Kenwood kac 820 repair

jol50

Member
2007-11-08 2:14 am
Thought someone might remember these old things, and one resistor I need to find out value of.

I fired this amp up and it had 5v offset on one channel and .5 on the other...so I thought input or power for is not right. Of course they are put together a little different, but get it apart and find a burnt resistor next to D25 and it took the number off the board, D25 is blown too, and both have been replaced at some time. Also D17 is shorted. These are next to a transistor Q17 that is a D1266 and I am guessing maybe a power regulator? This is on the input board, I have to get it back partly together to run it to verify what power is there and/or trace back to main board. D17 looks like it goes to ground. Did not see anything else obvious like damaged outputs/etc. It is next to the metal box/shield in middle of input board.
 
The model numbers for a given year are generally 820, 920, 1020 (as an example). The amps for a given year are typically very similar (same basic design, scaled up/or down for power). I have the manual for the 1020 and the reg is relatively complex (attached). If the caps across the zeners are rated for ~10v, I'd think that the reg is similar to the 1020. If the caps are like a more common reg, I'd expect for the caps to be rated for 16v or more. I should have a manual for an 820 tomorrow.

The regulator transistors are complementary in the 1020.


kac1020reg01.jpg
 

jol50

Member
2007-11-08 2:14 am
The caps are 50 and 25v. Someone replaced the transistor with a plug and a different transistor. Replaced the diode....bad part is it blew again so I'm thinking unless they used the wrong parts something else is trashing it. Other side looks good but did not test yet. This should be my +/-15v right?

[IMGDEAD]https://usera.imagecave.com/sho305/ken82052.jpg[/IMGDEAD]
 
That could be stock. I repaired a 820 about 2 years ago and they used this plastic spacer for the black transistor, too. the burned stuff around could be burned glue I belive. I remember the curcuit board was very dark in my 820 as well , the transistor gets very hot and on mine the soldering was very crumbled already. It could be that the transistor outsoldered itself and therefore burned , but the diode seems to be replaced.
 

jol50

Member
2007-11-08 2:14 am
I took the two diodes out and the little shorted one D17 crumbled, the larger one seems good. I think diode is a IN4742A and tests at .734. Think then the transistor is shorted not the diode...assuming it should not be a few ohms. Maybe that is a spacer it does not want to come out of there and yes looks like crusty glue. Burnt resistor is pretty open. Both caps check ok. Not sure if transistor was replaced then if that is stock. Someone resoldered a lot of stuff on the boards neatly, but the diode and resistor were kind of hacked in there with the wires sticking out sideways. I better look up that transistor and see what it is. J54 resistor should be the same to replace smoked one; looks like they put a larger one in that still burned. Unless they didn't figure the transistor was bad, but that seems too simple a reason. I'll still have to figure out what D17 is.

This amp has to be close to old school isn't it? Funny the outputs/PS is about identical to a newer S726 I looked at. Only bad part is they put RCA leads on it, not sure I can fit terminals back in it.

Not sure I like 4 output transistors for a 75-100w rms x2 amp, when I hammered the newer one it got hot but thats typical. I installed these back in the day though, they worked ok and did run hot. Could have had a little more power, the LP and RF had more. This would run a set of 10s pretty good.
 
aehm that transistor is supposed to be larger than the green one (I know that def. because I was also wondering 2 years ago). I think Perry is right with the transistor to be a 2SD1266 . From the schematics - D17 is a 20V Zenerdiode.

Not sure if the whole curcuit could be replaced with a LM7820 but I would stay with the original curcuit anyways. I´d suggest when you insert the new transistor scratch some paint away from the curcuit lines connecting the transistor , bend the transistor legs onto the curcuit line and make sure you do a good soldering job. Like I said, the transistor gets very hot during operation.
 

jol50

Member
2007-11-08 2:14 am
Thanks for the tips and zener ID, I will have to order parts for this one. Aside from the 1266/D17 going bad might there be anything else I should look for that would burn that resistor? Circuit is no longer grounded once I took D17 out.

Ah, if it lasted this many years I will not bother to change components. I don't know year but they have not used that black brick style sink for some time so it has to have some years on it.

I could try to fit a piece of heatsink to that thing, if it would help. The heat must be why they have the stand on it.
 
Here's a layout of the reg area of the board. It's strange that this amp has two D17s ans two Q17s.

820reg03brd.jpg


The larger NPN transistor is OEM.

Many times, when the board is damaged by too much heat, it's no longer strong enough to hold the components. If that's the case, make the connection from the transistor to the board with wire and mount the transistor elsewhere (preferably on the heatsink). If you can find a narrow clip-on heatsink, you could mount it in the area to the right where the diode outlines are. Use a high temp adhesive to hold the sink to the board. Silicone is probably the best choice. Apply it so that you can slide the transistor out of the sink if it needs to be replaced in the future. I'd use a 2n6488. The metal tab will transfer heat to the heatsink better than the fully encapsulated part. Use heatsink compound between the clip-on sink and the transistor.


This sink should work well:
Mouser pt#: 532-576802B31G
 

jol50

Member
2007-11-08 2:14 am
Where it is now there is no room above it but there is around it excepting the metal box/shield next to it. I could mount to sink but not sure I can get to anything to do it, I mean it goes in with a board covering it will have to investigate.

I think the board is ok there, that is old crusty glue for the most part around that base. The resistor did burn but underside is not loose. That layout is right from trace side of board. I'll stand the new resistor up, put some epoxy in there.

What did they do, reuse numbers on each board? (Q17, etc)
 

jol50

Member
2007-11-08 2:14 am
I was going to put a dab under that resistor, but can just leave it. That board is mounted near sink/case like normal amp, then the main board has sink risers and mounts at bottom of amp over this one. If I could put wires out of bottom of board I could mount that transistor to the main sink somehow but then how do I assemble amp. Could mount to a riser but they are on other board same deal, maybe would have to put a clip on one and slide it in on assembly. Otherwise just put a sink on it, it points down at the main board in an open area. Last option is if I had about 2.5" of wire on it, I might be able to put it on one of the riser attaching screws with a longer screw...that would keep it cool. Or put a clip on that screw to mount it in, if there are clips like that.

Thanks Sdoom, sent you a message.

I looked at it more, if I can get a wider sink I can just cut off one side if needed and it should work. Can't go much taller it will hit the main board.