Need help with Kenwood KR-4200

I recently picked up a nice condition Kenwood KR-4200 that is having a relay issue. The speaker protection relay is clicking off at random intervals. I have hooked up my multimeter to the speaker inputs to the relay and am getting a stable reading of 18mv, even when the relay clicks off.

On the relay power side I am getting 27.2v until the relay clicks off and it then goes to 0 for a second or two and then resumes normal operation.

I don't think it is a problem with DC offset as it is stable at the relay and doesn't drift at all.

Any idea where I should start poking around. What would cause the signal to the relay to disappear for a second or two at random intervals.

The receiver is all original and works great otherwise.

Thanks in advance!

I should also add the receiver hasn't been used in years and the problem seems to be becoming less frequent the more I use it. Could it be dried out caps?

yes it could be the caps reforming ..
or possibly a dry solder joint ..or flaky transistor
run it for a while and then see how it is .

cptnCanada said:

.... The speaker protection relay is clicking off at random intervals.

.....What would cause the signal to the relay to disappear for a second or two at random intervals.

Click to expand...

Is it the relay contacts opening up, or loss of the music signal?

The relay is opening up due to the signal to the relay cutting out.

I have found this reply in the archive section of AK that seems to be a good starting point. The original poster described the same issue I am having. A member by the name of sregor replied:

"Been a long time since I opened one of these. Basic trouble shooting - 1st make sure there is no DC to begin with. Pins 1 and 8 are the outputs before the relay. Should be less than 50 mV. but probably has to be higher than 600 mV before relay shuts down.
If the outputs are too high or drifting a lot, usually because of bad input stage transistor - Qe 1 and 3 or 2 and 4, depending on which channel is bad. Usually changed in pairs - you may find originals at bdent.com, but usual substitution is with ksc1845 from mouser. It can be other components....
If the offset voltages are low and seem stable, check for current limiting voltage. Check r44 (10k) If getting voltage, and no DC on output, then problems with qe15, 16, or 17. If zero volts, then problem should be in q19 20 or associated circuit. If everything is good, 19 should be in cut off (no voltage on base and emitter, high voltage on collector) and 20 should be "on" - small (a few tenths of volt) on the emitter, .6 to .8 on the base and collector should be around half a volt. Also, look on this area on the circuit side of board for cold solder joints and evidence of overheating (darkened PC board.) Hope this helps without getting to technical."

Im fairly new to diving this deep into electrical repair, but I am eager to learn. My question is, how do I use my multimeter to test the resistor and transistors in circuit with the power on as the poster describes? Is it simply set the meter to the appropriate DC volts and have one pin on the part to test and the other on a common ground? I have highlighted the above part that I believe most describes the issue I am having.

- I believe the term "signal" is usually used when referring to the actual music signal. There are two kinds of electrical voltage/current happening at the relay. There is a DC current that controls opening and closing of the relay. And when the relay is closed there is AC music signal that moves through the contacts of the relay and out to the speaker terminals. The relay contacts are controlled by the coil but they are electrically separate from the coil.


- If the problem does not clear up on its own and you can't find any bad solder joints, here is what I would do: There is a transistor whose job is to control current to the relay coil and keep the relay closed (contacts engaged). And there are a few caps that are part of that control circuit. Both that transistor and those caps age and can eventually fail. This is a common and well known failure mode.

While the cause of the problem could be other things, replacing those caps and that transistor would be a good place to start. They should be freshened up in any case, and there is a good chance that could solve the issue.

No testing, just replace those parts. Find the relay on the schematic, locate the transistor whose C or E leg (not sure in which order) look like they are in series with the coil of the relay. Identify any capacitors in the circuitry that supports that transistor.

You will probably have to sub in a different transistor because a lot of them from back then are NLA. Get the transistor model number from the schematic or parts list (or the transistor itself) and post back here asking for a good sub. And any caps in that part of the circuit replace with 105° caps such as Nichicon PW or HE, or Panasonic FM, FC, or FR.

Replacing the relay itself is not a bad idea either, while you are in there and ordering parts anyway.

I re-read your posts I did not see any mention of the Service Manual. Do you have one?

That is something that you really need to be able to identify circuits and components, for bias setting procedures, etc.

Unfortunately there seems to be no service manual for this model, but I do have a schematic that I have been familiarizing myself with today. I am sure it is a DC current relay control problem. One wire for the relay power comes from one board and the other comes from another board of course to complicate things. I think the longer I look at the schematic the more I am understanding it. Unfortunately the schematic doesn't show voltages at the transistors either, or transistor parts numbers.

I have done caps before and have a few lying around so I'll see if I can match them up and replace. If not I can order some and do the whole board. They are cheap enough.

Hoping I can get this fixed as I really like the looks of this old kenwood. Classy and refreshing compared to a lot of other classic brands that I tend to find look the same.

Transistor numbers are usually on the schematic as well as the transistor itself.

Is your schematic hard copy or .pdf?

pdf.

I got it here http://elektrotanya.com/kenwood_kr-4200_sch.pdf/download.html

cptnCanada said:

Unfortunately there seems to be no service manual for this model, but I do have a schematic that I have been familiarizing myself with today. I am sure it is a DC current relay control problem. One wire for the relay power comes from one board and the other comes from another board of course to complicate things. I think the longer I look at the schematic the more I am understanding it. Unfortunately the schematic doesn't show voltages at the transistors either, or transistor parts numbers.

......

Click to expand...

Thanks for the link to the schematic. Interesting that the relay is on a different part of the schema (right middle) from the circuit that controls it (right bottom). To do measurements you would usually (not always) attach the black lead of your meter to ground and probe the part under test with the red lead. It is good to insulate the red lead with electrical tape as close to the tip as possible and have just the very end of the probe's metal exposed.

I found the thread you quoted and the problem there does sound similar to what you describe. The transistor that I suggest needs replacing, Qe20, is one of the ones mentioned. It is the one that actually drives the relay coil and is most likely to fail. What happens is that over time the gain (aka hFE) of that transistor diminishes because it is always on, like a switch, whenever the amp is powered up. When the gain diminishes far enough the transistor is no longer able to keep the relay closed. It is a common failure mode. Supporting caps also age and can fail.

Please note that this discussion, as well as troubleshooting suggestions in the other thread, assumes that the relay is opening up when the sound cuts out. Also assuming that you are unable to locate any bad solder joints. If you are willing and able to troubleshoot per the suggestions in the other thread go for it. Another way would be to simply replace Qe20 and Ce21 and see if the problem goes away.

In the scenarios mentioned, the potential culprits are Qe15, 16, 17, 19, and Qe20. In any case you should replace Qe20. You could proceed by changing only Qe20 (what I'd do, lol), or you could test and possibly find other culprits, or you could shot-gun the lot of them (overkill most likely, but these are all 2sc1213A so you could order 10 and have spares).

I would definitely also replace supporting cap Ce21. I found one cap completely shot in the relay circuit of my KA-7100. In your relay circuit I was only able to see one obvious candidate for replacement and that is Ce21. I might have missed something with the circuit being spread over different parts of the schema, but I would for sure replace that one. Please note that relay circuits often have different types of caps such as bipolar or low leakage. I can not tell for certain from the schematic exactly what type of cap Ce21 is. If it is bipolar it would say so on the sleeve of the cap (I don't think it is because the schema shows it as polarized). But what I can not tell from the schema is whether or not it is low leakage. If Ce21 is orange in color it is low leakage and should be replaced with a Nichicon KL. Otherwise (not bipolar and not LL) use Nichicon PW or HE or Panasonic FM, FC or FR.

As for Qe20 it is a 2sc1213A (B or C) the last bit referring to the hFE rank. This can still be had from B&D, but they do not specify hFE rank in their ordering page so you won't be sure what you get unless possibly you call and ask them. Personally I would prefer not to stick a "B" rank in for Qe20 (although it would probably be much better than what is in there now) and would request a "C" rank for that one. A good and stronger sub is KSD1616A (Y) available at Mouser and Digikey. I used this sub to replace 2sc1213A in my KA-7100 relay circuit and it is working fine.

Hope this helps.

Thanks for the incredibly informative post! I just got back from travelling now and haven't had a chance to do any work on the Kenwood. I'll try your suggestions and let you know if I have any luck.

I have been fortunate to have received a lot of assistance here, so I try to give back when I can. Sorry for being so long winded. I'll try to summarize more briefly:

There are two general types of issues that could be causing your relay to cut out. First, a fault somewhere could be causing the relay to open. This assumes that the relay and related protection circuits are functioning as they should. This type of fault could be many things and would require general troubleshooting. And this type of issue is beyond my abilities to diagnose. sregor could, or EchoWars of course, and others.

But the second type of issue that could be causing your relay issues is failure of components within the relay protection circuit itself. This is fairly common in older Kenwoods. The most likely failed component is the relay driver transistor Qe20. As mentioned before Ce21 is a prime target for replacement as well.

Just by replacing these two components there is a pretty good chance that the problem could be solved. These parts would be around $1. And if money is a concern Digikey even has a free shipping option when you pre-pay with check or MO. Even if refreshing these two components did not solve the issue, it is something that needs to be done anyway, and you would have at least eliminated two possible causes.

good luck with your project :thmbsp:

you could narrow down the problem by monitoring the base of the relay driver .if it stays around 600mv or more whilst the relay is off the problem is the driver or a bad solder connection between relay and driver . if the voltage drops enough to turn the driver off its possibly the cap or something else .
i haven't looked at the schematic but i know how this part of the circuit works .
probably just as easy to change the caps in the protect circuit first and look for poor solder connections whilst you are at it .

Fixed! I replaced Ce21 and it hasn't tripped the relay once. I didn't have a 33uf @ 50V cap so I put a 47 uf @ 50V in there. Do you think this is OK or should I order the proper cap. I don't want to risk damaging anything.

Thanks for all the help folks. You have helped me rescue another vintage gem from extinction!!

That's great :thmbsp:

Always good to save one from the trash man. IMHO all this vintage gear will come to be appreciated more and more as years go by and manufacturing processes evolve.

RE the value of C21: I am NOT an expert on circuits, and could be corrected on this, but I wouldn't think that a value of 47uF replacing 33uF is cause for concern here. Getting the voltage right, and you did, is important I think.

If you have relay issues in the future don't forget about that transistor Qe20...

Damn spoke too soon. Looks like I will need to replace some transistors. I think I will shotgun replace Q15,16,17,19,20 as they are so cheap and im sure new ones would last longer.

One thing that is strange. In the receiver, Q17 has installed a metal can transistor labelled 2SC1416. According to the schema it should be a 2SC1213a. Did somebody go in there and put a wrong transistor in and could this be what is causing the problem.

Also when ordering replacements, will KSD1616A (Y) be good for all these?

cptnCanada said:

One thing that is strange. In the receiver, Q17 has installed a metal can transistor labelled 2SC1416. According to the schema it should be a 2SC1213a. Did somebody go in there and put a wrong transistor in and could this be what is causing the problem.

Click to expand...

It is not uncommon for parts on a schema to differ from what is installed. In which case the installed part, if original, always trumps. Does not rule out someone having changed it though. But I believe that both of those parts are older. Does the soldering look similar to the other transistors? Did the problem with the relay develop over time or has it always been the same since you owned the receiver?

Comparing transistors' parameters and/or subbing is something that I am not qualified to do. A lot depends on the role the transistor is performing.

Also when ordering replacements, will KSD1616A (Y) be good for all these?

Click to expand...

That is a question that I can not answer. You need someone more knowledgeable like EchoWars for that answer.

But, as mentioned before, I believe that Qe20 is the key part here. It is the one that actually holds open the relay and it has had a much harder life than the others in the circuit. Look back through previous posts in this thread.

IMO it is worth a shot to just change Qe20 to KSD1616A (Y) which is available via Mouser and Digikey (free shipping option at DK). Did changing that cap help? If so, just changing Qe20 might just get it done.

Personally, if I was going to shotgun the transistors mentioned, in absence of guidance from EW, I would get the original parts from B&D. Not sure about the 2sc1416, though you can download spec sheets at this site and compare all of the relevant parts.

EDIT: KSD1616A (Y) is not the only possible sub for Qe20. It is a good one (better than the original), and one that I am fairly certain about having used it. But if you have others on hand then possibly something else might work. But you will need verification from someone other than myself if you want to use a different sub for Qe20.

I'll mail you a KSD1616 if you like. Just to keep things simple.

I can tape it to a piece of cardboard and fit into a regular envelope. Can't cost me that much for regular mail to Canada, can it? If you PM me your mailing address I'll send the part.

If you could do that I would be ever grateful! I'll keep my eye on the forum and if there is anything I can help you out with, you can count on it. I'll pm you my address.