Pioneer SX-1250 Low Bias on both channels after rebuild

I've had a 1250 not meet ops bias spec using original semis. The problem was oxidized contacts on the connectors going to the ops bjts. Cleaned them up with some deoxit/contact cleaner to solve that problem.
If that does not work then increasing the pot to 1k is an option.
Its also possible if you use mje15032/33 for q8,9 it might change or increase the adjustment range and allow it to meet spec. Its just fine tuning the design a bit.
 
rcs16 said:
I've had a 1250 not meet ops bias spec using original semis. The problem was oxidized contacts on the connectors going to the ops bjts. Cleaned them up with some deoxit/contact cleaner to solve that problem.
If that does not work then increasing the pot to 1k is an option.
Its also possible if you use mje15032/33 for q8,9 it might change or increase the adjustment range and allow it to meet spec. Its just fine tuning the design a bit.
Click to expand...
Thanks for the reply rcs16. I will try cleaning up the connectors a little better. I used the eraser trick that I’ve seen recommended but maybe that’s not cutting it. I’ll give it a shot. I am having zero issues with the functionality of the receiver which is making me lean toward the type of a solution you mention. If I still can’t get to 100 mV do you think it would make more sense to swap the trimmers to 1k or to try swapping q8 and q9 first?
 
Use the 1k trimmer if you are not going to change the semis
If you try mje15032/33 leave the 500-ohm trimmer and then if that does not work then try above.
Another thing to do is calculate the current in the VAS stage, measure the voltage across the 620-ohm tail current resistor and you might want to check it's value too. It should be around 2.8mA
 
rcs16 said:
Use the 1k trimmer if you are not going to change the semis
If you try mje15032/33 leave the 500-ohm trimmer and then if that does not work then try above.
Another thing to do is calculate the current in the VAS stage, measure the voltage across the 620-ohm tail current resistor and you might want to check it's value too. It should be around 2.8mA
Click to expand...
I tried cleaning the pins more than I already did before and that didn’t do anything, but here’s an interesting thing I tried.

I tried swapping both power amp boards to see if each board would read the same maximum idle (really to see if it was a power amp problem or not), and after swapping them, I still got the same reading of ~80 mV left channel and 62 mV right channel… The idle measurements didn’t follow the boards, meaning it might not be a problem on the boards and might be something else?

I pulled the outputs previously and tested them and they all tested fine on the diode checker on my multimeter. I followed the guide on here to check them. All read right around the same voltages when I pulled them.

Any ideas?
 
With both amps (Channels) at 65mv's idle. Read the voltage across pins 16 and 8.
Post the voltages.
 
I messed up.
It wasn't a complete waste.
Same readings, same idles. Across pins 11 and 16.
 
Inspect all the wire wrap connections from the molex plugs to the output sockets.
Make sure the wire connections to from the big filters to the output sockets are secure and the unregulated voltages at the filters are to spec.
Lastly:
Install new or known good outputs. (I might have some originals, Free)
 
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Another thought. Check ALL voltages on both power amp boards that the service manual lists.

Compare with the service manual and compare channel to channel. Any discrepancies?
 
Another simple check. You say the 1250 is working fine. Can you check the temperature of all 8 output transistors? Are the temperatures pretty close?
 
In order to determine current sharing in each output device, you measure across each emitter resistor and if you want ultimate accuracy you perform a 4-wire ohms measurement on each emitter resistor to determine a accurate value for the calculation.
I offered my advise, we've been down this road before. Its not a very good bias generator design, what else can I say, it only works for the exact semis in the original design.
 
Thank you all for the advice. It’s very helpful. I have to put it away for now and probably won’t get back to it till later in the week. And yes, I tested the receiver again yesterday and it’s working completely fine… I will address all items you all have suggested. I really appreciate all of your time you took to help out a stranger :)

A side note: Zebulon1 I am suspicious of the wire wraps on the socket connections to the output transistors now that you mention it. I had 2 break on me that I had to repair, so I’m wondering if that also could be an issue.
 
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