KR-710 extremley weak right channel idle current

[Zrooney]

Hello all,

I have a Kenwood KR-710 that has an extremely low output on the right side in regards to the idle current. The service manual says the Idle Current should be 30mV for the right channel, but I am only getting -.10 to -.15mV which is incredibly low. I tested all transistors out of circuit between Q16 and Q36 and all of them tested good, no dead shorts and all were within a .5 - .650 on the diode test. I tested caps in this area as well (out of circuit) using the capacitance function on my multi meter, and all capacitors tested above rated capacitance, i don't know how to test ESR at the moment. All resistors were within +- 5% tolerance, and all diodes tested good.

When I turn VR2 all the way to the right (at its maximum of 200ohm) the mV rating drops closer to 0.

I am kinda lost at this point of what to test / trouble shoot next. Is the problem possibly further up stream, or am i not diagnosing the amplifier correctly? Attached is a picture of the readout on the multi meter, and a screenshot of the part that i have attempted to troubleshoot from the schematic just in case i mislabeled this section.

Any help is always appreciated. This amp has been on my workbench for (literal) months, and I just want to button this bad boy up. What started as a simple fuse replacement quickly turned into a flower pot project.

 

[petehall347]

you should be across both the big white resistors . they are in series .

 

[Zrooney]

I checked across the resistors, R134 and R132. Positive probe is hooked up to R134 and common is on R132. I am reading a -.1 mV reading, which is severely low.
I am going to take a look at the schematic and try to test some voltages. Will report back later tonight.

 

[petehall347]

look at collector voltages . then resistors

 

[Zrooney]

The service manual says to look at the emitter of Q28 and Q30 to set the idle current. When i do that, with the positive probe on Q30 and the neutral on Q28, i get between -.10 to -.15mV.


I attempted to probe the collector of Q30 and Q28, and i just let out the magic smoke (again)...... R128 is singed to a crisp. I am really bad at probing i guess

 

[petehall347]

tape or shrink wrap on your probes helps a lot .

 

[Zrooney]

tape or shrink wrap on your probes helps a lot .

I have the little plastic ends that snap onto the probes to keep the point relatively small.
Uhhhh, what do I do now. You want to help me trouble shoot another amp, I have about 10 more that need work. I think this one is dead jim.

It looks like I shorted the power transistors again, melted one of them clean.... I think I am calling it quits on this one. I am not going to put another $20 in transistors again into this.

 

[petehall347]

we all short things out sometimes . i slow down when this happens and take a break .

 

[Zrooney]

Yeah, I put everything away for the night. I am not going to repair this one. I keep burning through power transistors, and they are to expensive to justify putting any more $$ into this amp.