Peavey TNT Fuse blowing

[lowell]

This amp came in w/ fuse blowing. Found fried output transistors and replaced all 4, both drivers, and Q2 and Q6 as well (not sure what those are). I plugged it in w/ lightbulb limiter and the bulb is very bright. I tested all the components in the 25v and 16v supplies and they seem good. I also tested CR15-20, R27/28, C12/13 and they're good. .33 ballast resistors test good as well. Both 45v supplies have a decent resistance reading to ground and are not shorted.

I'm not familiar w/ this PA arrangement w/ what looks like an output transformer, though I'd guess that it's more of a center tapped inductor since there is no secondary? I think the signal leaves through the center tap and goes to the L1 inductor, then to the speaker. C10 provides some feedback to U1A?

Anyway I am finding something suspect. The resistance from CR5 cathode to ground is 15k while CR4 anode to ground is 9meg. It seems to me that those should measure equal resistances. Would appreciate some guidance on this. PA schematic is on pg 5.

[Enzo]

Q2,6 are limiters.

Output transformer? There is a power transformer, the center tap of which is used for the speaker feed. The "output" of the amp stage is grounded.

Pleaase do not use a speaker load until the amp has settled down.

If the outputs are all OK now, then look to the bias string for an open. If CR8,10,11,12 are open, then both sides of the output conduct at the same time, blowing fuses.

You replaced a lot of transistors, but which ones were actually bad?

Important to check R14,15 for opens, especially if Q3 or Q7 was bad.

[lowell]

Ha!! CR11 was not soldered, no evidence of ever being soldered so whomever was in here before forgot to do it! Anyway soldered it in and all is good! Thanks Enzo.

Question, is this diode string another type of bias arrangement INSTEAD of a Vbe multiplier type of biasing?

[Enzo]

To pevent crossover distortion, the V+ side and V- side transistor bases must be up just at the point of conduction. The net result of this is that the bases of the two sides need to be a couple volts apart. Run a little current through a string of diodes, a transistor, or even some resistors, and the voltage drops across them can be used to set the bases of the drivers.

So yes, some amps use a transistor, and often include a pot to set its conduction, other amps settle for some diodes. it all works.

[lowell]

right on... oh and if you have a minute can you give me a quick tutorial on those 2 (Q2,6) limiters?

[Enzo]

First, for testing, you can eliminate them simply by unsoldering one end of CR9, CR14 and lifting them.

Limiters. Let's look at the positive side, Q2. The negative side works the same.

Q4 is an output xstr, and all its current flows through R16. Ohm's Law tells us the more current through R16, the more voltage dropped across it. The voltage at the top of R16 is connected to the base of Q2 through R13. If the voltage across R16 gets high enough, it is enough to turn on Q2. Q2 in turn then turns on and grounds out the base of driver Q3. Well that then stops Q3 from driving.

So when enough current flows through the output, the voltage developed makes Q2 shut down the driver. The idea is that it prevents the amp from overdriving itself. All well and good, but it won't help when parts fail.

[lowell]

Ok makes sense, thanks for taking the time to write that out for me. It seems kind of like an auto-bias feature in a way. Cool! So the more "on" a transitor is the more it conducts from collector to emitter. What's the use of CR9 on the limiter's collector?

[Enzo]

A little breathing room and some isolation.