Re: LF: Decca 5501

[Johan Bodin <jhbodin@tele2.se>]

Yes, D7 will charge C8 when drive is present, and the time constant of C8, R15, R16 and RV3 is about a millisecond, so IC1b is most likely a drive supervisor.

The 29T winding together with D11 clamps the flyback pulse to the +27V rail when the stored magnetic energy is "discharged". This is a quite common method used to make the drain/collector of a single-ended switcher swing symmetrically around around the +V rail. The 27T / 29T difference was probably choosen because of non-perfect coupling and perhaps also the forward drop of D11. I think the zener D28 is there to "eat" the energy in the narrow flyback drain spikes kicking back from the leakage inductance of the 27T primary.

I'm not sure about the reason for moving the clamp from +27V to GND (via D10/TR1) when drive is lost. Maybe the transformer is big enough to have plenty of energy storage margin and the final "discharge" is routed through TR1 to GND instead of causing a looong final pulse to the PA FETs?

73 de Johan SM6LKM

gw8glo@aol.co.uk wrote:

OK thanks Andy I do have the circuit diagrams but I understood that the clamp only operated if the drive was lost. The description was not that clear to me but a probe with the scope may clear that up. I have added resistors across the fet gates  so that if the transformer fails the fet should turn off. I think I was lucky when the transformer failed and the fet was not  killed! The insulation in the transformer is only what is on the litz wire and it has sleeving out of the coil. It failed as the ferrite is earthed and it had degraded the insulation over time. It was a pig to wind but was worth it, no spares are probably available unless from a scrap unit.
Dave

-----Original Message-----
From: Andy Talbot <andy.g4jnt@gmail.com>
To: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Sent: Wed, 13 Jan 2016 10:45
Subject: Re: LF: Decca 5501

I've just looked again, and there's more to to the drive circuitry than that.  It is single ended drive for current into the inductor, as I stated, but there is another IRF520 used as an active clamp (the circuit diagram was drawn a bit ambiguously, so didn't spot the arrangement straightaway).  So when these two FETs are driven alternately there will be a proper equal amplitude symmetrical drive waveform generated.

It is definitely used as an energy storage inductor though, hence a gapped core is essential. 

'jnt