Murphy V530 repair. - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

murphyv310 · 22nd Mar 2011, 7:49 pm

Hi Frank.
When the set is on Radio the heater voltage required is 48.3v, to do this all the relevant TV valve heaters are shorted out and a 441 ohm 25w watt resistor is then is in series. The rest of the dropper though and thermistor is in circuit whether on radio or tv these add up to 199 ohms which I am leaving in the set, so the only resistor I am replacing is the 441 ohm for the radio function.
I am now confused!

Hi, I have just used Paul's with all the correct parameters and it cam back with 4.46uf, not too bad as I had tried it with 4.47uf and the heaters seem to be the correct brightness!

ppppenguin · 22nd Mar 2011, 8:09 pm

The reactance of 7.2uf is indeed about 441R but it reactance, not resistance. If you addding reactance to resistance it isn't a simple addition, it's a pythagorus SQRT(R^2 + X^2) calculation. Paul's spreadsheet takes this into account and it's explained on the web page. http://www.vintage-radio.com/repair-...per-calcs.html

murphyv310 · 22nd Mar 2011, 8:15 pm

Hi Jeffrey.
I had forgotten the correct calc! I have forgotten too much in the way of theory and probably as for years in servicing really didn't use an awful lot of it with the exception of ohms law.
Its now blatantly clear and my original value was pretty close!

22nd Mar 2011, 7:49 pm	#21
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,422	Re: Murphy V530 repair. Hi Frank. When the set is on Radio the heater voltage required is 48.3v, to do this all the relevant TV valve heaters are shorted out and a 441 ohm 25w watt resistor is then is in series. The rest of the dropper though and thermistor is in circuit whether on radio or tv these add up to 199 ohms which I am leaving in the set, so the only resistor I am replacing is the 441 ohm for the radio function. I am now confused! Hi, I have just used Paul's with all the correct parameters and it cam back with 4.46uf, not too bad as I had tried it with 4.47uf and the heaters seem to be the correct brightness! __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member Last edited by murphyv310; 22nd Mar 2011 at 8:10 pm. Reason: Added another line

22nd Mar 2011, 8:09 pm	#22
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Murphy V530 repair. The reactance of 7.2uf is indeed about 441R but it reactance, not resistance. If you addding reactance to resistance it isn't a simple addition, it's a pythagorus SQRT(R^2 + X^2) calculation. Paul's spreadsheet takes this into account and it's explained on the web page. http://www.vintage-radio.com/repair-...per-calcs.html

22nd Mar 2011, 8:15 pm	#23
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,422	Re: Murphy V530 repair. Hi Jeffrey. I had forgotten the correct calc! I have forgotten too much in the way of theory and probably as for years in servicing really didn't use an awful lot of it with the exception of ohms law. Its now blatantly clear and my original value was pretty close! __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member