Halcyon Super Screen Grid 4 (1929)

This lovely piece of  carpentry came on-line for sale last Month, a chap in Dumfries, Scotland sold it to me, not much information can be found on the London company of Halcyon, this magazine advert (above) dates from 1928 and has a five valve version. The super screen grid refers to the newly developed high power RF screen Heptode valve introduced to the market around then (replaces the need for 2 triodes RF stages.) Thus reducing the overall cost of the radio. The church looking speaker grill conceals an early 12" Celestion cone type loudspeaker, that I will show you later. Nice door at top, slides up and then hinges flat on the top of the set, unlike to earlier one in the advert that just folds down with risk of breaking. The set also has a turntable base and an internal aerial coils for LW & MW, so quite a top of the rage set for its day, would have cost more then a Months wage for the average working man around then.

The repair

No found circuits have been published for Halcyon sets, so I had to reverse engineer one, although later I found a similar one without band switches from a late 20s published book. 

The wiring inside I found a little altered, possibly somebody tied to repair the set in the past and gave up. There was no supply to the Heptode V1 anode, I concluded that an RF (HFC) coke had been found faulty and removed. I had a spare 1920s choke in my spares box and fitted that. I also found a wire missing on the volume control and had to make an educated guess where that went. It was a 100pf capacitor wire in feedback like C3 in above diagram

I think the large TCC capacitor (top left in above) had been fitted to replace the choke but wired in the wrong place. These chokes were used to stop the RF generated by the valve traveling along the HT rail to the next stage, but later large electrolytic capacitors were developed and used instead. The heptode valve was also u/s along with 2 other triodes, luckily I have other 20s working sets to test the valves in quickly, most were 10% gain left. 

I have a few period valves in my stock room - needed today!

Next I built a power unit for the set in a sandwich box. For the HT I connected 13 pp3 (9v) batteries in series, tapping off a lower voltage for the RF stages and higher for the audio output. The 2 grid bias's came from a number of 1.5 v batteries arranged for 12 volt and 1.5 v negative. The fiddly voltage is the 2 volts for the heaters they require a total of 3/4 of an amp, so I used a 3v battery fed to a regulator I/C (LM2596 or similar) to give me 2V, can be fund for just a few pounds.

  The one drawback of this arrangement is you need to fit an on-off switch on the regulator and a LED to remind you it's still on. The set came with nicely labelled connector wires with spade connectors that I attached to some nuts and bolts on the side.

Close up view of Celestion speaker mechanism, held in place by cast 'x' bar that I painted later. Careful with the cone it's only painted paper, I glued a tiny rip that I found. The fixing nuts can be tight.

Next came the speaker, it was working but very faint, so I dismantled it and gave it a good clean and oiled any moving parts .It's basically an electro magnet that moves the long copper tuning fork that then is fed to the centre of the speaker cone to push more air when vibrating. There is an air gap like in the horn type speaker that can jam-up so an adjustment bolt is fitted to the front of the unit and outside the cabinet when installed to open this gap. The loudest sound is when the copper rod almost touches the magnet.

The carry handle was rusty and the leather was missing, so I rust treated that and spayed metal gold after, then cut two pieces of buffalo leather (found on-line) marked two stich lines and hole possessions using masking tape. Then punched enough holes to stich some thick thread through, quite pleased with the result. 

Masking tape gave me a straight line but was a nightmare to remove after

Video shows inside and an all round view of this 92 year-old beauty, now restored to live another century.