Hi all, I have just rebuilt a channel on the pioneer spec 4.....almost all of the transistors were shorted and associated resistors were burnt behind recognition....after repair I can set the dc offset to 0.....and the bias is adjustable but I cannot get it down to spec.....it is running approx 80mv.....the trim pot has been replaced and is at the low end at its stop....I have the amp setup on the bench where I can get to everything under power and I see no reasons for this.....have any of you guys seen this and do you have any recommendations on how to resolve this issue.......thanks in advance.....
Spec 4 Bias
- Thread starter Avron
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What transistors did you use to replace each one?
Are you sure you didn't miss a damaged component?
Are you sure you didn't miss a damaged component?
I have checked all 4 diodes.....they are not open nor shorted......I have 4.3v on collector Q5 and Q4 I have a -4.3
.,.....I swapped Q1 with the working channel with no effect......Q2 and Q3 have been replaced.......honestly I have spent hours scanning all xistors and resistors and can find nothing wrong.....eventually I'll get there thanks for your response
I have checked all 4 diodes and confirmed their polarity....I have a -4.3v on Q4 and +4.3 on Q5.....they raise with the trim pot as I increase the idle current.
.,.....I swapped Q1 with the working channel with no effect......Q2 and Q3 have been replaced.......honestly I have spent hours scanning all xistors and resistors and can find nothing wrong.....eventually I'll get there thanks for your response
I am beginning to think the bias circuit if functioning properly......here is the list of semiconductors iv installed......
Outputs Q1,2,3 and 4 MJ21195, MJ21196
Drivers Q10 and Q11 MJE15033 ,MJE15022
Pre Drivers Q8, Q9....onsemi C3503 , A1381
Q6, Q14....2N3904....
Q7, Q13.....c945....
Q2, Q3.....c1845
Thanks for your attention...well appreciated
Q6 and 14 should be KSA1015. Otherwise looks good. Make sure there's no solder bridges and that everything is installed correctly.
I pulled the output xistors from the working channel....the gain was 1/2 of the onsemi transistors...... the gain was
on the Onsemi was 120
On the Toshiba was 63
I have been chasing a bias issue thinking my substitutions for bias transistors was the issue....or I missed an open resistor or bias diodes.....turns out these onsemi are just too hot for the spec 4....now what finals do I go with????at 20 bucks a pair I hate to miss it again......thanks much.
on the Onsemi was 120
On the Toshiba was 63
I have been chasing a bias issue thinking my substitutions for bias transistors was the issue....or I missed an open resistor or bias diodes.....turns out these onsemi are just too hot for the spec 4....now what finals do I go with????at 20 bucks a pair I hate to miss it again......thanks much.
I did.....the bias set up with no issues with the Toshiba outputs installed in the rebuilt channel......I checked the gain on the onsemi xistors.....they were within 20% of each other ....then I checked the gain on the Toshiba and they as well were within 20% of each other.....but the gain was nearly half as much....I used the mj21195 and 96 because of being lazy.....I watched a video X-ray tony rebuilt the same amp and used these outputs......go figure.....I had a set of mj21193, 4 the gain almost matched the Toshiba so I used them.....the only other issue was locating a sub for the 2sa733.....I finally found a ZTX553 in stock at mouser......they will be here tomorrow......then I can set this unit up and ship it.....thanks everyone for your input......
I find locating a decent substitute from my vendors for these vintage amps is as challenging as choosing it......almost every sub listed is discontinued or not shipped to my area. It would be interesting to see what everyone else does....thank you again
I find locating a decent substitute from my vendors for these vintage amps is as challenging as choosing it......almost every sub listed is discontinued or not shipped to my area. It would be interesting to see what everyone else does....thank you again
The biggest problem I had ( this is many many years ago) was receiving a blown amplifier and the replacement transistor were not with original transistors. The choice of the pre drivers were the ones that prevented idling currents to not adjust as in the recommended the service manuals. So if I did not get the idling current correct I would look at the original transistor data sheets and choose another pair that came close or exceed the originals. Today choices are getting worse. You may now have to start using chip transistors as replacements?
Just saying.
Just saying.
