1930's tube radio various large solid colored resistors?

Thread Starter

sdowney717

Joined Jul 18, 2012
705
All yellow, green, etc... Any way to ID them? Radio does not work of course.
What way to make it work for starting replace all caps? Do resistors like these go bad?
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I dont know if they are bad or not.
here is capacitors
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here is the radio add and the radio

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MrChips

Joined Oct 2, 2009
24,609
Don't replace anything for now.
This is more than a repair job. It is a restoration job. It probably has more value (sentimental and historic) as a collector's item (museum collection) than a working radio. Decide why you need to have it working.

If you really want to have a working radio take the following steps.

1) Search for a schematic.
2) Failing that, reverse engineer the radio and draw the circuit schematic.
3) Take photos (wider view) of top and bottom from different angles so that we can see what you have.
4) ID and list all the tubes.
5) Measure resistances while still in circuit.
6) Don't replace capacitors unless you are sure that they need replacing.
7) Since you have already powered the unit, get a voltmeter and measure voltages.
 

LesJones

Joined Jan 8, 2017
3,570
The way the colour coding was done on resistors was different to the way it is done today. The first digit was the main body colour of the resistor. The second digit was the colour of one end of the resistor, The third digit (Decimal multiplier.) was a spot (Or sometimes a band near the center of the resistor. The relationship between colours and numbers was the same as it is today.

This is a possible source for a schematic. https://antiqueradioschematics.org/schematics-montgomeryward.htm


Les.
 
Last edited:

AlbertHall

Joined Jun 4, 2014
11,594
Old paper capacitors tend to go leaky so where they used to couple anode of one tube to grid of the next it puts a positive bias on that grid and messes up the DC conditions. If that second tube has the audio output transformer in the anode it can make it draw enough current to blow the transformer.
 

Thread Starter

sdowney717

Joined Jul 18, 2012
705
ok, got more pics
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He epoxied the wires going into this large condenser...as the insulation had fallen off. First he put on heat shrink, then epox, I dont know why the epoxy.
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LesJones

Joined Jan 8, 2017
3,570
It looks like it is older than anything I saw even as a child and the resistor colour coding is different using a single colour.
The schematic provided by MrChips will be a great help to get you started. I suggest starting by checking all the DC voltages.
It is a TRF ( Tuned radio frequency.) receiver and the volume control seems to control the gain of the first two RF stages rather than being in the audio path.

Les.
 

Thread Starter

sdowney717

Joined Jul 18, 2012
705
It looks like it is older than anything I saw even as a child and the resistor colour coding is different using a single colour.
The schematic provided by MrChips will be a great help to get you started. I suggest starting by checking all the DC voltages.
It is a TRF ( Tuned radio frequency.) receiver and the volume control seems to control the gain of the first two RF stages rather than being in the audio path.

Les.
What is the chance some of the tubes are weak? and how would you check them?
 

sagor

Joined Mar 10, 2019
511
Hm, I see the 500,000 ohm resistor at the final stage as the volume control. Other potentiometer may be RF gain.
The capacitor may have been epoxied because many old capacitors had oil in them, and the epoxy may be preventing any leaks.
 

LesJones

Joined Jan 8, 2017
3,570
Hi sagor, I took that 500K pot to be a crude tone control providing top cut but it could be the volume control depending on the source impedance from the driver transformer.
sdowney717, I have no experience of valves (Tubes.) before the WW2 era but I think it is likely the valves have degraded, If you let us know if you just want to make it work or you want to restore it to be close to original for historical purposes then it will help with the methods that we suggest. For example it might be possible to make plug in adapters to use B7G valves. There used to be some that were used in portable radios that had 1.4 volt filaments which could be used with a resistor in series with the filament to reduce the voltage. (I can't remember if these were directly heated which would cause a problem.) If you used more common B7G valves you would need to find a way to provide 6.3 volts for the filaments. I have even seen suggestions of using FETs to replace valves. These methods would not be suitable for historical restoration. I remember seeing methods to extend the life of cathode ray tubes (Which would probably work with valves.) which I think exposed new cathode material. I think it involves running the heater at a slightly higher voltage and drawing a higher than rated anode current. (This also had the risk of destroying the CRT / valve.)
I think you will probably find better advice than this forum if you search for something like "Restoring old radios"

Les.
 
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