Technics SU-CH7 / Panasonic SC-CH7 amplifier high tones are dimmed, spongy.

Thread Starter

Shores

Joined Aug 7, 2016
13
Hello! I've followed your advices here to renew capacitors for my SU-CH7 which had the same problems as yours.

Now it's back to working order, but something strange happens: when using the DAT input, if I connect my headphones to the front jack, the sound is perfect, but when I hear the music with the loudspeakers it seems the high tones are dimmed, spongy.

Any advice on what may be causing this behaviour? Remember that using headphones the sound is perfect!

Thanks in advance for any hint,
Manuele



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Please don't hijack other member's thread, now you have your own.
This thread was split from --
Technics SU-CH7 / Panasonic SC-CH7 amplifier issue
https://forum.allaboutcircuits.com/threads/technics-su-ch7-panasonic-sc-ch7-amplifier-issue.120179/#post-1111452
 

Thread Starter

Shores

Joined Aug 7, 2016
13
I've discovered that simply the amp sometimes decides to mute high speaker outputs, but moving around cables to the speakers sometimes make it appear again...

It seems that something in the amp chooses not to send the high speakers output, maybe there's a circuit to sense if bi-wired speakers are connected?
 

Thread Starter

Shores

Joined Aug 7, 2016
13
Shit! I've made a mistake while trying to solve this, and shorted the positive lead of C702 in the AMP to ground...

Now the amp doesn't event turn on... :( :( :(

I've got the schematics, can anyone tell me what I could have damaged with this error? Should I Try and replace C702 itself?
 

dl324

Joined Mar 30, 2015
10,714
I've got the schematics, can anyone tell me what I could have damaged with this error?
Not without a lot more information. Use your eyes and nose to look for components that may have been damaged. If you find some, post the schematics.
Should I Try and replace C702 itself?
Not unless it looks damaged. By shorting that cap, you damaged something; but probably not the cap.

The reality of this situation is that it's unlikely anyone here will be able to help you. Professional help will be expensive. Is it worth the money? If not, time to buy a replacement amp.
 

#12

Joined Nov 30, 2010
18,210
moving around cables to the speakers sometimes make it appear again...
By the way, I've tested cables to the speakers and they're all ok..
That seems contradictory. The first thing I would do is renew every connection in the speaker wiring. An intermittent cable is usually bad at a connection. That might be the entire cause of the high frequency failures.

Even if you've roasted the amplifier, you will still need excellent cable connections after the repair or replacement.
 

Thread Starter

Shores

Joined Aug 7, 2016
13
That seems contradictory. The first thing I would do is renew every connection in the speaker wiring. An intermittent cable is usually bad at a connection. That might be the entire cause of the high frequency failures.

Even if you've roasted the amplifier, you will still need excellent cable connections after the repair or replacement.
I know that seems contradictory... :)

Changing cables would be a difficult operation, since they pass behind a wall mounted book shelf; would it be a good test if I check continuity with a tester?

Besides that, they're not high quality cables, but they worked perfectly for years before these problems begun, so I'm confident their quality is not the culprit of the bad sound...
 

Thread Starter

Shores

Joined Aug 7, 2016
13
Not without a lot more information. Use your eyes and nose to look for components that may have been damaged. If you find some, post the schematics.
Not unless it looks damaged. By shorting that cap, you damaged something; but probably not the cap.

The reality of this situation is that it's unlikely anyone here will be able to help you. Professional help will be expensive. Is it worth the money? If not, time to buy a replacement amp.
Ok, I'll reopen the Amp and look around for damaged components; are there components which may be more fragile that I should keep my eye on, or viceversa, component types I could safely ignore since they're usually very strong?

In any case, I attach the schematics...
 

Thread Starter

Shores

Joined Aug 7, 2016
13
... Professional help will be expensive. Is it worth the money? If not, time to buy a replacement amp.
:( It may not be worth the money, but this system is a college degree gift from my father, and I love it... ;)

Anyhow, thanks to you both for any help, it'll be very appreciated!

I'll go and look and smell around the amp, and report back!
 

Thread Starter

Shores

Joined Aug 7, 2016
13
By the way, following the paths of the pcb from the cap would be a good way to search for broken components?
 

#12

Joined Nov 30, 2010
18,210
Changing cables would be a difficult operation, since they pass behind a wall mounted book shelf;
It's the connections that go bad, not the middle of the wire...unless something very unusual happened...like rats in the wall.:eek:
I'm just saying, go OCD on the connections. There is no such thing as: too good, too shiny, too clean, too secure, too safe, or too stable for a wire connection.;)
 

Thread Starter

Shores

Joined Aug 7, 2016
13
Hooray!

We're back to "normal" functioning, and we can get back to solving the missing high tone speaker output!

I'll tell you what was happening: the PCB of the amp is very old, and has one area (nearby C702) where the tracks have detached from the board, so I had to rebuild them with jumpers as to make Q706 connected again with the surrounding components: so I recreated the tracks with jumpers, but when I was trying to find cold connections on the board, it seems I had de-soldered one of the jumpers!

I mistakenly thought that I had shorted the cap nearby, since I left a pin of the cap a bit too long when I replaced it, but happily it wasn't that, just a de-soldered jumper!

ME HAPPY! :) :D

OK, I should stop jumping around as a mad wallaby, let's get on with your helpful hints...
 

Thread Starter

Shores

Joined Aug 7, 2016
13
OK, I've tested the cables again, all have impedance lower than 1 Ohm, so they seem in perfect order...

By the way, I also tried to swap high tone and low tone cables (sure I did this on both sides, amp and speaker) to test if one of them was bad and the other OK, but this didn't change anything: bass/mid working OK, treble is in fact completely silent...

I'm thinking again that the amp might have a sensing circuit, and it might erroneously think that tweeters aren't connected...
 

#12

Joined Nov 30, 2010
18,210
There is a lot more to check. You might have smoked the tweeters with an ultrasonic oscillation that you were never aware of.:eek: But it's more likely in the circuitry of the Panasonic amplifier.
 

Thread Starter

Shores

Joined Aug 7, 2016
13
I'm happy to say that another part of the restoration has succeeded: another problem with the system was the CD player not reading discs any more, but I've managed to find another SL-CH7 unit which wasn't turning on, but the laser pickup, when inserted in my unit, works flawlessly!

So, the only real problem with the system is the missing high tone speaker output...

Anybody has any hints? I was looking at the muting circuit, but I don't know where to begin to check for failures...

Thanks for any hint!
 
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