VintageKustom.com: Repairing Kustom Amps

Home » VintageKustom.com » Repairing Kustom Amps » Kustom 150 Problem (Low Output and Distortion)

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

Kustom 150 Problem [message #17344]

Mon, 06 August 2012 16:34

Electro-Nick
Messages: 4
Registered: August 2012
Location: Portland

Junior Member

Hi, well here's another story of me thinking I can fix anything and getting myself in a time eating troubleshooting situation.

The anything at hand is a kustom 150 that a friends father owned and after the father passed away he really wanted to hear it play again..

I tested the filter caps and they are putting out 44vdc and -44vdc. Which I believe is correct. I figured it would be the main power transistors and replaced both of them with the NTE130 equivilents.. no dice.

Problem didn't change. I went through the power board and tested the transistors and resistors for correct values and function.

The problem is the same for both inputs on both channels. The tremolo is functioning but the reverb is non-exsistant.

I tested the reverb tank and found the output side transformer to not have any resistance so I figure its a goner. I had a smaller reverb tank laying around so I hooked it up and still no audiable reverb..

I am testing with a guitar, and sound is only present with the volume pot being at 50% or higher..

Also I notice that when I am playing through the amp and flip the on/off switch(off) I hear a bit of undistorted sound right before it completely turns off..

Any ideas would be welcome as I would really love to fix this thing but am losing patience.

Thanks in advance,
N

[Updated on: Mon, 06 August 2012 16:38]

Report message to a moderator

Re: Kustom 150 Problem [message #17345 is a reply to message #17344]

Mon, 06 August 2012 17:44

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

Hello and welcome.
If the reverb pan you popped in does not test out resistance wise at between 175 and 195 ohm on both the input and output jack then it may not be compatible for the amps circuit.

I would first test/check out all the plug in Molex connectors that go from board to board as these get intermitant with age , alot of times due to the aluminum getiing tarnished.
Making the female reciver pins tighter and then loading the male ends in a couple of times can help.
To test out the function of the driver board and output stage you can feed in a preamp signal from another amp, stereo or a headphone out of a walkman or whatever, but just make note that driving a good output stage with a headphone output will make for a good amount of volume.
The record output jack can also be used to test the output stage by pumping that with signal.

Report message to a moderator

Re: Kustom 150 Problem [message #17347 is a reply to message #17344]

Mon, 06 August 2012 21:14

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

Welcome to the place! What version of 150 do you have a plexi front combo or a metal slant front?

In any case, do you have the schematic for the power amp? It sounds like there may be a problem with the bias. If you do have the schematic check the voltages and compare to those on the schematic. Also check the electrolytic tantalum caps and as Steve suggested the Molex plugs if the amp has them.

Report message to a moderator

Re: Kustom 150 Problem [message #17348 is a reply to message #17344]

Tue, 07 August 2012 01:41

Electro-Nick
Messages: 4
Registered: August 2012
Location: Portland

Junior Member

Its a metal slant type. Thanks for the advice.. I took the plugs out de-oxit'd them and tightened all the female ends. I'll check conductivity of the plug wires as well..

The schematic is on the lid of the head and I have been slowly and carefully checking voltages.

No luck yet...

But thank you for the quick replies with advice and welcomes..

Report message to a moderator

Re: Kustom 150 Problem [message #17352 is a reply to message #17344]

Thu, 09 August 2012 12:06

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

Here is some other things to try.

First off I do not think you have anything major wrong with the driver/output stage. otherwise you would be blowing fuses and or hearing a loud hum from the speaker due to having 40 VDC from the power supply hitting the speaker.

The K150 metal face amps do not have a module output jack like the K250s so you can not drive them with a test tone from any jack on the rear of the amp, but you dive them and hence test out the driver/output stage without much trouble from jumping test tone inside the amp to drive the output stage, and feed preamp signal out from the amp and into another amp to test out the other three boards.

I would frist start by pulling the 3 pin molex audio input connector on the right side of the driver board.
The Black wire is ground, and the orange is signal in hot from the 2068 effects board.
To pump a test tone into the board you can use chassis ground or the Black wire and the signal hot can be tack soldered onto the 3900 ohm resistor to the left of the connector if you can not jimmy rig a way to hook to board pin end where the orange wire went to.

I have found that it only takes .968 mv of audio drive signal from the effects board to drive the amp to its full RMS output power of 81watts, this will be some 24.48 VAC into a 8 ohm load.
The power supply at this point will have dropped down from its idle voltage across the green and red wires at the two main filters from 85 VDC to 76 VDC, this is with 117.5 volts coming in from the wall socket.

If you have clean output to the speaker than its time to back up to the input of the effects board to check if that is getting clean output from the 5066 preamp board ( the channel without selectone), but first we need to check the power supply rail the feeds all the circuit boards in the amp.

With the amp at idle the terminal strip that has the 3 green and 3 red wires on it should check out at 25 VDC, if it does lets move on.

The input (blue wire) to the 5068 effects board should check clean when looked at on a O-scope, if you have no scope then rig up a way to pull signal off of the blue wire and chassis ground and feed it into another amp to check if its clean.
If its not clean then we need to check the output on the blue wire of the 5067 preamp board and see if thats clean.
If that checks out clean than your problem is very likly to be the 14 pin mixer IC on the 5066 board, as this outputs both the auido signal from the 5067 board and its self!
If you have a O-scope you should see a clean 11.40 volts of audio signal going into the output stage driver board if all is well with the 5066, 5067 and the 5068 board.
I was using a 1k test tone at -10db, or around .120 mv to drive the channels, if you use much more you may clip the input of eitheir channel and foul up your testing.

I hope some of this helps out, let use know your results!

[Updated on: Thu, 09 August 2012 12:08]

Report message to a moderator

Re: Kustom 150 Problem [message #17353 is a reply to message #17352]

Thu, 09 August 2012 16:06

Electro-Nick
Messages: 4
Registered: August 2012
Location: Portland

Junior Member

Well I pulled the orange and black wired molex plug, and hooked up a signal generator to it.

Unfortunately it fried my speaker instantly upon powering on. It was just a little 8" 8ohm tester speaker so no big deal..

I am curious if this is a situation where I mis-read you or perhaps a cause for alarm regarding the power amp section of the amp..

thanks again for the help,
N

Report message to a moderator

Re: Kustom 150 Problem [message #17354 is a reply to message #17353]

Thu, 09 August 2012 16:26

Electro-Nick
Messages: 4
Registered: August 2012
Location: Portland

Junior Member

This just in! Now the amp is blowing the 3a fuse upon startup. I am hoping this a case of things getting worse before they better..

Still attemping to stay positive, perhaps I shall take a small break on this thing....

Report message to a moderator

Re: Kustom 150 Problem [message #17355 is a reply to message #17344]

Thu, 09 August 2012 17:13

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

Sounds like there are shorted transistors now. Check the output and drivers for shorts. Do not connect a speaker to the amp until you have the power amp straightened out.

Do you have a light bulb limiter?

I would not recommend powering up the amp with the three pin Molex plug pulled. The board is only grounded through the black wire.

Report message to a moderator

Re: Kustom 150 Problem [message #17364 is a reply to message #17344]

Thu, 16 August 2012 07:17

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

I am sorry for your new problem, I ment well!
I was replying to your post from work while doing 7 other things and I had wanted to post that if you pull the connector off you can load some 22ga stranded wire with 4 to 6 strands taken out to the female side and them slip the connector back on and hook your drive test signal to the two short 22ga wires and prove out the output stage.

Report message to a moderator

Previous Topic:	Kustom 1200 - Sudden Death Syndrome
Next Topic:	kustom -x-stereo power amp

Goto Forum:

-=] Back to Top [=-

[ Syndicate this forum (XML) ] [

] [

]

Current Time: Sun Apr #d 21:55:22 EDT 2024

.:: Contact :: Home ::.