Author Topic: Sony TA-F770ES and 707ES repair  (Read 3565 times)

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Offline cheeseit

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Sony TA-F770ES and 707ES repair
« on: March 09, 2016, 11:57:36 pm »
I finished repairing a Denon PMA-520 a few days ago so I figured that it was time to get to the main event, which is repairing a 770ES and a 707ES. Both have the same symptoms, which is intermittent distortion and/or loss of one or both channels. Same thing with the Denon by the way, and that one was fixed by thoroughly cleaning the speaker A/B output switches, the source select switch, the output relay and reflowing some dry joints on two TO220 transistors in the supply stage that was literally flapping in the breeze. In the end I think it was mechanical and not dry joints though. Not a great amp but I'm happy that it now works. :)

It has been really, really hard to locate the failure point(s) in both Sony's, because it is so intermittent. Sometimes they work for hours and then suddenly it appears randomly, then comes and goes or disappears again. Working temperature does not seem to have anything to do with it, and sometimes it can be alleviated by switching the speaker output back and forth, switching the stereo/mono back and forth, or switching the source direct on/of. Often all that switching does jack and none of the tone controls or other switches ever does anything.

I'm starting with the 770ES and have been in there before a few times. I think it might well be mechanical on this one too.

All pots, except motorized volume pot, have been cleaned and are absolutely noise free and smooth. I wouldn't rule out the motorized input selector but I'm not totally comfortable taking it apart to clean it. All belt-driven switches has been taken to bits and cleaned. They were heavily oxidized and for a while that seemed to do the trick but no dice. This has been done in stages so now it is time to clean all the switches and the output relays. I'm about to order new relays but figured that I could try cleaning the contacts by sliding some clean and dry paper between them first. I know that they are delicate but it worked fine before and on the Denon so why not.
I have as said had it open a few times before but didn't do the remaining switches, because they are such a pain to get to and because I would like to isolate the problem without doing too much at once so I'd know where to start on the 707ES.

The 770ES was bought cheap as not working, which it kinds isn't, and it has had some trauma previously. I'm guessing the output has been shorted and someone has done a mediocre job at fixing it; what looks like regular and lower wattage carbon resistors instead of fusible, mounted a couple of really old power resistors instead of some 2W metal strip resistors, and put in a CapXon(!) capacitor and left a charred resistor and diode on top of a bit carbonized board - I would like to try to remedy this also.

I also figured that I might as well recap it while I'm at it, since it is close to 25 years old judging by a few 91-92 date codes, and this is where I would like to ask for some help:
I have for now chosen Panasonic FC to replace some Nippon Chemi-Con AVF-series on the output, power supply and preamp boards. Then some ELNA SILMIC II to replace old SILMIC on the preamp board and finally some Nichicon UPW to replace some Nichicon Muse (polarized) on the power supply board but I'm not sure those UPW? (Muse is supposedly audio grade - whoohoo - why use those on a power board?)
I'm a noob and capacitors are a jungle to me so I would like some opinions on the above, are they suitable or are there others that would be a better fit and cost about the same?

I can post some pictures of various bits if there is any interest and I might do it anyway if not. ;D Sorry for the wall of text but I wanted to be thorough and might have had too much caffeine.. :D

Comments and advice on how to proceed are appreciated - I'm learning by doing!

Service manual
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Offline Thomas1974

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Re: Sony TA-F770ES and 707ES repair
« Reply #1 on: May 07, 2021, 12:14:46 pm »
Hello Cheesit

I hope you found the problem 5 years ago. I just found your post, because I´ve come by a very pretty TA-770ES, but it seems like it might have the same problem, as the one(s) you describe. It played fine (though not engaging) the first day I had it, the next day it suddenly said: "FFOUMP!!", and the sound became really distorted and muffled in both channels, and  I turned it off. I let it rest for an hour or two, and the same thing happened again after a short while. I didn´t check all the pots, but I noticed, that the volume knob had become noisy. It wasn´t at first.

I hope you read this, because It would be really nice to have an idea what to do.

Kind regards

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