'74 Sony TA-1055 Recapping Project

[jwicaksana]

Hi fellas,

I decided to start a thread about my project to do some parts replacement on this amp. I am looking for insights and suggestions from you, and maybe help others with similar project in mind. I'd like to start by saying a big heap of thanks to Bob (trileru) for walking me thru the entire process up until now. You rock big time.

Schematic and service manual:
Dropbox - Sony TA-1055

I re-upload some photos via flickr for your reference.

So here's the amp:









The board:


Little did I know the final transistors were replaced, and one of the drivers are not original:


So I started compiling a parts list, anything under 10uF has been replaced with film caps, and most of the caps are replaced.








Bringing the whole thing to look like this:


Not too shabby, eh?

HOWEVER, DC offset is high on one channel and now the amp keeps blowing fuse. The story continues on page 6.

 

[jwicaksana]

I did some cleaning on the amp, getting ready for recapping.





Output transistors are not original, and one of four driver transistors is not original


Transformer looks untouched, so that should be good


First step is removing the transistors

 

[jwicaksana]

We created a list of material, this is a placeholder post, I will update with the complete list.

 

[Mooly]

So you are replacing all the transistors as well ? It should certainly be do-able but you may run into issues of both stability and also basic issues such as being able to obtain the correct bias current in the output pair. It looks like this is a fixed bias design and so its especially critical of the parts used. This is because of manufacturing differences between old vs new semis and the slightly different characteristics in things such as absolute turn on voltage.

Restoring old gear is great... if you dismantle it further then its even possible to wash the main PCB and chassis etc. Hot water and detergent. It would all come up like new.

 

[jwicaksana]

Mooly, I'm replacing 4 driver transistors and 4 output transistors, simply because the ones installed are not correct for this amp. There are smaller ones that I think I'll just leave as is. I'm replacing the caps as well, putting film caps in place of 'lytics, and also some diodes as well to be safe.
I'll see about removing the board altogether. Right now I'm fairly content with cotton swab and alcohol, working slowly around each part I'm replacing. Thanks for stopping by.

 

[Mooly]

OK and good luck.

And when you power it for the first time use a bulb tester.

 

[jwicaksana]

Thanks Mooly.
The parts started to arrive, yay. A few things, the legs of the big caps were spaced at 7mm and the board is designed for 13mm. A few twist and turns with the plier and we're good. And the TO220 insulation kit I bought uses 3mm screw and the heatsink accepts something like 2.5mm? So that means a trip to the hardware store tomorrow.

 

[Mooly]

Sounds like you are going to have some fun doing this Just take it slow and steady.

 

[Trileru]

Yeap. Also remember, unplug before working on it. Good luck and keep us posted with the progress.
Divide into steps, replace a set of parts, test the amp, then replace the next step, test it again. If you screw up, you have a small part count to check.

 

[kct]

Looking really good, nice job. Just for the fun of it, see if you can pair the transistors a little bit.

 

[jwicaksana]

Here's the bill of material, these are what I can find locally, with two Wimas coming from eBay.



@kct: Thanks for that, I tried pairing the power transistors as described here and found:
#1 .612
#2 .613
#3 .612
#4 .613

I'll let them sit together and re-test later in the afternoon, see if that changes anything.

 

[jwicaksana]

Okay so I let them sit in an airconditioned room, tried not to touch them or expose them to heat source, measured once every one hour, and the result is:
#1 531
#2 530
#3 535
#4 532

How do you suggest I pair them? With the heatsink facing me, left to right is 1-2-3-4, does that necessarily means left and right channel, two transistors per channel?

 

[Trileru]

Considering the schematic, I donmt think pairing them has any measurable impact. You should focus on installing the parts correctly

 

[Welcome]

Matching is stupid, don't even bother attempting it.

 

[Mooly]

Not stupid, just not appropriate in this situation.

 

[kct]

Matching is stupid, don't even bother attempting it.

Gain matching power devices is generally good practice. With amplifiers it helps to improve performance, so conclusion: it is not stupid, it is smart.

Sure the amp will work ok without matching anything just not a good as it could work.

 

[Trileru]

I don't think it's worth the trouble in this application, considering the old carbon resistors and schematic. In itself, matching is not stupid. But if it's efficient or not is another thing.

 

[jwicaksana]

Okay thanks for enlightening me on transistor matching. Still waiting for the parts to arrive, hopefully will be able to replace the driver, output transistors, and electrolytic caps within this week.

 

[jwicaksana]

I've taken some photos of the board showing old and new transistors. I'd be thankful if somebody could verify that this is the correct placement, especially of the new driver transistors before I solder them.







TIA ^^

 

[Mooly]

Its never easy working off pictures.

I would have thought that the collector of the originals would have been the centre pin... obviously check that, don't take my word for it.

Use your meter and check that the collector on the board (the print) goes to where you think it goes on the circuit.