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Working on a 1978 K300 PA Head [message #19537]

Mon, 14 October 2013 14:05

revolvoer
Messages: 12
Registered: October 2013

Junior Member

Hi - brand new to the forum and have been reading a LOT of the previous posts. Sounds like there is a lot of technical talent on this forum.

I have had this K300 PA head for about 10 years and have pretty much just lived with its issues and used it only occasionally - mostly it just sits and is admired by kids who've never seen anything that old up close!

Last week, I took it completely apart to fix the power switch which had broken in the expected/usual place. Following some advise I saw on the forum, I was able to repair the switch with a bit of SOOPERGLOO and two zip ties.

Also, today, I replaced the power cord with a grounded/3 wire new cord (again, following some instructions and recommendations I found on this forum!) and it's working just fine.

So, to my question/issue: I tested all 6 channels of the PA and #6 is hosed. All of the others seems just fine and are loud, clear and all functions seem to do what they should.

This thing seems to be in pretty fair shape for 35 year old analog gear.

Channel 6 outputs some signal but you have to boost the volume to 100% and it's pretty faint - a loud consistent hiss is present when you crank it to that level. Also, when I turn it up full volume and work the bass, treble and reverb knobs (on that channel, not the masters), I'm getting strange squeaks, feedback and sci-fi type warbles.

Also, I note that there is a dark brown area on the board behind the control face - just in one place - centered around two black egg shaped objects that have a small dab of yellow paint on their tips. this area seems to run hot when I run the amp for some time.

Anyone have any suggestions on if this is fixable by an electronics novice like myself? I can use a multimeter, but not very well - I can solder and usually do a good job at that.

Any comments/suggestions or feedback will be most appreciated!

S. Davis
Pearl, MS

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Re: Working on a 1978 K300 PA Head [message #19539 is a reply to message #19537]

Mon, 14 October 2013 15:26

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

Welcome to the place. Glad to have another Kustom fan around here.

A single dead channel can be caused by any number of things. If your amp still has the Molex connectors that connect the boards to the master control board, then try and clean the two connector parts and see if the signal is back.

If the wires are soldered directly to the boards, then try and flex them to see if anything has come loose. This will also help in finding a loose or broken lead off of a capacitor or resistor.

If you feel safe in doing it, you can carefully push and prod the parts on the board with a wooden dowel or chopstick while the amp is turned on, to see if the signal comes back. If you do try this last approach be very careful as there are voltages in the amp that can do you bodily harm.

The worst case will be a dead IC chip or some other discrete component. To find this you will need test equipment and soldering skills. What sort of skill set do you have?

As for the discolored section of the master control board, that area is probably near the Zener voltage regulator circuit, where the voltages are reduced from the power amp to a lower level for the preamp circuits. The Zener diodes get warm or hot, so the board will show signs of heating.

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Re: Working on a 1978 K300 PA Head [message #19542 is a reply to message #19537]

Mon, 14 October 2013 20:19

revolvoer
Messages: 12
Registered: October 2013

Junior Member

Thanks for the detailed info chicagobill. I will try some of the diagnostic methods you outlined. My skills are very limited....I can wiggle some wires and possibly solder something if I find that something is disconnected or loose. Beyond that, I might just have to live with the dead channel.........or perhaps learn some new electronics tricks!

S. Davis
Pearl, MS

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Re: Working on a 1978 K300 PA Head [message #19543 is a reply to message #19537]

Mon, 14 October 2013 22:24

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

I'll check my parts box, I think that I have a few of those preamp boards. Worst case I could check one out and send it to you to replace the dead one. Is that something you could handle doing?

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Re: Working on a 1978 K300 PA Head [message #19544 is a reply to message #19537]

Tue, 15 October 2013 06:30

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

Bill/revolver I have many spares of those boards if you do not have one so let me know.

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Re: Working on a 1978 K300 PA Head [message #19546 is a reply to message #19543]

Tue, 15 October 2013 09:06

revolvoer
Messages: 12
Registered: October 2013

Junior Member

chicagobill wrote on Mon, 14 October 2013 22:24

I'll check my parts box, I think that I have a few of those preamp boards. Worst case I could check one out and send it to you to replace the dead one. Is that something you could handle doing?

I would darn sure give it a try! I will poke around on it today and see if I can find anything wonky.

S. Davis
Pearl, MS

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Re: Working on a 1978 K300 PA Head [message #19547 is a reply to message #19544]

Tue, 15 October 2013 09:09

revolvoer
Messages: 12
Registered: October 2013

Junior Member

stevem wrote on Tue, 15 October 2013 06:30

Bill/revolver I have many spares of those boards if you do not have one so let me know.

Thanks stevem for jumping in here with that offer. If I find that a board replacement on that Sixth channel is what it takes to fix it, I'll be happy to pay either you or chicagobill for the part. Thanks guys.

S. Davis
Pearl, MS

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