Tuning shift over fifths and low volume

[njosnari]

I built my x0x some years ago and put it aside. Lately, I've pulled it out and set about finally tuning the thing. After some octave doubling and a new set of thermistors in r100, my x0x is more or less in tune. The problem I have now is that every 5th I get a tuning shift by about 50 cents. If octave C0 is in tune, all notes up to E0 are in tune, then F0 shifts sharp about 50 cents. Then the next 4 are the same. Then C1 shifts back down 50 cents, returning to the proper tuning. Up another 5th, and the tuning shifts up another 1/4 step. Another 5th up and the tuning returns to normal.

I also have very, very low volume output. Both in the mains and headphone out. it's so low that I need to use a mic preamp to get a decent level. All gains maxed gets me almost half way up the VU meter.

I am not sure how to troubleshoot either of these. any help is greatly appreciated!

[MMM]

I built my x0x some years ago and put it aside. Lately, I've pulled it out and set about finally tuning the thing. After some octave doubling and a new set of thermistors in r100, my x0x is more or less in tune. The problem I have now is that every 5th I get a tuning shift by about 50 cents. If octave C0 is in tune, all notes up to E0 are in tune, then F0 shifts sharp about 50 cents. Then the next 4 are the same. Then C1 shifts back down 50 cents, returning to the proper tuning. Up another 5th, and the tuning shifts up another 1/4 step. Another 5th up and the tuning returns to normal.

This hints at a problem within R74 to R90. Please check again using semitones (C, C#, D, D# and so on). Did you tune it so that C0 to C1 is a clean octave; or did you tune C0 to C4?

[njosnari]

When I check with semitones, the shift is at F0, C#1, A1, F2, etc.
When I tuned it, I sequenced C1, C2, C3, and C4 and tried to tune it.

[MMM]

When I check with semitones, the shift is at F0, C#1, A1, F2, etc.
When I tuned it, I sequenced C1, C2, C3, and C4 and tried to tune it.

This is very likely the resistor network then. Measure the following resistors: R90, R87, R79.

[njosnari]

Sorry for the question...

Do you want the voltages across them? I can't get any resistance measurements from them in the board.

[MMM]

Sorry for the question...

Do you want the voltages across them? I can't get any resistance measurements from them in the board.

They should measure in the board (unpowered) just fine.

[njosnari]

My mistake;

R90 200k
R87 200k
R79 143k

[MMM]

My mistake;

R90 200k
R87 200k
R79 143k

R79 is definitely off; this should be 200k as well. You may want to check if the color coding differs from the other resistors around it (R74 to R90 are all 200k).

The tuning precision of the xoxbox can be significantly improved by using resistors with 0.1% tolerance for R74 to R90.

[njosnari]

The codes indicate that they are all the same. When I test them, most give me less than 200k.
R74 85K
R75 85K
R76 85K
R77 85K
R78 143k
R79 143k
R83 85K
R84 85K
R85 100K
R86 100K
R88 100K
R89 100K
R82 190k

[MMM]

The codes indicate that they are all the same. When I test them, most give me less than 200k.

That's partially normal since a number of them are in parallel.

The following should read (in-circuit) 200k:

R78
R79
R80
R81
R82
R87
R90

The rest should read 100k.

Now, interesting enough, a 143k resistor with a 200k resistor in parallel equals roughly 83k.

So maybe there are a number of mislabeled resistors in there.

[njosnari]

I started to pull out r78 and r79 to replace them with new components. when i pull out r78, the rest of the resistors (r74-90) return to normal readings, 100k-200k. r78 reads 200k when out of the board. i replaced r78 with a new resistor anyway, and the readings return to 85-143k.

what would be causing this?

[MMM]

This is interesting (to say the least).

Let me think this through; and can you measure R80 and R81? Those are still missing from the list.

So far it appears to me as if the bottom portion of the whole network is off but the top portion is ok (you may want to look at the schematic for this). Maybe something's wrong with IC9; but right now I don't know.

[njosnari]

R80 and 81 both read ~200k

[MMM]

Can you try to remove (or lift on one side) R79 as well? And can you measure then the resistance between pin 12 and pin 16 of IC9?

[njosnari]

Pin 12 - 16 reads 10k