KR-4200 Static on L channel

[KeithD]

Had background static on a KR-4200 I picked up. Followed HopJohn's advice and replaced Q1-4 (2SC1345) on the Main Amp board with Zetex ZTX694B. That cleared much of the background static issue, but static remains on the Left channel. Inspecting the board, I noticed chemical leakage from C24 (220uF/35V). Replaced that with a Nichicon 220uF/63V cap I had on hand. This did not effect the background static.

Measured DC offset on the L and R speaker outputs:

Channel: High, Ave, Low
L: 208.8, 45.5, -84.3mV
R: 1.3, 0.7, 0mV

Suggestions on what to look at to narrow down the problem? Not seeing any other transistors on that board that appear on the known bad transistor list.

 

[KeithD]

Check the following transistors: Qe7-10. 7 & 8 are 2SA743A and 9 & 10 are 2SC1212. Appropriate replacements are KSA1220 for 7 & 8, and KSC2690 for 9 & 10. Also take a peek at Qe15 (2SA1213A -- replacement KSC2383) & 16 (2SA673 -- replacement DC327 >> different pinouts). The schematic I'm looking at doesn't have voltages listed at the trans pins, so you might have to settle for a diode test. I'd listed parts to be checked for both channels since normally, whatever I do to one channel, I also do to the other. I'm also suggesting checking both channels because all parts designations in the schematic are using odd numbers for the L CH and even numbers for the R CH ... until the 'Main Amp' stage! If the schematic I'm looking at is in error, it wouldn't be the first time.

Thanks for the input. I guess what I am looking for (and don't know myself) is what to check (voltages) to narrow down where the problem is. I too always do symmetric replacements (both channels); however, I don't have any of the replacement transistors you mentioned, so replacing one by one to see which one solves the problem is not an option without placing an order for all of the replacement transistors on the board. At that point I might as well rebuild the board. Do you think that is what it will take? If anyone had normal voltage readings for key points on the board, that would be helpful. Would it work to measure voltages at the same point on each channel to see if there are discrepancies?

 

[KeithD]

Understood. I understand the benefit of replacing all e-caps, and its very easy to do. However, transistors are not as straight forward. Often there are not exact equivalents, in which case it may be better replacing only what's necessary. If I can't narrow down the problem with moderate effort, I'll have to decide whether it is worth rebuilding the board and keeping it or selling it as-is.

 

[hopjohn]

because all parts designations in the schematic are using odd numbers for the L CH and even numbers for the R CH ... until the 'Main Amp' stage! If the schematic I'm looking at is in error, it wouldn't be the first time.

Nope not an error, that's the way it is. Smart, eh? I was thinking the same thing when he was talking even numbers being replaced, now I know why.

Keith, if it were me, I'd be trying to narrow down the source of static with the controls first to determine if it's in the preamp or the amp. Work smarter not harder.

You'll wanna hope the trouble is in the amp. If the problem is before the amp input the troubleshoot gets more difficult. The first thing I'm doing is hitting the mono button to see if the static moves to both channels and hoping like hell it doesn't.

If it does what would you think that would mean?

If it doesn't what would think that would mean?

This is how you need to be thinking.

 

[KeithD]

Thanks John. I agree with your approach. Pushed in the mono button and static is still on L channel only.

The schematic is indeed confusing. Best I can tell, looking at the Main Amp board, pin 8 (which is labeled OUPUT B Ch) is the output for the L channel and pin 1 (which is labled OUTPUT A Ch) is the R channel output. I can work my way back from there measuring voltages. Here are the pin voltages:

Pin: Max, Ave, Min
8: 96.2, 29, -23 mV
1: -0.5, -1.5, -2.3 mV

 

[KeithD]

Pin 6 (L) and 3 (R) appear to be the amp inputs. Both are at 0V, so in my mind the problem is on this board in the L channel path.

 

[hopjohn]

Pushed in the mono button and static is still on L channel only.

You'll be working on the even numbered ones in the Left channel (Nomally they are odd numbered in the L channel, but this amp is an exception to the norm.) You've already replaced Q1-4 ( you're sure you did the pinouts correctly?) , if so leave those alone. In my experience, the most likely to fail among the transistors you're looking at are 2SC1212A and 2SA1213A. Order all of rjsalvi's replacements and go one at a time and test. There's no reason not to when you have full access to the amp board with the bottom plate off. Where rjsalvi said DC327, should be BC327 (typo).

Edit: When ordering the BC327 get the 40 suffix parts BC32740BU which have the highest assured gain.

 

[KeithD]

OK, thanks for the advice. I have double and triple checked Q1-4 and those are correct.

I will be placing a Mouser order soon for a Pioneer SX-828 restoration and will get the needed transistors for the KR-4200. Will post again when I have the parts.

 

[KeithD]

Check the following transistors: Qe7-10. 7 & 8 are 2SA743A and 9 & 10 are 2SC1212. Appropriate replacements are KSA1220 for 7 & 8, and KSC2690 for 9 & 10. Also take a peek at Qe15 (2SA1213A -- replacement KSC2383) & 16 (2SA673 -- replacement DC327 >> different pinouts). The schematic I'm looking at doesn't have voltages listed at the trans pins, so you might have to settle for a diode test. I'd listed parts to be checked for both channels since normally, whatever I do to one channel, I also do to the other. I'm also suggesting checking both channels because all parts designations in the schematic are using odd numbers for the L CH and even numbers for the R CH ... until the 'Main Amp' stage! If the schematic I'm looking at is in error, it wouldn't be the first time.

Was able to look at the main amp board today. I checked my stock and did have one KSC2690 (2SC1212 replacement) and one KSC2383 (2SA1213 replacement). Replaced Q10 (2SC1212), but this did not clear up the static. You listed Q16 as being 2SA673, but this is incorrect, it is actually 2SA1213 as was on my board and is shown in the schematic. So, I replaced that with KSC2383. Again, this did not clear up the L channel static. So, at this point I will need to order replacement parts. Q18 is 2SA673, but Q19 and 20 are also 2SC1213, which as John said, are among the transistors most likely to go bad.

When I get to this, I will likely order enough parts to also do the right channel, so that the replacements are symmetric.

 

[KeithD]

Edit: is it also staticy with headphones?

Yes, that is how I was testing it yesterday, with headphones. I've also heard static through the speakers in past tests, so I don't think this is a switch issue. Besides, I cleaned the speaker selector switch twice a while back, thinking that might be the issue.

It may be a while before I get parts. I'll need to wait until I have a larger order. Thought I was going to be placing an order soon for another project, but it turned out I had the parts I needed on hand for that project.

 

[KeithD]

Was looking at the board and the schematic again. Q5 and 6 are 2SA621A. What is the recommended replacement for these? I'm seeing some old posts about 2SA1127, but Mouser does not have those. Not sure they are still in production.

 

[KeithD]

No necessarily suspect, but if I'm placing an order anyway, might as well get all the transistor replacements in the signal path on that board. I will likely get the needed capacitors also.

 

[hopjohn]

For 2SA621 (EBC pinout, the tab on the case indicates pin 1 the emitter, look at a datasheet if you're confused) I'd probably use ZTX795A (CBE pinout) as a replacement, but only if they prove faulty. The originals are mounted in a triangular pattern so the pinout differences shouldn't create any problem. The small dash inside the circle represents the collector , the line through the circle is the base and the line attached to the outside of the circle is the emitter. Don't trust the board marking though , make sure they are correct before you pull the originals. Take photos.

 

[KeithD]

For 2SA621 (EBC pinout, the tab on the case indicates pin 1 the emitter, look at a datasheet if you're confused) I'd probably use ZTX795A (CBE pinout) as a replacement, but only if they prove faulty. The originals are mounted in a triangular pattern so the pinout differences shouldn't create any problem. The small dash inside the circle represents the collector , the line through the circle is the base and the line attached to the outside of the circle is the emitter. Don't trust the board marking though , make sure they are correct before you pull the originals. Take photos.

Finally got parts, replaced Q8, 18, and 20. No change in static on L channel.

Did not end up getting the replacement for Q6. Not sure why, it was a large order, so may have forgotten that one. However, thanks for the instructions on the board markings on Q6, it was the same for Q18, which otherwise would have been confusing. Why did Kenwood use two different marking systems on this board??? In fact, why don't they use standard transistor markings??? I've not seen the circle with lines system before.

At a bit of a loss for what to do next. If this problem is with one of the output transistors, I don't have any of those.

 

[dr*audio]

Do you get static with the volume all the way down?

 

[KeithD]

Is the static present only when you're sending a signal through it, or also when it's idle? If it's only when you're sending a signal through it, have you tried different inputs, different interconnects, or different CD players/turntables? And yes, it could be dirty contacts or potentiometer. I had to go back to hopjohn's original thread to see where you started your queries because it was unclear to me that you'd Deoxited your pots. Have you?

There is static with no input signal and volume all the way down. It does not matter which input is selected, L channel static is the same. I did Deoxit all pots and switches including the two trimmers on that board. To me, that points to a bad transistor putting out background static, which is what I was working on, but none of those replacements changed anything.

One additional piece of information. Initally, I had put the replacement for Q18 in wrong because the markings are different for that transistor. When I had the unit on, the static was gone, but no sound at all. When I put the transistor in correctly, the statice on L channel was still there. That might or might not mean the circuit after Q18 is not the problem. I’m not sure...

 

[KeithD]

Thanks for the clarification. Hmmmm...what voltages do you have for Qe15 & 16? They should match.

Voltages: E,C,B
Q15: -4mv, 28V, 1mv
Q16: 22mv (variable 70 to -38mv), 28V, 24mv (variable 56 to -5mv)

Also checked the relay. Bypassed the left channel relay by de-soldering the output wire and connecting it directly to pin 8. Static was the same, so this is not a bad relay contact.

 

[dr*audio]

Static with the volume down means the problem is in the power amp. Get some freeze spray and spray a small amount on each transistor, one at a time. The bad one will cause the noise to increase when you spray it.

 

[KeithD]

Static with the volume down means the problem is in the power amp. Get some freeze spray and spray a small amount on each transistor, one at a time. The bad one will cause the noise to increase when you spray it.

Yes, agree, problem must be in power amp. Will pick up some freeze spray today and give that a try. Should help narrow down which transistor is the problem at this point.

 

[KeithD]

Qe16 ... were those voltages drifting with heat? Re26 & 28 are the .47 ohm emitter resistors ... what do they measure, as well as Re42 - 680 ohm? ...and do a visual check of the solder joints under the L CH components board.

No, the voltages were not drifting with heat that I can tell. Lots of jumping around as indicated by the range.

Measurements (all in line, resistors not removed):

R26: 0.62 ohm
R28: 0.58 ohm
R42: 536 ohm

Tried some freeze spray. Q19 seemed to respond, so replaced that with KSC2383. No change in L channel static.