VintageKustom.com: Repairing Kustom Amps » Troubleshooting a 250 model 1 Bass amp

Re: Troubleshooting a 250 model 1 Bass amp [message #27405 is a reply to message #27393]

Sat, 29 June 2019 00:15

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

I use DeoxIt, but any control cleaner should work for cleaning the connector pins. Most times just removing and replacing the plug will clean up the connectors.

I can't tell if the voltage on the mosfet Q3 is correct or not. The tone controls and IC should not be a factor in lowering the voltage as all of the voltages should be blocked by the tone and coupling caps.

I have never seen one of the mosfet transistors go bad, but that doesn't mean anything. I would look to a bad chip or two. The later version of those chips is an XR4739, which is also no longer made. NTE used to sell a replacement which was a remarked XR4739, but it will cost a lot of money.

What I typically do is rewire the board to take a modern 8 pin dual opamp like a 4558 or a 4560, etc. I use an 8 pin socket to make future replacement easy or to allow for changing chips for best performance.

Whatever way you decide to go, I would replace the one on the 5066 board first and then move to the 5067 once the 5066 is fixed.

Shorted opamps will show up as voltage on input or output pins. The thing to watch out for is that a voltage applied to an input pin will also show up as a voltage on an output pin. I usually test the chip by removing it from the board and reading the voltage on the empty pc points. If mystery voltages disappear, then I pretty much know that the chip is shorted. If the voltages remain, I will look for where they are coming from. If the chip is okay I will replace it using a socket to help reduce any possible damage to the chip from soldering.

By the way, these output voltages are only true for chips that are powered from bipolar power supplies. When a chip is powered from a single sided supply, the inputs will be biased to 1/2 the power supply voltage to fool it into working without the second supply.

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