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Yamaha A1 restoration 2021

TR 135 and TR137 readings on a cheap transistor tester..
 

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Probably just a bad connection with your testers spring clips.

index.php
 
Hifibythekg said:
BUT the SM states:
R549 56,000 ohms
R547 47,600 ohms
R548 47,000 ohms
R550 56,000 ohms
Click to expand...

Emitter resistors test ok (not open)
FR503, D509, 511, R531, R543 seem ok

R547 and R549 are in parallel, also in parallel with C515.
In circuit measurements can be tricky due to alternate paths for
(MM) current flow. When you attempt to measure you are measuring
the total of the 3 above components, ie, 47k in parallel with 56k
gives 25.5k then in parallel with C515 gives??? Also note that
R547 and R549 gives the same result (16.6k), no issue there.

C523 tests ok however I'd replace since you've removed it, MKP or MKS film

I'm not seeing how -I can be +4.7V to +8.7V. The obvious path is via +I and
the bias cct. Double check voltages at +I and -I, monitor voltages for 2-3 minutes
careful with +/- sign.

Maybe consider replacing TR519 2SC1953 ecb with KSC3503 ecb.
Monitor voltage at D511 anode (TR519base)
 
Thanks MBZ, and appreciate the working explanations.
Later I'll get on to this. In stock I have one KSC3503D, that should be fine?
 
Just tested voltages at +I and -I.
At start up getting:
+I +454mV
-I -448mV

After a few minutes have passed:
+I has risen to +484mV
-I has dropped to -421mV

Haven't yet replaced TR519 2SC1953.
Should I still do so?
 
Hifibythekg said:
Just tested voltages at +I and -I.
At start up getting:
+I +454mV
-I -448mV

After a few minutes have passed:
+I has risen to +484mV
-I has dropped to -421mV

Haven't yet replaced TR519 2SC1953.
Should I still do so?
Click to expand...
Voltages look fine, suggest problem is intermittent, makes fault finding "fun".

Wait on replacing any parts, TR519 would be a candidate if +/-I voltages were negative. Difficult for me to see
how -I pin is +ve and -I > +I voltage. When problem next occurs need to carefully check +/-I pins. If both neg 2-8V
then replace TR519, KSC3503D is ok. If +/- pins are both pos 2-8V and -I pin > +I then need to look elsewhere maybe
a solder bridge or similiar...
 
Thanks MBZ.
My gut feeling is that I mistakenly wrote
- I 4.8V to 8.7V
Should probably have been
- I 480mV to 870mV
but will do more checks.
Bias reading between TP1 and TP3 remains at 0.00mV and doesn’t respond to adjusting the Bourns VR. Makes me think that a solder bridge or similar is at fault. If so, should I be looking solely at the Drive board or the Elec Cap board?
 
On power up it starts at -36.5 mV and decreasing very slowly. Eg, -33.1 mC after a couple of minutes.
This is with red lead on TP1 and black lead on Ground / Chassis.
 
Hifibythekg said:
My gut feeling is that I mistakenly wrote
- I 4.8V to 8.7V
Should probably have been
- I 480mV to 870mV
but will do more checks.
Click to expand...
-I needs to be a/near negative voltage for correct control of driver TR136. The variation in voltages may be caused
by TR136 toggling between "off and on". With the multi-turn trimmers you can get lost/confused which way to turn
the adjuster. Try monitoring the -I voltage along with TP1,3 and adjust the trimmer so the -I reduces to zero then goes negative. Somewhere -400mV would be nice through noticed right channel was only -50mV.
 
Since the last update I’ve thoroughly cleaned and checked for solder joins etc on the left channel of the Drive board.
TP1 to Ground/ Chassis now reading -20.5mV
 
Hifibythekg said:
On power up it starts at -36.5 mV and decreasing very slowly. Eg, -33.1 mC after a couple of minutes.
This is with red lead on TP1 and black lead on Ground / Chassis.
Click to expand...
Use black speaker post for GND connection. Think it was avionic who posted that circuit GND was connected to chassis via a cap.
 
"Use black speaker post for GND connection."
Sure, will do.
Also, I can't see TR136 anywhere on the diagram.. apologies if it's really obvious.
 
O, and still with the same connection, it's climbing.. now at -12.2mV but hovering there..

Anyway, I'll get on with the other tasks.
 
"Try monitoring the -I voltage along with TP1,3 and adjust the trimmer so the -I reduces to zero then goes negative. Somewhere -400mV would be nice through noticed right channel was only -50mV."
So,
Black lead on negative speaker post.
On Elec Cap board..
Red on -I.
Turn Bourns VR102 which will adjust the Right Channel which is already working OK-ish.
Correct?
 
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I'm thinking a small positive voltage on TR136b will shut it down so no current flow through TR2 output transistor.
TP1 voltages of xxmV are fine, no harmful voltages like xxVolts. Amp is close to working.
 
Hifibythekg said:
Black lead on negative speaker post.
On Elec Cap board..
Red on -I.
Turn Bourns VR102 which will adjust the Right Channel which is already working OK-ish.
Correct?
Click to expand...
My mistake, good pickup. Left channel is faulty. Red probe on -I on left channel (near LO), adjust VR101
EDIT: Driver TR135 on left channel may be shut down.
 
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