1929, 3 valve home build (restoration)

 

I was lucky to find this beauty at a reasonable cost along with BTH horn speaker from a guy in Wednesbury in West Midlands. They were in an unrestored grubby state, but with no real damage to the cabinet that would have been professionally made as an upgrade after a home build had been built. The problem with home builds is that a circuit can never be found and I had to reverse engineer one for myself. I was quite surprised how sophisticated it was. With 3 separate HT supplies, 1 grid bias battery and a 2 volt heater accumulator required to fire the giant up. I later found the reproduction period hall table to put it on - that I found on Facebook sales in Erdington.

The repair

All valves were pretty much exhausted and needed replacing. The screened grid PM12, the top cap broke away and I had to etch the glass back (with a grinding tool) to re-solder a wire to it. I made a 2v power unit with a 'buck converter' i/c and fitted that a 2 x 1.5v batteries inside a plastic box (originally the store box for a JVC car stereo front) nice clip lid to give access to replace batteries. The i/c's are found on line for a 'buck', lol

2v power pack I made with 'Buck' i/c, a switch and an LED to remind me it was on.

The grid leak battery was still in it's holder on the rear, so I made a carboard copy and installed a 7.7v battery inside (lot's of 1.5's in series.) Didn't re-use old one it had been tar filled.

Grid leak batteries new and old

The 3 HT units I made with 10 x PP3s in series fitted in the Kodak boxes they were posted in, Only gives 90v per rail, but most calves will work down to as little as 30 HT. To save space I fitted the 3 x HT battery boxes in the screened can middle unit at rear.

This photo shows the inside after a good clean-out of spiders and webs, as you can see the radio is constructed in 3 compartments; left: screen grid, RF amp and detector unit, centre: feedback and tuning, right: audio amp and tuning.  A is where I etched the wire back to glass top of valve, B is hidden behind screen can but needed a 1uF the was open circuit, HT decoupling, and C was the Inter Stage audio transformer(TX1), 3:1 ratio, that was open circuit, luckily I had some period parts to replace them. The plug in coil in the centre area I moved forward a little to give room for battery box's.

Even the inside of the cabinet had been varnished, so made cleaning years of dust away a doddle. The hole of the inside slid out of the cabinet with only the bias battery fixed to the rear. Because the cabinet was so dark, died a mahogany colour I added a collection of cigarette cards from the mid 20s showing and naming all 1920s radio components for the constructer, this brightens the whole look of the unit considerably.

Even the aluminium screening can, un-seen when radio running was given brush engraved finish and a removable lid for the top was also added. The BBC had insisted these units were better screened by the late 20's due to interference being caused with neighbour's radios, due to local oscillators. 3 plug-in coils were fitted and would need to be changed for LW reception if needed. Many terminals at rear for all the batteries to be wired plus speaker with more for aerial and earths. I touched them up with Tipex to highlight symbols on them.