NADY PM-200A troubleshooting help blowing fuses and smoking

[Zappacat]

I need some help with this combo amplifier.  I purchased it recently knowing it was damaged in some way.  The fuse was blown so I replaced it with proper one.  First power up blew the fuse again instantly and the unit started smoking inside.  So far I've removed the amplifier chassis assembly(all the electronics) from the wooden speaker enclosure and the attached pictures are what I'm looking at.  What's the best way to test the toroidal transformer without damaging it?  Is this where I should start?

I don't see any visual clues that would indicate what was burning.  The only odd thing was something that looked like ash next to the led assembly on the front of the chassis.  Should I risk throwing in another fuse with the chassis out of the cabinet so I can possibly see something burning? 

Any help greatly appreciated.

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[phatt]

Hi Zappacat,
                  Looks like the secondaries are Red/Black/Red.
On the end of those wires is a white plug.
So set DMM to read ACV and you should get 25VAC between two reds and the black.

If low or no reading then yep it's probably a blown tranny.
If the tranny has gone *short circuit* (which will/should blow the mains fuse) then it will probably hum like crazy and get hot very fast.

My guess the smoke maybe comming from the rectifier section.
(that bank of electro caps where the 3 primary wires terminate on the white plug)
Most likey cause of smoke is a dud filtercap.
If big filter caps go open then you just get a lot of hum but the amp still works,
but if they go *Short circuit* then bang they blow the fuse.

Without being able to test everything your best bet is to replace all those big caps and replace the diodes to be safe.

Put it back together bit by bit.
Rebuild rectifier section then isolate it from the rest of the Amp.
Test it on it's own and establish your working volatages are right.
Then connect the poweramp back up again,,(Use the mains light bulb trick before you power it all up again).
Good find,, looks like it will make a lot of noise.
Cheers Phil.

[Zappacat]

Phil, I can't tell you how much I've appreciated your help over the last year or 2 since I've been a member here.  Your expert advice has really helped me in trying to learn about electronics and given me the enthusiasm to keep plugging away at things when I was about to give up.

[phatt]

Oh you are to kind. 

Learn as much as you can absorb about Power supplies it will open a lot of doors for you.
Very good stuff here by people who have far greater understanding than myself.
http://www.ssguitar.com/index.php?topic=60.0

Phil.

[phatt]

Oh just a little warning about mains voltage when working on any open chassis.

These things often have no shrink wrap on the mains terminals and it's Dead easy (pun) to inadvertanly grab the chassis and your finger can touch the bare Mains wires.

If I'm working on something like this My first port of call is a roll of 2 inch wide masking tape and I tape over all the bare mains terminations as it's just so easy to have a momontary lapse of reason.

So work safe. 0:)

No watches, finger rings or dangle neck chains. Never work in bare feet.
I'm on a concrete floor so I also stand on a 20 mm thick solid rubber Matt.
Phil.

[Enzo]

Look at your second photo, at the circuit board on the right.  ALong the bottom are two white stripes showing.  Right above each stripe appears to be a pattern of about 15 pins.  On the other side of the board, are there two 15-legged ICs mounted to the heat sink?  the white stripe being heat sink grease showing underneath?

Possible number on the things might be TDA7293 or LM3886 maybe?

If so, it would be one or more of those that I suspect burnt up.  usually the smoke comes out of them around the legs.

Your transformer is the last thing I'd suspect, though like anything else, it is not immune to failures.