Hartke HA3500 Messed up!!

[Maynard_K]

Hi, let's start my first post with a dark problem.....what happens when you plug the output of a HA3500 to the output of another amp?

Well that's what I got on the table, a nice HA3500 whos confused owner plug to the output of a Peavey and turn both on. He told me the peavey smoked hard and the HA3500 got really hot, and blowed one of the fuses.

Checking out the HA3500 I replaced the fuse and two power transistors shorted (one in B+ and one in B-, both with shorted base-colector-emitter), nothing else looked damage. Turn amp on and everything seem to work ok, but the power transistors got really hot while the fan got crazy trying to reduce inside temp. After turning it off I let it cool down for ome checking.
Turn it on again and try to measured NPN emitter voltages when the fuse blowed again and also 2 more transistor (one replaced before in B+ and another one in B-).

So, I unsoldered power transistor base and feed the PCB with a +15&-15 in order to avoid working with too much power anblow something else.
I took some voltage readings and it all looks messed up. The driving NPN is getting -15 on its base and there is no sense voltage drops like 15V in a diode etc etc

I have checked this other thread wich was very interesting but got no usefull hint yet:
http://music-electronics-forum.com/t3338/

Anyone could help me or give me some hints were could be the problem? By the way, the schematic is in this address. If you can download it please let me know, I can e-mail it.
Hartke HA3500 Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics

The power module is on the up-right corner. Any suggestions? thanks!

[Jazz P Bass]

I would gut the output section & replace the output transistors , the drivers & the bias transistor.
All of the resistors should be checked as well as the protect transistors.
There are a number of different schematics on this one amp.
Here is one of them.

[Maynard_K]

Thanks Jazz, actually that schem fits mine but using 2SC5200 & 2SA1943 for power transistors. I have checked all resistor, capacitors and diodes, working now on checking all the bjts. Din't find anything wrong and still getting all that nosense voltages, I guess the board has a short somewhere but uneable to find it yet. Also I suspect IC301 is bad, I have to take it off the PCB and check all the SMD components.
The initial idea was replacing all the power and driving transistors, but before that i want to get normal voltage readings in order to avoid blowing them just for turning on the amp.

[Maynard_K]

The HA3500 is back to life! After replacing power BJTs I got to repaired some comps in the DC detect ckt and replace the bias bjt wich got an open base-collector. I did test it on with power transistor paired in couples just to avoid blowing any of them again. Sounds good so far and power module temp is steady warm.