Need help with a KR-6050

I recently acquired a KR-6050 receiver. I've always liked the looks of this series. The one I found was extremely dirty, lights out, peeling laminate, dirty pots, and chewed-up power cord-- but it was functional and cheap. Popped the hood and looked around inside and powered it up with a light bulb in series long enough to verify that it was a good candidate for restoration. The power switch was sizzling, and there were a couple of caps that didn't look too happy, so I ordered up some replacement electrolytics and one of the "Alternistor" triacs, and some other misc. parts.

Long story shorter, I recapped most everything except some of the tuner board, adjusted bias and offset, got everything back together and amp works great EXCEPT that -on the left channel only- when I turn either of the tone knobs, I get a little popping noise as the offset jumps around. If I switch on the subsonic filter, I get about .5V offset (I think it's negative relative to speaker polarity, if I've got the leads on + to + and - to -). When the tone circuit is set to flat, everything is fine, and the right channel is fine anyway. Engaging anything in the tone circuit does cause small changes in DC on the right channel, but small being +/- 10mv, I consider to be normal.

I'll try to attach a readable picture of the relevant portion of the schematic.
I changed the 2.2mF coupling caps C7/C8 to Wima film, and I also used film caps for C9/10, C11/C12, and C15/C16. After discovering this problem, I swapped channels with C19/C20 which made no difference. I swapped Q1/Q3 with Q2/Q4 and problem stayed with the left channel. I also changed out Q5 and Q7 with no effect. I've checked or swapped channels with most of the parts in the tone circuit and can't find anything that makes any difference. Does anyone have any ideas on what might be causing this? I'm running out of ideas and would really like to get this thing buttoned back up because I've got it looking really nice! I'll try to attach picture later.

You've done pretty much all the things I would normally try. It's just a shot, but maybe leakage from C3 might introduce a surge when switching the low filter. Other though was maybe spillage of something on the PCB, or something with the input pair FET's of the amplifier section, but would expect other problems if that were so. Good luck.

did it do this before you started ? if so check resistors and solder joints ..if not still check these plus check for backwards caps .

That's really the question: Did it do this before I started? I don't know. I don't think I tried the subsonic filter. The pots were noisy... I thought about swapping those small value electryolytic caps that I replaced with film caps back in, in case that made some sort of difference, but if that was it, it should be both channels. One other thing:
C7, C8 are shown on the schematic and parts list to be bipolar capacitors. They weren't though. They were regular old 2.2uF/50V polarized capacitors and were almost certainly original, as there's no evidence that anyone had done any work on this receiver before. I swapped them to a Wima 2.2/50 stacked polyproplyene film cap, figuring that couldn't do anything but help. Again, though, both channels should be affected.

I've checked and double-checked for backwards caps and soldering errors, broken traces, etc.. I'm stumped. I've put a lot of work into it now. Gotta figure this out! As far as I can tell, the only thing the subsonic filter does is direct the signal through C19/C20. I did NOT change C19/20, but I did swap them L-R for testing.

Here's a picture of it now. I put mahogany veneer on it.

Can't help with the electronic problem, but wanted to say you did a fantastic job on the cabinet! Wow! That's a sweet looking receiver.

Thanks! I'm kinda proud of it myself! As I said in the first post, I've always thought that particular series of Kenwoods was one of their nicer-looking designs. Plus, I'm a sucker for the meters! I'm still trying to decide whether to light it up with the original-type bulbs or LED's.

Are we certain C19 polarity is correct?

If you suspect the FETs at the front of the tone amp, I replaced them in one of my 6050s with BF256Bs with success.

BTW - Your mahogany looks much better than the original vinyl.

Thanks for the replies! I'll try to swap some more stuff around later today. I did suspect the FET pair, but I moved the left channel pair to the right side and nothing changed. Good to know about your success replacing them with the BF256B- I was afraid that a suitable replacement for those might be hard to come by. And I'm thinking I was wrong in my first post about the polarity of the offset: I believe that it kicks the woofer cone outward, which I think would indicate a positive offset.. I'll check it later and verify in case it makes a difference in troubleshooting it. I actually sounds to me like a bad connection or broken trace but I sure can't find anything...

Thanks again!
Jason

That KR-6050 looks beautiful! :tresbon:

You did an amazing restore job on it. :yes:

Too bad you didn't take a 'before' picture of it. :scratch2:

Thought I'd bring this back up in hopes that someone else might have an idea. I'm completely stumped. I've checked voltages in the tone control amp, and they all agree pretty closely with the values on the schematic, and they're the same left and right. I've removed and tested (or swapped parts from right-to-left and left-to-right) nearly everything in the tone control area. I've noticed that even with the tone controls flat and no filters on, the DC offset on the left channel drifts around some, even with the unit fully warmed up. Don't know if that's indicative of another problem, but the right channel doesn't do that. It will still stay within 24mv or so, but it won't stay at zero. Applying any kind of boost or cut with the tone controls, however, makes the offset on the Left channel climb considerably (80mv or so). Barely a change at all on the right channel. The subsonic filter, though, is what really does it: When I tested it this evening the unit had been on for a couple of hours. Offset was reading 23mV left and 3mV Right. Switching the subsonic filter on made the left channel go up to .873V (steady) and the right channel went to 13mV. That offset is positive with respect to the speaker terminals.

I'm confident when I say that there are no caps backwards or wrong value, etc. I've double-double checked all my solder joints. Don't know what the next troubleshooting step should be, so I'd appreciate any help!

Oh, and I did find a "before" picture... sort of. I had already pulled off the knobs and done some cleaning.

possibly somewhere in the feedback circuit is playing up .
does the bias also go up ?
you could try swapping the left and right inputs to main amp to see if it is indeed the pre or main at fault .

What kind of voltages are you getting around Q5 and Q7? If the emitter of Q7 were negative, would cause leakage through C3 (100uF elctrolytic). Just thinking out loud.....

Input: Aux
Volume: Min
All filters and tone controls off, or flat.

All voltages measured to ground on the case of the unit.

Q7 E: -82.5mV, C: 23.55V, B: .505V

Q5 E: 16.05V, C: .505V, B: 15.51V

for reference , here's the right channel:

Q6 E: 16.18V, C: .502V, B: 15.65V

Q8 E: -76.6mV, C: 23.56V B: .505V

Switching the subsonic filter "on" doesn't seem to affect the above voltages. But switching the subsonic filter "on" DOES cause a positive 38mv to appear at the negative terminal of C19, while nothing changes on C20.

I'm trying to trace that out right now, to find out where that's coming from.

Thanks again for the help!

jasoncauthron said:

Switching the subsonic filter "on" doesn't seem to affect the above voltages. But switching the subsonic filter "on" DOES cause a positive 38mv to appear at the negative terminal of C19, while nothing changes on C20.

Click to expand...

I think that's telling me that the problem is not coming from the tone control amp... Am I thinking about that correctly?

Okay, now maybe I'm getting somewhere. I removed R39 that couples the preamp to the input of the amp. I've got 38mV on the side towards the amp, AND with that resistor removed I'm seeing about .8V at the left speaker output.

The good news is that I think I found my problem. The bad news is that it's the 2SK150 dual JFET. Though they behave similarly when I check them with the multimeter, I swapped the two of them around and the problem follows the one that came from the left channel initially.

Any ideas on what can be used to replace that?

Thanks,
Jason

Fixed!

Thanks for the help in diagnosing my problem! I started to just try to match up a pair of 2SK117, but after much consideration, I broke down and bought the 2SK150A from Littlediode. A bit pricey, with the exchange rates and such, but I think it was worth it. The receiver functions just fine now, bias is right on spec, DC offset stays below 10mV... Sounds great!

This was in interesting problem, to me at least. I'm no electronics tech, but rather, just a self-taught hobbyist, and this experience helped me learn a few things. Thanks!

Jason