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Old 19th Feb 2012, 9:34 am   #17
matspar
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Join Date: Aug 2009
Location: Portsmouth, Hampshire, UK.
Posts: 230
Default Re: Pye VT2 EHT problems

Time for an update! if anyone's still reading!

I re-fitted the original iffy lopt yesterday, crossed my fingers and hoped, but unfortunately there was no improvements, was worth a try I suppose.

I also experimented with a lopt from a 17" Dynatron TV38 (1959)
A very common type (I've had Ekco and Ferranti sets in the past which used the same chassis)

I chose this one as it looks fairly simple and only had 3 connections, 2x scan coils and what I think is the A1 connection. I didn't have a circuit diagram to hand but I know I have one somewhere.

With this lopt fitted there was considerable improvement, I now have a very tiny spark on the top caps of the PL and PY81, and getting on for about a half inch spark on the rectifier anode. Nothing on the screen however. The A1 voltage has shot up to about 400v. Too high but I can probably reduce this. I haven't used the A1 connection from this lopt, I've only connected the scan coils.

I think the EHT its still low with this one installed, I imagine the HT was a bit higher in the original set. At least it proves a point about the original lopt however.

My plan of action now is to either - remove the overwind from the original lopt and go down the Tripler route. I know the basic theory on how this is done but my only concern is if I can generate enough EHT to light the tube up.

The pye needs 7.5Kv - do you think I can get anything like this from the anode of the PL81 using a tripler? I'm not convinced personally, but I've not attempted this before so all this is unchartered waters for me.

Alternatively could I raise the EHT using the Dynatron lopt which is showing some signs of life? there is a large mica capacitor between one side of the scan coils and the boost diode anode - think I've read somewhere that you can play around with the value of this to vary the EHT?

Any thoughts on either of these would be very welcome! I'm not going to give up on this set!

Matt

Last edited by matspar; 19th Feb 2012 at 9:41 am.
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