Need Help With Marshall 8040 Frying Resistor

[BrianC]

I'm working on a Marshall 8040 Valvestate amp. I have a 10 ohm 1/2 watt resistor, R66 in the schematic, that is overheating and starting to burn if I leave it on. It is on Pin #5 of the TDA1514 IC on the line to the speaker. Rail voltages--+21 & -21 v look normal, as do the +12 & -12 v supplies. Any ideas of what is causing the resistor to over-heat? Here's a link to the schematic.....
http://www.drtube.com/schematics/marshall/8040-iss8.pdf

[Enzo]

Your power amp is making very high frequency output - way above audio. The amp is oscillating.


Does the amp make sound?

[BrianC]

It does amplify the guitar some, but no bottom, and very tinny sound....

[Enzo]

OK, that is consistent with my diagnosis. The am,p is cranking out RF for all it is worth, and your poor audio signal is trying to modulate the RF, with minimal success. WHen this is happening, I usually find myself describing the sound as weak and thin with a hummy aspect. I'd be checking all the ground connections and expectially any gain controls.

[Jazz P Bass]

I had one of these for repair a while back.
R66 got hot.
The soldering of the main board was horrible.
Redid that, replaced R66 AND C54.
Plus it had a intermittant connection on the FX Return jack.

[BrianC]

Thanks Enzo and Jazz...Found a couple bad solder connections on the board and on the input jack, went over the rest, and replaced R66. Good to go now!!!