Are your TIP41Cs TIP41Cs  ?
C is rated at 100V, A at 60V and no suffix may be 40V.
Peak voltage is ~~ 2 x Vsupply plus leakage inductance energy effects.

How old are the cores?
What are they made of?
Old powdered iron cores have increasing losses due to binder degradation du=
e
to heat. This leads to more heat and more degradation and ...
 Try swapping cores (whole inductors) between a good and a bad one.

How hot are the cores getting. Are the cores in bad units getting much
hotter than in the good ones.



  Russell



On 14 February 2011 09:19, Bob Blick <bobblick@ftml.net> wrote:

> On Sun, 13 Feb 2011 12:47 +1000, "cdb" wrote:
>
> > These are simple devices - 24v @ 620mA in,  uses two TIP41's with each
> > transistor attached to it's own centre tapped winding - in other words
> > there are two CT windings one for each transistor on the same
> > transformer,
> > two chokes in the power feed to each, one 27nF cap for LC tuning and on=
e
> > 100nF on the secondary side.
>
> I would look at your 24 volt power supply - perhaps the output
> capacitors are bad, allowing the inverter too much freedom and that
> causes the transistors to overvoltage, or maybe get insufficient
> feedback and there is double switching.
>
> You could try tacking on a biggish electrolytic cap on the 24 volt
> supply and see if that makes a difference. If it improves things,
> something's wrong.
>
> Cheerful regards,
>
> Bob
>
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