Help sourcing two resistors

[hotrodjohn71]

Hi group.
Can you help me source 2 resistors I need for my 57 Ford truck radio?
Ive searched for about a half hour but am coming up short.

One is a 680k 20% 1/2 watt
And one is 15M 20% 1/2 watt

Thank you

 

[Lightium]

https://www.mouser.com/c/passive-components/resistors/through-hole-resistors/?power%20rating=1%2F2%20W%20%28500%20mW%29~~500%20mW%20%281%2F2%20W%29&resistance=680%20kOhms%7C~15%20MOhms&rp=passive-components%2Fresistors%2Fthrough-hole-resistors%7C~Power%20Rating

 

[hotrodjohn71]

Thank you. I find a 1% and a 5%, but not a 20%.
Is it acceptable to use a smaller % in place of the 20%?

 

[Lightium]

The symbol is a measure of tolerance. The smaller the better. A resistor with 20% tolerance would be plus or minus 20% in ohms value.

 

[Diver300]

If you measure resistors in circuit, the reading you get can be lower because of other components on the board.

Please tell us the outcome.

 

[drtechno]

Hi group.
Can you help me source 2 resistors I need for my 57 Ford truck radio?
Ive searched for about a half hour but am coming up short.

One is a 680k 20% 1/2 watt
And one is 15M 20% 1/2 watt

Thank you

20% resistors are not made anymore and the circuit would operate better with the modern lower tolerances.
But I doubt the resistors are bad in the first place because they are grossly oversized in the first place wattage wise so the resistor's Johnson noise is not predominate in the circuit to be amplified.

The only way to test resistors is to lift one leg and test it out of the circuit. the one only measuring 210 ohms might have a transformer winding in parallel or even a potentiometr in parallel.

Which one is it?
The model number is usually stamped on it, sometimes in a corner. But looking at your pictures I'm going to guess its a 75MF. Someone will have a schematic of this as these things did come in the TV repair shop to fix every once in a while. So its either in the Bettman or the Sams photofact manuals.

 

[Nigel Goodwin]

20% resistors are not made anymore and the circuit would operate better with the modern lower tolerances.
But I doubt the resistors are bad in the first place because they are grossly oversized in the first place wattage wise

Possibly not, old resistors, and particularly high value ones, were notorious for drifting high in value - or even going O/C.

Obviously 1% resistors would be perfectly fine as replacements, but it's unlikely the circuit will operate any better, as it was designed to use 20% tolerance parts.

However, it's obviously dead easy to test them, lift one end out and check on an ohm meter - but while you're unsoldering one end, you may as well unsolder both and simply replace them with more reliable modern resistors.

 

[aliarifat794]

Did you contact digikey?

 

[hotrodjohn71]

Yes. I have the resistors coming. Thank you for the reference.

This is a 1957 Ford truck radio. I found the diagrams online through a group called Radio Museum.

I am a novice in electronics so I appreciate the advice.

There is a guy who did a rudimentary step by step repair of the radio that has been a big help.

He stated that there werent any parallel circuits so I didnt remove the components to test them.

I also have a working radio that I did component by component testing side by side.

Im awaiting the resistors with high hopes.

 

[Nigel Goodwin]

He stated that there werent any parallel circuits so I didnt remove the components to test them.

And what did the resistors read in-circuit?

 

[hotrodjohn71]

Power off, (2) resistors tested bad.
One was a 15M (20%) that showed infinate resistance, and one was a 680K (20%) that was down in the 200-300k ballpark.

 

[Nigel Goodwin]

Power off, (2) resistors tested bad.
One was a 15M (20%) that showed infinate resistance, and one was a 680K (20%) that was down in the 200-300k ballpark.

Two things:

1) Does your meter go up as high as 15M? - if not it would just show as over-range.

2) Such resistors rarely go low, if it's reading 200-300K then it's probably reading through something else in-circuit. However, you've got new (better) ones on the way, so test those before you put them in (to see how they read), and then test the old ones when you've taken them out. Mainly as a matter of interest.

As for resistors going low, I only remember one instance of it ever happening in my 46 years of repairing TV's, and that was a 39K resistor in an old GEC B&W TV, I seem to remember it was part of a screen feed potential divider (possibly on the video amplifier?), and was a common failure on that one particular chassis. Once you replaced it, with a better quality resistor, it never failed again.

 

[drtechno]

Obviously 1% resistors would be perfectly fine as replacements, but it's unlikely the circuit will operate any better, as it was designed to use 20% tolerance parts.

Tube circuits are not that critical with resistor values. That is why a lot of consumer tube electronics have 20% resistors back in the 50s and before.

 

[drtechno]

One was a 15M (20%) that showed infinate resistance, and one was a 680K (20%) that was down in the 200-300k ballpark.

The 15M resistor can only be tested with ohm meters that can go that high, otherwise its open (which I doubt) The 680K has something in parallel because resistors don't short on failing and I doubt any resistors are bad. Usually all you have to do with these car radios is recap them because the paper and oil caps would have went bad by now and you would just stick some polypropylene caps and electrolytic caps and move on.

 

[iggnator]

Yes. I have the resistors coming. Thank you for the reference.

This is a 1957 Ford truck radio. I found the diagrams online through a group called Radio Museum.

I am a novice in electronics so I appreciate the advice.

There is a guy who did a rudimentary step by step repair of the radio that has been a big help.

He stated that there werent any parallel circuits so I didnt remove the components to test them.

I also have a working radio that I did component by component testing side by side.

Im awaiting the resistors with high hopes.

Nice to see a restoration of this antique radio, for a classic truck. Interesting there was a component vendor change between the two radios.
Have you measured the power supply output? I think for tubes, they used a vibrating relay, that fed early solid state selenium diodes and capacitors to make voltage double-ers, or fed pulses to a transformer. 40volts DC is stuck in my head as a plate voltage for the tubes/valves
Could you post a pdf or jpg of the schematic.

 

[hotrodjohn71]

My meter goes to 20M, but I have a new and better meter on the way.

I changed out the capacitors (two were foil/wax, and one was a tall cannister style) with equivalent new caps.

After doing that, I still was not getting reception and thats when I checked the other components.

Thank you for sharing the info.

 

[hotrodjohn71]

My apologies as I am not able to convert these PDF diagrams to jpeg.

I checked the tubes by swapping them into the working radio.

In all, I have 3 radios. 2 work and one does not.

 

[Nigel Goodwin]

Tube circuits are not that critical with resistor values. That is why a lot of consumer tube electronics have 20% resistors back in the 50s and before.

Neither were transistor circuits

The reason for 20% was cost, they were far cheaper than 10% or 5% ones.

 

[Nigel Goodwin]

If the 680K is R5?, then it's likely either C2, C3 or C4 are leaking making it read low.

And if the 15M is truly O/C it would upset the biasing as it's the grid leak resistor for the AF valve, and it's pretty non-critical as regards it's value, anything over 2.2M/4.7M shoulsdbe fine, O/C could be an issue though.

 

[iggnator]

My apologies as I am not able to convert these PDF diagrams to jpeg.

I checked the tubes by swapping them into the working radio.

In all, I have 3 radios. 2 work and one does not.

Thank you, I can see the schematic, but somethings missing as I must be blind as I can't find A5, A6, A7, A8, tuning inductors. I see it's all tube type, until the final audio output which is a PNP, And everything runs on 12VDC input, not voltage doublers or other plate voltage boosters. The heaters in all the tubes are 12volt, So the final output transistor is an Germanium 2N285 PNP. Pretty good schematic notes. It's an old Sam's Phtofact schematic.

The pdf's you attached were protected, so I used a cutepdf writer program to print them to a file, so I could grab the graphics and copy paste into this reply.
As well you could get a cheap RF generator from eBay to do the alignment. But I didn't see the generator setup in the notes. Do they want some tone signal with a amplitude modulated RF carrier in the bandwidth of the AM radio band? An old AM radio repair guy would know this. I'm just a wanna bee RF technician.