Pioneer SA-8800 high bias

[Depaj]

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).

 

[JonSnell Electronic]

I would fit the original type output and drivers. The Vbe is probably different.

 

[Depaj]

Those are actually quite hard to find (and a lot of fake ones out there) and very expensive, is there another way to make this work ?

 

[JonSnell Electronic]

If you place a 1n4006 diode across D9 and D10, does the quiescent current go to zero?

 

[Depaj]

I don't have those. I could go and buy some but once more, if there is another way...

 

[JonSnell Electronic]

Any diode will do to test.

 

[JonSnell Electronic]

Ensure the voltage between the two bases that set the bias is less than 2.9V

 

[llwhtt]

Are you using all 4 bias adjustments, two for the 56mv and the other two for 70mv?

Craig

 

[Depaj]

Hello Craig,

Yes I am, both turned completely counter-clockwise and I'm still getting 78mV on one and a little over 60mV on the other. other than that the amp works and I manage to adjust dc offset to 0mV.

 

[llwhtt]

Try reducing R95/96, 4.7k to the next value, 4.3k. Or put a large value resistor in parallel with R95/96. You want to get the bases of Q39 and Q41 closer together, same for Q40 and Q42.

Craig

 

[Depaj]

If I'm not mistaking putting à 50K resistor in parallel would be the same as changing them to 4.3K resistors right ?

 

[llwhtt]

4,296 Ohms, close enough, just want to see if that brings the current down. Try the alignments after that.

Craig

 

[Depaj]

I tried with just one 51k resistor in parallel and then even with two so about an equivalent of 3.96R and I only got down from 60mV to about 56mV (that is with a bulb tester so lets say from 78mV to 74mV without.

 

[llwhtt]

Have you compared the specs of the original outputs and the replacements. I wonder how different they may be. Is there a better replacement?

Craig

 

[llwhtt]

Over on AudioKarma there are some good posts about the Pioneer NSA amps, look for posts by MarkTheFixer. They look to be a real pain in the ***.

Craig

 

[Depaj]

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

 

[llwhtt]

I've never worked on an NSA amplifier so I have no idea what it will take.

Craig

 

[east electronics]

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]

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.

 

[Depaj]

Well, replacing those didn't do a thing, so I'm guessing only the vbe can change something now.
What exactly should I look for ?