Home » VintageKustom.com » Repairing Kustom Amps » Lead III cutting off (My lead III will run for about 3 minutes then turn off.)
Lead III cutting off [message #26097] Mon, 08 May 2017 23:13 Go to next message
veldarc is currently offline  veldarc
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Registered: May 2017
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My lead III will run for about 3 minutes then turn off. I'm not sure what's wrong, but wanted to try to repair it myself. Does anyone have any suggestions or a schematic.
Re: Lead III cutting off [message #26098 is a reply to message #26097] Tue, 09 May 2017 06:00 Go to previous messageGo to next message
stevem is currently offline  stevem
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Registered: June 2004
Location: NY
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These amps like many other model Kustoms are built with thermal circuit breakers that on the schematics are called thermostats.
These are designed to open up the AC wall voltage going to your amp if the running temperature of the output stage exceeds 182 degrees F.
This is the likely cause of your amp shutting down after only 3 minutes of bring played, or even just being on in some caes.
Can you read a schematic, post back a reply and I will explain to you how to proceed.
This is a easy fix, so don't sweat it!
Re: Lead III cutting off [message #26099 is a reply to message #26098] Tue, 09 May 2017 22:12 Go to previous messageGo to next message
veldarc is currently offline  veldarc
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Registered: May 2017
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Yes I can follow a schematic, but it's been a while since I did. Let's give it a try. Thank you for your help. John
Re: Lead III cutting off [message #26100 is a reply to message #26097] Wed, 10 May 2017 06:00 Go to previous messageGo to next message
stevem is currently offline  stevem
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Registered: June 2004
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The thermal breaker is a bolted down black button with two white with a black stripe wires on it.
It's mount in the middle of the heat sink that the output Transistors are on.
If you turn the amp on and blow on it with a hair dryer I bet the amp will shut right down!
If that's the case you can just jump it out for now and use the amp , just first check that the amp is indeed not running hot which would indicate a bias problem.

[Updated on: Wed, 10 May 2017 06:02]

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Re: Lead III cutting off [message #26101 is a reply to message #26100] Wed, 10 May 2017 21:28 Go to previous messageGo to next message
veldarc is currently offline  veldarc
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Registered: May 2017
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didn't have a hair dryer so just bypassed it and amp ran fine, when i went to turn it off it was too hot to touch though. Scared it might be a bias problem. Dang that seemed to work.
Re: Lead III cutting off [message #26102 is a reply to message #26101] Wed, 10 May 2017 21:51 Go to previous messageGo to next message
pleat is currently offline  pleat
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Registered: June 2004
Location: Belding, Mi
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I had a K150-7 combo amp and it did the same thing, run for a few minutes and shut down. Turned out in my amp the diode had slipped out of the clip on the heat sink. Snapped it back in place and never had any problems after that.
pleat
Re: Lead III cutting off [message #26103 is a reply to message #26097] Thu, 11 May 2017 08:14 Go to previous messageGo to next message
stevem is currently offline  stevem
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Registered: June 2004
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Good point Pleat!
Yes, if that two lead diode in that clip next to the thermal breaker in out of its clip , or not held tight in its clip then the amp can run hot and shut down.

Be careful when moving that diode, as it's leads are fragile, and also make sure that it's leads are not touching the clip.
If your thermal breaker is ok and the bias diode is held in the clip well then the amp should be able to sit on and just idle for hours with the rear of the chassis only feeling warm to the touch, not hot!
Re: Lead III cutting off [message #26109 is a reply to message #26097] Fri, 12 May 2017 08:19 Go to previous messageGo to next message
stevem is currently offline  stevem
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Registered: June 2004
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A replacement T stat can be had from Comstat, model TK-24A-85 for 17 bucks.
This one has solder terminals, no slip on as on the original.
Re: Lead III cutting off [message #26111 is a reply to message #26103] Fri, 12 May 2017 15:38 Go to previous messageGo to next message
veldarc is currently offline  veldarc
Messages: 4
Registered: May 2017
Junior Member
I adjusted the diode and the amp is running cool. Thank you all for your help. This is the first forum I have ever joined and needed help. It was more than I could have hoped for. A working amp with just 20 minutes of my time. Wow! Thank you again, John
Re: Lead III cutting off [message #26115 is a reply to message #26097] Sun, 14 May 2017 05:50 Go to previous message
stevem is currently offline  stevem
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Registered: June 2004
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Your welcome, enjoy the amp!
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