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I believe my K200 A5 PA may be overheating [message #18831]

Sun, 09 June 2013 15:10

janalex
Messages: 11
Registered: May 2013

Junior Member

Hi,

Just got a Kustom A5 K200 PA it sounded great for the first 2 hours of rehearsal through twin Kustom PA towers however after a little over 2 hours of moderate volume use (5-6), it started to sound raspy and quiet through all channels. I shut it down. I waited an hour and turned it back on and it worked fine for a quick test. Today I will try to keep it on idle for a few hours and see what happens. Is this a common issue with Kustoms? Any ideas what may need replacing? Cap job?

Thanks

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Re: I believe my K200 A5 PA may be overheating [message #18832 is a reply to message #18831]

Sun, 09 June 2013 18:18

pleat
Messages: 1452
Registered: June 2004
Location: Belding, Mi

Senior Member

You want to make sure that you are using a speaker system that presents a 4 ohm total load to the K200. If the impedance is less than 4 ohms the amp will heat up and shut down.

Another thing that could cause the problem is a bias diode inside the amp. It is a silver diode that clips into a heat sink clip on the chassis. If it has popped out of the clip could be another cause for the amp to heat up. I bought a kustom amp from a guy who was having the same problem, worked great for a while then would fade and was hot. I got it home and saw that the diode was out of it's clip. Snapped it back in place and the amp has worked great. Be careful with the diode, the leads to the board are brittle and can break very easy.
pleat

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Re: I believe my K200 A5 PA may be overheating [message #18833 is a reply to message #18832]

Sun, 09 June 2013 19:37

janalex
Messages: 11
Registered: May 2013

Junior Member

Thanks for the reply. I will check the clip. Regarding the speakers, I'm using the Kustom towers from the same period that were designed for this amp.

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Re: I believe my K200 A5 PA may be overheating [message #18834 is a reply to message #18833]

Sun, 09 June 2013 19:54

pleat
Messages: 1452
Registered: June 2004
Location: Belding, Mi

Senior Member

Well that's a good sign using the original speaker cabinets. The one thing your not sure of, if someone before you changed the wiring or replaced some defective speakers with a different impedance. After 45 + years, it's hard to tell what might have been done to speaker cabinets. I always remove the backs just to do a visual inspection and make sure the wiring is correct. The cabs use 8 ohm speakers wired in series/parallel so each column is 8 ohms. You will want to check that each speaker is working correctly.
pleat

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Re: I believe my K200 A5 PA may be overheating [message #18835 is a reply to message #18831]

Mon, 10 June 2013 07:11

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

Vintage Kustom amps like yours where built with a thermal circuit breaker mounted next to the bias diode that Pleat spoke of.This breaker ( hooked up to the two black and white striped wires) is made to go open at 185 degress F and save the amp from truly burning up its output section, but if your diode is out of its clip, or one of its two leads have broken off, problems like you are having will take place.
Make sure this round black breaker that is held down with two machine screws is still in your amp, as I have worked on many that the breaker has gone bad, and someone just jumped it out!Let us know how you make out.

[Updated on: Mon, 10 June 2013 07:18]

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Re: I believe my K200 A5 PA may be overheating [message #18836 is a reply to message #18835]

Mon, 10 June 2013 09:37

janalex
Messages: 11
Registered: May 2013

Junior Member

Thanks again. I will look at the circuit to make sure the diode is in place and breaker operational. Can someone email me the schematic for the K200 A5?

janeubig@gmail.com[/email]

Also, as I understand it, if correctly working the protection circuit will cause a sudden shutdown of the amp at the given temperature with absolutely no sound coming through the speakers when engaged??

[Updated on: Mon, 10 June 2013 09:39]

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Re: I believe my K200 A5 PA may be overheating [message #18838 is a reply to message #18836]

Mon, 10 June 2013 11:31

Kustom_Bart
Messages: 601
Registered: October 2010
Location: Greenville, MichiGUN

Senior Member

yes, it should just shut it down like a regular breaker in a fuse panel in your home, until you re-set it. If you go to the main page http://www.vintagekustom.com/ you should be able to look up the Schem for your amp under the Technical button on the left hand side of the page.

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Re: I believe my K200 A5 PA may be overheating [message #18840 is a reply to message #18831]

Mon, 10 June 2013 12:12

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

I will add a few things here.

The bias diode that Pleat mentioned is designed to lower the bias to the output stage as it heats up. If it isn't thermally connected to the output transistors (it comes loose from the clip) the amp can start to overheat. When this happens, the power amp can start to distort and act as you have described.

If the output transistors get hot enough (185 degrees) the thermoswitch will shut off power to the entire amp until it cools off. When the thermoswitch opens, nothing will light up and there will be no sound from the amp. The thermoswitch will reset itself once the amp has cooled down.

When checking the thermoswitch, be sure to unplug the amp, as it is connected directly to the ac line.

Be careful when handling the bias diode, as the leads after all these years are somewhat fragile. If the one of the leads should break, there will be no bias voltage and the power amp will blow up.

[Updated on: Mon, 10 June 2013 12:20]

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Re: I believe my K200 A5 PA may be overheating [message #18904 is a reply to message #18840]

Sun, 23 June 2013 19:06

janalex
Messages: 11
Registered: May 2013

Junior Member

Hi guys

Just an update. All connections in the head appeared intact. Speaker column impedances measured 8 ohms. We ran a test tone from a synth through the amp for 5 hours at moderate volume and no issues. Then at band rehearsal yesterday with 3 mics running on about volume 5 the amp started to cut out after about 20 minutes. It starts with intermittent cutting out and then complete silence or a low output distorted output. We shut it down for 5 minutes turned it back on and it would work for a few minutes until the same occurred again.

Is it possible that the thermostat may have corroded over the years and may be malfunctioning? What other components may be suspect?

Thanks

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Re: I believe my K200 A5 PA may be overheating [message #18905 is a reply to message #18831]

Mon, 24 June 2013 07:17

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

Unfortunaly the fact that the amp was still outputting sound means that the thermal breaker was closed and that the power output stage atleast was not overheating, how hot was the chassis of the amp when it failed, over 150 degrees?.
That being said it would seem that the amp has a thermal problem somewhere.
One easy frist thing to check is that the screws on the rear of the amp that bolt down the pre-driver/driver board are all tight, as one or two of those screw (6/32") make the boards ground connection.
You may also first want to dismount that board and check for bad solder connections.
One thing is for sure, and that fact is since all the channels are acting up it will not be a problem with eitheir of the 4 channel preamp boards, your problem is on the reverb board, or the driver board, or maybe something weird with the output stage.
One thing you cab do before the next rehersal would be to get a can of cool down spray from radio shack, then take the amp out of its case and cover it and do the reshersal, when the amp acts up, take the freeze spray and spray it across these two boards slowly, when the amp clears up your on the right board.
It would also pay to have a hair dryer handy set on low to get the problem to reapear.
In this way you will atleast pin down what section of the amp the problem lies in. You can than report back to us and maybe we can help you rectifie the issue.
Good luck to you.

[Updated on: Mon, 24 June 2013 07:20]

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Re: I believe my K200 A5 PA may be overheating [message #19068 is a reply to message #18905]

Fri, 09 August 2013 18:50

janalex
Messages: 11
Registered: May 2013

Junior Member

Just wanted to leave an update that we found a cold solder joint on the driver board that was probably intermittently opening up with increasing heat from an hour or two of use and vibrations from the soundwaves and nearby drum set. Last 3 rehearsals there has been no issues.

Thanks again!

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Re: I believe my K200 A5 PA may be overheating [message #19095 is a reply to message #18831]

Mon, 12 August 2013 06:51

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

Yup, thats what it was starting to sound like the fix was going to be.
glad you stuck with it to get it resolved, enjoy!

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