Technics SU-G90 no display or sound

Hi!,
This unit had intermittent output and I tried to help a friend by expecting to find dry joints. I found a few, but the fan was not working so while I was tracing the circuit my hand slip and I now have no display or audio. I have voltage on pin 4 60volts and lights on the function buttons. Q705 transistor seems to run hot as the circuit board is discoloured. I have no schematic and would appreciate any help I will give more info if needed. Looking forward to someone pointing me in the right direction.
Thank you
Tony
 
i only have available su-g 91 schematic and you are welcome to have it .... just e mail me ... Please notice that most of these models have a circuit with ventilator with a sensor ...meaning that if the speed of the fan is low for any reason of failure or simply cause the fun is broken the amp goes on protection mode

in your case obviously there is going to be more than that.... I epect schematics to be the same only 91 might be more powerfull

kind regards sakis
 
The fan circuits in both amplifiers work. It appears the problem exists within the connections between the front panel and the main mother circuit board. It must have been a common problem because both these power amplifiers exhibit the same condition. If you tap on the box the interment goes and comes back! I am wondering if anyone else has found the source of the problem?
 
I know I know, old thread, blah blah.........


Anywho, I have the fix for this problem! Q705 most likely has died. The problem is a loss of +5 volts to the front display. I've had two su-g90's and an su-g75 on my bench for this. Instead of the original 2sc3940a, I used a bd139.

-Blazer40
 
No sound, no display

I have a similar problem with my SU-G75, but I am unable to find the root cause. There is no display and no sound but I hear a click from the relay when I press the power button.

You were talking about a Q705, but there is only Q701 and Q708 on my board.

Anyone have a solution or something to help me.

Thanks you!
 
SU-G75 Service Manual

Hi darko911, here is the service manual:

Technics SU-G75 Service Manual

Schematic is rather complicated, as usual with Technics, however I would start with checking the power rails (there are lots of power regulators there) and protection circuit / relays, it is well-recognizable on the schematic diagram. Start from the output (page 8). Oscilloscope would be helpful... Also check the "Muting circuit" (page 17).

You can also try to find where the power is comig to the display and understand why it is not powered - may be related to "no sound" issue as well.

Hope this helps,
Valery
 
Anyone can help?

In a microcontroller based amp any problems with the controller board will usually kill the sound because the inputs are selected and electronically switched by the UController.
Similarly the display will usually blank.
I have a thread on this problem in a Harman Kardon AVR 230.
Unfortunately I can't tell you how to fix it.
At least the Technics is simpler and better built than the HK.
Better manual too.

Best of luck.
David
 
I know I know, old thread, blah blah.........


Anywho, I have the fix for this problem! Q705 most likely has died. The problem is a loss of +5 volts to the front display. I've had two su-g90's and an su-g75 on my bench for this. Instead of the original 2sc3940a, I used a bd139.

-Blazer40

Hi!

I own a SU-G90 amp, and I'm facing some problems too...

I've posted the situation in a new thread, so I would thank a lot if you take a look to it and great if you can help me.

It is Technics SU-G90 with problems

Thanks!
 
Hello, i am looking at a similar issue. Amp was working fine, then got some low volume, and noticed display had disappeared. Restart, no display, check all 3 board fuses good. My main board does not have the Q705, 706 as in the technics schematic. They are nowhere to be found on the main board. Could it be located on another board? I also don't see any hot spots, which areas on the main board produce the most heat. Thank you, Bj
 

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@Bjjbj I don't have my unit here to look at it but I remember they were hiding under one of the heat sinks...I think the smaller one so you'll have to pull that off first. Also rather than removing the whole main circuit board for the repair, I just cut about a 1" hole in the bottom of the receiver sheetmetal with a Dremel so that it could be easily accessed.

Neil
 
Hobo, well i cut the hole, didn't see any cracks. I re touched the solder spots of Q705 and Q706, along with the small adjacent cap. Turned on, and nothing??? I flipped unit over, and tested Q705 at the pins, 12 volt at the center, and 6 volt on each end, hmmph. The when I went to flip it back over wa-la, the display was on, all the buttons worked, including the FET coupling light which never worked before. BTW, when i re-touched pins, the solder sucked down and left gaps, so adding solder seemed necessary. Stoked so far, off to test for sound, fingers crossed. bj
 
Semi good / bad news. Display was working, played for an hour, then display went out. Still had the music playing with working bass, treble, and balance controls. No volume, bass boost, lights, or input buttons worked. Re soldered all nearby to Q705, and display back on, but after an hour same exact issue. Tested Q705 and found 12 volts at center , but no volts on the outer pins. Could be q705 is failing when hot, or i have to remove bottom and do a complete solder check on the whole board. Suggestions? I can replace the transistors without removing the bottom. bj
 
Hello again! Sounds like progress. Did you check for continuity between the pins around q705 where there should be continuity? I remember I found there was no continuity between two pins where there should have been. Because reheating the joins didn't fix it I had to jumper a piece of wire between two pins...
 
Well after lots of testing and repairing a few traces, it worked again for about 10 minutes. Voltage from Q701 is 14+ volts, so voltage in to Q705 is the same. When the unit kicks out, I am seeing that the input voltage of Q705 is still 14, but the output voltages are gone, hence the no display power. So gonna say its the transistor on the blink. Ordering one up and we will see. Unless someone out there knows of a better replacement. bj