14-07-2013, 06:49 PM
I've now finished fettling up this set, apart from awaiting some 6.5V 0.3A dial bulbs. I've got a few 6.5V 0.15A ones, but they don't light the dial up enough. (It only uses one bulb, which is sited over the edge of the glass so as to project the light into the glass. I've ordered ten bulbs off e-bay as I'm sure they'll come in handy for other sets.
I'd mentioned in another thread that what looked like a 10 Watt 2K7 resistor in the HT smoothing had gone high and measured 4K15. As this is an AC only set, it's not a 'dropper' as it might be in an AC/DC set, and Gary doubted it would be 10 Watts and he was right. Once I'd got the set working I checked the voltage dropped across the resistor, which was 75 Volts. 75V/4,150Ohms = 0.018 Amps (18 MA). 0.018 x 75V = 1.35Watts, so whatever the wattage of the resistor, (5W, 10W whatever), it was being under-run. I temporarily added a 7k5 resistor across it to bring it down to around 2k7 and measured the voltage drop, which reduced to 70V, so wasn't appreciably lower. That upped the current to 25mA and the Wattage to 1.8 Watts. Although the resistor had gone high by 54%, the voltages on the screens and anodes were more or less as they should be and the set works fine, so I'm not going to bother replacing the resistor. It does though demonstrate that in some situations, even when a resistor is more than 50% above what it should be, it doesn't follow that it will necessarily have a drastic effect on voltages or performance.
I mentioned earlier that I abhor the flimsy two-core flex which was the norm even on AC only sets which could have been earthed, and was restrained by tying tight a knot in it. That seems to me to have been a shoddy, penny pinching, corner cutting and dubious practice even back in the 1950s, and I'm just not prepared to pass on sets to others with that shortcoming. Hence, I did as I said I would - I fitted a new three-core flex, enlarging the entry hole in the rear chassis apron to accept a cable restraint. I earthed the chassis and left plenty of slack in the three cores. I've attached before and after pics. Using a drill to enlarge the hole risked damaging the mains transformer unless I took precautions, so I clamped a piece of plywood to the rear of the chassis apron behind the hole so the drill bit didn't shoot through the hole and rip into the mains transformer.
The speaker was rusty so I cleaned it up and sprayed in with grey primer and dove grey gloss to make it look a bit more respectable.
The cabinet has cleaned up nicely, but the set would have benefitted from new speaker fabric, but it's passable.
Some pics attached.
Not a very exciting set, or anything other than a bog standard undemanding restoration, culling waxy caps and the odd resistor, but enjoyable nevertheless.
Hope it's of interest.
I'd mentioned in another thread that what looked like a 10 Watt 2K7 resistor in the HT smoothing had gone high and measured 4K15. As this is an AC only set, it's not a 'dropper' as it might be in an AC/DC set, and Gary doubted it would be 10 Watts and he was right. Once I'd got the set working I checked the voltage dropped across the resistor, which was 75 Volts. 75V/4,150Ohms = 0.018 Amps (18 MA). 0.018 x 75V = 1.35Watts, so whatever the wattage of the resistor, (5W, 10W whatever), it was being under-run. I temporarily added a 7k5 resistor across it to bring it down to around 2k7 and measured the voltage drop, which reduced to 70V, so wasn't appreciably lower. That upped the current to 25mA and the Wattage to 1.8 Watts. Although the resistor had gone high by 54%, the voltages on the screens and anodes were more or less as they should be and the set works fine, so I'm not going to bother replacing the resistor. It does though demonstrate that in some situations, even when a resistor is more than 50% above what it should be, it doesn't follow that it will necessarily have a drastic effect on voltages or performance.
I mentioned earlier that I abhor the flimsy two-core flex which was the norm even on AC only sets which could have been earthed, and was restrained by tying tight a knot in it. That seems to me to have been a shoddy, penny pinching, corner cutting and dubious practice even back in the 1950s, and I'm just not prepared to pass on sets to others with that shortcoming. Hence, I did as I said I would - I fitted a new three-core flex, enlarging the entry hole in the rear chassis apron to accept a cable restraint. I earthed the chassis and left plenty of slack in the three cores. I've attached before and after pics. Using a drill to enlarge the hole risked damaging the mains transformer unless I took precautions, so I clamped a piece of plywood to the rear of the chassis apron behind the hole so the drill bit didn't shoot through the hole and rip into the mains transformer.
The speaker was rusty so I cleaned it up and sprayed in with grey primer and dove grey gloss to make it look a bit more respectable.
The cabinet has cleaned up nicely, but the set would have benefitted from new speaker fabric, but it's passable.
Some pics attached.
Not a very exciting set, or anything other than a bog standard undemanding restoration, culling waxy caps and the odd resistor, but enjoyable nevertheless.
Hope it's of interest.
Regards, David.
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
