Musical Instrument Amps damaged by power surge or drop

[yards]

hi,
i was recently using my bass amplifier on a riverboat that cut its A/C power then powered up again. We were instructed to turn off our equipment prior to this, which we did. When the the power was turned back on my Bass amp and another guitar amp both had No Sound. The amps turn on and the fuses on the amps did not blow , but no sound (the bass amp has a preamp clip light that now stays oN)
I know an A/C surge or drop can damage an audio power amp but i was wondering what components i should look at first. Nothing looks or smells burned .
Thanks

 

[kellys_eye]

When reporting or discussing faulty equipment it is always wise to include the make and model you are referring to. It helps us enormously by giving us the opportunity to look up the technical details/schematics and resolve any problems quicker. However....

If the mains input fuses are ok then check for fusees in the power supply section and on the amplifier boards themselves.

If you find a fuse that is blown, report back here with details of HOW it has blown (i.e. check the inside and see if the wire has simply 'melted' or if the fuse is all blackened). If it has 'melted (or just disappeared) then simply fitting a new fuse may fix the problem.

If it blows again immediately or the fuse was blackened internally then you've got bigger problems.

So, what make/model is the equipment?

 

[yards]

Hi , thanks for the quick reply.

This is an Eden WT550 500 watt bass amplifier.
There is a fuse where the A/C power cord plugs in but it didn't blow. Somehow this surge (or drop) affected the amp even though the power switch was off . The guitarist was using a fender combo amp and it has the same symptoms. (Now I see the importance of surge protectors , especially at events where they're using portable generators )

I looked at the Eden Amp forum and there were cases of the square white ceramic resistors failing, and possibly a diode. I don't see any other internal fuses, but found a schematic for some details. I am going to take it apart for a better look at the boards and to poke around with a multimeter.

Thanks again for suggestions

 

[shrtrnd]

Somebody here may be looking for a schematic to try to assist you.
Just wanted to input that the ceramic resistors you describe sound like wire-wound types. If they took a power hit, it's possible the wire inside 'open-circuited'. While you're in there with your multimeter, make sure the resistors aren't open-circuit, and heeding the Eden Amp forum suggestion, the diode-check to ensure they're not open.
As a side-note, that was a good idea to check with a forum dedicated specifically to your gear.

 

[kellys_eye]

I've got a schematic of the amp (very poor quality but .... meh...).

First thing to check would be the positive and negative DC supply rails to the output stage. The PA stage schematic is as follows:

 

[yards]

I really appreciate the help with this. The eden amp forum is mostly musicians without tech knowledge.
I have limited skill with electronics but can read values, replace components and understand their basic function. I may be over my head reading voltages from the schematic information. (thanks btw, for the attached schematic , it may be done by hand, but it has more detail than one I have).
Is it more likely that components like diodes and power resistors fail by opening instead of shorting through (continuity) from a surge ?

 

[kellys_eye]

Component failure isn't 'guessable' and depends on the fault conditions at the time. If the main supply fuse is ok then check the main supply DC rail supplies. If you can find the main power supply section (photos would help) I could point to the places to measure at.

 

[tedstruk]

http://www.edenamps.com/wt550.html

tech support available-user manual available.

 

[yards]

i'm doing some diagnosis before trying some of the more radical suggestions, but thank you.
That may be a great help Kellys, if i attach a photo of the board and it can be pointed out to me voltage check points that correspond with the schematic.
I agree that a clue is that the main fuse doesn't blow (wouldn't a damaged transformer blow the fuse?). I did just find a large resistor that is scorched and reads 'open' and a transistor in line with it that i suspect .
will post some photos of the board and my findings. thanks

 

[kellys_eye]

Sounds like one of the output transistors has blown which would give you the symptoms you currently experience.

Pictures and some voltage measurements (that we can suggest) should localise the fault and we can take it from there.

 

[yards]

I found a few reports of the EXACT symptom on the same power board - faulty A/C mains at a venue cause smoking resistor R101 - Eden amp 15V power supply ( board P/N 110226 )

I (also) replaced the resistor and it immedietely burned again.

QUESTION : The board is on the bench and i am wondering how much of it i should rebuild before trying a 15V reading ? (with the preamp P7 unplugged )


suggestions and actions taken from some older forums:

"I replaced the 22 ohm resistor with a same rated resistor, replaced the TIP42C (shown as TIP32C on the schematic) . I turned on the amp and the same resistor smoked again. I've checked all of the diodes and resistors in both the negative and positive sections of this stage of the power supply and they are good."

"Possibly concerns within view. leaky C50? That 1N4746 D101 is protection. It is an 18v zener across the 15v rail. If its zener point has dropped, it may be trying to shunt the supply. Or if the 15v rail has climbed over 18v it is just doing its job. testing the diode with a meter will not reveal that. Likewise, a bad D56 could look OK on a meter but have its zener point up to 20v, which the 15v rail would try to become as well."

"I am working on the same amp with the same fault, in my case caused by AC mains problems at the venue. R101 smoked. In addition to Q55, Q54 and D56 were bad. Since I had the board out I pretty much rebuilt the circuits on both rails. I am working on the outputs now. All were blown."

 

[Cannonball]

It looks like you are on the right track and you have received good advice. You might can save some time by checking these with an Ω meter looking for a short or a very low reading. You can check these readings by comparing each reading in the - voltage supply with the readings in the + voltage supply. I believe you have it under control. All reading should be referenced ground.

 

[yards]

thanks,
i am trying some resistance checks
I'm also assembling a parts list to order from digi-key, and may try just rebuilding the ( - ) rail and output since its where the damaged components are.