Remove the tube shield and replace the tubes.
Take care not to use certain tubes in the V2 position. JadedFaith says at mtsforum.grailtone.com/viewtopic.php?t=19525:
- I noticed several Tung Sol's being used in V2 (the tube on the right). That is the cathode follower (CF) position. Anything manufactured by New Sensor (ie: Tungsol and Mullard reissue, EH, Sovtek etc) have a lower heater to cathode rating. They will either completely die, go half gain/volume or fluctuate in volume if used here. JJs, Chinese and NOS preamp tubes will be fine for the CF position.
Want to know what the cathode follower is? Have a watch of this video from about 9:00 on - www.youtube.com/watch?v=BatwDYFJ9ug. Basically, the cathode follower feeds the module's passive tone stack with a low impedance signal (high current, low voltage) which minimises the overall gain loss through the tone stack.
I have had three modules come with bad pots. The bad pot issues have manifested themselves as follows:
- high pitched squeal;
- inconsistent working of the pot;
- noisy pot;
- inconsistent volume or loss of gain.
I've been able to identify the at-fault pot using the following approach:
- disassemble module so you are left with just the board;
- re-insert module into chassis and power up amp - take care not to touch anything inside the amp;
- tap, jiggle, rotate each pot and you will soon find the one that is causing problems.
- power down the amp and remove the module. Take care removing it as you need to pull on the unsupported pots to get the module out. Do it gently so as not to damage a pot or the PC board.
Optocouplers / signal transistors
Module lost its output / no distortion? Jaded Faith says:
- There is at least one opto and possibly the signal transistors dead - mtsforum.grailtone.com/viewtopic.php?t=20632.
The module has two signal transistors in the top left hand corner. They are cheap and relatively easy to replace. If you experience these issues, replacing them will be a good place to start. One is a 2N4404 and the other is a 2N4401.
If that doesn't fix the problem then you need to replace one or more of the optocouplers. They look like IC's in DIP package with 6 pins. You can test these out of cicuit, but basically, once you go through the trouble to remove one, you may as well replace it. They are available off Amazon - H11F1