Bias problem on a KA-7100, new trimmer does nothing

Also please verify the basing of Q9, Q11, Q13, Q23 and Q25, vs the silk screen on the board and check the silk screen against the actual connections on the board. Checking 2 leads of each part will be sufficient. There could be an error in the silk screen or you could have mounted a part wrong.
sorry, dr, I thought you meant check the voltages. I will check pinouts again. I have even compared originals to replacements in the Trans.tester to check. no luck so far.
 
I have listened in headphones and speakers. It actually sounded good. I do not play it long because it heats up. Now the L mains are pulled out.
 
headphones only at low levels is critical especially with outputs removed .. all the current is being supplied by the drivers with no outputs there .. they are not designed for high current so will hot up if driven hard with low ohms loads such as speakers ..
anything but clear low level sound with headphones like distortion says there are problems .
sorry if i ranted a bit then .
 
To me... and I know nothing...but to me the most notable or confusing reading is the lack of any voltage at VRe3, the bias trimmer. I measure like 340mV on one side (with wiper) and like -345mV on the other, what I would call the "out". The Right side has 1.24V and 1.16V. With no voltage here, i would expect no voltage available for controlling the bias, so I'm not surprised there are 000 readings on the drivers and the outputs. That might be all screwed up, but thats my mechanical logic

I don't know enough to read a s/m and be able to trace the path of current, neither do I understand how a transistor affects that path. I have simply replaced one part with another. Where does that current come from?(VRe3)
 
Got to take a little break for the wife. Thank you everyone for being patient and helpful! I'll keep at this in an hour
 
[QUOTE="petehall347, post: 12069842, member: 121225"..
anything but clear low level sound with headphones like distortion says there are problems .
.[/QUOTE]

Which is why I asked - wondering if he's been hearing it play only through the drivers all along, which can sound good through headphones at low vol
 
yes a bad connection on the board can cause this for sure .. never easy if it isnt right in front of you is it ? and sometimes it can trick the best of us .
 
Could an output transistor failing to open C-E under operating voltage cause it? IOW, passes on diode test, but fails in the real world. Would be an odd failure
 
Sometimes not recommended, but i could swap the R and L main outputs once they were restablished. The amp still has the 1K's in place on the Left, but easily changed back to the 330 r's. Whenevr i tested the mains they were similar R to L. i did not record the figures.
 
never swap outputs channel to channel .. always keep one working if you can to compare voltages not marked on the schematic .
 
that's what I've been doing, keeping the good R side for reference and comparison. I was reluctant to swap sides for fear of spreading a problem.
 
+/- .36V across the bias pot is about a diode drop. This is significant. Sounds like there is a diode connection where there shouldn't be, making me think a transistor is installed wrong, or one or more of the transistors I asked if you replaced or tested are bad.
 
I checked Q9,11,13,15,17,19,21 for orientation and matching the silkscreen on both sides. Diodes 1,3,5,7,11 too. I checked the originals for pin layout, compared them to the replacements. I followed traces on the PCB to verify connections matching the s/m. I did some diode testing in place using the DVM. Most diodes tested OK this way, except the zeners, which would show voltage both directions. Hopefully I was doing it right. i did not find anything wired wrong. (yet!) I know there has got to be something wrong somewhere!

I have a some good magnifiers I have been using to check traces and soldering on the PCB. Nothing unusual noticed. I should probably try resoldering some joints. I have changed many parts, so there is lots of new soldering. I can offer pictures of my work.

I have enough stock to replace any of these if I find a bad one, so I can go through the line again, pull them out and check each one. There might not be another way. I'll start with diodes D1, 3 5, and 7. Simple enough.

Should I leave the 1K resistors in place at R57, 59?
 
sorry I talk so much, I just try to explain what I do so those of you across the world can watch over my shoulder. and I sure appreciate it!
 
Leave the 1Ks in place, and the outputs removed. Please test Q15, Q17, Q19 and Q21. They are protection transistors and they will kill the bias if they are shorted.
 
Removed and tested with transistor tester Qe15,17 (PNP - ECB - A733) and Qe19,21 (NPN - ECB - C945c)

All tested OK.

I purposefully shorted leads E x C, E x B, B x C, to see what the tester would do, and the tester displayed the shorts each time as two diodes, in various config's

I changed diodes De9, De11, and De15. All removed and new diodes tested correctly. I notice the s/m makes a distinction between IS2076 and IS2076A. Do you know what that is? Maybe wattage? I used 1N4149 for all replacements. A 1N4148 is 200mW, a 1N4149 is 500mW.

I checked De1 and De3 in circuit. Reversed they would bounce up to about a volt, then go OL. This was the same on the Right side De2,4.
I used a 1N5252B for the 24V Zener De1, and i used a 1N5255B 28V Zener for De3.

I checked pin out orientation and what each side connects to by following the traces on the PCB, then comparing that to the s/m. All silkscreening seemed correct, matching the top/bottom and the s/m on all diodes and transistors checked.

Nuts!

I noticed the PCB bottom transistor diagrams for Qe31 and Qe32 had an ECB orientation to the flat side, just like all other transistor diagrams found underneath. The transistors, MJE15032 and33 are actually pin out backwards, at BCE. This makes the transistor mount "backwards" to the picture if you use that convention, but it still mounts to the E, the C and the B of course. I would expect a catastrophic failure here if these were wired wrong. Made my heart jump when I saw it!
 
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