Repair Help- TFM35

Welcome to the forum david. There should be some others chiming in who have a lot more knowledge on repairs. Did you flip the mono switch while the unit was powered on??

Hi all. I am trying to fix a TFM-35 amp that has a strange problem.

I bought it used and "working".

I played each channel for a little while and concluded that it worked fine. Then, for some reason, when it was idling, I flipped the BRIDGE switch on the back and "BANG". Cap C506 exploded, along with 2 of the 8 output transistors on the right channel. I completed the repair to the output transistors and replaced the cap (6800uF, 35V).

I am using a variac to power up slowly, so the cap is no longer exoloding, but it is still getting too much voltage.

The current story is that there is a white wire that ties the board with the bridge switch and input jacks to the chassis gnd. When that wire is lifted, the voltage across c506 is less than 35V. When it is tied to chassis gnd like it is supposed to be, the voltage on cap C506 goes higher than 35V and BANG.

Why is the voltage on Cap C506 too high? Does anyone have an schematic in PDF?

Thanks,
David Countryman
davidc@bellsouth.net

I'm not home right now, so I can't get you that pdf, or even refer to it, so I can't getinto details either.

C506 is popping (must have been impressive eh?) because the triac control is running full tilt. Why it's doing that is what you have to discover. Having switched the mono switch during power on is Not good. Odds are high that at least something in the input board went bad.

If you replaced the two outputs you found bad, and the 35VDC rail is still getting too high. First try removing the three cables to the input board. If the rails behave with the input board removed then it alone is your culprit.

If 35VDC is still trying to climb too high, then it's either another shorted rail or the triac control circuit itself. Look at the 60VDC and 100VDC rails as you raise the variac. If they are way low, then one of them is shorted somehow, and this is what's opening up triac conduction to full bore, trying to lift the rail.

If the other rails are also going too high, then the triac timing itself is what's fubar