Yamaha A1 restoration 2021

[Martin.Ca]

Metal film resistors would have better temperature independence, but I would go with original specs.

 

[Hifibythekg]

Thanks Martin. Will the originals be 1/4 Watt?

 

[Hifibythekg]

My apologies to @avionic
Have just discovered that somehow.. I must have managed to select 'ignore content' in my settings!
Corrected this now and can see all his incredibly helpful and timely comments on this thread alone, what an own goal.
Anyway, thanks Avionic for all your help, hope you're keeping well and very glad I've actually discovered this error.

 

[Hifibythekg]

Hello
I am sorry but these transistors are on my schematic (see my picture)

TR138 = 2SC1213A and TR140 = 2SA673A

View attachment 1036709

Hi...
What's the best replacement transistor at this TR139, TR140 location, it's not in the common SM..

I can use KSA708Y
or KSA1013Y

Which is best?

 

[avionic]

What's the best replacement transistor at this TR139, TR140

I'll answer for Patrice @clinic-audio
2SC1213A and 2SA673A

https://www.bdent.com/2sc1213a-hitachi-low-frequency-power-amp-transistor.html

https://www.bdent.com/2sa673a-hitachi-transistor.html

 

[Hifibythekg]

Thanks Avionic, but I’m in UK and can’t easily use bdent unfortunately I think...
Are either of my two ok in this location do you think?

 

[avionic]

2SC1213A >>> KSC2383YTA
2SA673A >>> KSA1013YTA

 

[Hifibythekg]

Thanks Avionic, terrific.

 

[Hifibythekg]

Amp working and sounding very good... BUT
Idling Current has an adjustment issue.

Any advice for this issue?

TP1 - TP3
The left channel will only adjust between -0.3mV and -2.9mV.
-2.9mV is achieved by turning the Bourns (VR101) clockwise to it’s fullest limit.
DMM black lead on TP1, red lead on TP3.

TP2-TP4
The right channel’s range is between 0.00mV and -0.8mv.

The DC Offset is great at a 0mV to 10mV gentle fluctuation on both channels.

 

[avionic]

TP1 - TP3
The left channel will only adjust between -0.3mV and -2.9mV.
-2.9mV is achieved by turning the Bourns (VR101) clockwise to it’s fullest limit.
DMM black lead on TP1, red lead on TP3.
TP2-TP4
The right channel’s range is between 0.00mV and -0.8mv.

Your looking for 7.2 to 7.6 mv

Check your emitter resistors R113 thru R116 .47Ω @ 5watt

 

[Hifibythekg]

Thanks Avionic, especially appreciate your help on this one.
Those Emittor Resistors are all brand new and pretty sure I tested them before putting them in..
On the Drive Board I replaced TR 525,6,7,8 with those TC resistors you recommended too.
TR137,8,9,140 were also replaced.

 

[Hifibythekg]

Quite a mystery as so many components have been tested and / or replaced..

]

 

[Hifibythekg]

I went by this, attached below.
From where should I attach test leads for reading the idling current? The SM has so many errors...

 

[avionic]

Does you meter have mini grabbers ?

 

[Hifibythekg]

Yes

 

[avionic]

I went by this, attached below.
From where should I attach test leads for reading the idling current? The SM has so many errors...
View attachment 2402023

Nevermind I was looking at the wrong board..

 

[avionic]

Yes

Set your meter to volts
Clip your meter leads to each end of R114 and adjust VR101 for .0074 volts.
Then clip your meter to each end of R113 and adjust VR102 for .0074 volts.

Or set it the meter to mv
7.4 mv is target.

This is the second optional test points I was referring to yesterday.

 

[Hifibythekg]

Thanks.
For R114, is it definitely VR102 that gets adjusted?
Only asking as I’m thinking it’s the VR that’s on the other side of the board.

 

[Hifibythekg]

 

[Hifibythekg]

So its’s the Bourns nearest to the white Emittor resistor with the clips attached that I should be adjusting?