Marantz 1060 low bias and center voltage

[grindfix]

This amp had Left channel outputs and pre-drivers blown due to speaker wires touching pos to neg. at the speaker. Right channel was working well and responded to trim pot adjustments correctly.
While amp board is out for repairs I decided to replace all electrolytics as well.
I got parts list from several threads and suggestions from other members.
Here is what I used:

C701 - C702 .47uF 50V .47uF 63V 505-MKS20.47/63/5 Wima Film
C703 - C704 47uF 50V 270uF 50V 661-EKZE500ETD271MJS
C707 - C710 47uF 50V 56uF 50V 667-EEU-FR1H560
C711 - C712 1uF 35V 10uF 35V 647-UPS1V100MDD
C717 - C718 22uF 35V 22uF 50V 667-EEU-FR1H220
C727 - C728 100pF 505-FKP2100/1000/5 WIMA FKP

H701 - H702, H705 - H706 KSC945CGBU
H703 - H704, H709 - H710 2N3440
H711 - H712 2N5416
H707 - H708 KSA733CGBU

R747 - R750 .24ohm 2W resistors

Cleaned trim pots with deoxit and turned amp on with DBT and attempted to adjust center voltage and bias as I usually do after rebuilds of such extent. There was no DBT flash so I assume there are no shorts. All 4 trim pots are maxed out and even connected directly to the power outlet center voltage is maxed at 31v (72v supply at J709 / J710) and bias across emitter resistors is barely 1.5mv. These readings are approximately identical on both channels. Amp plays through headphones with both channels. There is some distortion, of course, level is very low and obviously outputs are not even in the equation due to such low bias.
I've rechecked all transistor datasheet layout against markings on the board and can't see what I could've missed or miss-installed to cause this situation. Mis-print on the board?
Anyone run into this situation?
Any advice would be greatly appreciated.

 

[grindfix]

Very discouraging error in Fairchild datasheet. KSC945CGBU and KSA733CGBU are listed as EBC polarity while they are ECB. I failed to verify polarity and trusted the datasheet. Never again. Replaced all 6 of them and was able to adjust center voltage but bias is still low. Trimmers are maxed out at about 6.2ma. Spec is 7.5ma
Can anyone suggest where to look for this discrepancy?

 

[paul_ral]

This datasheet is good. You don't read that note about "C" suffix

 

[grindfix]

This datasheet is good. You don't read that note about "C" suffix

I did. After the fact, obviously. This type of difference should be pictured side by side with original configuration. Easy to make this mistake.

 

[grindfix]

I found something. There is a resistor on the foil side of the amp board parallel to R729 and R730 trimmers as well as H713 and H714.
I did not find those resistors in the schematics and thought it could have been added by someone to compensate bias in the past, but I have another 1060 that needs work and that one has those resistors on the foil side as well.
logically, I could increase resistance of those added resistors to compensate for my bias discrepancy. correct?
I'm measuring bias current across R747 and R748 as per manual and only achieving about 5.8mv after about an hour of playing the amp. Heatsinks are barely warm.
Can someone advise?
Thanks in advance.

 

[catrafter]

I have seen those resistors on the foil side, I think they might be R761 and R732 on the schematic.
One thing to remember on this amp is that the DC balance adjustment will affect the bias adjustment. The procedure should say to adjust the bias, then the DC balance, then the bias again until you are satisfied that you have reached a good compromise.

A better way to set the bias would be to measure across both high wattage resistors R747/R749 and adjust R723 for 15mVDC. For the other channel, measure across RR748/R750 and adjust R730 for 15mVDC. Volume down, of course.
To set the DC balance without a scope, you can set R723 for about 29VDC at J715 and R724 for about 29VDC at J716 (black meter lead to chassis). Volume down, of course.

DC balance will only effect the sound quality as you approach clipping.
5.8mVDC across .24Ω is 24.2mA of bias current. 7.5mVDC across .24Ω (or 15mV across .48Ω) is 31.25mA. I seriously doubt that you can hear a difference between the two.

Good job getting it going!

Tom

 

[grindfix]

I have seen those resistors on the foil side, I think they might be R761 and R732 on the schematic.
One thing to remember on this amp is that the DC balance adjustment will affect the bias adjustment. The procedure should say to adjust the bias, then the DC balance, then the bias again until you are satisfied that you have reached a good compromise.

A better way to set the bias would be to measure across both high wattage resistors R747/R749 and adjust R723 for 15mVDC. For the other channel, measure across RR748/R750 and adjust R730 for 15mVDC. Volume down, of course.
To set the DC balance without a scope, you can set R723 for about 29VDC at J715 and R724 for about 29VDC at J716 (black meter lead to chassis). Volume down, of course.

DC balance will only effect the sound quality as you approach clipping.
5.8mVDC across .24Ω is 24.2mA of bias current. 7.5mVDC across .24Ω (or 15mV across .48Ω) is 31.25mA. I seriously doubt that you can hear a difference between the two.

Good job getting it going!

Tom

I was actually meaning to ask why Bias is measured only across one of the resistors.
Thank you for your answers. I will recheck everything when I get back home this afternoon

 

[grindfix]

My supply voltage is 71.8v
Should I still adjust DC balance to 29v? And not the center?
I understand that lower voltage at output transistors will raise current.
I do have signal generator and a scope so I can actually check and properly adjust clipping. I just thought that adjusting DC balance to half of supply voltage would be a good center point.

 

[grindfix]

I couldn't get clipping on the screen of my oscilloscope. I just got it and not sure I figured it out yet. I've settled on around 33v on DC center which allowed to adjust bias to 14.5mv across pair of emitter resistors. Amp sounds great and I'm sure will sound even better after caps break-in.
Thank you for your help Tom!

 

[catrafter]

You got it! Theoretically, the balance point should be at exactly half of the 71V power supply or 35.5V, but this is the real world, 33V is fine.

Good work!

Tom

 

[grindfix]

You got it! Theoretically, the balance point should be at exactly half of the 71V power supply or 35.5V, but this is the real world, 33V is fine.

Good work!

Tom

Tom

Do you possibly have some kind of a visual tutorial reference to adjusting clipping with oscilloscope?

 

[catrafter]

No visual. What I do is feed a 1kHz tone into Aux input, monitor the amp output on a dummy load with an O'scope. Increase the volume to the point that either the positive or negative 'peak' of the waveform begins to clip, then slightly adjust the DC balance while running the volume up and down slightly until both the positive and negative peeks clip at about the same volume level. Then go back, reset bias and do it again.
Do the same for the other channel.
I try to get it the best I can, but frankly, unless you routinely drive the amp near clipping, close is good enough.

Tom

 

[fromnowon]

A better way to set the bias would be to measure across both high wattage resistors R747/R749 and adjust R723 for 15mVDC. For the other channel, measure across RR748/R750 and adjust R730 for 15mVDC. Volume down, of course.

Hello, I've been reading 1060 posts. Really, really appreciate all the knowledge shared, and I'm learning a lot from Catrafter and others. It's awesome; what a great 'body of knowledge' about this amp. Anyway, I think I noticed a typo in this thread: In #6 above, I am pretty sure that the reference to R723 should be R729.

Like this:
For bias, connect your meter from J713 to J717 and adjust R729 for 15mVDC.
For the other channel, connect your meter from J714 to J718 and adjust R730 for 15mVDC

Right?