Smoking resistor

[rulerboyz]

Smoking resistor (solved)

I almost had my Kenwood KR-4140 fixed until I ran into another problem on the left side of the amplifier section. There is a 33 Ohm resistor located next to the output resistors that smokes when I turn the unit on. I know that the bias is high on that side, yet I don't want to leave it on long enough to try to turn the bias pot because that resistor will burn up if the receiver is on longer than 10 seconds.

I spotted a 1uf 50v capacitor on that same side that had a gob of solder that had fallen on it and was connecting it to a neighbouring resistor. I removed the cap and cleaned of that solder of and tested the cap which was fine, then put it back in. I turned it back on for a second, long enough to see that the smoking problem was still there. I just put a new 33 ohm resistor in and don't feel like burning it up as well. Anyone have any ideas where the trouble could be? All the transistors appear to be fine.

Also, this unit has two diodes on either side near the output resistors. On the circuit board each of these two diodes is labelled as though it represented 2 different diodes in series. 1 and 3 on one side, and 2 and 4 on the other. The diode test reveals a value of 1.5V or so which seems like an odd value for a diode.

 

[EchoWars]

You need a current-limited supply to troubleshoot. Perhaps something as simple as a 100W lamp in series with the power cord.

The diodes are likely multiple-junction diodes used in biasing. Sometimes they are mounted on the heatsink for temperature compensation. What does the schematic say they are doing?

 

[rulerboyz]

Ok, I've built the 100W lamp in series with a power cord. What exactly will that do when I plug my receiver into it?

The diodes do in fact cross over a junction. There is actually holes in the circuit board that would allow you to install 2 diodes instead of one. Do you think it would be an improvement to replace it with 2 silicon diodes (just a thought)?

This diode appears to handle the current coming directly out of the bias potentiometer, based on the schematic. I replaced the original single turn pots with 500 Ohm multiturn ones on both sides.

 

[EchoWars]

The diodes are there to set up the proper voltage between the bases of the driver transistors. If they are working, then there is no advantage to replacing them.

The lamp will allow you to quickly power up and see if something is majorly awry. If it glows brightly after the power suply caps charge, then there's a short. On a properly working unit, it will glow bright for a few seconds, then dim as everything is charged up.

 

[rulerboyz]

Ok, I guess I must have made a mistake with the lamp set-up. When I plug it into a properly working unit the bulb stays bright.

 

[EchoWars]

Bulb is in series...so all the current that flows through the amp or whatever must flow through the bulb.

 

[rulerboyz]

I think I'm going to need the bulb tester for dummies version on how to put it together.

 

[rulerboyz]

I wish the photos were still there. My command of electrical parts lingo is non existant.

 

[EchoWars]

Like this..

I use an outlet and a lamp mounted to a chunk of plywood, wired as such.

 

[hpsenicka]

Is there any preference for whether the lamp should be on either the line or neutral side of the circuit?

 

[EchoWars]

Makes no difference...current must flow through it, so it is still the limiting factor in the case of a short.

 

[rulerboyz]

Ok, I have it set up so that the black wire connects to one lead of the lightbulb socket. And the other lead connects to the continuation of the black wire towards the outlet (a new black wire) so that the current must go through the bulb. I can see the outlet light go on (meaning it's getting power) but the bulb doesn't come on (with nothing plugged in). Does the bulb only come on when something is plugged in?

I still must have the wrong set-up since the bulb doesn't come on when I hook it up to a working receiver.

 

[hpsenicka]

A bit difficult to visualize your situation....

The light bulb should illuminate only when the device you are testing is turned on.

Does the working receiver power on when plugged into the receptacle "upstream" of the light bulb socket?

If the light bulb is removed from the socket, the receiver should not have any power at all...

 

[rulerboyz]

Okay, you are right about that, it only powers up when the bulb is in the socket. Maybe the bulb will only light up when I connect it to a receiver with problems.

 

[rulerboyz]

I just took out the 100W bulb and put in a 40W bulb and tried again. The bulb lights up for a couple seconds and then dims out (sound like it's doing what it's supposed to).

I plugged my troubled Kenwood receiver into the bulb tester with a 40 Watt bulb. The bulb lights up and stays lit (the receiver is prevented from turning on).

 

[rulerboyz]

I tested DC with the 100W bulb in place. I am noticing that across the biasing diode on the good side I get a positive voltage. Across the diode on the bad side I get a negative reading with the same lead orientation.

I picked up a new multimeter that seems pretty good, however it says that it can only read forward voltage drop of a diode up to 1.5V, and I know that these diodes are greater than 1.5V. It can only read capacitance up to 99uf.

 

[SolderIron]

I love the small of burn resistors in other people's gear

>It can only read capacitance up to 99uf
What kind of multi meter is this that can read caps? VOM I know does not have this function.

 

[rulerboyz]

It's a talking multimeter with leads that light up when you press a button. It's about the best one that The Source by Circuit City has, but there are some better ones that are able to read large capacitors in the 10000uf range, and larger forward voltage drops for diode test.

http://www.thesourcecc.com/estore/P...=Online&category=Multitesters&product=2218202

 

[rulerboyz]

Power Transistors?

I've had my first dim-light bulb tester success story. My KR-4140 has gone from lighting the bulb to starting up without lighting the bulb. The problem components were 2 power transistors and one small PNP transistor.

When I originally started working on this receiver, and didn't know any better, I replaced a lot of things which certainly didn't need to be replaced. Among those things were the 2 power resistors. The original ones were:

4 Watts 560 Ohms
6 Watts 150 Ohms

I replaced them with:

5 Watts 680 Ohms
5 Watts 150 Ohms

I now understand that these were not an exact match. I will get some ones that I can heatsink to the chassis.