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Technics SU-8600 No Relay/Rail Voltage on Output

goldswimmerb

goldswimmerb

Failed out of engineering, but look at me now!
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Hey guys, I have a Technics SU-8600 on the bench and am hoping for some help with it. Unfortunately I am used to Marantz, with easy to work on and very detailed service manuals, so working on this Technics is like a foreign language. Hoping maybe someone with more experience with these can weigh in.

Pins 434/435 are showing an excess of 40v, but at least on the DBT there doesn't seem to be a dead short anywhere. It seems to me it may be a PS issue as it would be unlikely for both channels to blow outputs simultaneously.

What I've noticed so far is that the DBT shows almost no idle power draw, and the negative rail does not discharge after power off, the positive rail drains on its own as expected. Could I have a bad output tranistor in each channel? If anyone's got any ideas please let me know.

Thanks!
 
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Oh come on! This is a straight forward amp just like any decent Marantz! The schematic is all cut up so you will have to glue everything together, whether with tape or digitally (with Gimp). All the important voltages are marked.

It is unusual but not impossible for two amps to fail simultaneously. Test if the output transistors TR411/2 are shorted. If so remove them all. Or just disconnect all collectors. No need to jumper anything. Now you can check if the driver and VAS are OK. Then go on from there.
 
PE9ZZ said:
Oh come on! This is a straight forward amp just like any decent Marantz! The schematic is all cut up so you will have to glue everything together, whether with tape or digitally (with Gimp). All the important voltages are marked.

It is unusual but not impossible for two amps to fail simultaneously. Test if the output transistors TR411/2 are shorted. If so remove them all. Or just disconnect all collectors. No need to jumper anything. Now you can check if the driver and VAS are OK. Then go on from there.
Click to expand...

Got it figured out! Was an exploded blue cap in the power supply for the low voltage rail, thing was reading about 60nf and screwing everything up.

I had a hard time following voltages on the Technics schematic, but I was able to make my life easier by pulling up the board image with all the voltages, and schematic side by side to trace my issue out. Thanks for the reply though :).
 
Huh? The low voltage PSU (+/- 24 V) doesn't have anything to do with the power amp. Which C had gone phut?
 
PE9ZZ said:
Huh? The low voltage PSU (+/- 24 V) doesn't have anything to do with the power amp. Which C had gone phut?
Click to expand...

If I recall it was C506, I swapped all 4 caps in that area after finding it, they all had leaked a little but that one had completely popped it's bottom.

I'm noticing now that my input to my regulated -48v supply is lower than my +48V supply by around 11v or so. Positive supply is showing 57v and negative is showing -46v.
 
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Binkman said:
G'day
Either your testing of the PS is incomplete or haven't posted it. The final rail V's are +/-48.3 and you need to check the other v's in the view pdf 9/19 How about transformer AC35V? There are transistor V's to check, too. Blown p.s caps ain't good.
https://audiokarma.org/forums/index...-8600-no-relay-rail-voltage-on-output.913822/
Click to expand...

Both my positive and negative main rails, on the outputs of the main filters checkout fine. I'm seeing the discrepancy at pins 508 (57v) and pin 509 (-46v)
 
So, digging further I found that C507 was bad as well. Swapped it out to no improvement. Something is dragging down the negative supply. I pulled and tested TR504 and TR502, both tested fine, also went and tested my zeners, both if which tested fine as well. While I was in there I also swapped out the rest of the power supply Electrolytic Capacitors, to no improvement.

Bias dials in fine, DC offset is acceptable, but it starts distorting at around 10w
 
Just to close this off (for now) I found my issue. Both R435 and R436 had gone open or drifted significantly. Replaced them and the negative rail is well within specification now.
 
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