Pioneer Sx-850 amplifier board issues

[Mitchellk]

Hello,

I am having an issue with a Pioneer sx-850. R38 and Q6 on the power amplifier assembly is running very hot. So hot you can't touch either within a couple of seconds of turning on the power to the unit. The radio was playing fine when I smelled smoke and found that VR3 had a burn spot on the side of the pot when I pulled the cover. I have used the sx-850 recap lists to recap the power amplifier board and replaced the bias pots and recapped the power supply. I also replaced Q1,Q2,Q3, and Q4 on the amplifier board with KSA940TU and KSC2073TU.

I pulled R38 and it tested at 14.89 on my ohm reading.

This is my first time working on stereos, and would appreciate any help on how to repair this issue. I'd like to save the old girl if possible, and I have printed out the service manuals. I just don't have much experience with this.

 

[Mitchellk]

After installing KSA940TU and KSC2073 they are running hot as well. Where should I test for incoming power on that side? I will continue reviewing previous posts to see if I can find anything to help guide me.

 

[rcs16]

I also replaced Q1,Q2,Q3, and Q4 on the amplifier board with KSA940TU and KSC2073TU

Should be ksa992f to replace 2sa726s, note pin-out differences.

I just don't have much experience with this.

Measure all the parts for value/function, isolated from the circuit, replace as necessary. Check the channels against each other for verification that they are the same.

 

[Watthour]

According to my documentation, R38 should be 150Ω, not 15Ω.

 

[Mitchellk]

Thank you for the help. I stupidly listed the wrong numbers above. I replaced Q12 and Q10 by using the attached image I found on one the sx-850 treads with KSA940TU and KSC2073TU. I am going to order the remainder of the parts for this board on Monday. I have already recapped it.

 

[Mitchellk]

This is what I used to base the recap and transistor replacement lists on so far.

 

[Mitchellk]

I ordered a new Fluke electrical tester as my Ideal tester doesn't seem to be reading Ohms correctly. Should have it tomorrow and I'll recheck number R38 again. Thank you both for the help!