Kenwood KR-6340 Right Channel Issue

[braunerf18]

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.

This took me a bit to figure out, but if you are looking at the receiver from the back end (looking at the RCA jacks and speaker terminals), the amp board closest to you (or the rear), is the right channel board. The board in front of it is obviously then the left channel board. If you're looking at the right channel board, the left side of the amp is for the rear right channel, and the right side of the board is for the front right channel. The same logic applies for the left channel amp board. Left side of the amp is rear left, right side of the amp is front left.

You can also run the amp with one board installed at a time to help testing. The issue I had was on the right channel amp board, so I found taking out the left channel board and moving the right channel board forward a slot helped provide more space to diagnose the issue. As EW recommended to me, running the amp in 4-channel mode and putting test speakers on all 4 terminals definitely helped me isolate the issue.

If you haven't already, I'd also recommend recapping the amps. It significantly helped tighten up the overall sound of this amp. Next is the power supply for me, but this thing can really put out in 2-channel mode.

 

[ringading]

Thanks EchoWars. I ordered a service manual today which should explain it if braunerf18 doesn't chime in.

 

[ringading]

Thanks braunerf18! Time to dive in and start checking for the gremlin. At least the boards are removeable - right?

 

[braunerf18]

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).

Hi EchoWars, I took a closer look at the power supply and it looks like I have enough room to work around it without removing the leads. I've attached a picture of the board. If you don't mind guiding me in ordering the correct parts from Mouser, that'd be much appreciated!

1. You can see the resistors that produce a fair amount of heat. The white one is labelled "RSU5, 150Kohm, FU-353 and the green one is labelled "4(A-type symbol)32-5W10%, 270ohm. What would be suitable replacements for these? You will also see there is a red resistor in the middle of the board. It's labelled "RS2B, 4.7Kohm". Should I replace this as well?

2. The only axial cap is the 63V 470uF one. There is a 50V cap coming off the power supply leads that has two wires attached (pic attached), I'm not sure if I need anything special for this one or if I just snip the wires and solder them to the leads of the new cap.

3. Based on your recommendations, would you replace these caps with Panasonic FMs or FCs?

4. I've also attached a pic of the pre-amp. Would you recommend the Panasonic caps here as well or should I stick with the Elna's I recapped the amps with?

If there's anything I've missed that you recommend replacing, please let me know.

Thanks again!

 

[EchoWars]

Late to the party, but I've had a pretty horrific two weeks, and a rough 3 or 4 months. But I'll see if I can help...

1. Both resistors are 5W. The Vishay parts would work well. #594-AC05W150R0J and #594-AC05W270R0J would do. Mount the new ones 1/2" or so away from the board. (Mouser #'s) The rust colored resistor is also a power resistor, but from here I don't see any scorched board. But it would be nice to have it away from the board. I can't for sure, but I think it's a 4.7 ohm 2W Rk1. If so, you could use #594-AC03W4R700J.

2. #594-2222-118-18471 for the 470uf 63V axial. Yes, it's expensive, but it'll last damn near forever. Remember, with axial caps, one side will have the metal cap with the lead welded on...that's the negative side. As far as the 470uf 50V cap, the method of attachment is yours to choose. Personally, I desolder all the original leads and try to make the new cap look like it was born there.

3. The first 470uf axial, I've answered. The other one ought to be a Nichicon PW or HE, or the Panasonic's that you mentioned.

4. If you already have the Elna's, use 'em.

 

[braunerf18]

Thank you for taking the time to reply. My sincere condolences to you and your family during this difficult time.

...

1. I noticed the Vishay parts are 18mm instead of the 40mm that are currently on the board. I obviously don't question the spec, but is this ok regardless? As for Rk1, the Vishay part is 3W and the schematic says 2W. Is this ok?

2. I'm looking to replace Rk10 and Rk11 as well. They measure 65ohms in circuit, but should be 100ohm according to the schematic. They appear to be carbon comp. resistors. Is this metal film part from Vishay ok? - #594-SFR16S0001000FR5

3. I'm not sure if this amp has a bias adjustment on it. Nothing obvious from what I can see. If it does, would you recommend setting it after replacing these components? I know previously we discussed Vr1 & Vr2 on the amp boards. You mentioned they are to adjust idle current. What values should these be set at, or is it best to leave them alone?

Thanks again.

 

[EchoWars]

1. Modern parts are commonly much smaller than those in the 70's. As long as the leads can get to where they need to be, your golden. That's also the reason I went with a 3W part instead of a 2W part.

2. Does the color code on the parts in place correspond to 65 ohms (or so), or 100 ohms? It's far from uncommon to see small changes in parts values from a schematic to a factory unit. And given that modern parts are generally so much smaller than the originals, I'd probably put a 1W part there.

3. Schematic says there's a 500 ohm bias trimpot, but your pic on the first page of this thread shows a 100 ohm trimpot. What I'd do is to carefully measure the trimmed value of the trimpot (so you can re-adjust to the same value after we're done fooling with it), shoot the trimpot with a squirt of DeOxit D5, work the trimpot through its range a couple of times, and carefully adjust to the original value you measured. Then you are power everything up (using a DBT), and adjusting the bias while still on the DBT to confirm that it's functional, and then adjust it to a minimum bias reading before removing the DBT and powering straight from the AC line. IF, after powering straight from the AC line you are able to adjust the bias current (by measuring the across one of the emitter resistors) and the trimpot is somewhere close to the middle of it's travel, then you have confirmed that the 100 ohm trimpot is appropriate for the job. If you are unable to get about 25mV across one of the emitter resistors, then the value of that trimpot may have to be adjusted.

 

[braunerf18]

2. Does the color code on the parts in place correspond to 65 ohms (or so), or 100 ohms? It's far from uncommon to see small changes in parts values from a schematic to a factory unit. And given that modern parts are generally so much smaller than the originals, I'd probably put a 1W part there.

The colours are slightly hard to make out, but from what I can see it's Brown, Black, Brown, Silver which seems to be 100ohms at 10%. The brown looks more tan, however... What's your gut feeling on this? Perhaps I should take it out of circuit and check.

3. Schematic says there's a 500 ohm bias trimpot, but your pic on the first page of this thread shows a 100 ohm trimpot. What I'd do is to carefully measure the trimmed value of the trimpot (so you can re-adjust to the same value after we're done fooling with it), shoot the trimpot with a squirt of DeOxit D5, work the trimpot through its range a couple of times, and carefully adjust to the original value you measured. Then you are power everything up (using a DBT), and adjusting the bias while still on the DBT to confirm that it's functional, and then adjust it to a minimum bias reading before removing the DBT and powering straight from the AC line. IF, after powering straight from the AC line you are able to adjust the bias current (by measuring the across one of the emitter resistors) and the trimpot is somewhere close to the middle of it's travel, then you have confirmed that the 100 ohm trimpot is appropriate for the job. If you are unable to get about 25mV across one of the emitter resistors, then the value of that trimpot may have to be adjusted.

This will be a first for me, so I appreciate the guidance in advance... You are correct, both pots show 100ohms and are currently measuring about 75ohms of resistance (powered off). I will build a DBT. Your instructions are clear to me, however I just have a few questions around that before I dive in

1. Volume at 0 with speakers off, correct?
2. Minimum bias implies turning them up to 100ohms?
3. I'm assuming the more resistance I apply, the more I trim the bias and the brighter the 100w bulb will glow?
4. What is the safest way to do this given I have two amp boards with a total of 4 trim pots? Ie. Should I measure one board at a time with the other removed from the unit? (relay allows for this). Do either side back and forth or?
5. Which emitters should I probe on the board while I'm checking for the mV output?

 

[EchoWars]

1. Yessir.
2. With this design, minimum bias will be with the trimpot at minimum resistance.
3. See #2.
4. By all means, tackle one board at a time. And on each board, tackle one channel at a time. One channel should be set for minimum bias while you're fooling with the other.
5. The emitters of the output transistors. Because this is a quasi-complementary output stage, you can only measure the voltage across one of the two output transistor emitter resistors. Doesn't matter which one. I dunno what the spec is supposed to be, but with a 0.47 ohm emitter resistor, adjustment to 20 to 25mV voltage drop on each resistor ought to suffice.

You really need minigrabber leads for your meter. If you do not have them or are not sure what I'm referring to, pls ask.

 

[braunerf18]

Ok I removed Rk10 and it reads 115ohms right now. Is this 1W Vishay part from Mouser ok? #71-CPF1100R00FKE14

Trying to wrap my head around this one as I keep reading bias adjustments posts and confusing myself further...
2. So just to confirm, after I confirm existing output, I want to set the pots to 0 ohms and go from there? Will this not overload the output transistors?
5. I saw a post with Kenwood here stating bias should be set to 20mA for this receiver. So for example, if I'm working on the right channel amp on the front right output, my positive lead should be placed on the emitter of either Qf12 or Qf14, and the negative lead on the opposite side of Rf36 or Rf38 to properly read the mV value, correct?

You really need minigrabber leads for your meter. If you do not have them or are not sure what I'm referring to, pls ask.

I do not have these so I may as well get it in with my Mouser order. I read your post regarding what equipment you recommend for audio repair. Is this part and it's red counter-part still your recommendation? 565-3782-48-0

 

[EchoWars]

Mostly, a resistor is a resistor. Your PN is fine.

On this amp, the bias is set by the voltage between the base of Qe8 and Qe10 (we'll concentrate on one channel for clarity). Supply current flows through Re22 and Re28 and down through the multijunction diode (De1). The current then splits through the trimpot and Re26. The current then flows through Qe6. That current will flow no matter what, and any adjustment of the bias trimpot makes almost no difference in that flow. What it does do is to change the voltage you will measure across the trimpot/Re26 combination. Set the trimpot to minimum, and you have a lower voltage across it. Set to maximum, and you have a higher voltage across it. The higher voltage will cause the Qe8 and Qe10 driver transistors to conduct more, which in turn cause the output devices to conduct more.

Adjust bias to read 20mV across either Re36 or Re38. Your choice of meter probes are appropriate,

 

[braunerf18]

Thank you for teaching me! I will get the order in and report back in a week or so.

 

[braunerf18]

Ok I've finished recapping the power supply with Nichicon PWs and the pre-amp with Elna's. It's amazing how much smaller the modern components are. So far so good!

Following your steps, I used a DBT and I was able to confirm the 100ohm trimpot is the correct value for setting the bias. I had no issue making the same adjustment on direct AC power. That being said, I'm looking for a little guidance on getting it just right. I let the amps run for about 10min (no load) and then start adjusting values to 20mV. After another 10 min or so, it could be at 25-30mV. As you mentioned previously, I understand the boards have a bias circuit that checks the heatsink temperature and reduces the bias current as they get hotter. After I've had load on the amps for about half hour or so, bias is sitting around 17-20mV. I'm assuming this is the diode adjusting to prevent thermal runaway. Would this be the best time to adjust bias, ie. once the amp is good and warm after I've had load on it? My worry is if I adjust it to 20mV at this stage that current will rise to high while the outputs are warming up.

What do you think is best?

PS. These Minigrabbers are great!