SWR Workingman 300

[Techknowman]

I just got a SWR WM300 thats blowing fuses.
All the 2SC3264 transisters in the outputs are shorted.
Some base to collector. some emiter to collector ect.
The funny thing is nothing else is damaged. No drivers, diodes resisters or anything that I can find.
I know its about the circuit not the component, but I'm wondering how you could blow 4 transiters like this without other collateral damage.
If you look at the schematic below, you will see what I mean. I couldn't find a WM300 schematic but the basic black looks like its correct from what I've traced out.

http://techknowman.com/Electronics/P...k_poweramp.pdf

I can't believe all the transiters would short out like that, seams like one would take the hit.

Now I have to find a good source for those 2sc3264's prices are $4 - $18 each

[Enzo]

When you have a row of transistors in parallel, and one shorts from collector to emitter, think about what that does to the other parallel parts. it removes power from them. What used to be say 50v on the collector and zero on the emitter is now shorted together so there is essentially zero on the collector, until the fuse blows. That won;t usually hurt the other parts in and of itself.

However, if one of the row shorts collector to base, that usually burns out the entire row of them.

What CAN happen is that parts open, leaving smaller parts to try to carry the failure current - and when they can't drivers burn up. And other similar scenarios. But it is by no means necessary that lost of parts burn up when outputs fail. I have repaired a zillion PV amplifiers with nothing more wrong than a shorted output or two.


B&D has them for $4.68.

[Techknowman]

Thanks.
I haven't ordered from B&D before. Have to check them out.
And your right checking closer some of the transisters are shorted collector to base so it does make sense.

[Enzo]

I don;t use them often, but B&D has SO FAR not sent me any fakes. I generally use them when looking for Sankens I can;t find elsewhere.

B&D Enterprises - Electronic Components Distributor

[Techknowman]

Thanks I have added them to my favorites when looking for Japanese transistors.
I did a quick search and they carried a lot of the more common ones at competitive pricing.

I had already ordered 10 of these from a vendor on Ebay I have bought from before.
10pcs,SanKen 2SC3264 C3264 TRANSISTOR NEW - eBay (item 220672424569 end time Nov-19-10 23:39:57 PST)
I have had good luck with these guys so far.

[Techknowman]

I got my parts in.
Installed them and I thought it was going to have a problem.
Using my light bulb limiter (60W) the unit would drop about 6VDC to the speaker lead and the lamp would glow fairly well.
I traced, checked resisters everything. Then I switched to a 100W bulb it then balanced out and works.
So this amp is a bit touchy with brownout voltages.
Also my earlier post about the SWR basic black amp schematic being close was 50 / 50 its close but not correct. the real schematic for a Workingman 300 amp section is a SWR2000 Power Amp Module.
This schematic is available free from Peavey. Got mine and its about 95% correct. I did find a few differences but they were minor.
The real pain is the fact that the amp PCB has no reference designators for what Q1 or R1 is and the PCB is also double sided so tracing in a powered up circuit is fun!... Not!