CFS-77 Weak left channel

Just bought a mint SONY CFS 99 locally and found that the left channel is very weak.

Fixed the deck, and now the problem is a weaker left channel.

I cleaned all the sliders and the record bar, but still the left channel (left one when you face the box) is very weak.

Also I cleaned the headphone jack as usual. It is weak on radio and tape. The VU works perfect on both channels.

Any help with this problem is greatly appreciated.

Royce check the caps on the amp board . I know that's a generic response but I seen many an amp go south because caps on the amp board are bad even if they look OK.

I have 3 super expensive car amps sitting around because I can't afford to recap them all.
The usual sign of the caps failure is weakened sound output mixed with distortion.

Thanks Floyd! The design of this box is not user friendly to work on. The front face cover with speakers needs come out from bottom when the top section is kind of on slots locked in place and controls on top also grabbing it n it. What a dumb design! With some pain i fixed the cassette and later realized the weak channel issue. So many caps and need to check one by one and see which are bad ones. I need to get the schematics also. Didn’t break anything so far and got the feel of removing the face cover.

Royce is channel weak with Headphones too? My crown 950 had channel issues to. Turned out headphones jack was messed up and traces going to one of the speaker jacks were bad.

baddboybill said:

Royce is channel weak with Headphones too? My crown 950 had channel issues to. Turned out headphones jack was messed up and traces going to one of the speaker jacks were bad.

Click to expand...

Thanks Bill, I was about to check that and lost the adapter somewhere in the house. I will check that tomorrow. It is weak on sound when the right one blasts. So I loved the balance all the way to the left and checked it and found the volume was slowly increasing, but weak.

Floyd, You were right!

Found the culprit! I hope that is it. I will replace that and see how it works. In that process I will look for more leaky caps.


One 100uF/16V cap near the power ICs is leaking.


It is the cap connected to the Channel 2 of IC 302.

There are 4 caps of the same value around the IC
C 124 (leaking)
C 125
C 225
C 317
I will replace all of those 100uF/16V caps.

BoomboxLover48 said:

Floyd, You were right!

Found the culprit! I hope that is it. I will replace that and see how it works. In that process I will look for more leaky caps.


One 100uF/16V cap near the power ICs is leaking.


It is the cap connected to the Channel 2 of IC 302.

There are 4 caps of the same value around the IC
C 124 (leaking)
C 125
C 225
C 317
I will replace all of those 100uF/16V caps.

Click to expand...

Replacing leaking caps is a no brainer but caps rarely ever fail catastrophically unless overvoltage, overheated, etc. In generally, they fail slowly and gradually over time. The point is that if you have failed or failing caps, keep in mind that they are all the same age, operated in the same operating environment meaning same power levels, same ripples, same temperatures, etc. Replacing only those that affect one channel with fresh ones can leave the sound imbalanced between the channels, for better or worse. Not always but frequently, new caps can change the sonic characteristics. I recommend replacing them all, or at least replacing the ones that affect one channel AND the same sister caps that affect the other channel too.

I hear you Norm! My plan is to replace them all on both channels in the proximity of IC302. Thanks, Royce

Norm is right because in the power chain its only as strong as its weakest link.

Floyd, I totally agree!
Actually I ordered all the caps in the power amp stage yesterday and will get it end of this week.
~Royce

I find I have to keep reminding people this....

• Do ONE at a time.
• Place the removed cap on the workbench.
• Install new cap after verifying that it has same specs as old cap.
• Remove old cap from workbench.
• Repeat.

= won't make stupid mistakes.

Superduper said:

I find I have to keep reminding people this....

• Do ONE at a time.
• Place the removed cap on the workbench.
• Install new cap after verifying that it has same specs as old cap.
• Remove old cap from workbench.
• Repeat.

= won't make stupid mistakes.

Click to expand...

That is correct! Great advice! Thanks Norm

Received all caps and this evening I removed the dial cord assembly and the deck. Now have to remove the main PC board. I will post pics as soon as I am all done.

Main belt flat belt is elongated. I believe I have a 10.5 medium flat belt somewhere. bad planning.... I should have ordered belts also before dismantling it. Anyway belt changing is not that difficult on these SONY CFS models.

Good News!

Both channels are working so good now after a complete caps replacement in the power amp section.
IC 302 is running nice with no over heating. It was running hot (angry) with the leaky cap/s.

Even though I have done recapping a lot, I still appreciate all the comments and advices. It is so easy to make a mistake on polarity and also put the wrong cap. Thanks to NORM.

I learned a lot to work on this CFS 77. I will post all pics that might help our members when they get a chance to work on it.


The next hurdle is the belt replacement on this. These SONY decks are a nightmare to work on. I've worked on all the CFS decks, but I am always nervous working on it. The top plate for the flywheel will not come out easily. One has to remove several parts and components to get there.

I am waiting for 11" and 10.7" belts.
Original belt was slightly elongated. It measured 11.5" as it is.
Sadly our belt reference guide gave wrong numbers...10.5".

I did a complete recap....

This sounds the best!

The deck needed new belts and now I know it is also one of the toughest deck to work on.

Difficulty level working on the deck was a step above Sanyo M9998.....

The sound is .........AWESOME!

https://www.ebay.com/itm/Vintage-Sony-CFS-77-Boombox-Stereo-In-Box-Recorded-Only-Partially-Working/183987391057?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649

What a deal!

Oh my!

This is a place right under my nose....
Very kind guy!
Offer free shipping even.
What can you ask for?

Nice one Royce! Did you take any pics of your work on the inside?

caution said:

Nice one Royce! Did you take any pics of your work on the inside?

Click to expand...

Hi Eric,

Yes I did! I will post them soon.

~Royce

Pics...



Replacing with new caps one by one...


Excellent radio reception and nice sound quality. Decent bass at low volumes! One great Sony to have in the collection.
Now I have most of the CFS Sony in my collection.

.... I can see no one is interested in a SONY post....

Sad story of life

I was watching this post for a while.......


...fun..so much fun.... never seen a pic of my boombox pics here on Boomboxery!

...live and let others live...

I am hurt!