Calling all Yamaha A-1 Owners. Need Re Recapping Info

I had to solder in plain (but appropriate) diodes, in the correct configuration. 4 legs into 3 holes as it were.
 
Update on the restoration so far.

The 4 output trannies and the 2sk100 ic chips tested good.
All transistors with heatsinks including the 4 outputs had old thermal grease removed and new thermal grease applied.

All boards scrubbed down with acetone and cleaned.
Look like new again.

All up, 2 transistors, 1 diode and 1 resistor needed replacing.
All have been replaced with exact replacements or similar equivalents(1 transistor).

Honestly, before starting this restoration, I expected to find alot more fried components.

Delivery for the filter caps and bourns trimpots should be Monday. These are the only parts needed now to complete restoration.

The only thing I have not tested so far is the transformers. I suppose we will find out when I plug the amp.
Then the fun begins. Power her up with the dim bulb tester and see if she behaves or explodes like a nuclear bomb.
 
Last edited:
I have come across into a possible issue with the power supply board, the one that looks really messy in the photo of the original post of this thread.

Resistor R206 is 33k, but in circuit it measures 1k. I removed it from the board and it measures 33k with my MM.
Replaced with a new resistor, and it also measures 1k in circuit.

But R206 from both other A-1's I have measure 33k in circuit.

So I figure there must be something wrong with the board that measures only 1k.

Maybe a bad component somewhere on the board is causing this.

I looked for any solder bridges but it looks ok there.

Need expert opinion here as to what may be causing this and how to track down and fix it.
 
Maybe pull and check TR204. Also D206 and have a close look at C206.
 
Maybe pull and check TR204. Also D206 and have a close look at C206.

Thanks Avionic

You certainly know your way around electronic circuits.

D206 OK.
C206 OK
TR204 FAILS diode test badly on all legs!

Looks like I must have missed another transistor on this problematic PS board.

After removing TR204, R206 now measures 33K like it should.

I decided to remove all transistors on this board and check them again just to be sure(first time I just checked them in circuit and pulled out only the ones that were suspect, but obviously missed TR204).

Just that rogue TR204 it seems was the only one left that was shorted.

Slowly but surely coming closer to finishing restoration of this A-1.
 
Last edited:
I have come across into a possible issue with the power supply board, the one that looks really messy in the photo of the original post of this thread.

Resistor R206 is 33k, but in circuit it measures 1k. I removed it from the board and it measures 33k with my MM.
Replaced with a new resistor, and it also measures 1k in circuit.

But R206 from both other A-1's I have measure 33k in circuit.

So I figure there must be something wrong with the board that measures only 1k.

Maybe a bad component somewhere on the board is causing this.

I looked for any solder bridges but it looks ok there.

Need expert opinion here as to what may be causing this and how to track down and fix it.

Generally you can't measure a resistor in circuit, because you're measuring the resistance of everything else at the same time.
 
Generally you can't measure a resistor in circuit, because you're measuring the resistance of everything else at the same time.

Yes thats true. But in this case the low reading of the resistor prompted me to think there was something wrong within the circuit, and as it turned out with some guidance from Avionic, a transistor was shorted which was responsible for the resistor measuring low.
 
Hi folks,

I just found another dead transistor, 2sc1953(get a voltage reading between CE in diode function with DMM).

Cannot find an exact replacement, but have done some searching here on AK, and the following list has been recommended as substitutes in various posts by people seeking a suitable substitute.

The following 3 transistors I can get from my supplier. If anyone knows which would be most suitable to replace 2sc1953, it would be appreciated as I have no idea which is best.

2sc2632
2sc2911
2sc2682
 
Last edited:
I ended up replacing 2SC1953 with 2Sc2682.
I hope it works out to be a good match.

I finally finished pulling out every transistor from each board and checking them carefully this time round.

Another transistor was found to be bad, 2SA914.
I read in one of Echowars posts that he recommended KSA1142 as a good substitute.

I can get 2SA1142 from a local supplier, which
I hope this is the equivalent of KSA1142.

I am also getting wierd readings for the four 2sk68 devices in the tone control board using the diode function of DMM and not sure if they good or bad. Are these fets or normal transistors?

In the sm it shows pinouts as ecb left to right looking at the flat side of case indicating normal transistors.
Googling 2sk68 shows these as jfets with pinout from flat side as d-g-s.

I am confused guys. I need advise here on what these 2sk68 are and how to test them.
 
Last edited:
I ended up replacing 2SC1953 with 2Sc2682.
I hope it works out to be a good match.

I finally finished pulling out every transistor from each board and checking them carefully this time round.

Another transistor was found to be bad, 2SA914.
I read in one of Echowars posts that he recommended KSA1142 as a good substitute.

I can get 2SA1142 from a local supplier, which
I hope this is the equivalent of KSA1142.

I am also getting wierd readings for the four 2sk68 devices in the tone control board using the diode function of DMM and not sure if they good or bad. Are these fets or normal transistors?

In the sm it shows pinouts as ecb left to right looking at the flat side of case indicating normal transistors.
Googling 2sk68 shows these as jfets with pinout from flat side as d-g-s.

I am confused guys. I need advise here on what these 2sk68 are and how to test them.

here is how I do it

http://www.4qdtec.com/mostest.html

Nashou
 
Back
Top Bottom