Intersound IVP Preamp Repair

[garytoosweet]

I picked up one of these in problematic condition.

The problem is, when you play through the amp, there is a quiet distorted 'ghost note' in the background that tracks with what you play.

Audio Clip Here: https://www.dropbox.com/s/1oly0k57nx...Notes.mp3?dl=0

Note that I placed the mic very close to the speaker to exaggerate the effect. In person it is very quiet, but just enough to be annoying.

The ghost note is not affected by any of the controls, EQ,

Originally thought this was an issue of the Distortion channel bleeding over to the clean, but it's still present when playing on the distortion channel.

I have just recapped the power supply.

I've attached the schematic and the full service manual is here: https://www.dropbox.com/s/riv8h0j3co...anual.pdf?dl=0

Any ideas?

[Jon Snell]

Could it be cross over distortion? With a 1kHz sine wave signal, start from Q10 emitter and work backwards until it clears.

[garytoosweet]

Thanks Jon. Scope on the output and there is crossover distortion present. It is visible on Q11 emitter but not Q10.

D10 and D11 check OK. R54 and R57 check OK.

Voltages are:

Q10
B: 1.883VDC
E: 5.443VDC
C: 14.64VDC

Q11
B: 0.714VDC
E: -4.037VDC
C: -14.54VDC

[Jon Snell]

Q10 b/e is a junction and there can be no more than 0.7v across it likewise Q11.
In what you have measured, I would replace Q10 and Q11.
Assuming there is no distortion on C12.

[garytoosweet]

Swapped Q10 and Q11, no change.

No distortion on C12.

Distortion is present on U1(6).

R56 checks OK.

R55 reads open. Replaced with a new 10ohm.

All is well. Transistor voltages check OK, crossover distortion is gone.

Jon, thanks for your help in narrowing this one down!

[The Dude]

For sake of clarity, voltage across a B-E junction can be more than .7V if the junction is reverse biased, which was the case here.