VintageKustom.com: Repairing Kustom Amps » K100 transformer getting hot

Home » VintageKustom.com » Repairing Kustom Amps » K100 transformer getting hot

Show: Today's Messages :: Polls :: Message Navigator

K100 transformer getting hot [message #18842]

Mon, 10 June 2013 16:12

BassGuy97x
Messages: 4
Registered: June 2013

Junior Member

Hey guys, just got here and i figured this would be an appropriate place to ask my question Laughing

I'm working on resurrecting my dad's old k100, starting with the basics, filter caps and such, and after i fired it up today the chassis was getting real hot. i asked the old man if this happened before. he said it always did that, so i didn't think much of it. so after about a half hour of playing my axe thru it it started humming. it was like an annoying buzz kind of hum, not very deep, and the chassis was red-hot. so i shut it off and popped it out of the ol' tuck and roll box. i examined the output transformers thinking they could just be bad, and they are overheating in some way, but they seemed not too hot at all. so i accidentally kinda leaned my hand on the transformer and, WOW!, is all i gotta say. you could fry eggs on that transformer in no time at all. so... my question is, you know, why is that? is it a common thing for kustom's, does it happen at all? I'd appreciate any light you guys can shed on the subject Very Happy

Thanks

Report message to a moderator

Re: K100 transformer getting hot [message #18844 is a reply to message #18842]

Mon, 10 June 2013 16:42

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

Welcome to the place.

The power transformer should not get that hot, so you have some problem there. It could be circuit related or it could be the transformer itself.

I would probably test the transformer with no load to see if it heats up all by itself.

Tell us exactly which model amp you have so we can help.

Report message to a moderator

Re: K100 transformer getting hot [message #18845 is a reply to message #18842]

Mon, 10 June 2013 17:03

BassGuy97x
Messages: 4
Registered: June 2013

Junior Member

its a k100-1, thats all i can tell from the chassis markings

Report message to a moderator

Re: K100 transformer getting hot [message #18846 is a reply to message #18842]

Mon, 10 June 2013 17:41

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

So it is the basic two channel head with no reverb or vibrato?

If you have the basic soldering skills, you can unsolder the two purple transformer wires that go to the rectifier bridge. This will remove the amp circuitry from the transformer and then you can turn the amp on and see if the transformer still gets hot.

If you don't want to unsolder the two wires, you can try this instead. Remove the red and green wires that connect the rectifier bridge to the filter caps.

Unscrew the terminal that has the red wires connected to the top of the positive supply filter cap. Find the wire that goes directly to the bridge rectifier and set it aside. Reconnect all of the other wires back to the cap.

Repeat this with the green wires on the negative supply filter cap. If you do this, be sure to tape off the wire connectors on the two loose wires before powering up the amp.

While this test does not remove all of the amp circuitry from the transformer, it removes enough to be a good starting point.

Remember that working on any amp that is plugged in can hurt or kill you. If you don't have the skill set to work on the amp, take it to someone that does.

Let us know what you find out.

One other thing that may help you, is to take digital photos of the chassis wiring before you start to remove anything. This way you will have a reference when you put things back together.

[Updated on: Mon, 10 June 2013 17:43]

Report message to a moderator

Re: K100 transformer getting hot [message #18847 is a reply to message #18842]

Tue, 11 June 2013 07:17

stevem
Messages: 4739
Registered: June 2004
Location: NY

Senior Member

If I may add my two cents in on this, a thermal gun is a tool that I wish I would have had at my disposal way back when these amps where made to use on problems like this.
If you have one, or can get a hold of one from a auto machanic freind it can be of help with this.
In lew of that and with the problem you have I would say that you have this going on with the amp.
1) the two can type main filters have gone bad and are somewhat shorted and pulling big time current thru themsevels and in turn thru the power transformer, and both are getting hot. You can check this by turning the amp on and letting it idle, when you feel the power transformer starting to get hot the filter should be warm, if not hot also indicating its filter replacement time.

2)This is the same thing, but once you replace the filters the power transformer is still getting warm at idle and hot when playing thru the amp after 15 minutes to a half hour.
If the PT is toast it could be due to bad main filters, or just a defect PT that bit the dust finally after all these years!

Report message to a moderator

Re: K100 transformer getting hot [message #18856 is a reply to message #18842]

Wed, 12 June 2013 07:04

stevem
Messages: 4739
Registered: June 2004
Location: NY

Senior Member

Here are some numbers to help you determin if your transfomer is sound, I measured these on my k100-1 with 118 volts coming in from the wall outlet.
Be sure to be carfull working around the fuse and on off switch area of the amp whe it is plugged in, not only when its on!
With a ohm meter set for DC volts I checked the voltages at the two main filters. with the meter hooked across the red and black wire on one filter I read +39.4 volts, with the meter across the other filters green and black wires I read -38.7 volts.
Next I unhooked the two purple wires bill posted about from the round rectifier bridge, across these wires I read 59.8 volts. this is A/C voltage so you will need to reset your meter to read AC.
While doing these checks note your voltages down and as things get hot recheck them, when the DC voltage you read starts to drop off at the filters, turn the amp off and unhook the two purple wires again, if the voltage is low its your power transformer thats going bad, if the volatge goes back up to your noted reading than its the main filters drawing current, or the amp circuit somewhere.
Note that you can do all of these checks with no speaker hooked up to the amp.
My test amp had a normal A/C idle hum voltage read at the speaker jack of 2.9 MV.
Once again, let us know how you make out.

Report message to a moderator

Re: K100 transformer getting hot [message #18860 is a reply to message #18842]

Wed, 12 June 2013 10:45

BassGuy97x
Messages: 4
Registered: June 2013

Junior Member

alrighty then...so after much diagnostics, it appears that the circuit is drawing current elsewhere. i disconnected the transformer front he circuit, and it did not get overly hot... as i began to re-assemble the amp i was finding some problems here and there, screws loose, bad solder joints, rusty connections stuff like that. once i cleaned is all up and fired it back up again, there were 2 places that were overly hot. the resistors on the front board for the lamp, i think that's because when my old man used it in the 60's and 70's he just used whatever bulb that fit and its not the one that it calls for, as well as the output transistors. i haven't been able to diagnose why the transistors are heating up, whether they are bad, or something in the circuit is taxing them or what... thanks for your help guys

Report message to a moderator

Re: K100 transformer getting hot [message #18864 is a reply to message #18860]

Wed, 12 June 2013 15:27

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

The correct bulb is a #1829, 28 volts. If you have a 6 volt, #47 in there the dropping resistor will get very hot.

The output transistors normally run cool in these amps at idle. If yours are hot, you may have a bad transistor, or the bias is running too high or the amp is oscillating at a high enough frequency that you can't hear it.

What speaker load are you using? Did you find the earlier mentioned bias diode in the clip on the heatsink?

The output transistors are a house numbered version of the venerable RCA 2N3055. Are your transistors original or have they been replaced?

I'd check the idle bias before replacing any parts. Check the schematic and compare the dc voltages on the driver transistor bases to what you measure there.

Report message to a moderator

Re: K100 transformer getting hot [message #18866 is a reply to message #18864]

Wed, 12 June 2013 16:56

BassGuy97x
Messages: 4
Registered: June 2013

Junior Member

chicagobill wrote on Wed, 12 June 2013 15:27

The correct bulb is a #1829, 28 volts. If you have a 6 volt, #47 in there the dropping resistor will get very hot.

The output transistors normally run cool in these amps at idle. If yours are hot, you may have a bad transistor, or the bias is running too high or the amp is oscillating at a high enough frequency that you can't hear it.

What speaker load are you using? Did you find the earlier mentioned bias diode in the clip on the heatsink?

The output transistors are a house numbered version of the venerable RCA 2N3055. Are your transistors original or have they been replaced?

I'd check the idle bias before replacing any parts. Check the schematic and compare the dc voltages on the driver transistor bases to what you measure there.

i used a 4, 8 and 16 ohm load cabs and the same temperature resulted. the transistors are originals, and i have the new transistors on order. ill take a look a the bias voltages and let you know what i find

Report message to a moderator

Re: K100 transformer getting hot [message #18867 is a reply to message #18842]

Wed, 12 June 2013 18:12

stevem
Messages: 4739
Registered: June 2004
Location: NY

Senior Member

Hook up a meter set for a/c volts across the amps output, if you see more than a 10 mv reading, than its like bill posted, the amps outputs are cooking due to the amp oscilating full bore at a frequency that you can not hear and the speakers can not reproduce.
This can take place due to bad tantalum caps in one or more of the amps circuits.

[Updated on: Wed, 12 June 2013 18:13]

Report message to a moderator

Previous Topic:	K100-2 reverb and treble boost mods
Next Topic:	Replacing rocker switch

Goto Forum:

-=] Back to Top [=-

[ Syndicate this forum (XML) ] [

] [

]

Current Time: Sat May 25 19:59:48 EDT 2024

.:: Contact :: Home ::.