170DC burning up left channel

[PhilLem]

Hello guys,

I am trying to repair a Marantz 170DC with a bad left channel, when left on for a while R728, R729 and R731 along with Q710 would burn up. Other people have worked on this amp and replaced many parts without success, so it is now up to me...
Another post suggested to add heat sinks to transistors Q707, 708, 709, 710 as the equivalent run closer to their thermal limits. So I did replace all four transistor, added heat sinks and replace the 3 burned up resistors, however the resistors went into smoke the second I turned the amp on so there is obviously something else wrong. I cannot see any other damaged components apart the 3 resistors, any clues on what I should be looking for/replace?

Thanks for your help!

 

[correiadesa]

Could you solve your recap issues. I'm having issues after issues with the left channel too. I've received a Marantz 170dc with all power transistors in the left channel burned and the pre driver transistors too. I've fully reccaped the board, replaced burned resistors and diodes and then it worked perfectly for around 5 hours. Then after some pops the left channel went away again. The left channel peak meter constantly on and DC voltage raises as I turn the left channel volume up. Then I found out that the manual has several errors about resistors values. I've replaced them with the original values(r745, r743). Now when I turn the amplifier on the dim bulb tester lights up and the peak led light blinks. I immediately turn the amp off. If I remove the left channel board everything is ok with the right channel. Can anyone give me some clues?

 

[soccerfan]

Remove and retest both the output transistors and drivers, pretty sure they are blown up. Where did you buy the replacement transistors from and which part number?

 

[KeyserSozee]

Luckily you do have one working amplifier board, you can compare resistances and voltages (if you’re willing to do some measuring with the unit on) with the working board.

Definitely sounds like potential fake transistors, that’s their typical life story

 

[correiadesa]

Remove and retest both the output transistors and drivers, pretty sure they are blown up. Where did you buy the replacement transistors from and which part number?

Thank you. I bought all the parts on mouser. I already checked the new power transistors and they are fine. Now it seems that q727 and q715 are burned. I'm waiting for new transistors to replace them, but I suspect that something will short when I replace them and I can't find out why. What transistors should I use in pre driver transistors. Could it be something related?

 

[rcs16]

I'd pull the outputs on the bad Channel and then figure out if the circuit biases up correctly.
Compare the bias voltages on the good working channel against the bad one.
Turn output stage bias pot to minimum, power up using the DBT, and then if the lamp dims down, you can short out the lamp to supply full AC.
Power off, attach dmm lead grabbers, power on, mark good channel voltages on a print out of the schematic, compare, highlight notable differences against the bad Channel, so you identify incorrect voltages.
Q715 is a 2sc945, use a ksc945c, center collector.
Q727 is a 1n1555 diode, use a 1n4148
As for pre-drivers q713,714?
do I not see tta-004b,q, ttc-004b,q installed for q711,112? so use the same, verify pinouts.
Good luck