Yamaha A1 restoration 2021

[Hifibythekg]

Thanks. But can't see a TR136 on the pcb.

 

[mbz]

Thanks. But can't see a TR136 on the pcb.

TR135 for left channel, TR136 for right. TR139 is a typo, should be TR136, right channel should have event part numbers

 

[Hifibythekg]

Wow. Ok.

 

[Hifibythekg]

- I reads -401mV

 

[Hifibythekg]

Climbed to -416mV and stable

 

[Hifibythekg]

This is -I, near to LO.
What should I do now?

 

[Hifibythekg]

-I, near to LO, now at -394mV

 

[mbz]

-I looks good, when you turned VR101 did it increase/decrease significantly.

-I will drift slightly with mains AC voltage drift and as the unit warms up.
EDIT: Bias still 0mV?

 

[Hifibythekg]

-I, near to LO, now at -376mV

 

[Hifibythekg]

Yes.
Turned VR101 and made it go up to 400mV

 

[Hifibythekg]

What to do now?

 

[Hifibythekg]

Sorry.....
minus..
-400mV

 

[Hifibythekg]

"EDIT: Bias still 0mV?"
so,
Measure TP1 to TP3 on Elec Cap board?

 

[mbz]

Bias/TP1,3 still 0mV?
If yes then post all voltages on left channel, ie, LO, +I, -I,,CE, CC
Wanting +O, -O at about +/-1V. Need to step out for short time.

 

[mbz]

"EDIT: Bias still 0mV?"
so,
Measure TP1 to TP3 on Elec Cap board?

More typos in the sm??? Seems like TP1,3 associated with R-channel which is "wrong"
Need TP's closest to LO pin, marked TP2,4 below, vaguely recall markings on board have been corrected.

 

[Hifibythekg]

Yes, thanks

 

[mbz]

Signing off,
check left channel bias, test points closest to LO pin.
If still no response/0mV to VR101 then record left channel voltages with black mm probe connected to black spkr post

 

[Hifibythekg]

LO -18.5mV
-767mV
-419mV
-O -1006mV
+O +1359mV
-E -13.5mV
+E -14.3mV
AE -395mV
CB 793mV
AE 5.2mV
AC -54.3V
CE -6.6mV
CC 53.8V

 

[mbz]

LO -18.5mV
-767mV..... +I +767mV
-419mV..... -I
-O -1006mV
+O +1359mV
-E -13.5mV
+E -14.3mV
AE -395mV....AB
CB 793mV
AE 5.2mV
AC -54.3V
CE -6.6mV
CC 53.8V

Little odd that CE is -ve and AE is +ve
CE should be same as +E


AE should be same as -E

CE to AE shows 11.8mV across emitter resistors while +E to -E shows 1mV
Seems like a disconnect between CE/AE and +E/-E.
With amp powered off try continuity/resistance check between CE and +E.
Repeat for AE and -E. Expect about 1 ohm in both cases.

CE to LO (1 emitter) shows 12mV, this is the same as measuring bias across the TP's
ie, meter should display 12mV at bias TP's. Check/reflow solder connections on jumper J123
(red), CE and AE wires and also at left channel TP's.

Seems like the amp is roughly biased(11.8mV) when using CE to LO but not ok when
using the TP1&3. Try to set bias (7.4mV) when connected to CE & LO (use minni grabbers)

 

[Hifibythekg]

Thank MBZ. So, today's initial measurements are..
CE -13.5mV
AE -13.4mV
+E -13.3mV
-E -13.4mV
these are all moving a bit, measurements taken about 5 mins after power on.