Matra 400Hz 3 Phase PSU repair updateOK after only 5 minutes probing around, I might found something....
I think that one of the 3 heat sinks / 2N3055 pairs, the one located at the back half of the chassis, is for the PSU part of things. It's un marked.
The other two H/S however are "strangely" marked 'A' and the other 'B'... just like the Amp boards are labeled A and B ! Hmmm !!!
So I make the reasonable assumption that that's how it is. Only two H/S, one for each of the two amps. Only two amps not 3.
so since I have two working phases and one down... I will make the assumption that one of the amps is working fine (fine "enough" at least, for our immediate troubleshooting needs..), and the other is not.
Also, the two amps looks strictly identical, judging by the looks of theuir respective boards. It's also would make sense that they are identical, in any case, doesn't it..
So I will make the assumption that I can compare voltages on both amps, and that they ought to be identical.
so, I probed the voltage on the metal case of their 2 TO3 2N3055 tranistors.
On one amp, one 2N3055 reads about 40V and the other about 20V. Which makes sense since these are the two voltages I see when I measure all the filter caps in the PSU section. Also makes sense because the two PSU boards are marked 40V for one, and 15V for the other. 15, 20... close enough.
SOOO when I measured on the OTHEr amp.... one of the 2N3055 reads 40V, fine, but the other reads only 6.6V !!! Hey..... getting somewhere.
So I meaured the voltage across all the (six) filter caps .. again. Again I find them at 40 or 20V heu... hold a minute, THIS ONE reads only 7 Volts now ?!
So maybe the cap is bad and overloading the transformer and voltage drops a lot. I guess the transformer is not happy at all and that might why it makes that horrible noise at power up ?! Hmmm...
OK so I will investigate in that direction. I guess I could desolder the wires from that cap, to see if the voltage at the wires goes back up to 20V or so as it should I think...
Stay tuned !!!