Thanks johnmarkp. I made assumptions on the REF 3, based upon the REF 2 MII and just eliminated the negative feedback FET circuitry from the REF 2 MII to create what I suspect is representative of the REF 3 (attached), showing one channel. The MII used 4 digital resistor IC's and the REF 3 uses 2 but adds two digital potentiometer IC's, so it's a little different, but similar enough. I left the Digital resistor/pot and mux the same as on the MII.
So it appears that a test I could do, would be to lift the 1K leads from each, the negative (DG409 pin 9) and positive (DG409 pin 8) input resistors and run a cross wire to swap the L & R signals?
If if the noise transfers to the R channel, it is clearly one of the following:
- MAX5437 (Digital Pot),
https://datasheets.maximintegrated.com/en/ds/MAX5436-MAX5439.pdf
- DJ409 (Dual 4 channel differential Analog MUX),
https://www.vishay.com/docs/70062/dg408.pdf
- DS1666 (Digital Resistor),
https://datasheets.maximintegrated.com/en/ds/DS1666.pdf
If not, I wonder if it is a tube biasing issue? But swapping the constant current FET's didn't change anything.
I wasn't provided the remote, so I can't check if the issue remains in Mono or with Inverted phase.
hmmm...
Here are some additional pictures, FYI. The LT1085 is on the power supply board, shown in picture.
Ok, so it's clearly coming from that digital trim-pot/differential analog Mux/digital resistor array on the left channel. I lifted in input side of all of the 1K resistors [the negative side (DG409 pin 9) and the positive side (DG409 pin 8) from each channel] and criss-crossed wires to route the circuit board L volume stage to the R side of the preamp tube stage, and visa versa.
The noise followed and is now on the right channel. So it's clearly one of the two MAX5437's (Digital Pot), or the DJ409 (Dual 4 channel differential Analog MUX) or one of the two DS1666's (Digital Resistor).
Looks like the DS1666-010's are matched (marking on them), but I suppose unmatched pairs will certain sound better than the noise.
The noise difference between the two channels is significant with an average difference of 10dB and with the spikes in the noisy channel, a difference of 16dB.
I'm surprised I couldn't find trace the noise with the scope... one would think it would be visually obvious.
I will update when I figure out which specific one went bad.
Any ideas on how I can get ahold of matched sets of the above and of the 0.0024uF polystyrene caps? I suppose the caps should be rated at least at 100V.