"Treble Max" Problem - Can you help with this puzzle?

[tusntuk2]

Hi y'all, hope you are staying healthy at home! My main home activity has been to recap/rebuild a Kenwood KR-5600, and things are going pretty well for a difficult layout - except the inevitable mystery problem has cropped up. Maybe someone else has heard of this? It goes weird when the Treble gets near the Max. The old "Treble Max" problem!

I recapped the Audio board and installed a repopulated Power Supply board. Old one was burned too badly. Still original main caps. I fired it up after the PS board, and heard the amp sound distorted in FM, one side, but clear in both channels in AUX. Sounded good, really, but then the inevitable. I had fired it up with all controls down and still plugged in the DBT, and was using headphones. As I turned the Treble control to the max, I suddenly heard that high voltage type bzzt-hum and the DBT glowed. Yeah, I know not to use the DBT to play, but the problem is a strange one, isn't it? I was glad to see the DBT glow, it saved the amp. Is it an oscillation?

I tried again with full power, and the zzzt remains, but only when I turn up the Treble to the max.
I have tested every voltage on the S/M and even compared it to another working 5600 I have, but everything checks until the Treble Max zzzt. All controls have been cleaned and seemed to test fine. Of course I have to back off the Treble to kill the buzz, so I can't get voltages under duress yet. I'll try.

Any clues where I can look? It's a good puzzle! I'll relate more of the work in another post.

Thanks for your thoughts! Stay safe.

 

[Thefamous]

Maybe a bad treble pot? I had a pot (on an old guitar amp) that was fine for about 90% but put out a nasty buzz when turned all the way. New pot and no problem. I think the wiper can wear through the resistance element sometimes, and this seems to show up more at full position. But that was a fairly simple circuit, so maybe something else is going on.

 

[tusntuk2]

Thanks for the quick reply! I did not take the pot off the board for testing. I have seldom had a pot wear to that extent, but anything's possible. Maybe the old girl has been through a lot of Deoxit. I have read of oscillation problems in this model before, I was guessing and hoping someone might have solved one of those and could chip in. And I still have old guitar amps!

 

[Thefamous]

I always try the easy fix first, bc it's easy and I'm lazy . Hence the suggestion of checking the pot. The next easy lazy thing I would do is check all the solder joints I did, especially on path to ground. I have had a bad joint (that I soldered) cause a buzzish type sound before because of lousy contact to ground. Showed up at higher volumes. If they all looked good I would start checking resistors next. You said you replaced all the caps?

Does it buzz with the loudness switch on? More a curiosity question then a diagnostic one. Troubleshooting oscillation is beyond my simple abilities. IIRC it's caused by feedback getting into the output circuit. Sometimes through the ground, and sometimes through a bad NFB circuit? I don't know if it could come in through the power supply. And it seems like if it was the power supply, then adjusting the treble wouldn't do much. But this is why I stick to simpler tube based circuits when I build.

I am in the process of restoring a KR-1400 for my young son. The circuit for that is much less complicated than the 5600. It was making my eyes tired looking through the 5600 schematic.

Best of luck to you.

 

[Artie]

Sometimes through the ground, and sometimes through a bad NFB circuit?

I can't help much, but it is worth noting that this is one of those amps where the tone controls are in the NFB circuit. So, maybe a bad cap, or resistor that has drifted out of value in that circuit.

 

[tusntuk2]

Thanks, the Fam. The S/M makes my eyes water too! Good luck with the 1400.

Thanks, Artie. I would hope it's not a bad cap, the electorlytics and the non-polars are new. I did not change any ceramics - could it be one of them? I will check resistors. The bias transistors are new too, although they seem to work correctly and I have used this replacement before many times.

 

[Lavane]

Still, check all the solder joints you put an iron to. Check for bridged solder joint. Also check the orientation of every electrolytic cap you replaced.
One or more could be in backwards. This is where you need a schematic to make it easier to check. I've restored a few stacks of Marantz equipment with sprinklings of other brands. So many times the silk screening on the boards was wrong. A few times I just flubbed it and put one in wrong. Now
I use a sharpie and mark the negative side of the cap on the PCB before removing the old cap.

 

[tusntuk2]

Good advice, Lavane, thank you. This is my 20th rebuild, so I am getting familiar with some of the more typical problems one encounters in these projects. And it's taught me to solder better, too. I am not an engineer, so my technique has always been to replace one piece at a time, whatever the part. Each part is photographed, marked, checked aganst the parts list, and sometimes tested too. Transistors get tested for pin orientation. I try to verify location and part number and do one piece at a time to get it right. I have rechecked the board more than once, but I know I can still make plenty of mistakes, even being careful. You can never check it too much. So far i can not find a physical problem. Maybe there is a value wrong?

I am curious about why everything sounds just fine until that last little click of the Treble control, when the hum/buzz starts and there is an obvious current draw. You would think that the problem would be there constantly, or would at least get gradually worse with the turning of the control. I have checked the control too, and it checks out fine.

There was mention of using an oscilloscope to help find oscillation. I now have a scope, although I am not trained yet how to use it. Maybe it will help.

Meanwhile, I will replace the big main caps and try installing some bypass caps, too. I will also replace electrolytics in the phono preamp and the FM muting circuit. And look for a broken tie wire too, these stacked boards take a twisting while recapping. Wish me luck!

Anybody wanna help me with the 'scope? Thanks!

 

[FluxT]

My Harman/Kardon 430 has a hum at certain positions on the volume pot-most noticeably at the lowest few degrees. Once you get it in the normal listening range (say, 8-12 o’clock), it subsides and is barely noticeable unless you’re really listening for it. I think there’s just some wear or stubborn dirt in the pot. Bugs me sometimes, drove myself nuts checking solder joints and transformers, but it seems to be going away as it gets used more. Switching to less efficient speakers effectively masks it.

 

[Thefamous]

You could try using a piece of wire as a jumper to bypass the pot, effectively making the amp see it at full position. If no hum then you know it’s something wrong with the pot. If the noise is still there it’s somewhere else in the circuit

 

[tusntuk2]

Meanwhile, back at the Lockdown... I think I solved my own problem! So I'm writing the info down, it might help someone researching a similar problem later.

I had totally recapped the PS and Audio boards on the 5600. I also changed the relay transistors, Qm 25,26, the Bias transistors Qm7,8 and the voltage regulator. I am not an engineer, but I narrowed my guesses to the (pre-drivers?) pair of transistors at Qm5,6, and the decoupling caps at Cm41,42, on the rails. I had replaced the rail caps with WIMA Film, but I reinstalled polarized 1uf/50v UPW caps thinking that the polarity may be important. I also triple checked polarity everywhere I did work, and transistor pinouts everywhere I had made changes. All voltages I could check were very close to spec everywhere, too. I rechecked the treble pot for shorts or dead spots, and it seemed fine. Its nice to see you did good work, but then again I would often like to find an obvious problem.

I noticed a large discrepancy in gain when testing Qm5 and 6. Qm5 tested at 85 hFe, and Qm6 tested at 183 hfe. The new pair matched well. With the new electrolytics installed on the rails I powered up and everything worked. I cannot say which of the fixes was right, or maybe they work together, but it works now.

I also had distortion on one side of the FM board. I changed the output transistors Qg6,7 from 2sc1222 to Ksc1845. The 1845 had a lower gain than the 1222's, which both tested fine, but the distortion had almost disappeared. It was pretty quiet, though, with a limited signal. I had planned on recapping the FM board, so I went ahead and recapped it. Yes! Distortion disappeared, gain and volume were restored, and the FM sounded great. Muting worked, stereo separation worked, threshhold worked for the stereo indicator bulb. Recapping everything, including the tantalums, had replaced whatever cap(s) had created the distortion problem.

Been listening for about 4 hours now, and it sounds great! I think the KR-5600 is one of Kenwoods better efforts. The sound is now very clean and detailed, with a smooth response curve that doesn't show obvious frequency peaks anywhere. Lots of power, enough bass but not too much, and a fine FM tuner for this grade of amp. The Advents are singing again!

Thanks for everyone's help! Stay safe.

 

[Thefamous]

Awesome you got it working. I would love to know what was going on electrically to make the treble go weird when turned all the way up like that. I’m no EE so the explanation would probably be over my head anyway.

Maybe it was pushing whichever failing component just past the point it could take it? Thanks for the follow up.

 

[tusntuk2]

I'd like to know too! There is some discussion of oscillation in amplifiers, and Kenwood is known to have oscillation problems, both in design and in how they manage to avoid it. I don't understand these discussions, but I do understand feedback, being a performing musician for 40 years. This problem presented like feedback, that sudden burst of overload that causes the amp to run away uncontrolled. It could have been thermal failure or cap failure, like the FM board.

Qm5,6 are in the signal path and I noted the gain difference, so I changed them out to KSC2690. I'm thinking that the gain imbalance was the problem, but I don't know for sure. I think I got lucky with the bad cap on the FM board. Some call it a "shotgun" approach, but I had previously decided to recap the FM. The clarity and dynamics of the signal were greatly improved, and recapping did not affect the tuning at all.

I am a firm believer in recapping these old amps. This was #21 for me, and maybe one of my best success stories. New parts were the key to making this amp sound great again. They always do!

 

[jnahanna]

I also had the same issue with the kr5600 when the treble was increased the amp would do a low freq oscillation about 2 Hz with a carrier of 100 KHz. The replacement of the coupling caps 1 uf did the trick and yes they had to be electrolitics for some reason. I tried film as well and it was still oscillating. Also anyone know of a good replacement for the volume pot. It's a 2 gang 8 pin 100k with the 50% loudness tap. I can't seem to find one at digikey or mouser.
Thanks, Jim

 

[Warren-Oates]

I think the film caps have no ESR, which can be great in most instances. I have heard discussion though that some circuits depend on that tiny bit of ESR to prevent "ringing".