Kenwood KR-2400 fuse blows at power on

iamthejeff

Active Member
Just wondering how I should troubleshoot this first. Is this a typical sign that the output transistors are blown? Or should I be looking at the power supply board first?

I picked this up with only the knowledge that it doesn't power on, so I don't know what happened to it.

Thanks
 
The first thing I'd do is a very careful visual, with bright light and magnifier. Look for overheated resistors and transistors with "blowout" damage. If everything looks good, I'd probably test the output devices with a DMM on "diode check" just because it's easy. If everything looks good, I'd go after the power supply rectifiers and filter caps next. If those tested good I'd make sure the bias pots hadn't gone open and the output stage wasn't drawing huge current. If no joy there, I'd go have a stiff drink and think about it for a while, worrying that the transformer might be bad.
 
i would check the outputs while still in circuit for shorting from C to E . if they appear shorted i would unsolder or pull them out if just screwed down and test again .
but if both channels look shorted i would then be looking at the rectifier . its unusual for both channels to be blown at same time . but not unheard of in a rare case .
if anything is burnt up you will likely smell it before you see it .
 
Three out of four outputs tested good C to E. One was open. I guess I'll start there and see how far I get.

After a close visual inspection, all of the caps and resistors look pristine on the power supply and amp board (the KR-2400 has a single board shared by both channels).
 
Kenwood made it very easy to remove boards on this receiver. All of the wires are connected with little plugs rather than soldered on. I was able to completely pull the amp board without desoldering any wires.

Just out of curiosity, I tried turning the unit on with the amp board removed and the fuse still blew (luckily I have a whole bunch of spares lying around). So now I am looking at the power supply.

Just poking around, I found both D2 and D4 diodes on the power supply board appear to be blown. I tested them in circuit, so my next step would be to remove them and test. They also appear to be slightly darker than their neighbors D1 and D3. Do you think this is a good place to start?

dDCV27e.jpg


I guess now would also be a good place to source equivalents. The service manual lists the diodes as V03C.

And the output transistors are the following:

2SC789
2SC1444
2SA489
2SA764

Does anyone have any experience replacing these parts?
 
Three out of four outputs tested good C to E. One was open. I guess I'll start there and see how far I get.

After a close visual inspection, all of the caps and resistors look pristine on the power supply and amp board (the KR-2400 has a single board shared by both channels).

if one was indeed open and not shorted then 3 are shorted .
maybe your idea of shorted is opposite to mine .

they need pulling to test now
 
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I think you're right. I am new at this, and I probably used the wrong terminology

I pulled D1, D2, D3 and D4 from the power supply board. D2 and D4 are definitely toast.

I also pulled all 4 output transistors, and Q22 is also definitely toast.

Time to search for replacements - any help is appreciated.
 
Sometimes I feel like this forum is bustling with activity, and other times not. Anyway I ended up sourcing replacements for the transistors and diodes.

For anyone curious:

2SC789 is being replaced with BD243C
2SA489 is being replaced with BD244C

The diodes are being replaced with 60S10-TP

I also ordered all new caps for the power supply and amp boards. I'll let you know how the repair goes.
 
I replaced all of the output transistors and the diodes on the power supply. I tested powering it on with a DBT, and everything looked okay. So I tried powering it on normally and I was happy to see it light up without blowing a fuse.

Only one problem remains—there's no sound at all. FM, AM and AUX are completely silent. No noise or hum, no matter how high the volume. I tested the DC offset and both channels read about 45mV.

The only sound I get is a little click when I turn the speaker selection knob back and forth between Off and speaker set A.

Any idea how I can troubleshoot this?
 
I'll check but I somehow doubt it. Nothing out of headphones either, except a dual clicking that sounds like a heartbeat and has approximately the same tempo.

I also tried adjusting the bias and I can't seem to get it up to 30mV before maxing out the trimmer pot. I think I am measuring it properly (currently hovering at around 16mV). Something has to be wrong.
 
if you are reading across both emitter resistors you are doing it right ..30mv .. or 16mv across one resistor
 
That's what I'm doing, so I guess I am doing it right. Since there is no sound out of the headphones, does that mean the problem lies elsewhere, maybe in the preamp?
 
did you check b+ pin 8 and b- pin 7 ?

preamp / tone control is a chip . icd1 . its underneath .

you could try inject a signal into main amp at pin 1 left pin 2 right ...or a wet finger should get you a loud hum .
 
Sorry, where's B+ and B- ?
b+ pin 8 and b- pin 7 main amp .
or power supply 1+ 2-
may as well get all power supply voltages so we know things are being fed properly .

pin 1 B+ 20v
pin 2 B- 20v
pin 3 ground 0v
pin 4 +B
pin 5 +14v
pin 6 ground 0v
pin 7 -14v
pin 8 stereo beacon ground whilst in stereo .
pin 9 ground 0v
pin 10 8v
pin 11 7v AC
 
This is getting a little out of my realm of experience. I measured 7 and 8 on the amp board and they both read 12V, but 1 and 2 on the power supply board read 24V. That can't be right. For both measurements I used pin 3 on the power supply as the negative terminal.

Edit: Nevermind. Looks like the amp board is getting 24V, but that 4 volt difference shouldn't be an issue, would it?

All the other voltages seem to check out fine.
 
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no issue as its unregulated ..maybe your line voltage is slightly higher than amp design voltage .. shouldn't be a problem .
did you try the wet finger trick on pins 1 and 2 ?
 
I just did, no hum or anything. could I have installed incorrect transistors? also sometimes there is a hum for maybe 1-2 seconds after switching the receiver on, but it cuts out with a pop immediately.
 
something aint right then ..
can you follow the schematic ?
if so could do with checking all voltages that are marked on the schematic . report back with any that are wrong . don't forget they are plus or minus and black lead goes to chassis ground .
voltages are marked for left channel right channel is a mirror image .
just a thought was the vol up a bit when you touched the pins ? if not turn to 1/4 vol and also try with tape monitor on ..
i have a hunch there is some bad resistors on the main amp board ....could be wrong though ...
try with some vol and touch the pins again ..also get voltages at pins 1 and 2 .
 
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