SS Fender Bassman 100 power transistor heat-sink insulation question

[tubeswell]

Working on one of these at the moment (Type PR420)- came in blowing fuses.



Changing the power transistors out and noticed that one of the T03P packages (Q14 - 2SC3263) had no heatsink insulation spacer between it and the heatsink (all the other transistors had insulation spacers though). The thing is that the heat sink is bolted directly to the chassis (from underneath) so the heatsink is at the same potential as ground potential. Are the fender assembly people doing something unusual?, or is this likely to be the source of the problem?

[Enzo]

Well, the collectors of all those output devices are wired directly to V+ or V- on the board through their center legs. So by not insulating them from the heat sink, we create a dead short across those supplies. And it ought not damage the transistor, because no current can flow through a grounded transistor.

That almost certainly is the cause of your problem.

[xtian]

Q14 and Q15 are identical parts. Does Q15 have an insulator?

To my amateur* eye, it looks like no element of Q14 or Q15 (B,C,E) should be touching chassis ground.

*FWIW. Heck, I recently memorized NPN = not pointing in, and have been putting it to good use!

[tubeswell]

Well, the collectors of all those output devices are wired directly to V+ or V- on the board through their center legs. So by not insulating them from the heat sink, we create a dead short across those supplies. And it ought not damage the transistor, because no current can flow through a grounded transistor.

That almost certainly is the cause of your problem.

Yes there was that and a cold solder joint on the -ve connector pad to one of the filter caps under the power board. Since I had the whole thing apart I overhauled it completely and put in new filter caps (amp was made in 1999) and new power transistors and driver transistors.

Amp all going nicely again now

[tubeswell]

Q14 and Q15 are identical parts. Does Q15 have an insulator?

To my amateur* eye, it looks like no element of Q14 or Q15 (B,C,E) should be touching chassis ground.

*FWIW. Heck, I recently memorized NPN = not pointing in, and have been putting it to good use!

Yes the other NPN had an insulating washer. I couldn't believe it! I was thinking 'WTF'?

[J M Fahey]

I bet owner swears: " I didnīt touch it !!!!!! I was playing fine, it stopped working, I brought it to you as is !!!!"

[g1]

I wonder if this was one of those that was replaced under warranty and supposed to be destroyed?
Either that or someone was in there before you.

[tubeswell]

I wonder if this was one of those that was replaced under warranty and supposed to be destroyed?
Either that or someone was in there before you.

I never knew. So there was a run of these amps that were recalled by the factory?

[g1]

No. I meant if that model was on the 'Do not repair' list.
Item's on that list are not repaired under warranty, they are replaced and the original is supposed to be disposed of. A lot of them get pulled out of dumpsters (or whatever) and end up on ebay or elsewhere as 'not working, for parts'.
If that unit left the factory like that, it would have blown the fuse as soon as it was turned on, and the buyer would have returned it for warranty.
If that is not the scenario, then it must have worked before and someone got in there at some point and removed the insulator in a repair attempt.
I suppose a third possibility is that there was heatsink grease acting as insulation until it shorted through.

[J M Fahey]

I suspect user intervention.

Or "his friend who knows how to solder".

I guess the DNR list is sort of a modern invention, with ultra cheap automated factories and cheap oriental labour, but this amp is from 1999 and might have ben made in Mexico; cheaper than USA but not "free" or almost.