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Old 4th March 2011, 10:05 PM   #1
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Default Problem with Laney 320 tfx

Picked up a Laney tfx 320. Trying my best to troubleshoot it, was hoping for some more ideas to throw into the pot.

This amp is Tube pre-amp with solid state main. The tube is a ecc83 (similiar to the 12ax7)

PRoblems: about 3 sec after turning on, it begins to hum. The hum stays consistant as long as the amp is on. none of the controls effect this hum.

The amp still functions despite the hum. The main volume/high/mid/low controls work. BUT, the preamp seems to have no effect. none of the gain/drive conrols do anything, and also the LED's that indicate drive/gain stages do not light up.

Heres some thins ive looked at already:
1: All fuses are fine- double checked them all
2: Annode/plate voltage for the tube is at 317(max) which is below the max rating for the tube
3:tube heater V are 13v (high, but shoudl be fine)
4:tube hums the same with or without the tube (pulled the tube to check this)
5:drop resistors on the preamp baord run very hot. They are 2x 470 5w. They are part of the curcuit that takes 50v ->drops/filteres to 17v for preamp board supply.


My next move will be to replace the filter caps on the pre-amp. The current filter caps are only rated 16v, while it seems 17.5v are present. This will require me to totally dissasemble the amp to reach these compents.
Before i dive in i wanted to see if anyone here had some ideas. Any little quick check or extreme idea you can think of will be appreciated.
I do have a schematic of the amp. could post a link to the PDF if anyone would like to see it.

If there is any other information you need to assist your thought, let me know and ill post it for ya. Thanks for reading.
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Old 4th March 2011, 10:06 PM   #2
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Here is the schematic Laney Tube Fusion
http://www.schematics.ca/modules/PDd...df&title=Laney Tube Fusion 300 schematics

about 1/2 way down on the right of page 3 of the shematic you will see r54 and r53 (they will have the FP fire proof label on them). These are the resistors that I said were running hot

Directly to the left of the resistor their are caps c34, c35. These caps are rated 16v. I am reading 17.5 V and am considering replacing them with higher rated caps.

Last edited by Tube Buildin Joe; 4th March 2011 at 10:12 PM.
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