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Forum 135

Murphy V280.

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Till Eulenspiegel
(@till)
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Removed the line output transformer today.  It's long time since I opened up one of these transformers, sometime in the 1960s and the last set to receive attention was a V240. The problem is removing the transformer from the can. Split the bakelite sealing ring? I kinda remember that's how the transformer was taken out of the V240 can. Messy job.

Posted by: @pye625

I think this set has the additional 500pf capacitor and then  a 470k resistor in series with the EHT output to the CRT.

Door stop capacitor.

MurphyV280 EHT cap

 The transformer.

MurphyV280 LOPTx

Till Eulenspiegel

 
Posted : 21/08/2020 8:40 pm
PYE625, Nuvistor, PYE625 and 3 people reacted
PYE625
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I wouldn't break the plastic ring. Try using a fine flat blade screwderiver to pry up the outer rim. Try to drain out as much oil as possible via the little bung hole first.

There is no doubt it is a painful and messy job, but take your time and all will work out fine. ? 

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 21/08/2020 10:58 pm
Till Eulenspiegel
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Managed to ease the transformer out of the can.

Could replace the U25, but what if it fails again, got to go through that messy procedure again? Much better employ a TV18 stick or more modern high voltage silicon diodes.

Meanwhile, the transformer is being subjected to the warming up procedure by passing a current through the windings. The resistance of the Murphy transformer windings are quite low compared with other line output transformers, two to three hundred ohms is typical. The V280 EHT winding has a resistance of only 50 ohms so a much higher current must pass through windings compared with other makers transformers. 

MurphyV280 U25

Till Eulenspiegel.

 
Posted : 25/08/2020 4:33 pm
PYE625
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Posted by: @till

Could replace the U25, but what if it fails again, got to go through that messy procedure again? Much better employ a TV18 stick or more modern high voltage silicon diodes.

I faced the same dliema, but as you probably know, I fitted a new U25. But it was the small heater coil that was open, the old U25 was in fact ok. Replaced it all the same though.

I agree, it would probably be more sensible to fit a TV18 outside the can thus removing a chance of future failure of the said items.

However, I did seal the can in such a way that it could relatively easily be opened again, but I would rather not have to !

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 25/08/2020 4:55 pm
MurphyV310
(@murphyv310)
Posts: 456
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Hi.

I use those little EHT diodes BY8210 which are rated at 10kv and designed for high voltage multipliers. two in series or even three will fit nicely in the can. The added advantage is the small reduction of time base power required as there are no heaters to run on the U25.

Cheers,
Trevor.
MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

 
Posted : 25/08/2020 7:17 pm
Till Eulenspiegel
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These high voltage diodes can be used as a substitute for the Mazda U25.

HV Diodes

 Till Eulenspiegel.

 
Posted : 26/08/2020 9:11 am
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