KA-3500 blowing fuse

[Stink Bear]

I just acquired a KA-3500, the previous owner said that it keeps blowing fuses.
I took the cover off and looked around, there has been something spilled in it at least once.
I did a little cleaning so I could see the components better, I don't see anything that looks burnt.
I used an ohmmeter on diode function to check the output transistors, I am getting different readings
between the same transistors on the different channels. One transistor seems to be shorted. Since
this was done with the transistors in circuit and I am new at this I'm not sure what to do next. I think I
should pull the board, remove the transistors and check them.
I'm wondering if there is anything else I could or should do before I start taking the board out. I
considered putting in a fuse and powering it up on a DBT to see if it might stay powered on and work
long enough to isolate to one channel but I think I could also cause more damage doing that.
Any suggestions?
Thanks,
Brett

 

[stentis]

I am not an expert but i would pull the transistors out of circuit to test them. Do you have a schematic ? If not I highly recommend you get one. I have a hobby electronics shop with test equipment up to and including an o-scope. I have gained my expertise from the burn-em up and try again school of electronics. A DMT is a great idea if you have the schematic. One word of caution about replacing parts is it has been the experience of myself and others that labeling orientations and pin positions and cross referenced replacement parts with markers or tape will save you from hours of frustration. When checking board components be methodical and write down observed values so when circuits change you will know it. And only replace two or three components at a time, then test your work / circuit before moving on.
You definetly have a dead short some where but I am not much help unless I can see a schematic which I may look for. Hopefully I can be a little helpful.

 

[Stink Bear]

Thanks for the reply Stentis, you've got some good tips there. I did download a schematic, my understanding of it is limited. As I said, I planned on removing the output transistors and checking them, probably other transistors as well. I will also make checks with components in the circuit and compare channel to channel.
I mostly wanted to know if there was anything else I should do before tearing into it. I didn't want to get it apart and then find out I should have done some other checks / tests first.
I have done some repairs to other receiver / amplifiers, some successful and some as you say, burn-em up and try again. I recently acquired an O-scope, but I got just a little training and use on one 35 years ago. It will take some education on how and when to use it.

 

[mikepick]

I just got a KA-3500 working: Batwing transistor questions (KA-3500) this thread has links to lists of transistor substitutions and other information. However, I did not have a problem with a short so I'm not sure how to handle it.

In my case one channel was bad, but there was visible damage to transistors, and also a couple of resistors in the area of the outputs.

I wonder if it would be worth pulling the output transistor assembly and then firing it up with the DBT.

Pulling individual transistors and testing could be tedious, especially since it's not an easy board to get at.

 

[Stink Bear]

Thanks mikepick. Having transistor substitution numbers will probably be good.
Pulling out the transistor assembly? The output transistors and the heat sink? I think that would be a good idea if the board were easier to get in and out, but I'm not so sure in this case since taking the board out looks like it might be somewhat involved.
Any tips on taking the board out since you have experience?

 

[mikepick]

I think there are 8 screws, 4 on either end of the power board that hold it down, plus one larger one that is attached to one of the transformer feet. Two of these screws on either end go through the heatsink. The heatsink is also attached to the board by 3(?) screws IIRC plus the screws that attach the diodes above the output transistors.

One you remove the screws that hold the board down, and you remove the screws from the sides of the front face (it's a sort of hinge) you can work on it by propping it on its side and folding the board up at a 45 degree angle, and the front another 45 degrees.

This picture is crap but its the general idea.



I found this useful for quick work but gravity made soldering a little more difficult. You can also do a similar arrangement by putting it upside down on the desk and propping the front and back up on angles, but that is a bit nerve wracking.

If you remove the board entirely, it's a good idea to tape and label, desolder, and pull the wire-wrapped posts from the board rather than undo the wrapping. I made a note with a full list of the posts I pulled, in order, so that I remembered to put them all back afterwards.

 

[Stink Bear]

Thanks again mikepick.
It looked to me like wires would have to be removed to get the board up much at all. You didn't say anything about removing wires to get the board up enough to work on it so I assume you didn't remove wires for that.
If I could just pull the bard up and remove the output transistors I could lay it back down and power up on a DBT without output transistors, without doing a lot of work.

 

[mikepick]

I didn’t have to remove any wires on mine, there was enough slack all around to open it into the position in the picture.

 

[Stink Bear]

I managed to pull the output transistors without removing any wires. two tested bad, one is shorted. I will probably stop there for the time being, other things to do.