Kenwood KR-6340 Right Channel Issue

[braunerf18]

Hi all,

I've acquired a 6340 quad to build my first vintage audio system. I've found invaluable information on AK's forums already, however I'm having an issue with the right channel on this receiver and I'm hoping someone is able to point me in the right direction and offer guidance.

I've paired this receiver with 2 Paradigm 9SE 4-ohm speakers. I'll try to keep my troubleshooting steps as clear and concise as possible.

When the amp is in 2-channel mode I'm getting an intermittent popping/crackling sound from the right channel. Left channel measures about 0.1-0.4mV and the right channel idles around 15mV. When I experience the crackling the right channel will jump to 40, 60, 100+mV, then go back to 15mV. There's no rhyme or reason as to when this happens. Usually the second I turn it on it will start crackling regardless of input and volume level, however occasionally it won't do it at all.

Troubleshooting steps:
1. Main caps off the power supply have been replaced.
2. All high voltage caps on both amp boards have been replaced.
3. Problem seems to follow the amp board. I swapped amp boards and the problem then existed on the left channel. Amp boards are currently in their original slots (swapped back).
4. When the receiver is set in 4-channel (quad) mode, the front left and right channel operate perfect with no issues. When I run my meter on the speaker terminals the rear right's channel voltage bounces around 20-80mV. The 3 remaining channels look good.
5. If I run the amp in 4-channel mode for a few hours then power off and switch to 2-channel mode, 9/10 times I won't experience the problem for at least 30min-1hr.
6. One day I turned it on in the morning in 2-channel mode and it was running for 4+ hours with no problem between FM and Phono. When a vinyl finished I spent about 2 minutes looking for another and the poppping/crackling started happening with no load on the receiver...
7. Cleaning the 2 to 4 channel switch at the back of the receiver and the volume pot with Deoxit seems to help alleviate the issue the most.
8. If i start to hear the crackling, then go to an input with no source and turn the volume up, I hear an audible hiss coming from the right channel speaker along with the crackle/pop.
8. Tried different speakers...

Offhand my gut is telling me there's something up with the rear right section of the amp and when it's bridged in 2-channel mode the issue presents itself. Where I struggle is with items 5-7 but perhaps that's just luck, I'm not sure.

Any ideas? I appreciate any help you can guys can offer!

 

[braunerf18]

I've had it running in 2-channel mode for nearly a week with no issues between any of the inputs until today. I had a vinyl playing softly and all of a sudden the issue showed up out of no where. I cleaned the volume pot and input selector switch with Deoxit again. At first it didn't make a difference but now it seems the longer I wait, the less popping I hear and it starts to stabilize.

DC voltage is showing the same as mentioned earlier. I put it in 4-channel mode and checked DC on each channel. FL, FR, and RL were around 7mV. RR was showing 15 and going up to 25mV.

 

[EchoWars]

As you've already figured out, each amplifier card has two channels of amplification. You already know which card it is, now you need to narrow it down to which channel on the card, if possible. If you can connect four speakers and run it in four channel mode, that would help. At any rate, what you describe is a classic symptom of a failing semiconductor.

I looked at the schematic and from the parts list indicated there, I see no known bad actors in the way of transistors. But any type of semiconductor can go bad, and Kenwood could have made production changes, so a list of the transistors used on the board would be good.

Freeze-spray may help by carefully hitting individual transistors and diodes.Care must be taken, as you do NOT want to spray the bias diodes (De1 & De2) as idle current will skyrocket, and while you can spray the differential pair transistors, know that doing so may cause offset to go high enough to trip the protection for a few seconds. Just take care to protect those rare TO-66 output devices.

 

[braunerf18]

Thanks EchoWars. I took a few detailed pictures of one of the amp boards to help show what I'm looking at. De1&2 appear to have wires coming off the board and are screwed directly to the heatsinks. I've labelled them with arrows on the picture. What could this mean?

There's also two adjusters shown on the top of the amp that are each labelled 100ohm. On the board they show as Vr1 and Vr2 and I'm not quite sure what these are used for either.

I'm gone for the rest of the week but when I'm back I'll connect all four channels and see which speaker exhibits the issue. My guess is it will be the rear right. I'll try freeze spray as well. Could I run the unit with only one amp board connected? This will give me more room to accurately use freeze spray.

Thanks again.

 

[Ohighway]

A non conductive implement can sometimes be used to diagnose intermittent problems. I had the big brother to your receiver, a 9340, that would occasionally kick on the speaker protection relays. I had no schematic or any equipment to use to diagnose the problem. I did have a #2 pencil. I opened up the receiver and touched the eraser end lightly against components on the boards one by one. I finally got to a transistor that was the culprit. Every time I'd touch that transistor the problem would occur. I desoldered the component, purchased another, installed and all was well from that point on.

 

[EchoWars]

What could this mean?

Those diodes are designed to lower the idle (bias) current as the power transistors heat up. Transistor gain rises as temperature increases, so the diodes are a means to provide a bit of thermal 'negative feedback' to prevent thermal runaway. Know that the leads on these devices can be fragile, and breaking a lead on one of these means the instant destruction of the output devices if it happens when power is on. If you break a lead on one of these diodes while the board is out, it must be repaired before attempting to power on again.

I'm not quite sure what these are used for either.

These are for adjusting the idle current. Leave them be (for now).

Could I run the unit with only one amp board connected?

As long as the protection circuit allows it (lets the speaker relay close even with a missing board), then sure.

A non conductive implement can sometimes be used to diagnose intermittent problems. I had the big brother to your receiver, a 9340, that would occasionally kick on the speaker protection relays. I had no schematic or any equipment to use to diagnose the problem. I did have a #2 pencil. I opened up the receiver and touched the eraser end lightly against components on the boards one by one. I finally got to a transistor that was the culprit. Every time I'd touch that transistor the problem would occur. I desoldered the component, purchased another, installed and all was well from that point on.

By all means, can try this as well.

 

[braunerf18]

Thanks EW for helping me understand what those diodes and adjusters are for. Ohighway, I did try this very carefully and I couldn't find the offender.

In 4 channel mode with 4 speakers connected, it is the rear right channel that has the problem. If I swamp amp boards, it then becomes the rear left. Before I get to far ahead of myself, I've ordered the rest of the low voltage caps that weren't replaced on the first round. I should be able to get those swapped this week and I'll report back.

Thanks again!

 

[braunerf18]

As you've already figured out, each amplifier card has two channels of amplification. You already know which card it is, now you need to narrow it down to which channel on the card, if possible. If you can connect four speakers and run it in four channel mode, that would help. At any rate, what you describe is a classic symptom of a failing semiconductor.

I looked at the schematic and from the parts list indicated there, I see no known bad actors in the way of transistors. But any type of semiconductor can go bad, and Kenwood could have made production changes, so a list of the transistors used on the board would be good.

Freeze-spray may help by carefully hitting individual transistors and diodes.Care must be taken, as you do NOT want to spray the bias diodes (De1 & De2) as idle current will skyrocket, and while you can spray the differential pair transistors, know that doing so may cause offset to go high enough to trip the protection for a few seconds. Just take care to protect those rare TO-66 output devices.

Success, I found the bad transistor. I swapped all the remaining caps and the issue was still there (as expected). Following the schematic, I knew it was the left side of the amp board I needed to troubleshoot. I picked up some freeze spray and carefully hit each transistor. The second I hit "Qf7" it started crackling/popping like crazy and protection mode kicked in as you expected it to. Letting the transistor come back to room temperature, I was able to repeat this another two times.

I then did some studying on your "Testing transistor guide" and this helped me confirm 100% it's bad. The trace shows B, [blank], C so I assumed this is Base, Emitter, Collector. I removed the transistor from the board and when I put my meter (diode mode) positive on the collector and the negative on the emitter, I'm getting upwards of .8-9V when this should be 0, correct? I tested the other transistors on the board and collector to emitter was 0V. Also, the label on the transistor shows a B with a circle around it, blank, then what looks like a crosshair. Oddly enough the way these transistors are soldered to the board, the B is actually the collector, not what I would think to be - the base. Does this make sense or is the label misleading?

Lastly, the schematic shows this transistor (and Qf6) to be "2SC1213A B or C". The actual label on the transistor says "C1213" on the top row, and "3--A" towards the bottom on the same side. What is a suitable replacement for this transistor? Should I also replace Qf6 or just leave it?

Thanks again!

 

[EchoWars]

Does this make sense or is the label misleading?

That is a 'lot' number and has nothing to do with lead ID.

What is a suitable replacement for this transistor?

The KSC945CY from Mouser (note the 'C' suffix, meaning a center collector part) would do well. The data sheet for the original Hitachi part and the C945 Fairchild part are both online.

Should I also replace Qf6 or just leave it?

L to R channel symmetry is good. I'd replace them both.

 

[braunerf18]

The KSC945CY from Mouser (note the 'C' suffix, meaning a center collector part) would do well. The data sheet for the original Hitachi part and the C945 Fairchild part are both online.

Perfect, thanks. The trace was labelled BEC and looking at the data sheet for the one at Mouser, it looks like the pin layout is EBC under the picture. Although under Features it says "Suffix “-C” means Center Collector (1. Emitter 2. Collector 3. Base)".

Just looking for clarification here as I'm somewhat confused by their description.

 

[EchoWars]

Simply means that parts with the 'C' suffix are ECB layout instead of the EBC layout of the 'non-C' suffix part.

 

[braunerf18]

Simply means that parts with the 'C' suffix are ECB layout instead of the EBC layout of the 'non-C' suffix part.

Thanks. So if I need one with an emitter in the middle, how do I choose that?

 

[EchoWars]

You don't. A KSC945 is a center-base part. The KSC945C is a center-collector part.

In the history of through-hole three-lead transistors, I don't think anyone has ever manufactured a center-emitter part. Center-base or center-collector pretty much cover every usage need.

 

[braunerf18]

Thanks again for clarification. I don't quite fully understand the transistor specifics, but I will go ahead and order 4x (2 for each board) KSC945CY from Mouser. That being said, it's probably the labelling of the board that I'm not seeing correctly. It appears it's B, blank, C but it would make sense it's BCE then.

 

[EchoWars]

Looking at the flat side of the original 2SC1213, the lead arrangement from L to R should be ECB. This should be the same as the KSC945C.

 

[braunerf18]

Ok I have all 4 transistors replaced and so far so good for nearly a week. I put my meter on the transistors and the Fairchild units were a reverse layout to the original. Thank you for your help!

Diving deeper here, I'm thinking I should recap the power supply. The 2 main 35V 8200uF caps have been replaced with Mundorf capacitors, however there are large caps on the power supply that have one lead coming out on each side. What is this referred to as? The amp boards were recapped with Elna audio grade caps. Should I stick to these or what would you recommend? Also, there are large (looks like ceramic) resistors on the PS board that look a little worn. What would be best to replace these with?

I figure I may as well recap the preamp while I'm at it as well.

Would you also recommend replacing any of the ceramic caps?

I appreciate any advice you can offer.

 

[EchoWars]

however there are large caps on the power supply that have one lead coming out on each side. What is this referred to as?

Axial caps. Radial = leads exiting the cap at the same end. Axial = leads exiting the cap at opposite ends. I do not have a foil layout for this receiver, so if you tell me the ones you need, I can make suggestions.

The amp boards were recapped with Elna audio grade caps. Should I stick to these or what would you recommend?

I like to use parts that are designed to do what I want to do in the circuit. If we're talking about a power supply here, then 'audio' caps are, IMHO, inappropriate. Panasonic FM and FC, and Nichicon PW and HE (for radial parts).

Also, there are large (looks like ceramic) resistors on the PS board that look a little worn. What would be best to replace these with?

If the resistors are mounted flush against the board, I like to replace them with something else and mount them 1/4" to 1/2" above the board to aid cooling and prevent the board from getting scorched.

 

[braunerf18]

Thanks for the explanation. Looking closer at the PS board, I see everything is wire-wrapped to leads so this is a project I'll have to put to the side for now as it's a little more work than I intended it to be.

 

[ringading]

Don't mean to hijack a thread but just looking for a simple answer and since this is a recent thread. . . . . I started working on a 6340 with noise in one channel and cannot figure out the layout of the two boards. Not sure if one is front left and right and the other is rear left and right. Once I get this figured out I will start looking for bad transistors.

Will start a new thread if can't diagnose once I figure out the board layout.

 

[EchoWars]

Well, if braunerf18 takes another look at the thread, he may be able to help.

@braunerf18