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Technics SU8080 amp -- no sound

A

AussieKen

New Member
Hello everyone, first post here. I have a Technics SU8080 amplifier which I purchased new back in the late 1970s. After an hiatus I decided to set it up again but have no sound. The speaker protection relay does not power on. Going through threads on this site ( and others ) I have gone as far as I can but need more help please. So far have checked the relay coil and contacts, OK. Checked resistor R416 OK and Diode D402 OK. Replaced transistors TR404 & TR405 , as well as Capacitors C401, C505, C506. This amp has some sentimental value to me and would like to get going again, any more ideas please.
 
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momolou

Thread 'Technics SU8080 Relay not Engaged'

i am try measure voltage between coil on relay, when i put multimeter on each coil , there is no voltage, but when i put one on ground ( have screw) and one on relay ,there is -50volt
First make sure all the AC fuses are OK. F1 through F8. Make sure clips are tight also.
Check for DC in the outputs. Not at the speaker terminals as they are after the relay. If no voltage on non ground side of C402, then probably ok.
If voltage, check points A on the amplifier on both channels.
If no voltages, look at voltages around TR402 to give you a direction. It gets a little odd (at least for techs) because relay is run off of the negative rail. (most run off the positive rail) Tip: put some tape around the tip of your probes so only a little metal sticks out. This will save you spitzensparken when probing voltages. This will give you a start.
 
Thankyou. I will do those checks and report back. To be clear --- there is -48V at the coil , but should be 24V DC, there is not.
 
AussieKen said:
there is -48V at the coil , but should be 24V DC, there is not.
Click to expand...
Relay will pull in when the voltage drop across it is 24 volts. One side is connected to -48 volts through a resistor, so when it is not energized, both sides of the relay coil will sit at -48 volts, but when energized, one side will sit at near zero volts and the other at around -24 volts going to the resistor.
 
I've worked on a couple of these and while they are GREAT amps, the speaker protect circuit is very fickle - there may not necessarily be much wrong for the circuit to not engage the relay.
 
First thing to do would have been to see if either channel is putting out DC and if the circuit is just doing it's job! Right at the relay contacts.

And then check power supply outputs.
 
Checked the 4 X power fuses , clips clean and tight OK. Checked C402, has -1.5V on the - side. Checked TR402 has -0.4V on B , -1.5V on E and -2.0V on C
I just need direction on where to probe for power supply outputs / DC outputs, channel A location.
 
AussieKen said:
TR402 has -0.4V on B , -1.5V on E and -2.0V on C
Click to expand...
Start here - the -2 is also on the base of TR404, what are other voltages on 404. TR404 should be on, and driving TR405 with about .8 volts on its base. If that voltage is lower, could be low gain or improper orientation of tr404.
I always verify base and collector with meter - sometimes they change and are different from the specifications. With an PNP transistor, the base will show up as about a .5 reading in diode test with the black lead on the base and red on the collector and with red lead on the emitter.
https://audiokarma.org/forums/threads/bipolar-junction-transistor-testing-basics.43186/
 
AussieKen said:
Hello everyone, first post here. I have a Technics SU8080 amplifier which I purchased new back in the late 1970s. After an hiatus I decided to set it up again but have no sound. The speaker protection relay does not power on. Going through threads on this site ( and others ) I have gone as far as I can but need more help please. So far have checked the relay coil and contacts, OK. Checked resistor R416 OK and Diode D402 OK. Replaced transistors TR404 & TR405 , as well as Capacitors C401, C505, C506. This amp has some sentimental value to me and would like to get going again, any more ideas please.
Click to expand...
Build a Dim Bulb Tester, that way you can probe voltages without the risk of shorting the whole unit.
Easy build, there are tutorials all over, the hardest part, these days, is finding an incandescent bulb
 
Power transformer LEFT --- orange 35V AC yellow 35V AC RIGHT --- orange 35V AC yellow 35V AC blue to terminal 415 on protection circuit 3V AC
These are the transistor values TR402 B -0.4V C -2.1V E -1.5V TR403 B -1.5V C -2.1V E -1.1V TR404 B -2.1V C -46V E -1.9V TR 405 B -1.9V C -47V E -1.5V
 
AussieKen said:
TR 405 B -1.9V C -47V E -1.5V
Click to expand...
TR405 emitter should be connected to ground! Try tightening screws on the protection board. they are ground connections. If not, look for cold solder joints or crack in ground trace.
1789708692108.png
 
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sregor , you are DA MAN ! Yep , you got it right. I disconnected the circuit board mounting screws to gain better access to replace components and did not put back = no grounding. Thankyou Sir. :thumbsup:
I'm going to put it all back together and test. Fingers crossed, let you know.
 
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