there are other tests that utilize the identical parts, but those tests pass (such as 407.2 and 410.1).

[coromonadalix]

Read this

you have the 4xx errors

https://xdevs.com/fix/kei2001/

in the k2001 repair manual   no schematics  attached in this thread

407 Series

              Front End:
407.1      2V Range
407.2      200V Range
407.3      750V Range


other thread :   https://www.eevblog.com/forum/testgear/restoration-glory-of-keithley-2001-dmm/

[J-R]

Yes, I read all that pretty thoroughly, but they are covering multiple errors and replacing a lot of parts without too much explanation.  The DMM was sold as working so I can return it for another unless there is a simple fix that I could take a chance on.

I did notice one oddity.  After running the 407.x test group manually, once I exit the menu the voltage display is frozen.  I have to change modes then go back to get it to work again.

[Kleinstein]

For some strange reason the amplifier is rarely used in the x 1 mode. The 2 and 20 V range use a gain of -0.5 and 5 and thus a different configuratio.  AFAIK this is only for the self tests, maybe AC and some resistance ranges.

There could be a problem with switching to the gain of 1. The other possile issue could be with the switches used to route the 7 V signal (reference) to the amplifier. It could also be a problem with calibration constants, as the x 1 case is not that commonly used.
AFAIK the normal auto zero cycle includes a reference measurement. So a drift in the ADC gain may get compensated there and a rarely relevant cal constant could be used for the test.

[J-R]

Well, thanks for the info.  Tracing the 7V signal sounds like fun, and a repair is tempting, but I think I am going to go for the return option.  It has a very deep scratch on the VFD cover that is annoying me among a couple other things.  So I'm going to try for a better unit.

[coromonadalix]

the only know so far reversed schematic for the k2000, on xdev's i think ??   but was not found there

not sure it reference well


and you have an no schematic SV manual w part lists  for the picotest m3500,  if i recall  it is supposed to be the 2000 brother or 2100 ??  (not sure)