Troubleshooting Technics SU-8080

sgp09c

New Member
I'm new to repairing vintage amplifiers. Originally this amp would only work if I ran audio into the 'Main In' inputs on the back of the amp. The amp sounded great, but it bypasses the preamp with the volume and tone controls. Whenever I'd try to run audio through the preamp I wouldn't get sound. I deoxit'd everything - got it almost working 100%.

The left channel input on any of the preamp inputs (phono 1, phono 2, aux, tuner) will not work. However, both speakers will play perfectly if I run it in mono via the preamp. It also plays stereo perfectly with "Main In" - but I lose tone/volume controls. I don't think it's an amplifier problem since I can get both speakers to play. The fuses appear fine.

Any ideas? Appreciate the help.
 
Try running a signal (cd player or similar) through the "Tape one" inputs.
The service manual is on HiFi Engine.
 
This is a great amp and worth the effort. My next project is resurrecting a Technics SU-V10 that I managed to partially blow up. Just another day for me really......
 
The manual/ service manual was very helpful!

Those switches are such a pain. How did you get to them? Did you have to take apart the front? I've got it so they turn on and off appropriately but they're still a bit scratchy.

Tape 1 and 2 work really well - stereo works and sounds awesome! I love this amp. I'm guessing the problem is either the selector knob for the aux/phono/etc or those switches... Waiting on my next shipment of deoxit!
 
I knew I recognized your username from somewhere! I ran into your rebuild pics while I was searching for the manual - great job man! If I tried to do that, it wouldn't go back together again lol.
 
In that pic of the switches, the one on the right has a cover over the contacts that can be slid back to get the spray inside. You can also spray into the openings on the front, and then stand the amp on it's back to let the contact cleaner run down the contacts.
 
I've taken that type of switch apart before. Use a white eraser to clean the stationary contacts and a folded piece of paper soaked in Deoxit, slid between the sliding contacts. On a top notch resto those switches get torn apart, cleaned and lubed. No worries for quite a few years.
 
Edit again: the other inputs work now! it's those damn switches. they're coming around though.

Edit: It's the via tone switch. In via tone position the sound in one channel is way lower / cuts out, while in "direct" mode it has balanced sound. How difficult is it to get to that switch to slide the top back? I assume I need to take off that whole upper PCB board, which makes me nervous haha.
 
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