PA amp dead ?

[dunebuggy]

Hi,
first time poster, not a pro but curious

Have a Jedia Mobile 60 PA amp I'm using the 70 volt tap for speakers.
Worked fine for 10 years.
Now it's not turning On, no LED.

Checked the 2 fuses, one on the AC connector, the other inside, all good.

The only weird thing I found is the voltage at the secondary of the power transformer reads 0 volt (?). I've unplugged the 2 wires from the circuit and I'm measuring in AC
If I measure the phase individually to ground it reads 20volt each.

Is the transformer dead ?

Thanks in advance
Eric

[Alex_Baker]

Do you have a schematic? if so post it here.

It is possible that it is the power transformer, they do go bad. They shouldn't really go bad for no reason but I have seen it more than once.

[Uunoctium]

maybe this fits
https://elektrotanya.com/jedia_l60as_amplifier_sch.pdf/download.html

[dunebuggy]

That schematic is for the L60 stereo amp

Mine is mono.

Might have the same power trasfo, Looking at the schematic now

thanks

[dunebuggy]

Dont see info on the secondary output except in D6 there is a 24volt.

might be out of my league...

[Alex_Baker]

That schematic does not appear to cover the power supply section.

[dunebuggy]

Here is a pix of an intriging sealed part screwed on the bottom of the chassis
2 reds from the secondary
1 black and 1 red going to B+ connector

Would that be some kind of rectifier ?

thanks again

[Alex_Baker]

pic didn't attach, is it too big?

[dunebuggy]

sorry
thanks

[Wallace Gasiewicz]

That is your bridge rectifier, Has four diodes in it. Should be two AC connections from transformer and two DC connections out.
Usually they are labeled  close to the pins
Notice the AC and the + label

[dunebuggy]

thanks !, makes sense

But i'm still puzlled by the fact that the secondary reads 0 volt between the 2 wires .AC mode used on the multimeter

[TheMG]

Measure the resistance of the transformer primary, it might be open-circuit. If it is, then it's likely the transformer has a built-in thermal fuse that has gone open.

Voltage readings between secondaries and ground are just stray capacitance between primary and secondary winding, doesn't mean anything.

[dunebuggy]

hmm , primary is measuring about 8M ohm, I removed the plastic and no hidden thermal fuse

until now never heard of the thermal protection fuse, thanks MG

Is 8M ohm means open (almost...)

[Alex_Baker]

8M ohms is certainly open, yes.

[dunebuggy]

Thank you for the replies
Éric

[TheMG]

The fuse is usually buried deep within the windings, as that is where the temperature is highest. Usually you can see/access the leads of the fuse where they connect (under the plastic insulating tape wraps). Replacing the original fuse is usually not possible, the next best thing you can do other than replacing the original one is to solder a new one with a lower temp rating to the outside of the winding, in parallel with the leads of the burnt out one.

[dunebuggy]

It's fixed, thanks again for all the infos
The thermal fuse was blown and buried very deep in the primary.

Eric

[Alex_Baker]

Cool, glad you were able to find it!

The other question is if there was a particular reason that the thermal fuse blew, like an overload condition of some sort.

[BrokenYugo]

In my experience they just fail sometimes, thermal stress, bad connections heating the wrong end, etc. I'm sure they do sometimes actually prevent fires, but I always find them blown with either no signs of heat (discolored magnet wire and whatnot) or they failed on the connection end.