Lance Neame
Tinkerer
Ham - Amateur Radio Operator, keen on valve / tube based technology
Posts: 51
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Post by Lance Neame on Aug 13, 2022 21:23:27 GMT 12
Hello to the group, Thanks kindly for reading this post firstly!
My Uncle in Blenheim has recently bought a Philco model 220 Radiogram dating from around 1931 from an antique store.
Apparently they used the same Chassis as the Model 20 Cathedral radio.
And I will travel up from Christchurch to attempt to get it running for him.
My question is does anyone have experience with these Model 20 Philco TRF sets?
And can perhaps advise if there were any particular problems I should be aware of?
I would love to receive any good advice please?
Once again my thanks for reading to this point!
Warm regards, Lance.
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Lance Neame
Tinkerer
Ham - Amateur Radio Operator, keen on valve / tube based technology
Posts: 51
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Post by Lance Neame on Aug 13, 2022 21:41:24 GMT 12
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Steve
Society Members
vintageradio.co.nz
Posts: 725
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Post by Steve on Aug 16, 2022 22:38:30 GMT 12
Hi Lance, I have no experience with those early Philcos, but I have just emailed you Ghirardi's troubleshooter section on the model 20 which might be helpful
Cheers, Steve
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Post by Philconut on Sept 13, 2022 22:04:06 GMT 12
Hi Lance, I haven't done a Philco 20 but I have done a couple of other TRF sets from 1930. As always, it is the capacitors that need to be replaced - including the electrolytics. The problem with both my sets was power supply chokes which unfortunately were potted with pitch in cans. It took a while with a heat gun to extract the bits. The designers of these seemed to like to put as much as possible into cans - I suppose they didn't give much thought to future restoration! Apart from this constructional difficulty, the TRF sets are reasonably simple to repair. Watch the biasing of the anode bend detector. It has to be just right and even then the distortion on music is quite noticeable. - they are HORRIBLE things!
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Steve
Society Members
vintageradio.co.nz
Posts: 725
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Post by Steve on Sept 20, 2022 11:28:51 GMT 12
Its funny - I only recently started playing with sets with anode-bend detection and I have to say that I quite like the sound - but probably just from a nostalgiac perspective - distorted? yes, but the effect is almost what you expect from an early set so it kind of sounds 'right'(ish). I did find, as you mention John, that the biasing is critical to getting the sound just right and some experimentation of the cathode resistor might be required
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Lance Neame
Tinkerer
Ham - Amateur Radio Operator, keen on valve / tube based technology
Posts: 51
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Post by Lance Neame on Sept 28, 2022 14:49:42 GMT 12
Turns out that extensive modifications have been done. A type 27 is in the shield and a type 42 is beside that. I am not sure exactly how to proceed??? Just repair the modified radio, or take it back to Christchurch and look at it properly? The underside has seen much work. THanks very much for your replies!!!
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Post by Richard on Sept 28, 2022 15:24:07 GMT 12
The 27 is probably the detector and this model used to have push-pull output with an interstage transformer providing coupling and phase splitting to the push-pull output stage. MY guess is that the interstage transformer went open circuit and someone decided to just install a single ended output stage using a 42. IMO,I would keep it "as is", otherwise you could be opening a pandoras box ? I'm sure the 42 output stage will be fine, just check hes got the appropriate output transformer installed? that most likely is open circuit ! try to draw out the circuit, as it stands.
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Lance Neame
Tinkerer
Ham - Amateur Radio Operator, keen on valve / tube based technology
Posts: 51
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Post by Lance Neame on Sept 28, 2022 17:44:38 GMT 12
Hi Richard, Yes, good advice thanks. The most practical way forward is as you suggest, draw the circuit out, and repair as found. The radio is now operating, but I haven't changed components for longer term operation. A broken solder joint on the replacement speaker was stopping it from producing sound. Onwards and upwards 🤣
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Lance Neame
Tinkerer
Ham - Amateur Radio Operator, keen on valve / tube based technology
Posts: 51
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Post by Lance Neame on Sept 28, 2022 19:18:57 GMT 12
Replacement Rola electrodynamic speaker With transformer attached
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Post by Richard on Sept 29, 2022 8:34:42 GMT 12
I just had a look at the schematic/chassis layout for this model, it looks like your radio has had an extra audio stage added as well, using a 27 tube? maybe to get enough gain to drive the 42?, otherwise there should be a spare tube socket on the chassis with the removal of the push-pull output stage by someone in the past! Drawing the existing configuration out and checking it, maybe a good option for you.
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Lance Neame
Tinkerer
Ham - Amateur Radio Operator, keen on valve / tube based technology
Posts: 51
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Post by Lance Neame on Sept 29, 2022 9:21:14 GMT 12
Hi Richard, Yes you are 100% correct!!! The 'spare, 71A socket now has an extra type 27 tube as a driver for the added type 42 tube and matching socket in place of the other type 71A , so quite a deviation from the original circuit. I have yet to measure the heater voltage on the type 42, but the wanted voltage would be 6.3v at 0.7a I believe. The original type 71A consumed 5v at 0.25A each. So the two type 71A's would be a good half Amp total at 5 Volts. I wonder if there is much difference in overall B+ consumption??? Thanks kindly for your help! Warm regards, Lance.
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Lance Neame
Tinkerer
Ham - Amateur Radio Operator, keen on valve / tube based technology
Posts: 51
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Post by Lance Neame on Sept 29, 2022 9:29:06 GMT 12
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Post by Philconut on Sept 29, 2022 11:04:24 GMT 12
that's interesting as there must have been a separate winding for the 71A heaters at 5V. How is the 6.3V for the 42 supplied? oOn 5V the emission and therefore performance of the 42 will be down quite a bit.
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Lance Neame
Tinkerer
Ham - Amateur Radio Operator, keen on valve / tube based technology
Posts: 51
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Post by Lance Neame on Sept 29, 2022 11:22:48 GMT 12
Thanks, on the Model 20 schematic, there can be seen a separate winding 'C' To supply the two 71A heaters.
I have just poked the multimeter on, and yes, the valve is getting a much lower voltage.
Mu reading is below 5v, as the type 42 requires 4.4 watts of power, whereas the two type 71A's consume a total of 2.5 watts if my rough calculations are correct.
I am wondering if the extra type 27 and 'new' 42 are putting unnecessary stress on the power transformer, and may be shortening the life of it in time???
Your thoughts would be awesome please? Many thanks once again! Regards, Lance.
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Post by Peter Walsham on Sept 29, 2022 11:39:43 GMT 12
You could wire that 5 Volt winding in series with one half of the tapped 2.5 Volt winding (in phase) to give you a heater supply of 6.25 Volts - which would be way better than 5 Volts. You will need to disconnect the resistor '25' from the centre tap of the 5 Volt winding first (if it hasn't already been disconnected) and leave the 5 Volt centre tap unconnected to anything. Personally, I doubt that the extra 2 odd watts of heater power consumption (assuming using a type 42 output valve) would overload the power transformer.
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