Technics SC-CH7 repair

Thread Starter

udhaka

Joined Jun 26, 2021
6
Hey guys, newbie here. Need help to resurrect my beloved TECHNICS SC-CH7 mini HIFI system. Had this system since forever and recently rediscovered it up in the attic and would like to have it working again. I might have to take it to a proper service guy, but until I find a reliable place, I will see if it's doable on my own. Also, I could give a few pointers at the repair shop based on AAC feedback.

The TECHNICS SC-CH7 mini hifi system consists of:

ST-CH7L (TUNER)
SU-CH7 (AMPLIFIER)
SL-CH7 (CD PLAYER)
RS-CH7 (TAPE DECK)

Seen related threads regarding this unit but didn't find anything related to my problem.

So when I found the system, it had all the interconnecting cables between the units there, so no cables missing (all cables looked to be in working conditions). Except for the power cable which was missing, but I was able to find a generic model to replace it with. When I connected the power cable and switched on the mains at the back, it wouldn't turn on. So I went to open the the unit that connects to the main power, which is the amplifier. Sure enough, I found one busted fuse out of the three fuses I could find. The replacement fuse didn't look anything like the original fuse but I made sure it had similar specs. Original fuse was T1 25A 250V and replacement is F 25A (it doesn't say 250V anywhere but I checked with the seller). See attached pic for comparison.

So after the fuse replacement, the amplifier turned on. The little red light in front was on, indicating that it's on standby mode. While on standby mode however, the TUNER (which has the power switch to turn on all the other units) should indicate it has power by showing the time on the display. In this case it doesn't and with that I'm not able to turn on the system. I'm guessing the TUNER unit is not powered? Could the fuse be the issue here?

I've attached a few images to show the overall condition of the system, see if you guys can spot anything or figure out as to what could be the problem. Thanks so much for looking into this post, any help & feedback is greatly appreciated.


burned part inside the amplifier which I saw on another thread as well:
View attachment 115195

replaced fuse:
View attachment 115197

inside amplifier:
View attachment 115198

inside tape deck:
View attachment 115199

inside tuner:
View attachment 115201

inside cd player:
View attachment 115202

signs of burns inside of tuner and cd cover plates:
View attachment 115203

original fuse (left) replacement fuse (right):
View attachment 115204

redlight on, standby mode:
View attachment 115205

system:
View attachment 115200
hi i would like to know if you fixed it and how please im having the exact same problem
please help me i have also looked inside the amplifier and the same place was burned
thanks 20210626_201406.jpg
 

MrChips

Joined Oct 2, 2009
24,609
Your first photo shows browning of the PCB under Q706. This does not necessarily indicate failure.
It shows poor design. Q706 drops 23V to 5.7V.
Altogether, R706, R709, R710, Q706 have to drop 33V to 5.7V. This energy has to be dissipated as heat energy.
That is the reason for the heat stress on the PCB.

Technics SC-CH7 Q706 photo.jpg


Technics SC-CH7 Q706.jpg
 

Thread Starter

udhaka

Joined Jun 26, 2021
6
Your first photo shows browning of the PCB under Q706. This does not necessarily indicate failure.
It shows poor design. Q706 drops 23V to 5.7V.
Altogether, R706, R709, R710, Q706 have to drop 33V to 5.7V. This energy has to be dissipated as heat energy.
That is the reason for the heat stress on the PCB.
thank you for the information and now i have to figure out whats wrong can you explain clearly where the failure is most like to happen
 

MrChips

Joined Oct 2, 2009
24,609
Tell us what are the symptoms on your SU-CH7 amplifier.
Tell us what test equipment you have.
Tell us what experience you have on repairing electronics equipment.

Then we can take it from there.
 

Thread Starter

udhaka

Joined Jun 26, 2021
6
Tell us what are the symptoms on your SU-CH7 amplifier.
Tell us what test equipment you have.
Tell us what experience you have on repairing electronics equipment.

Then we can take it from there.
its stuck in standby mode the light is on and when i press power it doesnt turn on and i have checked the fuses located near the supply area using the multimeter buzzer mode and other than that i havent checked anything i have many capacitors from an old sony HI FI system
 

abrsvc

Joined Jun 16, 2018
97
First of all, the photo shows that there are 2 transistors in the Q706 position. I know for a fact that this would not be factory. Someone has worked on this before. Correct that problem first and see what happens. From the photo that shows the amplifier IC, it appears that the capacitor on the left side may be swollen a bit which may indicate a problem. Change that out as well and report the results here.

If the unit won't turn on, it is possible that a problem with the amp IC can cause this. I don't recall any common problems with these units from when I was a warranty center for these.

Keep us posted.
 
its stuck in standby mode the light is on and when i press power it doesnt turn on and i have checked the fuses located near the supply area using the multimeter buzzer mode and other than that i havent checked anything i have many capacitors from an old sony HI FI system
Hello, I think there is an issue with the amplifier. It is preferable to give this to a professional service center rather than doing it yourself.
 
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