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Old 21st July 2015, 06:22 PM   #1
Ggizzy is offline Ggizzy  United States
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Default PT Failed Winding

I have a Brenell four valve amp pulled from a Mark 5 tape recorder. The PT has a 300-0-300V secondary and two 6.3V filament strings. One string for the amp and the other for the EZ80 rectifier.

In testing the transformer I disconnected all secondaries except grounds and measured voltages. HV and amp filament string were fine but I got nothing on the rectifier filament string. I tested with a DBT in line with the power supply and the bulb failed to even light. (15W 240V bulb)

I let it soak and the transformer never got warm or any other visible problems. My question is, is the rest of the transformer alright or is the bum filament string an indication the transformer is ready to get binned? Could I get a small 6.3V transformer to replace the bad winding? Thanks!
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Old 21st July 2015, 07:22 PM   #2
rayma is offline rayma  United States
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Originally Posted by Ggizzy View Post
Could I get a small 6.3V transformer to replace the bad winding?
Yes, I would go for it.
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Old 21st July 2015, 07:34 PM   #3
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Originally Posted by Ggizzy View Post
My question is, is the rest of the transformer alright or is the bum filament string an indication the transformer is ready to get binned? Could I get a small 6.3V transformer to replace the bad winding? Thanks!
I see no reason you could not use a second 6.3V filament transformer for the EZ80 rectifier provided you have the room to safely mount it. It is unusual for the filament winding to open up, so carefully check that your EZ80 filament is not shorted or partially shorted. The filament current for that tube is 1 amp so be certain to use about a 1.5 amp transformer for a little extra reserve. And since the original PT runs cool, I think it safe to still use it. With the transformer unloaded the bulb should not light at all. Another little test I like to do is to put my nose right down on the windings and take a good sniff for any burnt odor. If it smells burnt, replace it.
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Old 21st July 2015, 07:40 PM   #4
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Ggizzy View Post
HV and amp filament string were fine but I got nothing on the rectifier filament string.
Such a winding is short, thick, and generally very robust.
I do not see how it could burn without taking the rest of the transformer with it, or at least leaving some serious and obvious scars.
Unless it is a case of extreme oxidation having eaten up all the copper, I can only think of a mechanical issue (broken wire) or bad contact (dry joint, etc)

Before using another transformer, I would try to have a good look at this winding, near its terminations in particular
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Old 21st July 2015, 07:59 PM   #5
Ggizzy is offline Ggizzy  United States
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The transformer is the older paper type, so it's hard to see where the wire goes in between the layers. All of these voltages were taken with the secondaries unattached. Another thing occured to me. The rectifier heater string doesn't have a CT. Could this be throwing it off? Should I add an artificial one with 100 ohms to ground?
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Old 21st July 2015, 08:02 PM   #6
rayma is offline rayma  United States
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Originally Posted by Ggizzy View Post
The rectifier heater string doesn't have a CT. Could this be throwing it off? Should I add an artificial one with 100 ohms to ground?
You should still be able to measure the 6VAC across the secondary with a meter.
There's no need for a ct or resistors on the rectifier winding.
Don't ground the rectifier winding in any way, this must float with the rectifier cathode.

Last edited by rayma; 21st July 2015 at 08:15 PM.
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Old 21st July 2015, 08:03 PM   #7
Ggizzy is offline Ggizzy  United States
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The hum pot is on the other filament string. The rectifier string has no ground.
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Old 21st July 2015, 08:37 PM   #8
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Make sure when you test it, you are measuring the filament winding, and not each side to ground. You should measure resistance of the filament winding (make sure the power is off). It should read 1 ohm or just under.
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Old 21st July 2015, 10:08 PM   #9
Ggizzy is offline Ggizzy  United States
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I measured again this evening and still getting 0V across rectifier filament winding. Can I get a 300-0-300V with a single 6.3V secondary with higher current rating and run the rectifier on the same string as the other valves?
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Old 21st July 2015, 11:34 PM   #10
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A quick, minimum cost solution would be to disconnect the wiring to the EZ80 and wire in a pair of 1N5408 diodes in its place. You can connect the EZ80 filament to the other 6.3 Volt winding so it looks like it is still working.
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