Does anyone repair stereo receivers?

[chipper]

I have a Kenwood KR-A4040 stereo receiver that I've had for a long time and now finaly something went bad with it some smoke came out and blew the fuse. I replaced the fuse and tried turning it back on and it blew the fuse again! I took a look inside and I could not find anything out of order physicaly, however I'm good with a DMM and a solder iron. I'd like to fix it if I can I'm still buying a new thought . This was one great receiver when it was working excelent! But I don't know what caused it to fail? I also forgot to say when it powered on with the fuse before the second fuse blew a resistor had a orange glow then the fuse popped. Any help would be greatly apreciated, I just don't want to throw this away.

[Enzo]

In any solid state amp, if it blows fuses hard, it is usually a pretty safe bet that the output stage of the power amp has blown one channel. I don't know that recei9ver, I am sure you can find teh schematic, but looking inside, you either have discrete output transistors on heat sinks, or there is one or two power amp ICs. COuld be one IC per channel or some ICs are stereo in one package.

Look for the heat sink in the amp. If there is a dark colored block screwed to it with 10-15 legs soldered to the board, that is a power IC.

A lot less common but still possible is a shorted rectifier in the power supply.

And way less common than even that, but still possible, a shorted filter capacitor.

And other things are still possible and even WAY less likely. For example I see darn few bad power transformers in things like this.


Since a resistor has burned up, more will be wrong than just the transistors or ICs. That resistor that got hot enough to glow needs to be replaced at the very least.

I use a lot of different places for parts, but someplace like MCM would probably have any transistor or IC it needs if such parts are still on the market. B&D Enterprises sometimes has odd ones.

MCM Electronics: Home and Pro Audio/Video, Security and Test Equipment

B&D Enterprises - Electronic Components Distributor

[chipper]

Originally Posted by Enzo

In any solid state amp, if it blows fuses hard, it is usually a pretty safe bet that the output stage of the power amp has blown one channel. I don't know that recei9ver, I am sure you can find teh schematic, but looking inside, you either have discrete output transistors on heat sinks, or there is one or two power amp ICs. COuld be one IC per channel or some ICs are stereo in one package.

Look for the heat sink in the amp. If there is a dark colored block screwed to it with 10-15 legs soldered to the board, that is a power IC.

A lot less common but still possible is a shorted rectifier in the power supply.

And way less common than even that, but still possible, a shorted filter capacitor.

And other things are still possible and even WAY less likely. For example I see
darn few bad power transformers in things like this.


Since a resistor has burned up, more will be wrong than just the transistors or
ICs. That resistor that got hot enough to glow needs to be replaced at the
very least.


I use a lot of different places for parts, but someplace like MCM would probably have any transistor or IC it needs if such parts are still on the market. B&D Enterprises sometimes has odd ones.

MCM Electronics: Home and Pro Audio/Video, Security and Test Equipment


B&D Enterprises - Electronic Components Distributor

It has descrete out transistors label numbers on them are "C4467 SK" "A1694 SK" then there's two more that are the same label numbers and the resistor
that was glowing orange is a 0.22KX2 and has the ohm symbol near the '0.22' it has threw legs and is rectangular and white.

[Mooly]

Classic case of failed outputs by the sound of it. Measure those transistors you mentioned above on ohms and also on diode check on your DVM and they will probably read 0.00 (short). Measure center leg to the other two in turn

[chipper]

Originally Posted by Mooly

Classic case of failed outputs by the sound of it. Measure those transistors you mentioned above on ohms and also on diode check on your DVM and they will probably read 0.00 (short). Measure center leg to the other two in turn

Will I need to test them in the circuit or out of the circuit? And also is there any kind of IC that turns on the display screen on the receiver? Because when I tried to turn it on nothing turned on just a glowing resistor and that's about
it. I'm assuming it has to do with the output stage with a set of shorted outputs maybe the other pair?? But ill test them and anything else.

[Mooly]

You can test in circuit. Don't continue to power it up as it is. You can add a 60 or 100 watt mains filament bulb in series with the live mains lead to work on it. This saves further damage, if it's faulty the bulb lights rather than further damage being caused.

[Mooly]

It would really help to see a circuit. Good close up pictures of the output stage would help too.

You will probably find one channel OK.
If you remove the faulty output transistors the amp should power up normally.
It's highly likely that the driver transistors have failed too.
It worth checking carefully for dry joints etc on anything this age, particularly on any transistors/regulators that run hot.

[infinia]

Classic case of failed outputs by the sound of it.

'

Yup
You will be needing to replace at least 4 or 5 transistors on that channel if yer lucky. Plus we still don't know what caused the original failure yet. But certainly replacing the fuse after you've seen smoke wasn't too good on parts.
Not sure if you have the tools, test gear, time , and know how. You can spend the bucks trying to repair it, and then still end up paying more than if you didn't touch it. I know I'd charge 2X seeing a botched repair attempt. If you really want to keep the gear or learn a little of this skill is up to you.
You will need to locate a schematic first and then determine if suitable replacement devices and resistor can be had.

[chipper]

Originally Posted by Mooly

It would really help to see a circuit. Good close up pictures of the output stage would help too.

You will probably find one channel OK.
If you remove the faulty output transistors the amp should power up normally.
It's highly likely that the driver transistors have failed too.
It worth checking carefully for dry joints etc on anything this age, particularly on any transistors/regulators that run hot.

That's what I had in mine but the screen will not display anything if I try powering it up for a split second. I was wondering if there is any kindnof driver ic that controls the screen display or is it because the output is shorted. Also this particular amplifier has six transistors four big ones and two smaller ones screwed to the heat sink. But what if I disconnect the shorted side will it atleast power up again normaly because one side gets hot. I'll get some
close up pictures for you to check out just gimme a sec. Thanks

[Mooly]

Piccys will help. With a "hard fault" such as outputs it's unlikely to have other issues such as display problems etc.