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Technics SA-500 Help!

LazyB

LazyB

New Member
Can you guys give me some direction? The relay won't click on my Technics SA-500 receiver. If I take out the speaker fuses at the back, it clicks. All other functions work from what I can tell (line out to a separate amp plays both channels just fine). I've got the schematic, but am a bit lost. Any help would be much appreciated!
 
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might be a power supply fault then ..its unusual to have both sides bad at the same time .
check the internal fuses .
 
Need to check your +/- 44 vdc unregulated rails.At the output transistors collectors.
 
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LazyB said:
By output transistors, I assume those are on the power amp board by the heatsink?
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Yes. The transistors fastened to the largest heatsinks. Collector is the metal case..

2N3771-NPN-Power-Transistor-Pinout.jpg
 
Securely fasten your black test probe to the metal chassis. And probe the transistor cases with your red probe. Do not short the transistors case to the heatsink with your probe. Sparks will fly..And fuses will permanently " check out"..
 
avionic said:
Securely fasten your black test probe to the metal chassis. And probe the transistor cases with your red probe. Do not short the transistors case to the heatsink with your probe. Sparks will fly..And fuses will permanently " check out"..
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Okay will do, I'll go get some alligator probes tomorrow to make sure I have a secure connection to the chassis. Thanks. If the voltage is high, then the transistors are out? And if it's correct, then what?
 
LazyB said:
If the voltage is high, then the transistors are out?
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No.Not necessarily. I'm looking for equal plus and minus voltages. ie +44vdc and -44vdc..

Just verifying power supply voltages right now.
That is always the first step. Verify you have a working power supply.
 
That is the second step.

Edit: wait a minute! The protection circuit senses after the fuses! So your amp is prolly railing, removing the fuses will bring it out of protect. Railing means that its output is at or near the power rails, in your case either +44 or -44 V.
 
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PE9ZZ said:
That is the second step.

Edit: wait a minute! The protection circuit senses after the fuses! So your amp is prolly railing, removing the fuses will bring it out of protect. Railing means that its output is at or near the power rails, in your case either +44 or -44 V.
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So which part would cause that problem? The output transistors? I noticed for the day it worked after recapping, that the loudness switch would cause the right channel to drop. I assume this was a protection mechanism?
 
LazyB said:
So which part would cause that problem?
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Could be anything. Even something mundane as an electrolytic the wrong way around. Many people seem to do that. Swapping PNPs and NPNs is also popular.

But is your power OK? You haven't completed the first step yet. First poke around with your multimeter, then we'll talk.
 
Okay finally got around to this. The output transistors read +/- 45.2v on each side, equal all the way around.
PE9ZZ said:
Could be anything. Even something mundane as an electrolytic the wrong way around. Many people seem to do that. Swapping PNPs and NPNs is also popular.

But is your power OK? You haven't completed the first step yet. First poke around with your multimeter, then we'll talk.
Click to expand...
 
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