View Full Version : Kenwood kr 4200


oldmarantz
02-23-2003, 09:26 AM
Hi all
Yesterday I picked up an old Kenwood KR 4200 but this one is very ill,the speaker relay doesn t work.I think there is a problem in the power supply or amp. board.If somebody have got just the schem. it would help me.I take a (great) look but don t find... :dunno:

Thanks

Dom.

EchoWars
02-23-2003, 09:41 AM
There could be several reasons why the relay doesn't engage. Kenwood typically has a protection circuit on the relay which is designed to take the speakers out of the circuit if DC is detected at the output. This would result from some reasonably serious failure of the power amplifier section (such as output transistor or driver failure).

Also, it is possible that the circuit which engages the relay in the first place is faulty. Generally there is a transistor which is held to ground with a 100f capacitor and charged through a medium-sized (10k-30k) resistor which allows for a turn-on delay (to allow circuit energising without nasty thumps to piss off speakers). Capacitor or transistor could be shot. There is also a fast turn-off circuit with a diode and a small cap (10f or so) which is coupled directly to the unrectified AC from the transformer. There is a lot of ripple current on this small cap, and they tend to fail with regularity.

Lastly, the coil for the relay could simply be open, in which case you simply replace the relay.

You probably will have to bite the bullet and buy the prints for the amp if you want it fixed. There are many places that sell them...

oldmarantz
02-23-2003, 09:46 AM
Thanks EchoWars you are as quick as lightning ;)
I take a look immediatly at this little kenwood with your tips and I will give you the result...

Dom.

oldmarantz
02-23-2003, 11:54 AM
For instance nothing seems to be wrong with final amp Trs and drivers the relay is ok (no tension on the coil)... I found KR 5200 Schems this one is nearly the same (power is higher some tensions are differents but I think it s a good base)if you have some ideas...:rolleyes:

Dominic

EchoWars
02-23-2003, 08:15 PM
Topology is a little different for the amp, but basics are basics. Do not assume that what you have matches this drawing.

Qe16 along with De7-De10 provide DC protection for the speakers. If Qe16 is engaged, it provides drive for Qe17 and Qe18, which removes drive Qe19, opening up the relay.

The turn on delay is provided by Ce16/Re47. I do not see a quick turnoff circuit, but perhaps the voltages collapse quickly enough that it may not be necessary.

Qe11-Qe14, along with Qe15 provides protection from an overdriven condition by providing drive for Qe17.

When things are woking properly (normal conditions), Qe11-Qe14, Qe15, Qe16, Qe17, and Qe18 should all be off, and Qe19 (of course) should be on.

First, check for DC at the emitters of the output pairs (Qe7-Qe10). Any DC there will engage Qe16 which will shut down the relay. If that looks OK, then check EB voltage and CE voltage on Qe19. If transistor has no drive, then check Qe11-Qe18 and find out why.

Chances are Qe19 is dead and was killed by a failure of De12 to protect it from inductive spikes from the relay. Both Qe19 and De12 get replaced in a scenerio such as this (De12 can be replaced with a common 1N4004 or similar). Also check DC resistance of the relay coil, should be about 650 ohms or so. All Electronics (http://www.allelectronics.com/cgi-bin/category.cgi?category=search&item=4PRLY-24PC&type=store) sells inexpensive replacement relays which fit perfectly, although I do not know their overseas shipping policy (if it comes to that, we can work something out).

Again, I strongly recommend that you track down a schematic specific for your amp. It might cost a few bucks, but you will thank youself in the long run...

oldmarantz
02-24-2003, 06:14 AM
Thanks for your advices EW

I ve got a big problem on one channel I ve got more than 3V dc !!!
on output (tension taken on the relay) the other channel is ok with some mV...so sure the protection is engaged!but when I look at the Tr (drivers+final stage) they seem to be good (I verify with a fluke on diode test position):mad:
Well I follow my investigation I ll look at caps solder...etc

Dom.

EchoWars
02-24-2003, 08:40 AM
3V would generally be plenty to engage the DC protection (most kick in at about the voltage level of a diode drop, 0.6V or so).

Check your grounds carefully on both channels. Connect your DVM to the power supply filter cap ground, and measure the potential between there and various ground points in the channel that has the problem. If you see 0V (or something less than 10mV), then grounds are probably OK. If so, then you need to examine the input pairs. The job of the input pairs is to provide high gain and apply negative feedback. In your schematic, for example, on the right (top) channel, Qe1 and Qe3 are the input pair. Negative feedback is applied to Qe3 to correct for distortion, bandwidth limitations, and yes, DC offset. Generally, a mess of DC offset is caused by a bad transistor, or a leaky electrolytic capacitor (or a bad ground...double and triple and quadruple check grounds!!). The electrolytic to look at in this case would be Ce5 (using the right channel as an example). Replace it. Exact voltage rating is not important (I usually use 50V and 63V for almost everything), but here size is, so stick with a similar value (looks like 100f, hard to read).

If that doesn't do it, replace both input pairs. I'd go ahead and do it on both channels. You'll never find a 2SA620 (well, not here in the States anyway. Europeans may have a better selection of Japanese transistors than Americans get). I keep a supply of 2SA1085's around for replacing PNP input pairs. They would work fine here. DC offset at the speaker termnals should be less than 50mV.

You still need that schematic bro... :twak:

oldmarantz
02-24-2003, 05:12 PM
Aie!!! EW don t hit my head ;)

I ve got some Bad and Good news
The DC offset pb is solved it was the "differential stage" out the 2 TR was good with the DVM but in reality they were OUT...
but the relay doesn t work!!! I try to jump the relay (just to see) if the ampli works it s ok on the both channels and I ve got less than 30 mv on one channel and less than 10 on the other...so now I ll look at the "relay stage"... but it s not the end When I tried the Amp I saw there's a pb with FM....the sound is weak :( it s ok with a CD...
When I said it was Ill it was not just for fun:mad:
I ll give you some news...
Thanks again for your advices

Dom
;)

EchoWars
02-24-2003, 07:13 PM
Originally posted by oldmarantz
Aie!!! EW don t hit my head ;) LOL! :lmao:I ve got some Bad and Good news
The DC offset pb is solved it was the "differential stage" out the 2 TR was good with the DVM but in reality they were OUT... The problem is that they are 'unbalanced', not dead. It is not too uncommon in old amps to see a differential stage that has a pair of transistors that are no longer functioning together as a 'team'. but the relay doesn t work!!! I try to jump the relay (just to see) if the ampli works it s ok on the both channels and I ve got less than 30 mv on one channel and less than 10 on the other...so now I ll look at the "relay stage" Take a look at the driver transistor...in the schematic, Qe19.... also look at the charge capacitor, Ce16. If the transistor is bad, be sure to replace the protection diode also (De12)but it s not the end When I tried the Amp I saw there's a pb with FM....the sound is weak :( it s ok with a CD...
When I said it was Ill it was not just for fun :mad:
I ll give you some news...
Thanks again for your advices

Dom
;) The FM may require alignment, and almost certainly will require equipment that few people own. Get the relay fixed, and if you want to keep the receiver, shop around for a repair place that will do the work for a reasonable price. But do check the easy stuff, such as the connections to the antenna and to the RF unit.

You are doing well, young Jedi. :starwars:

oldmarantz
02-25-2003, 11:19 AM
Hi EW

Good news:)
it works... the relay problem came from Qe18 it was good when I tried it with VTM but not as good as it looked...Now it s the end of the game.the FM Tuner need to be realigned the AM works well.I bought on ebay this Sunday an AM FM Generator (National VP 8190) for 190 bucks.I m sure I will use it ;)

Thanks Again EchoWars... I m sure it s your job :cool:

Dom.

A pic of my new tool:D

EchoWars
02-25-2003, 04:21 PM
Nice work! Perhaps when you finish aligning yours, you can tackle mine. :)

oldmarantz
02-26-2003, 11:59 AM
Hi EW
It will be a pleasure to drink a good beer with you ;)
but it s a little too far even If I like your big Country (I came twice in US and work for a Americano-French company)it will be nice I will come back with a lot of Marantz and Sansui stuffs :cool:
Dom stop dreaming:D

Dominic

Buzz
02-26-2003, 02:19 PM
Now thats some serious shi* that just happened there. I am really amazed at how you two trouble shooted and fixed the receiver. Hats off to both of you. I could only wish to be that proficient at electronics.

oldmarantz
02-26-2003, 03:06 PM
Hi Buzz

With schematics it s not so complicated, I m sure EW has got a great knowledge in audio electronics stuffs.Concerning me I m also an electronician and repair medical automated systems to control blood, bactery...etc.So I ve got enough knowledge to do that but an automat for medical diagnosis is not a receiver;).The schem is a little specific.When I was younger I wanted to work in audio repair but no work at all so I did other electronic repair but it s not so fun for me:mad:
I like to do that if I could earn my life with that job I ll do it immediatly:cool:

Dom.

kikook
03-08-2009, 06:31 PM
Some years later.....

I have also some trouble on my KR-5200 with the relay.
The relay doesn't want to engage.
I found no DC, so I engage the relay by myself : the sound is good.
De12 is good
Found 34v between CE of Qe19.
If I make a contact between C and E of Qe19 the relay engage itself ans sound is ok.
All chemicals caps are new.
Qe18, Qe19, Qe16, Qe17 seems to be ok with VTM.


Any help is welcome !

kikook
03-09-2009, 04:27 AM
Culprit found.
Qe15 was dead (2SC983), replaced with 2SC3468, all is ok now.

:thmbsp: