hi
my son has just brought me a small PA amp,this had been shut down as smoke was seen,
on removing the lid i found the cause of the smoke was that the bridge rectifier had started to melt,
could anyone please suggest a reason for this, as they are normally quite robust devices.?

the internal mains fuse was intact,there is no visable problems either side of the board.
there are two high value caps adjacent to the bridge, they check ok with multimeter, should i replace these as well?

(the audio output looks a single push pull pair)

will try and get a pic when the batteries have charged.

thank you
dougal

 

...the bridge rectifier had started to melt,
could anyone please suggest a reason for this,

?

A short seems the most likely reason.

do some google searches with model numbers, etc. and see if you can find a schematic.

Are you familiar with how to use a volt/ohm meter to read Ac voltages?

 

thanks kermit
the DC side circuit with the bridge removed reads around 800R which i guess should be ok.

ive since found the amp was being operated with quite a heavy load.

the output pair are identical 2sc5197, so could be a driver/ main

the main? is reading almost zero on base/emitter in circuit...


dougal

 

What's the manufacturer/model of the power amp? Maybe one can find a schematic on the internet.

 

Was it seen smoking out behind the barn?

 

Your troubleshooting is on the right track. I'd add that with those symptoms and the fuse intact, I'd want to make sure it was the right fuse. Ordinarily, the beginnings of catastrophic failure will blow the right fuse.

 

It doesn't happen often, but I have seen equipment in which the rectifier diodes just 'blew' after many years of service.

I would suggest taking some resistance readings of the transformer(without power applied) to see if the primary to secondary is still isolated, and also check both primary and secondary to the transformer case. If this all checks good. Then measure the secondary AC voltage with power applied to the primary(the secondary must not be connected to anything at this point) If you have pulled the bridge rectifier, then that should isolate the secondary. If you get the proper AC voltage reading, I would try simply replacing the bridge rectifier and turning the amp on again.

Have the DC volt meter connected to the secondary after the bridge and filter caps. Watch it closely when you turn it on. IF the DC voltage is much lower than normal, immediately turn the unit off and we will start looking for a short elsewhere in the circuit.

 

hi again, you were right first time kermit,
have taken the power transistor out, reading a dead short collector to emitter on both polarities,and .047 on the rest,(i would normally read 800 on my meter on a good junction)
looks a pretty rare beast, here-
http://www.awatronic.co.uk/2sc5197/prod_18760.html

the other transistor seems ok

i havent seen this company before, they seem pretty good

dougal

 

ps a bit worrying the fuse didnt blow, its only a one amp.
the bridge is rated at 3 amp.

 

hi again, you were right first time kermit,
have taken the power transistor out, reading a dead short collector to emitter on both polarities,and .047 on the rest,(i would normally read 800 on my meter on a good junction)
looks a pretty rare beast, here-
http://www.awatronic.co.uk/2sc5197/prod_18760.html

the other transistor seems ok

i havent seen this company before, they seem pretty good

dougal

This transistor is (also) being manufactured from Toshiba. But according to the datasheet it's designed for an audio amplifiers output stage. And it's the complementary to 2SA1940.
Was this one in the power supply circuit or in the amplifier part?

What's the amplifier model? Did you try to find a schematic?

You may find the blown components measuring them one by one, but threr may be other blown components, bad contacts, resistors which changed their resistance, capacitors that lost capacitance etc.

But it's also possible that the amplifier was just overloaded.

 

hi praondevou
the transistor is in the audio outpt stage, one of a pair on heatsink.
they are not a complimentary pair so the output configuration is not known,
i'll try and get a schematic, be interesting to know this,

regards
dougal

 

i'll try and get a schematic, be interesting to know this,

If you tell us the model I can have a look too. On the internet