Help me fix a plate amp, please. (1 Viewer)

[Aaron_Morris]

It is an Apex Jr amp plate amp. It is basically the same (at least to my eye) as the 120w PE plate amp.

It decieded to bite the dust one day. It still has a thump when turned on and off. Only think that I can think possibly happened to it is a dead short between the leads (however, this is just a possibility).

I'd appreciate it if you fine folks would help me trouble shoot , find the problem and fix it. Thanks for any help in advance.

 

[Jonathan M]

Start by checking the output to see if the output stage is shot - test to see if you have DC present on the speaker outputs - if so then the output stage is blown. Do these things have overload protection on them or anything? The fuses sound as though they might be OK - does it have both a mains fuse as well as fuses on each DC line? If so, one of them might be blown (Indicates possible dead output stage again.)

A good visual check over for dead looking parts (burnt out etc.) and also a quick check whilst powered up (Check DC voltage rails on both amp and pre-amp section + check parts for heat generation - especially emitter resistors) can often help.

Let us know what you find. Do you have a schematic of the amp available?

 

[Aaron_Morris]

I think they have protection circuits on them. Also, it doesn't smell dead.

 

[Aaron_Morris]

Ok, now that I have this thing apart, where do I connect the DMM to check for DC voltage on the output stage?
Also, there doesn't appear to be any char marks or obviously blown caps or anything like that.

 

[Jonathan M]

Just connect the DMM to the speaker outputs - have it set to DC mode starting with the 200V or so range. If that reads close to zero, alter the range down to determine the offset.

Have you checked all possible fuses? There will be at least a mains fuse, and maybe a set of fuses on the DC rails (From the filter caps).

Also, while powered up, check for excess heat generation in any resistors+transistors etc. BE CAREFUL - these things run at fairly high DC rails and have the current available to cause serious damage. If you aren't comfortable doing this, then don't do it.

Also, see if you can find the link from the pre-amp section to the power amp section. If so, you could tap a couple of wires on and test the pre-amp section and power amp section independently.

Troubleshooting this sort of thing is fine if you know what you're doing. A basic idea of how an amplifier works will be essential. The schematic would be a great help as well.

 

[Erik M]

I would call Parts Express to find out if they will fix it. Their technician Eric is very knowledgeable and quoted me just $20 + shipping to fix a damaged PE Amp. He should be able to work on the Apex JR as well. I think he can even change out your boost while he has it if you want that.

 

[Aaron_Morris]

Thanks for the input. I called but he was on the phone. I emailed him, so hopefully something good will come of this.
Looks like either way this goes, I should have bought from PE, the little bit of money that I saved has come back to bite me in the butt.