Peavey XR600b with 400b/g output board OT replacement questions

[nosaj]

I have a Peavey xr600b with 400b/g board with one blown output transistor It is a sj6357.
I have 3 2n3442 TO-3 that I am wondering if would be suitable and 2 mj15001. What I've read from Enzo is since they work together I should replace all 3.

On that channel there were3 sj6357 and one mj15003. Going by the specs on the mj15003 the data is very similar except for current rating 20amps versus 10amps, plus the amp uses a 8 amp fuse so I think I'm safe there.
Thanks,
nosaj

http://www.onsemi.com/pub_link/Collateral/2N3442-D.PDF
http://www.onsemi.com/pub_link/Collateral/MJ15003-D.PDF400 BG module.pdf

[mozz]

I think i have one of these amps for parts, are you in the USA?

[nosaj]

I think i have one of these amps for parts, are you in the USA?

Northwest Florida just outside of Pensacola in a place we call LA. (Lower Alabama).

nosaj

[mozz]

Pulled the old amp out. It has 8 of the sj6357. They are date coded 8152, will have to unsolder to test. If you are interested in them and other parts, send a PM.

[nosaj]

Pulled the old amp out. It has 8 of the sj6357. They are date coded 8152, will have to unsolder to test. If you are interested in them and other parts, send a PM.

I went ahead and got 8 mj15003 for 30 bucks new. Thanks for the offer though.
Thanks,
nosaj

[Enzo]

The fact the amp has an 8 amp fuse has nothing to do with the current ratings of the output transistors.

[nosaj]

The fact the amp has an 8 amp fuse has nothing to do with the current ratings of the output transistors.

My reasoning was that if the transistors were at least 8amps or more and the voltage requirements of the transistors were not exceeded that they would be fine. Does that logic not work in this scenario?

Thanks,
nosaj

[J M Fahey]

The fuse you are talking about is NOT in series with any transistor but is the mains fuse.
Its rating is ABSOLUTELY unrelated with what the transistors can pass safely or not.

[Enzo]

When you look at the data sheet for a power transistor, scroll down to the graphs for one titled Safe Operating Area or SOA. A transistor rated 140v 20A is not good for both at once. It can handle up to 20 amps OR 140v. At 140v the current rating is way down, and at 20 amps, the voltage rating is way down. You will usually note the gain of the part is specified at some current, NOT the max current. In fact the PV part numbers contain this info. The 70484140 - the 4140 part means the gain is specified at 4 amps, and they consider it a 140v part. The actual specs for that part are much higher current, but that is only its max.

And that is why the amp has three parallel outputs even though the spec sheet might make you thin just one would be enough.

[nosaj]

When you look at the data sheet for a power transistor, scroll down to the graphs for one titled Safe Operating Area or SOA. A transistor rated 140v 20A is not good for both at once. It can handle up to 20 amps OR 140v. At 140v the current rating is way down, and at 20 amps, the voltage rating is way down. You will usually note the gain of the part is specified at some current, NOT the max current. In fact the PV part numbers contain this info. The 70484140 - the 4140 part means the gain is specified at 4 amps, and they consider it a 140v part. The actual specs for that part are much higher current, but that is only its max.

And that is why the amp has three parallel outputs even though the spec sheet might make you thin just one would be enough.

Thanks for explaining that cause I sure wasn't getting it.

nosaj

[nosaj]

In the interim I replaced the shorted output transistor but am still getting about 30v DC on the output. I have 8 new ones on the way to replace them all. But since the dim bulb is no longer going bright and the amp is not blowing fuses when off the dim bulb how does DC get on the output? Is it because one or more transistors are leaky and dumping DC on the output?

Thanks,
nosaj

[Enzo]

Look at the circuit, not the parts. it will work with one transistor missing on the output triples. But when a transistor blows, it often takes out a driver with it, and often opens resistors associated with it.

[nosaj]

Look at the circuit, not the parts. it will work with one transistor missing on the output triples. But when a transistor blows, it often takes out a driver with it, and often opens resistors associated with it.

Is blows the same as shorted or does that mean open?
Q1 and Q12 are the drivers correct?
The wirewound resistors on the output board all checkout one leg unsoldered.

I was just trying to visually understand how dc would be dumped onto the output. I was thinking maybe somewhere in the location of Q1 and Q7.
Thanks,
Jason

[52 Bill]

Is blows the same as shorted or does that mean open?

Either way. If a transistor shorts from Collector to Emitter, then there is a direct path for dc voltage to get to the speaker buss. If a transistor opens, then it cannot turn on when it is needs to and then the other side of the power amp takes control of the speaker buss.

The entire power amp is trying to balance the speaker buss between the two power supplies. If one side fails it will cause the speaker buss to be pushed or pulled to one side or the other. When an output transistor shorts it can cause too much voltage to hit the driver transistors or if it opens, then the driver will try and take its' place. So always test the drivers when an output goes bad. And remember that the driver transistors have emitter resistors as well.

[Enzo]

Blown is simply slang for defective in some way. Like a flat tire on your car, it could be from a leak, or a bad stem valve or a big nail through the tread or a hole in the sidewall.

If Q3,4,5,6 were to short E-C, then +52v would appear on the output. If Q13,14,15,16 shorted then -52 would show up. But that assumes they are bad parts, they can also be turned on by some problem further back. A shorted Q2 for example can turn on the whole Q3,4,5,6 string. SO could a problem at Q8,9 or even U1.

The drivers are Q3 and Q13. Predrivers are Q2, Q12. Q1,7 are limiters.

The output transistors are valves or gates for current. They connect the output to the two main power rails. So to get DC on the output, the output transistors either must be shorted or be turned on by earlier circuits.