Yamaha CR-600 Offset Issue

[ivandezande]

Maybe I've got this wrong but TP3 (left ch) is connected to TP4 (right ch)
via R743 and R754 (both 100k). So excessive Vdc on one channel, say TP3
will also appear at TP4, 2x100k resistors mean minimal current flow/volt
drop. Suspect fault on one channel.

Suggest measure dc voltage between TP3(red probe) and TP4(black), will need mV scale.
A positive result +xxmV indicates the fault is on the right channel, -xxmV points
to left channel.

All resistors are good.
TP3-4 is .36 that slowly went to .4, positive.

With all fuses removed, I see .1 volts positive on 3,5, .95 on 4,6 and the relay clicks. Cannot adjust the DC however.
Now we're getting somewhere!

 

[mbz]

Hope I got this the right way, voltage TP3-4= +0.4 indicates TP4 is the lower/more negative voltage, so problem
is on the right channel. Remove F802 and F804 to isolate right channel, F801 and F803 should be in place.
Power up, hopefully relay click, otherwise I've messed up the logic (try F801,3 removed and F802,4 in place)

 

[ivandezande]

Hope I got this the right way, voltage TP3-4= +0.4 indicates TP4 is the lower/more negative voltage, so problem
is on the right channel. Remove F802 and F804 to isolate right channel, F801 and F803 should be in place.
Power up, hopefully relay click, otherwise I've messed up the logic (try F801,3 removed and F802,4 in place)

Hmm I tried just having 1 and 3 or 2 and 4 and with both the relay doesn't click.

 

[mbz]

OKay... what about...
with F801, F803 removed, measure dc offset on both channels, ie, TP3&TP5 then TP4&TP6
Reinsert F801,F803 then remove F802 and F804 and repeat dc offset for both channels.

Relay clicked when all 4 fuses removed, you would have to be unlucky but probably a fault on each channel

 

[ivandezande]

OKay... what about...
with F801, F803 removed, measure dc offset on both channels, ie, TP3&TP5 then TP4&TP6
Reinsert F801,F803 then remove F802 and F804 and repeat dc offset for both channels.

Relay clicked when all 4 fuses removed, you would have to be unlucky but probably a fault on each channel

Been a while, but I'm back!

F801,2 Removed
TP3,5: -1.1 keeps rising at steady rate but small.
TP4,6: -35.4

F802,4 Removed
TP3,5: -34.7
TP4,6: 8.80, keeps rising by 0.01V slowly.

 

[mbz]

Most probable scenario is a fault in the power supply eg, rectifier,
open resistor, lifted track... The other alternative is a fault in
each power amp eg, transistor failed short (negative half) or failed
open (+ve half)

Suggest work on one channel at at time, eg Left.
Remove F802, F804 which supply the right channel

Measure Vdc to GND at R727, one side should be about +38Vdc

Earlier measurements suggest TR705c-e short. It was previously replaced
but with wrong orientation.

Suggest start again with TR707, measure b/c/e to GND. Expect
b = -0.4Vdc
c = 1.2Vdc
e = -1.2Vdc

If TR707c is -xxVdc then check R727 and R725, then replace TR705.
Could also be TR715c-b short

 

[ivandezande]

R727 has 28V on a DBT.
TR707 has voltages in the teens all around. 705 diode checks just fine and I'm guessing the issue is the same on the other side. I replaced 701-4 and it didn't do anything, not surprised.

 

[mbz]

TR707 voltages negative? Expect so, then it's either TR705 or TR711.

Maybe wait for others to chime in, fresh eyes needed.

 

[ivandezande]

TR707 voltages negative? Expect so, then it's either TR705 or TR711.

Maybe wait for others to chime in, fresh eyes needed.

Yup it's negative. Sounds good, I'll pull them and check and probably just replace.
I still can't get over the fact that the rails are basically shorted to the negative rail and it passes DBT. This thing will be the death of me.
Thank you for the continued help!

 

[ivandezande]

So, to wrap this up I installed Q6 and 5 wrong as unlike the rest of the transistors, they are EBC instead. Somehow I was blind to that fact even after directly talking about the pinout in this thread.
She's sounding great and works perfectly, thanks all and I apologize for my shortsightedness.

 

[avionic]

It happens to everybody at least once. Alls well that ends well.