Rudolph Airline 05gcb 1540-solved

Thanks Cliff, that had crossed my mind. I think that will be my best bet!
Rickey

It looks like the Airline name is depressed and could be filled in with gold or silver paint. You can also do the same to the frequency numbers. if the letters are raised you could use a product used as a rubbing to gilt frames, vases and such.

Well, ole Rudolph is looking good and playing better! I just need some ideas on filling in the label
lettering. Thanks again!
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Thanks for your help Cliff! I played it for a couple of hours yesterday. Now on to another one.

That doesn't mean it has to be negative or at 0 Volts. as long as it's negative in respect to the cathode from my understanding any way. As long as it works!

Most measurements are usually taken with the volume control at maximum and no signal in to the radio.

The values I gave are for a Generic set, different manufacturers have shortcuts to get things working. I wouldn't worry as long as it works. It might be of concern if it were a multifunction (Shortwave, AM, and FM) as the voltages will change (even ON the grids)

Or it were high end set for the Military. But general radios can have voltages as much as 20% because of using slack component tolerances. The only place I can think of that it might matter is the oscillator.

Great to hear its working.
I would do two things, 1st, put a polarized plug on it and a fuse, and 2nd, let it run for at least 5 to 6 hours just to make sure you get no surprises down the line.

Unless you have anything more on this set I will marked it as fixed or solved.

ok, wrong bulb. I put in a new one and it burns bright. Voltages are as follows, and all are dc positive:
pin 1 6.5
pin 2 1.0 :evil:
pin 5 1.0
pin 6 105
pin 7 121
Radio is playing great! A few resistors made a big difference. What would you do to get the minus voltage corrected?
Thanks, Rickey

What voltage do you get?
On pin 2 that is?

Taking the values from a general schematic here are the approximate readings you should get with a 20k Ohm/per-volt multimeter.


This is for 50C5
(pin 1 cathode-grid 3 6V)
(pins 2 & 5 grid 1 0V)
(pin 3 heater)
(pin 4 heater)
(pin 6 grid 2 95V to 101V)
(pin 7 plate 84V to 120V)

NOTE: Variations due to different Circuits
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This is for a 50L6 GT Just to compare against
Pins
1= 0V nc
2= 80VAC heater
3= 125VDC plate
4= 95VDC g2
5= 0V g1
6= 0V nc
7= 37VAC heater
8= 6VDC k-g3

Does the pilot light still glow dimly?
If it does then you may have the wrong bulb. Or there is also a resistor in series with the filaments that may have changed value-if it has then presumabley it went up in resistance and would decrease the curent needed in the filament circuit. Hence dim bulb.
If it glows dimly then you are not getting the filament voltage on the rectifier split correctly and one leg will have an over-voltage and will burn out your rectifier a lot quicker.
If the bulb is left out then the tube will burn out in just a couple of months.

Replaced the 10 Meg and a few other resistors. The radio is playing much better!
I still get low positive voltage on pin 1 of 50c5. I'm going to get another 50c5 and measure voltages again. The radio improved the most when I changed the 6.8 Meg resistor!
Thanks!

All caps were changed to begin with. I get very low positive volts on pin 2. The audio tube gets hotter than a 2 dollar pistol! The dial light glows very dim also? I replaced the cap going to the first audio again? Any thoughts would be appreciated!
Thanks

Thanks Cliff, yes just touching the probe on the cathode of the 50c5 caused a dramatic improvement, like it was adding capacitance. Maybe there is a clue in this.
Thanks

That's a clue. If probing caused the distortion to disappear then you could have a bad wire, cold solder joint, or bad socket contact.

This also may be evident in radios that don't work till probed, sometimes the effect of additional capacitance or minuscule voltage breaks down a barrier and then it will start working TEMPORARILLY.

I was editing my last post while you posted, so please re-read. Thanks.

Thanks Cliff, the distortion is like a hum, kinda like when an electrolytic is bad. I tried other electrolytics but no change. The only improvement is when I was checking voltages on 50c5.
Rickey

You should replace the 10meg also if its 20% tolerance 2meg variance is the limit, so 2-10=8megs minimum 2+10 = 12megs. maximum. Your at 17megs way too high that will limit the current and will seriously effect operation of the circuit.

I don't know what the actual voltages should be, because of engineering short cuts to standard designs also because of not enough parts at the factory or costs of parts changed so modifications were implemented.

Another point is make sure the right tube is in the right socket, it will make a huge difference in voltage readings.

About the only time you would use the AC volts measurement is on the power input to the power supply and filaments.

All other measurements would be DC with no signal and volume at minimum.
If you want to See AC voltages they would be of no help unless you want to see gain or distortion. An Oscope would be more helpful.

Just realize that you will see 0 volts ac on any capacitor that is connected to ground because caps are used to act as a shunt to ground for AC signals (bypass) but blocks the DC from going to ground.

You might find that the voltage will change with a voltmeter when you are measuring around High resistances, 10 megs or so as the volt meters are standardized at 20 megohms so it could load the circuit being an additional added resistance, at lower resistances this would not be the case however.

the resistors are beyond 20 percent in a couple of cases(30) percent.
Resistor 3 should be a 10 meg and it reads 17 meg! its on the 12at6 on pins 1 and 2, grid and cathode. I ordered a few resistors from mouser, but i dont think i ordered the 10 meg.
Thanks,Rickey

Now i have some voltages. The cathode on the 50c5 reads 5 volts dc. The cathode resistor reads 190, a little out of the 20 percent range, should be 150. I get low positive voltage on the grid pin ,1.2 volts. I should be on dc, correct? The radio improves vastly when i check the voltage on the cathode!
Rickey

Thanks Cliff, I'm learning!
Rickey

Signal coupling capacitors

Because capacitors pass AC but block DC signals (when charged up to the applied dc voltage), they are often used to separate the AC and DC components of a signal. This method is known as AC coupling or "capacitive coupling". Here, a large value of capacitance, whose value need not be accurately controlled, but whose reactance is small at the signal frequency, is employed.

Decoupling or bypass

A decoupling capacitor is a capacitor used to protect one part of a circuit from the effect of another, for instance to suppress noise or transients or even unwanted signals caused by other circuit elements is shunted through the capacitor, reducing the effect they have on the rest of the circuit.
Coupling capacitors are used from one tube stage to the next, such as from plate to grid to allow only the AC signal to pass. The grid circuit uses a resistor to usually be negative in reference to the plate then the signal rides on top of the negative voltage to control the amplification of its tube.
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Distortion can be caused by the wrong bias, overly strong radio signals, shorted or open caps.

Just a thought:
One thing that you may need to do is to adjust the IF trimmers, to bring them into alignment, if they are not centered at the correct frequency then you can get distortion, also if the signal is to strong, it maybe caused by the AVC not working.

I am starting to give theory here and I am trying not to go down that path as it gets hard to explain.

What is meant by bypass caps and coupling caps? Is that all the ones besides the electrolytics?
Thanks!

Thanks Cliff, I'm gonna do a lot of checking. The audio transformer can cause distortion, correct?
I have already put an audio transformer in this radio ,as the original one was bad ,and no audio.
What would cause these transformers to go bad? I will check the one that's in it now.
Thanks

Can resistors be checked connected?

If the resistor is connected to a tube with no other connections (In other words only one resistor lead is present at the tube connection and nothing else, you can remove the tube and test the resistor in place.) other wise lift one lead and test,
For resistors its best to heat sink the resistor as you solder or remove the resistor to prevent it from changing values, (just my leaning)

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And the same for capacitors. However for most this isn't possible, so lift the one lead of the resistor or cap and use an Ohms check to find its actual value.

The simplest test for capacitors is to short the leads together with a 1k Ohm resistor no power applied UNPLUGGED (and tubes removed) The resistor will safely discharge the capacitor and limit the current discharge to a safe value without stressing the capacitor. Leave the resistor across the capacitor for at least 10 seconds (just my opinion here). Then short directly with a jumper wire. After that observing polarity with the ohm meter red lead to positive connection and black to negative side. The Ohm meter should show infinity to start and then move towards 0 ohms or some resistance till it stops. It depends on the meter, but what is happening is the meter battery is charging the capacitor, that is a good sign that the cap is alright. If the meter shows infinity or high ohms or close to it the cap is probably OK.

If it shows Low ohms or 0 ohms the cap is shorted. If doesn't move the needle at all the cap is open (also bad)

NOTE: I would make sure of the polarity but you can jumper caps as long as the cap is no more than the cap value or less.
I would only do this for testing the cap in place.
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The distortion is usually caused by too much voltage on the tube elements. It may be a short in the audio transformer drawing down the voltage, or one of the bypass caps being open or shorted. If a resistor is bad then the circuit suffers from not enough voltage (LOW RESISTANCE) or lower voltage drop or higher resistance that causes not enough current flow.

ok y'all talked me into it! Put in lamp, and 35w4, and back to life again! It plays strong and loud ,yet distorted. I jumpered in electrolytics and no change. It only came with a straight wire hank for antenna, no loop. Tried all tubes from working set, no change. Can resistors be checked connected? Can i jumper in the other caps or resistors? Thanks for your help Cliff, im learning a lot!
Rickey

By all means go for it.

Use a Dim bulb that has at least twice the wattage of radio. What you want to see is the bulb glow brightly to start with and them dim down, then start to glow slowly until at about half brightness. If the radio is OK then the brightness will remain at a constant lower brightness. If the radio is bad then you will get full brightness all the time and this indicates a short.
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Will set work at all without lamp?
The set will work without the lamp but it will run at a higher voltage and shorten the tube life
The brown bead in the lamp indicates the current draw of the dial or pilot lamp.
S40 = 6.3V 0.15amps screw base
S47 = 6.3V 0.15amps bayonet base
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Is resistor 9 something special? It is a red metal one on a heat sink?
That is usually referred to as a candelohm resistor and may be a voltage divider for other usable voltages in the circuit.
I'm curious as to the black braiding covering one of the leads coming from the power cord to the on/off switch.
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On the rectifier tube pin 3 &4 are the heater connections with pin 6 as the tap for the pilot light.
So yes you probably shorted the filament causing the filament to open.
Pin 7 is the cathode
Pin 5 is the plate
pin 1 and 2 have no tube connections (and are used just as connection tie-points for other parts.)

-----------------------------------------
Also the brown metal resistor that goes to pin 6 on 50c5, is that r8?
Brown metal? it should be 1,200 ohms bands are brown,red,red if its carbon resistor, if its the older dog-bone carbon resistor body = brown , end band = red, center band or dot =red. Measure with an ohm-meter.
That is connected to grid pin so that is the B+ filter resistor. I am having to go back and forth to review the schematic.

Thanks Cliff, nothing really looks that bad or burnt. I have another bulb and rectifier. Would it hurt to try them with the dim bulb tester?
Thanks, Rickey

Its very hard from a distance to make general conclusions without seeing the physical components in place.

My understanding on the pilot lamp is, that it has a couple of functions, the rectifier filament in the tube acts as a voltage divider as its filament is center tapped. So for the rectifier, the tap has 17 volts of the 35 volts (17 1/2 V+ 17 1/2 V = 35V) to be used to light the bulb.

On second thought, though its more of a current provider to the B+ of about half the current or of the tube which is .15ma/2 or 70MA of DC.


This is from wikipedia "Many design tricks were used to reduce production costs of the five-tube radio. The heaters of the vacuum tubes were all rated to use the same current, so they could be operated in series from line voltage. The more-powerful tubes (the rectifier and audio output tube) required more-powerful heaters that dropped proportionally more of the line voltage. The rectifier tube had a tap on the heater to power a dial light. The plate current was routed through that portion of the rectifier heater, in order to make up for the current diverted to the dial lamp. If the dial lamp failed, that part of the rectifier heater would have a larger current which could burn out the tube in a few months. Early radios had a resistor network to minimize the problem but this was soon eliminated once manufacturers figured out that the cost of replacing the tube was not their problem."

Real problem now. I was trying to check voltage and shorted out at rectifier pin 4. Rectifier will not work in a working set. I also checked pilot lamp and it shows no continuity with meter. Will set work at all without lamp? The short tripped my gfc and panel breaker! Is resistor 9 something special? It is a red metal one on a heat sink? That resistor confused me for a while, as they used pin 1 as a terminal on the 35w4! It connects to 7 as a bridge correct? Also the brown metal resistor that goes to pin 6 on 50c5, is that r8? Sorry for the mess, i don't know where to begin! What is a brown bead?
Rickey
.

Time to check resistors and tubes, ones in the cathode and grid may effect distortion, with old sets resistors almost always climb in value just from age. I would be surprised if a cap went bad that fast, but who knows?

However to be thorough, I would check all resistors.

Well after a few days, I turned radio on and it sounds distorted! It was untouched ,as I had been working on the case. It sounded perfect the first day after all the caps were changed!
Thanks, Rickey

Thanks, Cliff!