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 Post subject: Power transformer specs
PostPosted: Feb Tue 06, 2018 4:06 pm 
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When I pull the 80 tube my radio pulls a large current and so I think the power transformer is shorted. Radio is a Westone model 40 using 2A7, 58, 2A6, 2A5, and 80 tubes. I saw a transformer rated at 950VCT. Would that work or is the voltage too high. The other specs are OK. If that voltage is too high could I drop it with a resistor?


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 Post subject: Re: Power transformer specs
PostPosted: Feb Tue 06, 2018 4:22 pm 
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Pulls a large current with the tube IN also? how much current?

With no load at all, the transformer can still draw a few 10ths of amps, but should not get warm.

Yes, 950 is WAY too high---you want something in the range of 600-700 volts center-tapped, rated at least 60mA

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 Post subject: Re: Power transformer specs
PostPosted: Feb Tue 06, 2018 4:35 pm 
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Draws about 2 Amps with the 80 out of circuit.


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 Post subject: Re: Power transformer specs
PostPosted: Feb Tue 06, 2018 4:37 pm 
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andrefranc wrote:
When I pull the 80 tube my radio pulls a large current and so I think the power transformer is shorted. Radio is a Westone model 40 using 2A7, 58, 2A6, 2A5, and 80 tubes. I saw a transformer rated at 950VCT. Would that work or is the voltage too high. The other specs are OK. If that voltage is too high could I drop it with a resistor?
That does sound awfully high, but unfortunately there is about zero information on the schematic for this radio http://www.nostalgiaair.org/pagesbymode ... 024469.pdf .

Normally I would not suggest this, but if the transformer is already toast there is not much to lose.

With the 80 pulled out and powering up the transformer only for brief periods (few seconds each time) to make measurements. What AC voltage readings do you get on the high voltage connections to the 80 plate pins relative to the center tap?

I am guessing one side will be low, but the other might still be reasonably close to normal and would give you an indication of what might be needed.

If the voltage of the potential replacement transformer is about 475-0-475 and the 80 voltage drop is about 60 volts. Then the worst case voltage at the filter capacitor could reach 1.4 x (475-60) = 581 VDC with no load. :shock: That just doesn't sound like a good idea with regard to your filter capacitor.

Personally I think you are more likely looking for no more than a 350-0-350 V transformer and possibly 300 would be safer. This should result in 1.4 x (350-60) = 406 VDC at the first capacitor which seems entirely more reasonable.

The problem with a simple series resistor is that, until there is a load (the tubes warm up), the filter capacitor is going to get the full voltage because there will be no voltage drop to reduce the impact on the capacitor. Alternatively you could precede the capacitor with a voltage divider (two resistors - one in series and another to ground) but that gets a little tricky to get right, but could be done (not something I would recommend).

Curtis Eickerman

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 Post subject: Re: Power transformer specs
PostPosted: Feb Tue 06, 2018 4:44 pm 
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andrefranc wrote:
Draws about 2 Amps with the 80 out of circuit.
Sounds like that transformer has a built in 200 Watt soldering iron inside of it. :shock:

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 Post subject: Re: Power transformer specs
PostPosted: Feb Tue 06, 2018 4:49 pm 
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You can also run the transformer on a low voltage like 12VAC or 24VAC on the input and see if the output voltages are 10% or 20% of normal. The low input voltage will keep you from doing more damage and makes the measurements safer.


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 Post subject: Re: Power transformer specs
PostPosted: Feb Tue 06, 2018 5:02 pm 
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It is also possible there is an external filament(s) circuit short.

You might pull some other secondary leads and test again.

Dennis

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 Post subject: Re: Power transformer specs
PostPosted: Feb Tue 06, 2018 7:24 pm 
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How positive is a transformer check with rectifier removed, ohm meter across 120V. legs and showing dead short, no resistance at all. I've had only one bad transformer in many years.


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 Post subject: Re: Power transformer specs
PostPosted: Feb Tue 06, 2018 7:35 pm 
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What happens with all the tubes pulled?

RRM


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 Post subject: Re: Power transformer specs
PostPosted: Feb Tue 06, 2018 7:51 pm 
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hoffies2 wrote:
How positive is a transformer check with rectifier removed, ohm meter across 120V. legs and showing dead short, no resistance at all. I've had only one bad transformer in many years.
I've only had 1 failed transformer ever on something that was mine (a Webcor 2130 tape recorder) and it was REAL obvious after filling the room with an obnoxious odor (no electrical test was necessary). There was no capacitor failure that triggered the event. The transformer just failed of its own accord.

It seems that they mostly fail in the high voltage windings. I suspect it is because the finer wire is more subject to damage than the heavier filament windings and primary windings. The high voltage winding also has more layer-to-layer voltage differential that can exploit a weakness in the insulation.

I recall some exceptions where a specific transformer filament winding would fail in a particular device and this was probably because the filament winding was really too small to properly support the load. Then in some cases it can be a construction issue where a particular winding has a propensity to develop a short to the case because of placement within the bell housing (pinched wire where insulation finally failed).

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 Post subject: Re: Power transformer specs
PostPosted: Feb Wed 07, 2018 1:21 am 
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I can only remember one radio with a power transformer failure and it was in the B+ winding. The B+ was the outside winding so I thought I would try to rewind it. I found the short in the outermost half of the winding so removed down to the center tap. At that point I got lazy and put the transformer back together with only the good half of the B+ winding. I ran that to the rectifier and added two solid state rectifiers to make a full wave bridge. I wasn't sure the single winding could take the higher current but it seemed to be OK and I know the radio operated for many years after the repair.


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 Post subject: Re: Power transformer specs
PostPosted: Feb Wed 07, 2018 4:18 pm 
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Someone asked what happens when all the tubes are pulled. Radio draws .8 amps and smells with all tubes pulled but can't tell where the smell is coming from.


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 Post subject: Re: Power transformer specs
PostPosted: Feb Thu 08, 2018 2:08 pm 
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andrefranc wrote:
Someone asked what happens when all the tubes are pulled. Radio draws .8 amps and smells with all tubes pulled but can't tell where the smell is coming from.

Power transformer....

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 Post subject: Re: Power transformer specs
PostPosted: Feb Fri 09, 2018 9:37 pm 
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JnTX said he had a bad power transformer once and he disconnected the bad side of the HV winding and ran the set off of just one half of the HV winding through a full wave rectifier. My transformer was arcing and smoking and turned out that half of the HV winding was bad so I disconnected that half and then made a full wave rectifier like JnTX said and it works. My set now runs by just using half of the HV winding. Don't know how long it will run that way. JnTX said his ran a long time and was ok. Thanks JnTX


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 Post subject: Re: Power transformer specs
PostPosted: Feb Fri 09, 2018 11:57 pm 
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andrefranc wrote:
JnTX said he had a bad power transformer once and he disconnected the bad side of the HV winding and ran the set off of just one half of the HV winding through a full wave rectifier. My transformer was arcing and smoking and turned out that half of the HV winding was bad so I disconnected that half and then made a full wave rectifier like JnTX said and it works.
There are two different ways that a transformer winding can go bad. It can either develop one or more shorted turns or it can develop an open winding. These are two fundamentally different problems only one of which can be addressed by continuing to use 1/2 of a winding.

In the case of one or more shorted turns you have the equivalent of a soldering gun inside of your transformer. Perhaps to understand this better, a soldering gun is actually a transformer with a shorted winding with its narrowest point being at the tip which therefore will get the hottest. A shorted winding in a power transformer is exactly the same thing.

Shorted turns in a power transformer cannot be bypassed or worked around by not using the shorted winding. No matter what you do externally that shorted winding will continue to heat up, melt down, fry, and smoke the transformer.

The only case in which you can sometimes work around a failed transformer is in the case of an open winding in 1/2 of a center tapped winding (as long as the open connection is not the center tap itself).

In that case you can disconnect the half of the transformer that is open. Then replace the full wave rectifier by a bridge full wave rectifier operating on the remaining 1/2 of the original winding. This is a little sketchy for one reason, but often works. The problem with this approach is that in a full wave center tapped transformer 1/2 of the winding is not supplying ANY power 1/2 of the time so it has half of the time to cool off during each cycle of the AC.

When you hang a bridge full wave rectifier on that 1/2 winding it is now required to supply power continuously and gets no cooling relief during a 60 cycle sine wave. It is doing twice the work that it was originally designed for (carrying current for every half cycle instead of every other half cycle). Often that is fine and the wires can take it, but sometimes it will exceed the capability of the wire and the remaining winding will subsequently burn out as well.

I will not go into the fact that the current is not actually continuous in any power supply transformer. So that even in the bridge rectifier case there is cooling, but it is still half of what it would have been. This is not intended to be a discourse on power supply design.

When someone has a power transformer that is consuming .8 Amps at 120 volts while connected to nothing (all tubes pulled), this is the sign of a shorted winding and is like having a 100 Watt soldering iron buried inside of the transformer (.8 A x 120 W = 96 Watts). This is not a candidate for disconnecting a wire and adding a bridge rectifier as a solution. The solution is to repair or replace the transformer (find and repair the internal/external short and hope that the insulation is not overly compromised by the heat damage, rewind the transformer, or replace the transformer).

Curtis Eickerman

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Last edited by Eickerman on Feb Mon 12, 2018 7:50 pm, edited 1 time in total.

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 Post subject: Re: Power transformer specs
PostPosted: Feb Mon 12, 2018 6:44 pm 
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I once had a transformer that pulled way too much current, and I was in the process of removing - but gave it one last try with one of the filament lines unhooked. Turned out that one of the dial light sockets had shorted, the transformer was fine. Sometimes pulling all the tubes isn't enough...


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