Pioneer SA-8800 high bias

Depaj

Member
2015-08-05 5:31 pm
lyon
Hello,

Just got a Pioneer from a friend sitting on my table. He complained about a crackling noise on one channel, some days after listening for a while, some days no problem at all. He has had it checked out by professionals a few times who changed some capacitors and the output transistors to 2SA1301/2sc2380 instead of the 2SA1075/2sc2525 that were the original ones.

When I first powered up the amp I immediately got a lot of heat coming from the output transistors. checked for bias and had about 160mv on one channel and 120mV on the other and over 150mV of dc on the outputs.
I guessed that nobody took care of readjusting after changing those output transistors. I was able to set dc to 0mV +/-2mV but even with the two trimpots turned completely counter-clockwise I still had over the 77mV of bias that the service manual aims for. I have had this problem with some other amps after changing the output transistors but there was a simple vbe multiplier that I could slightly redesign to get where I wanted. In this case I don't really see how to get there.

Any suggestions would be welcome, as well as a possible cause to the crackling (that is his description, I never managed to hear it with my own ears).
 

Depaj

Member
2015-08-05 5:31 pm
lyon
I looked at some of his posts and he does seem to know a thing or two about these pioneer models but so far I didn't find anything that could help me.

Here are the specs of both original and replacement pnp transistors :

Type Designator: 2SA1075 /2sa1301
Maximum collector power dissipation (Pc), W: 120 /120
Maximum collector-base voltage |Ucb|, V: 120 /160

Maximum collector-emitter voltage |Uce|, V: 120 /160

Maximum emitter-base voltage |Ueb|, V: 7 /5

Maximum collector current |Ic max|, A: 12 /12

Maksimalna temperatura (Tj), °C: 135 /175

Transition frequency (ft), MHz: 60 /30

Collector capacitance (Cc), pF: 300 /480
Forward current transfer ratio (hFE), min: 60 /55
 
here is what i would do first

For these amplifiers there is a number of resistors depending on the serial number that could go open( value drifts...up ) look carefully Often these resistors drift symmetrically in both P and N side so amplifier continues to operate but with problems . Unfortunately i don't have a a list to give you since this one died on a computer problem years ago .

Secondary power supply might be faulty but still partially operating check voltages according to the schematic Often there is problems on Q13 Q14 , the fets and the capacitors over there, together with soldering. Work with caution this is a tracking regulator though start with checking voltage .

Vbe might be far different (suggested before ) that will also require a recalculation of the operation of the Vbe multiplier , and since this amplifier biases with 2 circuits it probably require some work in both stages ...that might be tricky ...

Kindest regards
Sakis
 

Depaj

Member
2015-08-05 5:31 pm
lyon
I checked all the resistors on the main amp board and found R125 to R128 (15K) drifted to about 14K each. Otherwise they are all within 3% of spec.

I checked the voltages on the secondary power supply and found -50.6V on pin 14, -33.8V on pin 15, +32.6V on pin 17 and +49.6V on pin 18 which seems to be a little higher but ok.

I will start be changing those 4 resistors and report back.