25-09-2013, 06:19 PM
A little update.
I'm relieved to say that I've successfully rewound the field coil for the mains energised speaker on the Ekco AC77 radio that I'm restoring
As I said in an earlier post, the pocket calculator turns counter wasn't a success - it works OK up to about 200RPM, but anything faster than that, and the speed is too rapid for it to count. When I get a bit of time I'll see if I can fit a mechanical tally counter in place of the calculator for checking the number of turns - obviously when transformer ratios come into play, it's important to be able to count the turns. I've got a tally counter which I initially used on my 'Morris Gingery' hand coil winder (which I've never used as Litz wire is soooo expensive and I haven't needed to wave wind any RF coils, for which it was intended). A pocket calculator 'turns counter' works fine on that hand winder of course. The other thing I discovered about the calculator is that it has auto shut off, so within a few seconds of it not having any key presses, the blighter switches off so you loose the record of turns counted!
For the field coil, the number of turns was irrelevant - all that mattered was that the finished coil was as near 2,000 Ohms as could be, but I'd estimate that the 2.53 KM of wire I had to wind onto the former was in the region of 18,000 turns. The coil former is 1.25" diameter, the finished size of the coil is 2", so the average diameter is 1.625", which works out about 5" (12.5 cms) per turn on average, so that's about 8 turns per Metre. Th resistance of the wire is 0.85 Ohms/Metre, so for the 2,000 Ohms needed for the coil, that equates to 2,353 Metres x 8 turns per Metre = 18,824 turns. Whether or not I've got the sums right matters not in this instance! I just filled the bobbin with wire, then ran the soldering iron on the (solderable) wire to tin it for 5mm or so, and tested the resistance, intending to either add or subtract turns to get it about right. As luck would have it, it was 1,997 Ohms! The lead in/out wire, the silicone sleeving and 'fish paper' came in handy, so thanks for that. As the original coil was wrapped in black plastic, I put a couple of turns of black gaffa tape over the fish paper so it looked as near as possible to the original coil.
As to the two guide pulleys I fitted to the coil winder, they were not a success and were a needless complication. I had thought it might help me control the lateral movement of the wire back and forth across the former, but with wire of such a fine gauge any notion of trying to lay the turns side by side is fanciful. I therefore adopted Gary's method and fed the wire through my fingers using one hand to steer the wire as it came off the feed bobbin and the other to lay the turns of the coil former as neatly and evenly as I could, and it looked quite respectable. I had a heart stopping moment when the coil was at least half wound. Suddenly the motor ran at top speed and the speed controller failed to work. I quickly switched the motor off, and spent about half an hour untangling the wire that had spooled off the feed bobbin. Luckily, it didn't break or kink, though it it had, I'd have just had to repair the break and press on. The fault was caused by the suppressor cap on the speed controller going dead short in the foot controller. My fault really - I should have fitted a new X type cap at the outset.
The spring friction arrangement I fitted to stop the feed bobbin free-wheeling worked fine, but I don't think it was essential. The foot controller enables you to slow down the speed in such a way that the feed bobbin behaves itself. I was surprised how fast I could wind on the turns - the wire was far less fragile than I'd imagined it would be, and total winding time was well under an hour.
I've attached a pic of the speaker as found, and one of the restored speaker with the field coil fitted. I'm indebted to Gary Tempest for his advice and for supplying me with 'fish paper' to wrap the finished coil, wire for the connections to the coil, and silicone sleeving. He also tipped me off that Halfords Ford 'Dove Grey' paint is a very good match for Ekco grey as used on their speakers and chassis.
So, for my AC77 restoration, that's the power supply rebuilt and working, and the speaker too. I think the main chassis will pose more of a challenge. I'm rather miffed that through dithering, I missed out on an AC77 on ebay, untouched, neglected and in just as poor a state as mine, which went for under £30 and would have been excellent as a reference for when I strip this one down. Oh well, it's a hobby after all - I'm restoring radios in my scarce leisure time - I'm not putting rockets into space, so what the heck!
I'm relieved to say that I've successfully rewound the field coil for the mains energised speaker on the Ekco AC77 radio that I'm restoring
As I said in an earlier post, the pocket calculator turns counter wasn't a success - it works OK up to about 200RPM, but anything faster than that, and the speed is too rapid for it to count. When I get a bit of time I'll see if I can fit a mechanical tally counter in place of the calculator for checking the number of turns - obviously when transformer ratios come into play, it's important to be able to count the turns. I've got a tally counter which I initially used on my 'Morris Gingery' hand coil winder (which I've never used as Litz wire is soooo expensive and I haven't needed to wave wind any RF coils, for which it was intended). A pocket calculator 'turns counter' works fine on that hand winder of course. The other thing I discovered about the calculator is that it has auto shut off, so within a few seconds of it not having any key presses, the blighter switches off so you loose the record of turns counted!
For the field coil, the number of turns was irrelevant - all that mattered was that the finished coil was as near 2,000 Ohms as could be, but I'd estimate that the 2.53 KM of wire I had to wind onto the former was in the region of 18,000 turns. The coil former is 1.25" diameter, the finished size of the coil is 2", so the average diameter is 1.625", which works out about 5" (12.5 cms) per turn on average, so that's about 8 turns per Metre. Th resistance of the wire is 0.85 Ohms/Metre, so for the 2,000 Ohms needed for the coil, that equates to 2,353 Metres x 8 turns per Metre = 18,824 turns. Whether or not I've got the sums right matters not in this instance! I just filled the bobbin with wire, then ran the soldering iron on the (solderable) wire to tin it for 5mm or so, and tested the resistance, intending to either add or subtract turns to get it about right. As luck would have it, it was 1,997 Ohms! The lead in/out wire, the silicone sleeving and 'fish paper' came in handy, so thanks for that. As the original coil was wrapped in black plastic, I put a couple of turns of black gaffa tape over the fish paper so it looked as near as possible to the original coil.
As to the two guide pulleys I fitted to the coil winder, they were not a success and were a needless complication. I had thought it might help me control the lateral movement of the wire back and forth across the former, but with wire of such a fine gauge any notion of trying to lay the turns side by side is fanciful. I therefore adopted Gary's method and fed the wire through my fingers using one hand to steer the wire as it came off the feed bobbin and the other to lay the turns of the coil former as neatly and evenly as I could, and it looked quite respectable. I had a heart stopping moment when the coil was at least half wound. Suddenly the motor ran at top speed and the speed controller failed to work. I quickly switched the motor off, and spent about half an hour untangling the wire that had spooled off the feed bobbin. Luckily, it didn't break or kink, though it it had, I'd have just had to repair the break and press on. The fault was caused by the suppressor cap on the speed controller going dead short in the foot controller. My fault really - I should have fitted a new X type cap at the outset.
The spring friction arrangement I fitted to stop the feed bobbin free-wheeling worked fine, but I don't think it was essential. The foot controller enables you to slow down the speed in such a way that the feed bobbin behaves itself. I was surprised how fast I could wind on the turns - the wire was far less fragile than I'd imagined it would be, and total winding time was well under an hour.
I've attached a pic of the speaker as found, and one of the restored speaker with the field coil fitted. I'm indebted to Gary Tempest for his advice and for supplying me with 'fish paper' to wrap the finished coil, wire for the connections to the coil, and silicone sleeving. He also tipped me off that Halfords Ford 'Dove Grey' paint is a very good match for Ekco grey as used on their speakers and chassis.
So, for my AC77 restoration, that's the power supply rebuilt and working, and the speaker too. I think the main chassis will pose more of a challenge. I'm rather miffed that through dithering, I missed out on an AC77 on ebay, untouched, neglected and in just as poor a state as mine, which went for under £30 and would have been excellent as a reference for when I strip this one down. Oh well, it's a hobby after all - I'm restoring radios in my scarce leisure time - I'm not putting rockets into space, so what the heck!
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'
