Kenwood Kr-4200 Protection Relay Issue...

[Kenwood97]

Hey everyone,
I recently purchased a Kr-4200 stereo that needs some work before any further use. When I power on the stereo I can hear the relay click and audio comes through either the speakers or headphones (which ever is used). The stereo works well for a while until the relay starts clicking, and eventually cutting out the right channel. The time that it takes for the stereo to get to this point is pretty spiritic, it can be 10 minutes to 30 minutes. I've looked through threads hoping to find some information that could help me solve this issue, but there was nothing specific to my situation, so I figured I would start one. My initial guess would be some faulty transistors on the main amp board, but I would like to hear some other ideas before getting this project going!

Also, all the switches have been cleaned and lubed with de-oxit so I doubt that it is that...

 

[Semanon]

Likely suspect is a faulty transistor in the protection circuit.

But, the relay definitely clicks on and off, right? A poor relay contact can also give intermittent sound and poor quality in one channel.

 

[Kenwood97]

The relay definitely engages/disengages. I've opened up the receiver to check while this issue arises. I then proceeded to try and clean the contacts but that didn't seem to help the problem

 

[sregor]

Educated guess - (if you feel lucky) try changing CK5 on the power supply board. 10UF 63 volt. This filters the DC supplying the relay, and when goes bad, causes the DC supplying the relay to be low and relay becomes intermittent. They don't give voltages, and I'm estimating, but you should see around 40 volts at pin 8 of the power supply board, going to one side of the relay. You can go higher in voltage, but don't go higher in value. It is kept low, so relay will shut off right away when power is shut off.
There are other possibilities, this happens to be closest that fits your symptoms for this protect circuit. Losing just the left channel after acting up doesn't make sense, as both channels should be switched by the relay, but low voltage could account for poor relay contacts. As I said, educated guess....
Also , keep eyes open for cracked solder connections (cold solder, or dry solder connections). Some aren't easy to spot, and probably isn't your problem, but something to be aware of. Many techs routinely touch up any connections that don't look 100 %.

 

[Kenwood97]

Well I decided to refer to a great thread that was posted a while ago by Hop john/john hop. I changed out two input transistors on the main amp pcb board. Q2 and Q4 were both replaced with ZTX694B transistors. Both channels appear to be working now, I've been listening to The Doors all afternoon! Right channel has a pretty noticeable hiss, so probably could use some cap replacements as well! Any suggestions on where I should start with that?

 

[Semanon]

Download the service manual from HFE. Follow what @hopjohn did. Looks pretty meticulous to me. Plus there is a lot of good info on this scattered around AK and the web. It’s as likely to be transistors as it is caps. Because this one has separate boards, you can go board by board checking your work as you go.

 

[Semanon]

Took a glance at the schematic. There are some ‘known bad’ transistors in this unit. Check out the @dlucy sticky thread on the solid state forum about the bad boys of the semiconductor scene. Lots of good info there, too.

 

[Watthour]

I doubt that the "hiss" is a capacitor issue in this instance.

 

[Kenwood97]

Thank you Semanon, I'll take a look at that here soon! Despite the stereo working, I'd really like to dial it in to prevent any further issues from arising.

 

[Kenwood97]

Also, whenever I switch the speakers from off to a/b/a+b there is a noticeable pop. I've cleaned the switch with de-oxit thoroughly and was just curious if this could be the protection relay having a bad connection? If so, it may be worth it to just replace this part.

 

[llwhtt]

Check for DCV at the speaker terminals.

 

[tnsilver]

Please note capacitor Ce21 (33uF/50V) is wrongly marked on the main amp board layout. Its polarity is wrong. It's correct on the schematics. It goes to the speaker relay coil. If installed backwards it will definitely screw up the relay function. I've used 33uF/63V with the wrong polarity and it swelled but didn't blow up.