SA-706 Amplifier restore. Dead channel and more..

[streetlevel]

Hi all


I'm restoring a Pioneer SA-706 amp, my first real restoration project, and have come to a dead end.


The amp was Originally blowing the main fuse and lighting up the DBT.


I recapped the unit entirely and replaced multiple transistors. new Relay and filter caps also



I found that Rectifier diodes D1 and D2 were allowing current both directions so I changed all 4. This stopped the DBT lighting up.


Fuse was no longer blowing, and on full power the relay clicked in, and I was able to set the Bias on both channels to 30 mV.


Testing the output of the channels my multimeter showed 0 VDC on the left channel. ~5VDC on the right.

With the amp on the Phono setting, the right channel meter is showing a signal (nothing plugged into phono) which rises with the volume being increased.

I went back to adjust the bias again as the unit was warmed up and the right channel trimmer pot jumped of the scale and burned out, tripping the relay into protection mode!

So a couple of questions:

What should I start looking at for the dead left channel?

What could be causing the noise on the phono setting?

I'm going to order new trimmer resistor pots and new wire wound resistors 0.5ohm 5 Watt (one beside each output transistor). Anything I should add to that list?

I've been learning as I go with this, AK being one of the best sources of knowledge I've found.
If anyone has any suggestions to more reading for restoration of vintage audio electronics, I'm eager to learn.

Thanks all

 

[streetlevel]

Update on this:

Changed the trimmer pots and the wire wound resistors.
Left channel had DC at the offset pins but right channel is dead.

I took some voltage readings from a few places:

The relay has 32.5v on the left side and 13.7v on the right.

Pins of the IC board:
1: 0v
2: 0v
4:0v
9: 0v
10: 45V

Pins on the PA3004 chip:

1: 12.4v
3: 0.75v
4: 0.01v
6: -7v
7: 0.7v


I've been checking diodes and nothing so far.
Anything I should be looking at first?

 

[streetlevel]

Made my way through nearly all the components, checking with the MM and no luck.
Checked for cold solder joints and no luck there either.
Any ideas are very welcome, I would love to get this amp back in action!

 

[streetlevel]

Protection Relay has stopped engaging.
VU meter lights working ok.
DBT not showing a short.

 

[streetlevel]

Progress..
Found a crack in one of the traces running from the left side of the relay.
installed a jumper cable and presto, amplifier is back running again on both channels.

I've adjusted the DC Bias to 30mV on each channel.
Is this the optimum setting for the SA-706?

The service manual says between 10mV and 70 mV

Talked myself through this one, and learned a lot from this forum. old posts anyway