Pioneer SX-1250 Original Bias at Zero

HankM

New Member
I just finished my 3rd recap of Pioneer SX-1250. I recap 2 or 3 receivers per month, so I do a lot. After the recap and replacement of all 2SA725. 726, 750, and 2SC 1312 and 1313 transistors I naturally set the DC and Bias currents. What is surprising is that on both of the last 2 1250's, and an SX-850 I recently did, is that the DC is off, OK, but the Bias has been absolute ZERO! It is supposed to be 100mv! At zero, there should not be enough voltage to turn on the transistors! Was this setting not done at the factory on every unit? Or was it supposed to be set by the retailer before delivering the unit? I never worked in a Hi-Fi store in those days, I sold cameras. Is this something others have found?
 
Probably someone has been in there and wound it off for whatever reason, or the unit has a fault....
Did you check it before the recap? I always check all those things before I undertake such a job, just so I know what was happening beforehand.
Yes Pioneer would set the Stabiliser, DC offset, Bias at the factory as they would have no doubt tested the units before shipping.
As I said before I'd say someone has simply gone in there and wound the bias off for some reason. No way of knowing really unless you'd checked it before the work.
Do you replace the trimmers? I would...
Also it is not necessary to run the SX1250 at 100mV, 65-70mV is the standard we use in this forum here, thats all the quiescent current they need, this way they idle a lot cooler which is not a bad thing with the 1250.
 
Thanks for the reply, no I don't check anything, and you may recall I stated the most recent was DOA with a burnt up stabilizer board. Why would 3 units in a row be at zero on the bias, 2 SX-1250, and an 850. I just took in a 1050, I will take a look. I did set the bias on the recent one at 90mv to give it some room
 
Thanks for the reply, no I don't check anything, and you may recall I stated the most recent was DOA with a burnt up stabilizer board. Why would 3 units in a row be at zero on the bias, 2 SX-1250, and an 850. I just took in a 1050, I will take a look. I did set the bias on the recent one at 90mv to give it some room
Maybe someone has been trying to work on some of the problems and turned the bias down whilst diagnosing?

The first thing I do before I rip into a resto is to have a good look around the unit, whether it's working or not, and take note of odd things like for example, the bias pots turned all the way down....
You'd be surprised the things you find.

There is no way those units left the factory like that, they would have had to miss whole steps on the production line, I can't see it personally. It has to be someone tinkering.

You can definitely run the bias even lower on the 1250...as I said around 65-70mV is plenty.
 
The one I had no bias on one channel. It was the oxidized connectors on the pcb, the ones going to the heatsinks/bjts. Just that little bit of extra resistance will create a higher bias V needed. it is also unstable. So clean those contacts first, then check /adj bias again.
probably cleaning the TO-3 sockets would be a good thing too and re-grease the o/p's.
 
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