my friend vintage silverface twin reverb get this hum

[kwm488]

hi

my friend vintage silverface twin reverb get this hum. i never heard this hum before. do anyone know it, and give me a trip about what problem?

http://youtu.be/Xdxh6qC9fnw

thanks

[tubeswell]

That's what we call 'motor-boating'.


Its caused by a failed filter cap.


If its a vintage SFTR, its probably overdue to have the filter caps replaced anyhow.

[kwm488]

Hi
I find out the problem. It is the capacitor which mount to the 10m resistor of LDR.
I remove it. No motor noise now. But also no tremolo.

Blackface TW no this cap, just silverface. So i believe tremolo can work without that caps.
I tested that area resistor, replaced tube, still no tremolo.

I guess the LDR. How can i measure it?

[sluckey]

still no tremolo

You must have the footswitch plugged in and vib turned on for the trem circuit to work. If you don't have the footswitch you can put a temporary ground on the VIB footswitch jack to enable the circuit.

The LDR is actually two simple components inside that black heatshrink tubing. There is a NE2 neon lamp in one end. With the house lights off you should see it flashing at the tremolo speed if you peek in the end of the shrink tubing. If not flashing then either the circuit is not working, footswitch is not plugged in and turned on, or the lamp is bad.

In the other end of the shrink tubing is the actual LDR. When no light shines on it, it will measure several MEGΩ. When lots of light shines on it, it will measure much lower, maybe a few KILOΩ.

[kwm488]

Hi
I have footswitch. THe tremolo is work before i remove that capacitor.

Btw, the ldr is flashing when i turn tremolo on. BUt no tremolo function.

[sluckey]

Replace it. Hoffman sells them.

[kwm488]

hi
i replace it. everything is fine now.
thank you for your help