820 Left channel distortion

[Nuklhed]

Hey folks, thrilled by my new hobby and thrilled to be here. Dave from Sterling, NY.

Started troubleshooting left channel distortion. Found abnormally high DC voltages in left channel vs the right channel counterparts.
TR-705, all pins ~ 60v while TR-706 ~ 45v
TR-711, all voltages ~ 10 - 15v higher than right channel TR-712
TR-701 C ~60V while TR-702 is ~ 45V
TR-703 C ~60V while TR-704 is ~ 45V

I may have left out the +/-.. they are correct, but the magnitudes are way out. Any help is appreciated, and any feedback on my first post is certainly welcome.

 

[avionic]

Welcome aboard Dave


Dave

 

[avionic]

What is TR719 collector voltage

 

[Nuklhed]

Thanks so much Avionic, im a big fan!

TR-719 collector is 42.5 vdc

 

[avionic]

TR719 and TR721 Base voltage..

 

[avionic]

TR701 and 703 emitter voltages.

 

[Nuklhed]

Tr719 base 0.9vdc
Tr721 base 1.02vdc
Tr701/703 emitter 0.59vdc

 

[avionic]

TR723 and 725 base voltage.

Tr719 base 0.9vdc
Tr721 base 1.02vdc
Tr701/703 emitter 0.59vdc

Looks good..

 

[mbz]

R822 listed as 2.2K, 2W maybe someone replaced it with a 220ohm or...

 

[Nuklhed]

Tr723 base 0.44vdc
Tr725 base -0.46vdc
I will check that resistor, thank you!

 

[Nuklhed]

R822 is 2.2k

 

[avionic]

Try to set the idle current ( bias ) in the left channel.

 

[mbz]

TR-705, all pins ~ 60v while TR-706 ~ 45v

Puzzles me... TR-705 not switched on? Can you provide TR705 b/c/e voltages to GND/chassis(black speaker post)
to 1 decimal point, alsp note sign +/-

 

[Nuklhed]

Idle voltages set at 8mv....both channels are responsive to pot movement.

Thanks mbz, getting those voltages today.

 

[Nuklhed]

TR705 at zero volume
B -48.6vdc
C -48.1vdc
E -48.8vdc

at volume level 3
B -54.8 - -56.2vdc
C -54.6 - -56.5vdc
E -55.9 - -57.2vdc

 

[mbz]

Puzzling results. Have you replaced any transistors in the power amp or psu stages?
Replaced with what? Diode check D701 (powered off)

 

[avionic]

Remove TR705 and test for shorted junctions.

voltages should have been in this ballpark.

E - 42
C - 43.5
B - 43.5

Check the voltages around TR406 in the other channel and compare channels.

 

[avionic]

TR705 at zero volume
B -48.6vdc
C -48.1vdc
E -48.8vdc

at volume level 3
B -54.8 - -56.2vdc
C -54.6 - -56.5vdc
E -55.9 - -57.2vdc

All troubleshooting should be with volume at min. and no source selected.

 

[Nuklhed]

mbz - D701 checks out good. I replaced all electrolytic caps and transistors on the main amp board with the exception of the filter caps. Used Supernoobs 820 rebuild as a guide. Ran excellent for a couple months until this distortion crept in.

Avionic - pulled tr705...tested good. Question...did you mean TR706 in the other channel? Also, if I have input selected to Aux with nothing connected is that what you meant by no source selected?

 

[avionic]

if I have input selected to Aux with nothing connected is that what you meant by no source selected?

Yes