Gecophone BC2740 - UK Vintage Radio Repair and Restoration Discussion Forum

PJL · 22nd May 2022, 9:04 pm

I posted a wanted for a knob and thought it worthwhile giving a bit of background.

It was in a local auction with a fair estimate but I did not bother viewing as I could see it had cosmetic issues. I watched the auction live and when they reduced the starting price I lost self-control and pressed the button. The internal picture is much enhanced as it originally showed little detail.

PJL · 22nd May 2022, 9:10 pm

I was a bit disappointed to find the inter-valve transformer had been replaced with a bright orange RI one and the wrong knob on the right which should have connected to a volume control was just a shaft with nothing on the end. On the bright side it was fairly clean inside the box. The RI IV transformer had a 100K resister over the secondary and proved to be o/c, opening it up revealed just one bobbin is o/c. This uses the early style of laminations that are very then and have a diagonal cut on one side so they can be bent and slid out of the bobbins.

There is a circuit drawing of a later 3 valve AC version on the web that shows how the volume control works. The inter-valve connections from V2 to V3 and V3 to V4 are all in the metal box. Some checks with a component tester showed 2.4K from previous anode to HT, 1.4uF from anode to next grid, and 8H/800 ohms from grid to grid battery connection. I will try to open the box to make sure someone has not hacked it around but it should work as is.

PJL · 22nd May 2022, 9:14 pm

I started on the connector board at the back of the set as it looked so bad and all were seized solid, odd considering the insides are pretty clean. Not much finish left on the brass but a lot better than it was.

vinrads · 23rd May 2022, 7:53 am

Very nice! if the rest of the set is having the same treatment as the back connector board it's going to look fantastic , Mick.

PJL · 23rd May 2022, 2:38 pm

Scrolling around a bit and the BC2740 probably had a tapped inter-valve transformer with a multi-way switch for the volume control that selected the different taps. There was a thread on a BC4000 on here with one that appears to match the empty fixing holes.

PJL · 23rd May 2022, 10:08 pm

The metal box on which the 3 audio valves are mounted contains 4 transformers with the laminations just visible above the pitch that the box has been filled with. I am guessing that it is an anode choke that is capacitor coupled to a grid choke for each of the two couplings. The component tester shows the anode load as a resistor only but there may be a damping capacitor messing the reading up. No signs of tampering so I will check leakage from anode to grid and leave well alone if tolerable.

I had wondered why this radio was so heavy!

PJL · 24th May 2022, 3:51 pm

The four chokes in their pitch bath. Leakage anode to grid at 150V is about 0.5mA, not great but should be OK as the grid choke measure 800 ohms so just 400mV offset. It would drain the grid bias battery though.

Catkins · 25th May 2022, 3:35 am

Quote:

Originally Posted by PJL

I started on the connector board at the back of the set as it looked so bad and all were seized solid, odd considering the insides are pretty clean. Not much finish left on the brass but a lot better than it was.

What did you use to bring the colour/finish back to the panel? (ebonite I assume).

PJL · 25th May 2022, 9:28 am

Yes, ebonite but it had only turned brown in a few patches. I cleaned with soap and water rubbing with cardboard from a cereal carton, to fill the engraving I applied white mixed with yellow ochre acrylic artist paint with a small spatula, allowed to partially dry, then removed the excess with the cardboard treatment, finally once dry I rubbed over with a fine dry cloth and applied a beeswax finish.

PJL · 29th May 2022, 11:31 pm

Good news, I now have a replacement knob supplied by a forum member.
Not had much time spare and I am having big trouble taking it apart so I can tidy up the front panel as the brass fittings are all seized solid.
It does have the first early grid leak resistor I have found that still measures OK, marked 2ohms it reads 1.8Mohms.
I guess I could reproduce the missing tapped IV transformer and selector but I first plan to fix the RI transformer and get it going and may then try the 1M volume control used on later sets. My concern is even 1M load may cause reception issues as it will appear far lower at the transformer primary.

PJL · 1st Jun 2022, 12:15 pm

I have drawn out the circuit. The content of the pitch box is a bit of informed guesswork. The front-end is a bit odd, I have drawn the coils as they are laid out on the formers, 3 pairs of coils on one former with the reactance coil, and a larger coil pair perpendicular to the reaction coil. Each coil is paired with another and coupled with the tuning condenser.

The intervalve transformer is shown as wired with the replacement but the original would have been tapped with a rotary switch to select the required tap.

PJL · 5th Jun 2022, 2:03 pm

Finally got it apart, the brass of the screw fittings had migrated into the surface of the front panel. 5 out of 8 front panel brass screws sheared off but a soldering iron helped extract the broken ends.

I have stripped the escutcheon back to brass as it was so badly corroded, it originally had an 'antique' finish that I will attempt to reproduce. I am not too sure what the finish was on some parts as everything is now black, other examples on the web seem to have suffered similarly.

Bit of cleaning up, reassembly, and rewind of the IV transformer and it will be ready to go... just in time for the Burndept Ethophone delivery by my daughter.

PJL · 11th Jun 2022, 11:45 am

Ethophone delivered and it is in very good restored condition, line up is DER DER DE4 DE4. one DER and one DE4 have o/c filaments. It has the PMG BBC stamp and some notes that claim it to be 1924 and that it has been working using PM valves.

The BC2740 is reassembled, I used a megger on the o/c coil and it is now showing conductivity, 4K with 3K for the good winding.

PJL · 20th Jun 2022, 11:01 pm

The resistance of the 'repaired' winding kept increasing again so I have now rewound it. The fault proved to be about 2/3 of the way in and showed no signs of blue spot but corresponded to a patch where it looked like things had gone awry with loose and overlapped turns.

PJL · 28th Jun 2022, 9:39 am

After I found a poor connection to the V2 filament it is working of a sort, lots of whistles and not much volume. Detection efficiency of simple triode regenerative designs like this will have many dependencies so I will try a few different valves and HT voltages first.

PJL · 29th Jun 2022, 6:55 pm

Connected it to mains earth and it's much livelier although not the easiest to tune with lots of whistles but I now imagine a long wire aerial would result in reasonable performance.

I will call it a day on this one for the time being and move on to the Ethophone.

22nd May 2022, 9:04 pm	#1
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Gecophone BC2740 I posted a wanted for a knob and thought it worthwhile giving a bit of background. It was in a local auction with a fair estimate but I did not bother viewing as I could see it had cosmetic issues. I watched the auction live and when they reduced the starting price I lost self-control and pressed the button. The internal picture is much enhanced as it originally showed little detail. Attached Thumbnails

22nd May 2022, 9:10 pm	#2
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Gecophone BC2740 I was a bit disappointed to find the inter-valve transformer had been replaced with a bright orange RI one and the wrong knob on the right which should have connected to a volume control was just a shaft with nothing on the end. On the bright side it was fairly clean inside the box. The RI IV transformer had a 100K resister over the secondary and proved to be o/c, opening it up revealed just one bobbin is o/c. This uses the early style of laminations that are very then and have a diagonal cut on one side so they can be bent and slid out of the bobbins. There is a circuit drawing of a later 3 valve AC version on the web that shows how the volume control works. The inter-valve connections from V2 to V3 and V3 to V4 are all in the metal box. Some checks with a component tester showed 2.4K from previous anode to HT, 1.4uF from anode to next grid, and 8H/800 ohms from grid to grid battery connection. I will try to open the box to make sure someone has not hacked it around but it should work as is. Attached Thumbnails Last edited by PJL; 22nd May 2022 at 9:26 pm.

22nd May 2022, 9:14 pm	#3
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Gecophone BC2740 I started on the connector board at the back of the set as it looked so bad and all were seized solid, odd considering the insides are pretty clean. Not much finish left on the brass but a lot better than it was. Attached Thumbnails Last edited by PJL; 22nd May 2022 at 9:32 pm.

23rd May 2022, 7:53 am	#4
vinrads Dekatron Join Date: Feb 2009 Location: Chesterfield, Derbyshire, UK. Posts: 3,760	Re: Gecophone BC2740 Very nice! if the rest of the set is having the same treatment as the back connector board it's going to look fantastic , Mick.

23rd May 2022, 2:38 pm	#5
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Gecophone BC2740 Scrolling around a bit and the BC2740 probably had a tapped inter-valve transformer with a multi-way switch for the volume control that selected the different taps. There was a thread on a BC4000 on here with one that appears to match the empty fixing holes.

23rd May 2022, 10:08 pm	#6
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Gecophone BC2740 The metal box on which the 3 audio valves are mounted contains 4 transformers with the laminations just visible above the pitch that the box has been filled with. I am guessing that it is an anode choke that is capacitor coupled to a grid choke for each of the two couplings. The component tester shows the anode load as a resistor only but there may be a damping capacitor messing the reading up. No signs of tampering so I will check leakage from anode to grid and leave well alone if tolerable. I had wondered why this radio was so heavy!

24th May 2022, 3:51 pm	#7
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Gecophone BC2740 The four chokes in their pitch bath. Leakage anode to grid at 150V is about 0.5mA, not great but should be OK as the grid choke measure 800 ohms so just 400mV offset. It would drain the grid bias battery though. Attached Thumbnails

25th May 2022, 9:28 am	#9
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Gecophone BC2740 Yes, ebonite but it had only turned brown in a few patches. I cleaned with soap and water rubbing with cardboard from a cereal carton, to fill the engraving I applied white mixed with yellow ochre acrylic artist paint with a small spatula, allowed to partially dry, then removed the excess with the cardboard treatment, finally once dry I rubbed over with a fine dry cloth and applied a beeswax finish.

29th May 2022, 11:31 pm	#10
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Gecophone BC2740 Good news, I now have a replacement knob supplied by a forum member. Not had much time spare and I am having big trouble taking it apart so I can tidy up the front panel as the brass fittings are all seized solid. It does have the first early grid leak resistor I have found that still measures OK, marked 2ohms it reads 1.8Mohms. I guess I could reproduce the missing tapped IV transformer and selector but I first plan to fix the RI transformer and get it going and may then try the 1M volume control used on later sets. My concern is even 1M load may cause reception issues as it will appear far lower at the transformer primary.

1st Jun 2022, 12:15 pm	#11
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Gecophone BC2740 I have drawn out the circuit. The content of the pitch box is a bit of informed guesswork. The front-end is a bit odd, I have drawn the coils as they are laid out on the formers, 3 pairs of coils on one former with the reactance coil, and a larger coil pair perpendicular to the reaction coil. Each coil is paired with another and coupled with the tuning condenser. The intervalve transformer is shown as wired with the replacement but the original would have been tapped with a rotary switch to select the required tap. Attached Thumbnails

5th Jun 2022, 2:03 pm	#12
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Gecophone BC2740 Finally got it apart, the brass of the screw fittings had migrated into the surface of the front panel. 5 out of 8 front panel brass screws sheared off but a soldering iron helped extract the broken ends. I have stripped the escutcheon back to brass as it was so badly corroded, it originally had an 'antique' finish that I will attempt to reproduce. I am not too sure what the finish was on some parts as everything is now black, other examples on the web seem to have suffered similarly. Bit of cleaning up, reassembly, and rewind of the IV transformer and it will be ready to go... just in time for the Burndept Ethophone delivery by my daughter.

11th Jun 2022, 11:45 am	#13
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Gecophone BC2740 Ethophone delivered and it is in very good restored condition, line up is DER DER DE4 DE4. one DER and one DE4 have o/c filaments. It has the PMG BBC stamp and some notes that claim it to be 1924 and that it has been working using PM valves. The BC2740 is reassembled, I used a megger on the o/c coil and it is now showing conductivity, 4K with 3K for the good winding.

20th Jun 2022, 11:01 pm	#14
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Gecophone BC2740 The resistance of the 'repaired' winding kept increasing again so I have now rewound it. The fault proved to be about 2/3 of the way in and showed no signs of blue spot but corresponded to a patch where it looked like things had gone awry with loose and overlapped turns.

28th Jun 2022, 9:39 am	#15
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Gecophone BC2740 After I found a poor connection to the V2 filament it is working of a sort, lots of whistles and not much volume. Detection efficiency of simple triode regenerative designs like this will have many dependencies so I will try a few different valves and HT voltages first.

29th Jun 2022, 6:55 pm	#16
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Gecophone BC2740 Connected it to mains earth and it's much livelier although not the easiest to tune with lots of whistles but I now imagine a long wire aerial would result in reasonable performance. I will call it a day on this one for the time being and move on to the Ethophone.