Model 50A rebuild - Printable Version
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RE: Model 50A rebuild - GarySP - 06-30-2024
It looks like the shield on my model 20. There are a few of them on Ebay under philco tube shield. It might be a perfect fit to your needs, but if not they aren't that difficult to modify to fit. Just my dos centavos. Take care and BE HEALTHY! Gary
RE: Model 50A rebuild - Mtmattking - 07-06-2024
Thank for the explanation on the EM shield. This makes sense
I pulled off my variable cap and cleaned it. Now I have lost all reception but I still have an AC hum.
Any ideas?
I did note that both sides of my variable cap has both sets of plates when I reinstalled it. Is this normal for this circuit?
Thank you
RE: Model 50A rebuild - morzh - 07-06-2024
I am not sure what this means; a photo would help.
Meantime, see if you install it so that it shorted out.
I hope you were gentle when cleaning it. One bent vane will do it in.
RE: Model 50A rebuild - Mtmattking - 07-07-2024
After I had cleaned the Capacitor and reassemble, then I checked the vanes and put the VOM on it, and made sure that the Stationæry vanes were isolated from the moving vanes.
After reassembly and doing some troubleshooting one of the things I did, was put the vom on the variable capacitor and discovered that both the stationary vanes , and the moving vanes are connected electrically ie they are both grounded.
I checked the Internet and the electrical engineers say that some circuits have both sets of vanes ground out And other circuits are designed to have the vanes isolated from each other.
Which design should this Radio have?
RE: Model 50A rebuild - RodB - 07-07-2024
Hi Matt,
You're going to have to measure the resistance before connecting the capacitors in the circuit. Looking at the schematic, when the cap is in circuit, the secondary of each rf transformer is connected to the ungrounded terminal of a capacitor section. That secondary is probably a few ohms and looks like a short to ground to your VOM.
RE: Model 50A rebuild - Chas - 07-07-2024
The model 50 is a tuned RF receiver, two tubes progressively amplify the radio signal. To do that effectively each stage with one tube has one section of the tuning condenser connected mechanically with a common shaft called the rotor. The rotor is at "ground" potential, the fixed set of plates are called the stator, these are insulated and each section is connected to its respective tube. The third section is for tuning the detector. It was difficult to make a precise tuning condenser with multiple sections, so attached to the stator is a trimmer capacitor consisting of a blade of metal, a wafer of mica for an insulator an insulated washer and a nut or screw to compress the package to change the capacity.
What may have happened is that one or more of the trimmers was mal-adjusted or damaged during the cleaning process. By disconnecting the wire to the stator to each section of the tuning condenser an ohmmeter can determine if the trimmer has shorted. If the mica has been damaged it must be replaced. Mica can come from a salvaged tuning condenser, but if no mica can be found. A poor but workable substitute is a piece of PEET plastic taken from a water bottle, the thinnest and flattest as possible.
Plastic changes with temperature and moisture so it is not ideal but the radio will work at least until correct mica can be found.
With the tuning condenser now out of adjustment, if a station can be found around 1000khz, adjust the three trimmers to bring that station in the loudest. Any more precise adjust is possible, helpful is using an RF generator or a pocket transistor radio using its oscillator as a signal by local coupling. Follow any alignment instruction published by Philco. If a trimmer is tight and still not peaked (loudest signal the plastic is too thick...
Above assumes problem began with tuning condenser cleaning, but a short created during re-installation of the tuning condenser may have occurred too. Trace the schematic to the radio and do the measurements.
Chas
RE: Model 50A rebuild - Paul Philco322 - 07-07-2024
Used to find MICA sheets in old toasters.
Paul
RE: Model 50A rebuild - GarySP - 07-07-2024
There are hoards of mica supplies on Ebay as well. Take care and BE HEALTHY! Gary
RE: Model 50A rebuild - morzh - 07-07-2024
All three sections of the tuning cap are across the secondaries of their respective tuning circuits, so not only you should see a small resistance (I suspect units to couple of tens of Ohms) across each section, but also to the chassis.
As lon as it is not 0 Ohm, it is what it should be. Make sure your Ohmmeter can measure low Ohms.
If you see close to zero, unhook the wires to the respective section and re-measure: it should be open.
RE: Model 50A rebuild - Mtmattking - 07-15-2024
Cabinet question.
My 50a was picked up from a bus stop in Victoria, BC. Seriously.
The cabinet is delaminated to the extreme….say 60-80%.
I figure this is fine unit to start to learn how to repair and refinish the woodwork (plus this give me an excuse to purchase even more tools like a selection of C clamps).
It looks like a previous owner stained the cabinet. It might even have been painted OR was this entry level radio painted at the factory?
Opinions and experience most welcome!
PS: veneer is missing in sections. I will need to purchase new veneer to cut and fit. Should I purchase walnut or mahogany?
RE: Model 50A rebuild - Mtmattking - 07-20-2024
I have worked over the variable cap. It does seem to have the ohms cited above.
Still no sound.
Who would be able to supply me with the voltages going into and out of the tubes (the filaments voltage seem to all be stable as does the output of transformer).
There seems to be very little power going to the speaker leads.
Thank you
RE: Model 50A rebuild - Radioroslyn - 07-22-2024
Hi Matt,
Lets have a look over in the p/s and see what cooking over there. Measure the dc voltage from the chassis to the bottom of #32. Should see about -16v. Measure the top of #33. Should see about 250vdc+. This is the HV rail and if you follow it thru the circuit more than likely you can find the failure. Common plate voltage would be 200-250vdc except for the 80 and the 24 det tube. Screen grid voltage 75-100vdc but it's going to vary w/the volume control.
Common fails are rf coil pri/no plate on 24A plate, leaky/shorted caps in the HV rail, and out of spec resistor.
Happy hunting!
RE: Model 50A rebuild - Mtmattking - 07-22-2024
So, I have problems. Bottom voltage of #32 is -143VDC
Top voltage of #33 is 1.27VDC
Oh, heavens. What to do!
RE: Model 50A rebuild - Radioroslyn - 07-23-2024
Well I'm thinking your field coil is open. Should measure 1140 ohms or there abouts. The specs on the spkr are 7.5K to 1 ohm opt fc 1.4K @ 65ma. Philco model P,P2,and P16 should be good replacements.
You may be able to find the break in the coil if it's near the outside part of the winding. Or rewind it but may require pressing the pole out of the frame to remove the coil. If it has a nut on the back that holds the pole in place. Removing it and the pole will make it possible to remove the fc after removing the voice coil.