Set I'm working on is a Majestic 20. The power transformer had obviously been overheated in the past from the large amount of tar that had leaked out. Anyway, with the tar cleared out of the way I started making some measurements. There were no shorts between windings or to the chassis. I unsoldered all the secondary leads, then applied power slowly to the primary. All the secondary voltages tested within reason, and the two center-tapped windings showed equal voltage on either side of the center, so I thought everything was good. Next, I put it on my dim bulb tester with a 60W bulb. With no secondary loads connected the bulb glowed dull orange. I thought with no secondary loads the bulb would not light at all. What is the dim bulb test showing me about this transformer? Is this transformer shot?

:Set I'm working on is a Majestic 20. The power transformer had obviously been overheated in the past from the large amount of tar that had leaked out. Anyway, with the tar cleared out of the way I started making some measurements. There were no shorts between windings or to the chassis. I unsoldered all the secondary leads, then applied power slowly to the primary. All the secondary voltages tested within reason, and the two center-tapped windings showed equal voltage on either side of the center, so I thought everything was good. Next, I put it on my dim bulb tester with a 60W bulb. With no secondary loads connected the bulb glowed dull orange. I thought with no secondary loads the bulb would not light at all. What is the dim bulb test showing me about this transformer? Is this transformer shot?

Vinton:
What you probably have is a shorted turn or two inside the transformer, acting as an entire shorted winding. As you have described the transformer, I could not trust it even if it didn't have an internal short. I think you would be better off with a replacement transformer.
Lewis
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Sounds like transformer is a goner, might want to unplug remove rect tube. With ohm meter connect one lead to middle of center tap wire & other lead of meter to each side of end tap wires, should read about same if this winding is good.

:Sounds like transformer is a goner, might want to unplug remove rect tube. With ohm meter connect one lead to middle of center tap wire & other lead of meter to each side of end tap wires, should read about same if this winding is good.
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Ohm meter readings are seldom equal for a center-tapped secondary. The outside winding is on top of the other windings, so contains more wire to achieve the same number of turns. 5 or 10% difference in DC resistance is not unusual.
Rich

Thanks to all. So I guess I'm right and the 60W bulb should not light with all secondaries disconnected? I was afraid of that...

I don’t think you should condemn the transformer just yet. The primary of the transformer does make a complete circuit. It acts like a resistor in series with the light bulb. The light bulb should light dim. A bright light would indicate a short.
Remove the light bulb and bring it up slow with your variac. ( fused ) Now read the secondary voltages. Sometimes the transformer does get overheated due to bad rectifier tube, or anything else shorting in the B+ circuit.

Is this the same as a Grigsby-Grunow model 20?
I have a Grigsby-Grunow Majestic 90B that had a secondary shorted to the core and an open field coil. The transformer also fed 2.5 volt filament tubes with separate filament windings for the audio output tubes and rectifier. The transformer and caps were also potted in tar.
If it turns out that you do need a replacement transformer, Radiodaze has a reasonable replacement: RDX-200.
An idling transformer primary does draw some current.

:I don’t think you should condemn the transformer just yet. The primary of the transformer does make a complete circuit. It acts like a resistor in series with the light bulb. The light bulb should light dim. A bright light would indicate a short.
:Remove the light bulb and bring it up slow with your variac. ( fused ) Now read the secondary voltages. Sometimes the transformer does get overheated due to bad rectifier tube, or anything else shorting in the B+ circuit.
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This may or may not be appropriate to the GG 20, but recently found the light bulb test better than meters. It was an Edison 7R. With chassis upside down and terminals exposed, all measured fine. Transformer off chassis same. Right side up on or off chassis, a short between HV and 5V fils. Just a small area inside can, under the phenolic base plate had no tar, and the 2 windings touched. With the phenolic removed, I noted the glint of shiny copper on the 2 leads adjacent to each other, but separated. The weight of the xformer against the phenolic, in it's upright position pressed them together. Upside down, no contact. This had baffled a techie of yesteryear, so he butchered the chassis to install a second 5V transformer. I eliminated short, xtra xformer repaired butchered chassis, and set completed, awaiting an Edison cabinet or reasonable copy.

marv

: Is this the same as a Grigsby-Grunow model 20?
: I have a Grigsby-Grunow Majestic 90B that had a secondary shorted to the core and an open field coil. The transformer also fed 2.5 volt filament tubes with separate filament windings for the audio output tubes and rectifier. The transformer and caps were also potted in tar.
: If it turns out that you do need a replacement transformer, Radiodaze has a reasonable replacement: RDX-200.
: An idling transformer primary does draw some current.
:
::I don’t think you should condemn the transformer just yet. The primary of the transformer does make a complete circuit. It acts like a resistor in series with the light bulb. The light bulb should light dim. A bright light would indicate a short.
::Remove the light bulb and bring it up slow with your variac. ( fused ) Now read the secondary voltages. Sometimes the transformer does get overheated due to bad rectifier tube, or anything else shorting in the B+ circuit.
::
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Nice radio, to put all that effort of restoration & TIME into and not sure about the transformer, replace it.

: Nice radio, to put all that effort of restoration & TIME into and not sure about the transformer, replace it.
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Follow Warren's test first before you give up on the transformer, if you can find a way to split the power cord and connect a amp clamp as you increase VAC with the variac you can monitor the current, it should be a few hundred mA's with no load on the secondary windings.
The secondary voltage readings will be a little high as there is no load on them.

I have done these tests before to prove out a good transformer, if the amp clamp suddenly rises to amps, stop!

Thanks everyone for the input. As I stated before, I have unsoldered all secondary leads so there's no load at all on the transformer and when I apply power to the primary the transformer stayed cool for the 20-30 minutes I left it on. All secondary voltages tested fine. and I didn't find any shorts to the chassis.

However, it is sound advice that, with all the work ahead of me, it may be wise to just replace the transformer with a trusted one.

One more question: Lacking an AC ammeter I put a 10-ohm resistor in series with the ac line and measured 3.5vac across the resistor. That roughly equates to 350mamps, is that true?

:One more question: Lacking an AC ammeter I put a 10-ohm resistor in series with the ac line and measured 3.5vac across the resistor. That roughly equates to 350mamps, is that true?
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If there is no load whatsoever on the secondary, there should be only a few MA consumed by the primary. You have correctly observed about 350 MA consumption which means the shorted turns in the transformer are consuming over 30 watts of power!

E=IR, and W=EI; OHM yGod.

Add this to the load consumed by the radio and you have a space heater on it's way to becoming a fire hazard.

You have nothing to lose by taking the bells off the transformer and probing the connections and lead wires hoping to find arcing, etc... but it dosen't look good.

::One more question: Lacking an AC ammeter I put a 10-ohm resistor in series with the ac line and measured 3.5vac across the resistor. That roughly equates to 350mamps, is that true?
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:If there is no load whatsoever on the secondary, there should be only a few MA consumed by the primary. You have correctly observed about 350 MA consumption which means the shorted turns in the transformer are consuming over 30 watts of power!
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:E=IR, and W=EI; OHM yGod.
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:Add this to the load consumed by the radio and you have a space heater on it's way to becoming a fire hazard.
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:You have nothing to lose by taking the bells off the transformer and probing the connections and lead wires hoping to find arcing, etc... but it dosen't look good.
:
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Which goes along with his first observation that a 60 Watt bulb lit at about half-brightness when is series with the primary with no load on the windings.
Lewis

Thanks for the continued input, but I'm confused at one point. One fellow says the there should be several hundred ma current with no secondary load, while another says there should be just a few mamps. I guess the amount of primary current draw will depend on the particular transformer design.

Current draw seems a bit excessive, and reminds me of the Philco 60 I pulled out of someone's trash after a big flood. I wound up replacing that power transformer.

You could fuse it at 1 or 2 amperes and see how it works. I would not use it unprotected, however.

I read at least one article on the web that predicted unloaded current might even decrease when secondaries were loaded, depending on xformer design, which rather confused me, but may have some merit based on my own recent experience rewinding one. It was a HeathKit BC-1A AM tuner. Based on unloaded losses and heating, (approx 40W) figured my rewind was a loser. Even had an identical unit in a HeathKit FM-3A tuner, with unloaded losses about 60% less. Even though tube complement was different for the two, loaded current measurements were comparable, as was heating. Both sets have played reliably for several hours, with near equal heat losses (finger touch) Go figure.

marv

:Thanks for the continued input, but I'm confused at one point. One fellow says the there should be several hundred ma current with no secondary load, while another says there should be just a few mamps. I guess the amount of primary current draw will depend on the particular transformer design.
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