Pioneer SX-838 voltage problems???

[avionic]

wet finger on each jumper should make a nice speaker buzz .

Just kidding LOL Had to borrow John's avatar. @restorer-john

 

[Tomcow]

I’ve been working so long and hard on this receiver,I feel so stupid!The jumpers were in the tape 1 rec play jacks!!!! The receiver now plays out of both channels!Last problem now is left channel still showing 300mv dc at speaker output.Right channel is fine!

 

[Tomcow]

I was wondering if having different pairs of pre-drivers on the output stage could be a possibility.Right channel is running 2SA-1010 and 2SC-2344,Left channel has 2SD-358 and 2SB-528. Right channel bias and DC offset are perfect and sound great.Left channel,bias is perfect but can’t get dc below 300mv.Left channel plays but doesn’t have the quality and output of the right channel.

 

[rcs16]

what is supposed to be in there? 2SB,2SD devices are slower by definition

 

[Tomcow]

2SD-358 and 2SB-528 were the originals.

 

[Tomcow]

Pic

 

[avionic]

can’t get dc below 300mv.Left channel plays but doesn’t have the quality and output of the right channel.

Could be the differential long tailed pair for the right channel. Q2 and Q4 need to be Hfe matched as close as possible.

 

[avionic]

2SA726 ---> Mouser 512-KSA992FBU order 20 or so. Check the Hfe on each one. Select the two with the closest and highest Hfe values. The closer to identical Hfe values, the better.

 

[Tomcow]

PicView attachment 1304068

2SA726 ---> Mouser 512-KSA992FBU order 20 or so. Check the Hfe on each one. Select the two with the closest and highest Hfe values. The closer to identical Hfe values, the better.

you don’t think it has anything to do with the output transistors or the pre-drivers,2SC2344,2SA1010 on right channel,2SB528,2SD358 on the left?I can dial in the bias,no problem on both channels and right channel,can dial in 0 dc.Left channel,300-315mv with controller(adjustment) all the way off.

 

[avionic]

I can dial in the bias,no problem on both channels

Has nothing to do with DC offset/DC balance.

can dial in 0 dc.Left channel,300-315mv

DC offset

 

[avionic]

you don’t think it has anything to do with the output transistors or the pre-drivers,2SC2344,2SA1010 on right channel

It's a possibility that there is 300mv of leakage in one of those transistors.

 

[Tomcow]

Are you talking about the output transistors?2 2SA-679’s and 2 2SC-1079’s.They appear original and very Old!!IWhen I first started working on this unit,It was having this same problem.

 

[Tomcow]

I know bias and Dc offset are completely different.Just saying what they are.

 

[avionic]

Are you talking about the output transistors?2 2SA-679’s and 2 2SC-1079’s.They appear original and very Old!!IWhen I first started working on this unit,It was having this same problem.

Both the outputs and drivers. I'd try the LTP first ---Q2&Q4.

 

[Tomcow]

Ok thanks.

 

[avionic]

Just in case...

AWH-031 transistors
Q1: 2SA726: 512-KSA992FBU
Q2: 2SA726: 512-KSA992FBU
Q3: 2SA726: 512-KSA992FBU
Q4: 2SA726: 512-KSA992FBU
Q5: 2SC1451: 512-KSC3503DSTU
Q6: 2SC1451: 512-KSC3503DSTU
Q7: 2SD358: 512-KSC2073TU
Q8: 2SD358: 512-KSC2073TU
Q9: 2SB528: 512-KSA940TU
Q10: 2SB528: 512-KSA940TU
Q11: 2SC1079: 863-MJ21194G
Q12: 2SC1079: 863-MJ21194G
Q13: 2SA679: 863-MJ21193G
Q14: 2SA679: 863-MJ21193G

 

[Tomcow]

Thanks for the list.Tom