ST300 now working! - UK Vintage Radio Repair and Restoration Discussion Forum

phil.turrell · 11th Aug 2011, 11:44 pm

Hello,

In March 2010 I asked forum members for help in identifying a radio I had just bought from Spitalfields market for £35.

https://www.vintage-radio.net/forum/showthread.php?t=51721&highlight=ST300

It turned out to be a kit radio for a design by John Scott-Taggart published in The Wireless Constructor in February 1932. It's taken a long time to get it working, mostly because I didn't have a suitable power supply. However, I can say that it works very well indeed, especially on LW with a frame aerial.

From the start I thought it would be best to preserve the work of the person who originally made the radio, so I decided not to replace any of the perished solid core wiring. It didn't really matter that the insulation was perished as long as it didn't short out. So I simply added any extra wiring that was missing (such as the connections to the front panel on-off switch, the top cap of the SG valve and the grid bias). I also put some sleeving on the wiring in two places where it passed through the metal screen. Other than that I simply loosened the existing connections, scrapped them where appropriate, sprayed some contact cleaner and then re-tightened them.

As far as the components are concerned, the radio is running with all the original parts, as purchased, bar the valves! Unfortunately, the valves in the radio as purchased were not correct. I was advised that PM12 (SG HF), PM1HL (triode detector) and PM2 (triode output) would be good choices. I bought these from Gerry of Crowthorne tubes.

The first bit of remedial work was to free up the motion of the reaction variable capacitor. I needed to take it apart for cleaning. Unfortunately the grub screw holding the knob snapped off and so had to be drilled out. This was a bit tricky but after cleaning the reaction control is now fairly smooth.

The other areas that needed attention were the reaction choke (which was open circuit), the SG choke (which had a very high resistance > 1M ohm) and the spaghetti resistance which also appeared to be open circuit. The two 1uF block capacitors measured 2-3uF on my meter but were also rather leaky.

I had to drill out the rivet holding the top of the reaction choke. The pitch like substance on the underside simply fell out. The wires had become disconnected internally because the screw terminals had rotated. I extended them with some fine enamelled copper wire, and then resoldered and tightened up the terminals. I then filled the void around the base of the coil and terminals with glue from a gun (to replace the pitch). All was then well and the choke registed a suitable inductance on my meter.

The SG choke was more problematic. It showed a very high resitance between the two coils. I assumed that this was due to corrosion around the connection tabs. In preparation to resolder them I gave them a scrap, but only succeeded in breaking both enamelled wires off short. I then had to extend them (as with the reaction choke) and remake the connections. This done the choke still showed a high resistance! I then undid the bolts holding the two coils and found that the resistance problem was due to the way the coils were linked at the base. The enamelled wires simply emerged from the base of the coils and were clamped to a metal strip by the action of the bolts holding down the coils. Naturally, after 80 years the wire had oxidized. I scrapped the ends carefully and re-assembed the coils. The choke then showed a suitable inductance on my meter.

The spaghetti resistance had become corroded at the crimped ends. I opened out the crimping slightly, sprayed some contact cleaner and then re-crimped. The resistance then measured 22K, pretty close to the 20K on the compoent label!

I had assumed that the two 1uF paper dielectric block capcitors would need to be restuffed. However, I had recently read an article by Doug Postlethwaite ("Degenerate Capacitors, Radio Bygones, April/May 2011) in which he recommended submitting paper dielectric capacitors to hours of high voltage treatment, reversing the connections at intervals. He found that in many cases the leakage current went down to acceptable levels and that the effect appeared to be long lasting. So it proved with these 1uF capacitors.

Initially, at 150 volts the leakage current was around 1.8 mA (although one of them was a little better than the other). After hours and hours at 150V, with the connections reversed periodically, the leakage current declined to 200-300 uA. I was quite pleased with this and thought it would probably be acceptable.

Other than the electrical work above I've not done much to the radio externally, apart from to clean the face of the knobs with Brasso, clean the rear terminal strip and wax the case with Antiquax.

The power supply is largely based on an article that appeared in the May 2007 edition of Practical Wireless ("VariHV a valve power supply" by Stefan Niewiadomski). His design has a single variable HT output using a power MOSFET. I replicated part of this circuitry three times to produce three variable outputs. I also added regulated 2v and 6v fixed outputs, and a regulated 1-14 grid bias output (all using commonly obtainable regulator ICs).

For the ST300 I set HT1 to 60 volts, HT2 to 75 volts and HT3 to 120 volts. I set the grid bias to 7.5 volts.

The final part of the story is the speaker. The radio did not come with a speaker but was designed for use with either high impedance headphones or a horn speaker. The speaker would have directly formed the anode load.

I did not fancy using my high impedance headphones (The 120 volts either side of my head struck me as dangerous !) and horn speakers are difficult to obtain and command high prices. I therefore decided to use a minature mains transformer and 200mW 8ohm speaker. (I believe that the PM2 is only capable of about 100mW). The transformer is a 240 primary, 0-6, 0-6 secondary. I connected the secondary in series to become 0-12. With an 8ohm speaker this gives approximately the correct load impedance for the valve, although I think it's not critical anyway.

This arrangment produces quite good, if somewhat quiet audio, but this is really due to the smallness of the speaker. However, I found that mounting the speaker in a kitchen funnel greatly improves the volume (perhaps by as much as 50%) and the radio 4 is quite audible from the next room.

I know that this was a long posting but I wanted to share the fact that I've got this antique radio going with a fairly small amount of effort (once I'd made a suitable power supply) and the results are really very good, much better than anticipated!

Regards

Phil.

Paul LS · 12th Aug 2011, 6:47 am

Great Success Story Phil. I have a couple of early 1930 radios that I have been trying to get around to. It has taken me a good 8 or 9 months to build the multiple regulated power supplies, life has been busy, but at last they are up and running. Your article has spurred me on to get on with the restoration. Good job...

kalee20 · 12th Aug 2011, 8:30 pm

Yes, well done!

The only bit I'd feel uncomfortable about, is the paper dielectric condensers - I'd be wary about trying to recover these. However, if failure (leakage) has no damaging effects - and I expect in this set that leakage will put a drain on the HT rail but nothing more - then you can at least sleep at nights without worrying you'll over-run a valve.

It's nice to think you've preserved some anonymous, possibly long-gone, builder's work from the past.

11th Aug 2011, 11:44 pm	#1
phil.turrell Triode Join Date: Feb 2005 Location: Basingstoke, Hampshire, UK. Posts: 34	ST300 now working! Hello, In March 2010 I asked forum members for help in identifying a radio I had just bought from Spitalfields market for £35. https://www.vintage-radio.net/forum/showthread.php?t=51721&highlight=ST300 It turned out to be a kit radio for a design by John Scott-Taggart published in The Wireless Constructor in February 1932. It's taken a long time to get it working, mostly because I didn't have a suitable power supply. However, I can say that it works very well indeed, especially on LW with a frame aerial. From the start I thought it would be best to preserve the work of the person who originally made the radio, so I decided not to replace any of the perished solid core wiring. It didn't really matter that the insulation was perished as long as it didn't short out. So I simply added any extra wiring that was missing (such as the connections to the front panel on-off switch, the top cap of the SG valve and the grid bias). I also put some sleeving on the wiring in two places where it passed through the metal screen. Other than that I simply loosened the existing connections, scrapped them where appropriate, sprayed some contact cleaner and then re-tightened them. As far as the components are concerned, the radio is running with all the original parts, as purchased, bar the valves! Unfortunately, the valves in the radio as purchased were not correct. I was advised that PM12 (SG HF), PM1HL (triode detector) and PM2 (triode output) would be good choices. I bought these from Gerry of Crowthorne tubes. The first bit of remedial work was to free up the motion of the reaction variable capacitor. I needed to take it apart for cleaning. Unfortunately the grub screw holding the knob snapped off and so had to be drilled out. This was a bit tricky but after cleaning the reaction control is now fairly smooth. The other areas that needed attention were the reaction choke (which was open circuit), the SG choke (which had a very high resistance > 1M ohm) and the spaghetti resistance which also appeared to be open circuit. The two 1uF block capacitors measured 2-3uF on my meter but were also rather leaky. I had to drill out the rivet holding the top of the reaction choke. The pitch like substance on the underside simply fell out. The wires had become disconnected internally because the screw terminals had rotated. I extended them with some fine enamelled copper wire, and then resoldered and tightened up the terminals. I then filled the void around the base of the coil and terminals with glue from a gun (to replace the pitch). All was then well and the choke registed a suitable inductance on my meter. The SG choke was more problematic. It showed a very high resitance between the two coils. I assumed that this was due to corrosion around the connection tabs. In preparation to resolder them I gave them a scrap, but only succeeded in breaking both enamelled wires off short. I then had to extend them (as with the reaction choke) and remake the connections. This done the choke still showed a high resistance! I then undid the bolts holding the two coils and found that the resistance problem was due to the way the coils were linked at the base. The enamelled wires simply emerged from the base of the coils and were clamped to a metal strip by the action of the bolts holding down the coils. Naturally, after 80 years the wire had oxidized. I scrapped the ends carefully and re-assembed the coils. The choke then showed a suitable inductance on my meter. The spaghetti resistance had become corroded at the crimped ends. I opened out the crimping slightly, sprayed some contact cleaner and then re-crimped. The resistance then measured 22K, pretty close to the 20K on the compoent label! I had assumed that the two 1uF paper dielectric block capcitors would need to be restuffed. However, I had recently read an article by Doug Postlethwaite ("Degenerate Capacitors, Radio Bygones, April/May 2011) in which he recommended submitting paper dielectric capacitors to hours of high voltage treatment, reversing the connections at intervals. He found that in many cases the leakage current went down to acceptable levels and that the effect appeared to be long lasting. So it proved with these 1uF capacitors. Initially, at 150 volts the leakage current was around 1.8 mA (although one of them was a little better than the other). After hours and hours at 150V, with the connections reversed periodically, the leakage current declined to 200-300 uA. I was quite pleased with this and thought it would probably be acceptable. Other than the electrical work above I've not done much to the radio externally, apart from to clean the face of the knobs with Brasso, clean the rear terminal strip and wax the case with Antiquax. The power supply is largely based on an article that appeared in the May 2007 edition of Practical Wireless ("VariHV a valve power supply" by Stefan Niewiadomski). His design has a single variable HT output using a power MOSFET. I replicated part of this circuitry three times to produce three variable outputs. I also added regulated 2v and 6v fixed outputs, and a regulated 1-14 grid bias output (all using commonly obtainable regulator ICs). For the ST300 I set HT1 to 60 volts, HT2 to 75 volts and HT3 to 120 volts. I set the grid bias to 7.5 volts. The final part of the story is the speaker. The radio did not come with a speaker but was designed for use with either high impedance headphones or a horn speaker. The speaker would have directly formed the anode load. I did not fancy using my high impedance headphones (The 120 volts either side of my head struck me as dangerous !) and horn speakers are difficult to obtain and command high prices. I therefore decided to use a minature mains transformer and 200mW 8ohm speaker. (I believe that the PM2 is only capable of about 100mW). The transformer is a 240 primary, 0-6, 0-6 secondary. I connected the secondary in series to become 0-12. With an 8ohm speaker this gives approximately the correct load impedance for the valve, although I think it's not critical anyway. This arrangment produces quite good, if somewhat quiet audio, but this is really due to the smallness of the speaker. However, I found that mounting the speaker in a kitchen funnel greatly improves the volume (perhaps by as much as 50%) and the radio 4 is quite audible from the next room. I know that this was a long posting but I wanted to share the fact that I've got this antique radio going with a fairly small amount of effort (once I'd made a suitable power supply) and the results are really very good, much better than anticipated! Regards Phil.

12th Aug 2011, 6:47 am	#2
Paul LS Retired Dormant Member Join Date: Mar 2009 Location: Southampton, Hampshire, UK. Posts: 196	Re: ST300 now working! Great Success Story Phil. I have a couple of early 1930 radios that I have been trying to get around to. It has taken me a good 8 or 9 months to build the multiple regulated power supplies, life has been busy, but at last they are up and running. Your article has spurred me on to get on with the restoration. Good job...

12th Aug 2011, 8:30 pm	#3
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,081	Re: ST300 now working! Yes, well done! The only bit I'd feel uncomfortable about, is the paper dielectric condensers - I'd be wary about trying to recover these. However, if failure (leakage) has no damaging effects - and I expect in this set that leakage will put a drain on the HT rail but nothing more - then you can at least sleep at nights without worrying you'll over-run a valve. It's nice to think you've preserved some anonymous, possibly long-gone, builder's work from the past.