KR-4200 Static on L channel

R42 seems a bit light, but unless it's significantly drifting under load, I wouldn't worry about it. One way to tell would be to measure the voltage on either side of it. Do you have a replacement for Q16? ...or swap it with Q15? If that doesn't change anything, find the equivalent of a chopstick and tap on the L CH components -- specifically the carbon composition resistors -- and listen for a change.

Already replaced Q16 (see post #12). Will check the resistors...

Any chance this is a capacitor issue? I've only replaced a few caps on this board (ones I thought looked bad).
 
Tap test the components on the L CH amp board and give a shot of freeze spray to the L CH output transistors (if you haven't already).

Did both of these, and no change in static. Not seeing any broken solder joints. I've inspected the board multiple times looking for anything that could be bad.

May need to recap the board, but that will have to wait. I've got other projects that are more pressing. I've already replaced C3, 4, 13, 14, 24. I think I have replacements for all the rest on hand except for C21 (33µF/50V), I don't have any 33µF caps. If the recap doesn't solve it, I'll be looking for more assistance!
 
Replaced all E-caps on the power amp board except C21, since I don't have a 33µF replacement on hand. No change in static! :(
 
I made a mistake telling you it has to be in the power amp. Usually the volume control comes right before the power amp with no circuit in between. In this receiver, the volume control comes before the tone amp, a very very bad design, because any noise in the tone amp goes right to the power amp regardless of the volume control setting. Hit the 2 transistors on the tone board with freeze spray.
 
I made a mistake telling you it has to be in the power amp. Usually the volume control comes right before the power amp with no circuit in between. In this receiver, the volume control comes before the tone amp, a very very bad design, because any noise in the tone amp goes right to the power amp regardless of the volume control setting. Hit the 2 transistors on the tone board with freeze spray.
I will check that, but I have measured voltage coming from the tone amp board to the power amp board on pins 3 (R) and 6 (L), and both read zero with volume all the way down and no signal on the inputs (such as AUX 1). In this condition, I'm getting about 0 DC offset at the R speaker out, but about 30 mV on the L, which is quite variable. Seems to me that evidence points to the power amp board. I will likely look at Q 2 & 4 again, possibly replace with a new pair of KSC1845.
 
I think the 30mV offset may be unrelated. Also the noise in the tone amp would not show up on a meter, you'd have to use an oscilloscope to see it, and even then it could be a very small signal, there is a lot of gain in the amp. How loud is the static you hear? do you have to have your ear up to the speaker to hear it? If you are sitting 10 feet away from the speakers can you hear it?
 
Tried freeze spray on Qi1 and 2 on the tone amp board. No change in static. Those two transistors are 2SC1000BL. I have no idea what the replacements for those would be. Suggestion?

In all of my tests I am listening through headphones. However, it has sounded the same through speakers. The static is noticeable when playing music. Might be equal in volume to 2 or 3 on the volume knob. I can hear the static from 10 feet away from speakers easily.
 
Again, how loud is the static?
Just to be clear, the static does fade in and out a bit. It is not a constant background hiss or noise. The volume is about the same level as if music were playing at about 2 or 3 on the volume knob. If I turn volume louder than that, the static is less noticeable when sound is playing, but still there during silent patches. In my limited experience it is the kind of static sound that I have heard when a transistor somewhere in the signal path is going bad.

I had to re-read this thread, but it looks like no mention was made whether or not the static changes with volume? Does it get louder with volume? What happens if you gently tap the relay while the unit is on?

Edit: Fwiw...I'd also automatically replace any and all carbon composition resistors.
Static is independent of volume. With volume at zero, static is there on L channel (see post #22). Also bypassed the relay (see post #24) and static remains, so I do not think the issue is with the relay.
 
Replaced Q4 on amp board with KSC1845, no change in static. Replaced Q2 with KSC1845, STATIC GONE! I can only guess the Zetex I put in a month ago was a bad one?

Decided to replace Q1 and Q3 (R channel) with KSC1845 to keep things symmetric, and that went fine.

New issue I noticed a while back (but decided to solve the L channel static first), the speaker relay is engaging right away instead of the few second delay it is supposed to. I'm unclear why there is no delay. Looks like the solenoid in the relay gets power from pin 8 on the power supply board, but not sure what the check on that board.
 
Static is back. Mostly on right channel, but also on L channel. Only transistors I have not done on the main amp board are Qe5 and 6. Rjsalvi suggested KSA1220, which I have on hand. Hopjohn suggested ZTX795A, but I don't have those.

Also, need to look at the power supply board as my last resort. I see Hopjohn replaced Qk2 and 3 (2SC945) with KSC945CGTA, which I have in stock. What about Qk1 (2SD220)? In my AK search, I see someone replaced a 2SD220 in a Kenwood 700C power amp board with KSC2690AYS, which I do have in stock. Is that an acceptable replacement in this application? There is also a large heatsink on the 2SD220, will that be necessary with the KSC2690? Will look at recapping PS board, but I have to see if I have what's needed in stock.
 
1. Get out the freeze spray and start spraying transistors to find the noisy one. Stop guessing.
2. Look at the output with an oscilloscope at a higher sweep rate. maybe it isn't noise but oscillation.
 
1. Get out the freeze spray and start spraying transistors to find the noisy one. Stop guessing.
2. Look at the output with an oscilloscope at a higher sweep rate. maybe it isn't noise but oscillation.
1. Already tried the freeze spray, and there was nothing obvious. Will try again. I wish it was obvious. I'm tired of replacing transistors.
2. Will give this a try.
 
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