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Pionner Sx-1280 DC Offset / Pre-amp question

B

Bobs34969

New Member
Hello Pioneer Guru's! I am seeking some advice on my Sx-1280. One of my pre-amp boards will not adjust into spec for the DC offset. The "lowest" I can get it is -1.1V. That is with the VR turned all the way clockwise. The other pre-amp board will adjust no problem. I have replaced all the transistors on both boards, and even replaced the 979 transistors with matched 992's. I have been comparing voltages across the good and bad boards and it seems that the bad board has been very consistent with being about .7 - .9 volts off on the 733, 945, 608, and 628 transistors compared to the good side except on Q4 and Q5. My Q4 & Q5 voltages are as follows:

Good Board Bad Board Good Board Bad Board
Q4: Q4: Q5: Q5:
B = +78V B = +78V B = -76V B = -76V
C = +2.2V C = +.9V C = -2.2V C = -3V
E= +78V E = +78V E= - 76 E = -76

My question is what would cause such an imbalance at these transistors? I have also replaced a couple of the diodes on the bad board with 4148's to see if they were an issue and no change. Every transistor is new, most diodes except the varistors are new, and all resistor values match exactly to the good board and yet it still won't get into spec! I am at a loss here and could really use some help from the guys who know what they are doing Ha!. Any suggestions would be appreciated! Thanks for your time.
 
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The AWH-074 boards are the Power Amp boards, one for each channel; the Pre-amp section(s) are before the Pre-Out Power-In jacks/jumpers. The dc offset adjust (VR1) is on the power amp pcb.
Which is the bad channel, left or right?

Did you replace Q3 (2SC2291) with matched KSC1845 transistors?
 
Thanks for your reply merlynski! The board in question is the right channel. But at the moment, the output transistors are completely separated as a few of the original un-obtanium ones had blown. I replaced with the modern upgrade and made my own circuit boards to match, but haven't installed yet as I wanted to get the DC offset as close to 0 as I could get. I bought this unit as a "won't turn on, don't know why" unit and it was in terrible shape electrically. Especially the 157 board. But I got it all squared away up to the pre amp boards. Left channel DC offset will dial down the +- 15mv no problem, but right channel just wont go under -1.2V. I did not replace the 2291 with any 1845's as I didn't have any 1845's on hand. I did remove the 2291 and tested it, and it tested good on both sides so I re-installed.
 
The left channel has the o/p bjts installed but not the right ch. maybe if you install the right ch o/p bjts you’ll be able to set the dc offset correctly
 
If the output transistors are removed, I don't know if the unit would correctly setup offset?????????
 
The dc balance feedback path is closed through R28 and R29 (100Ω, 1W) even with the output transistors removed, so it should be adjustable. The BIAS cannot be set without Output transistors installed.
Look at Q6,Q7, Q8, Q9; constant current sink/source transistors Q4, Q5; as well as the predrivers Q10, Q12; and driver transistors Q11, Q13 if it will not balance.
 
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Thank for all the replies everyone! Merlynski thanks for the response. I also saw that R28 & 29 close the circuit so I figured it should be adjustable as well. I will be sure to check and test those transistors. Looking forward to cranking this thing up!
 
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