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Modifying the Fisher 400 with EFB

Just found this if you wanted to try this formula. Like Mr. Gillespie had mentioned the OPT would still be under the voltage it would normally be taking using your 125v wall socket.

Turns Ratio = V in (to the primary) / V out ( measured in the secondary) e.g. if we apply 5 Volts AC and we measure 0.21 Volts AC to the secondary the Turns Ratio of the transformer is : 5 / 0.21 = 23.8 That means a turns ratio 24:1

Knowing the Turns Ratio we can calculate the impedance ratio of the unknown transformer and the impedance in a given load to the secondary. The impedance Ratio is the square of the turns ratio:

24 X 24 = 576 that means an impedance ratio 576:1

So in a given load impedance of 8 ohms the transformer impedance is 8 X 576 = 4608 ohm ( it is about 4.5K)
 
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Excellent - thank you. I have a few loose OT's of known values I will test with first.
 
larryderouin said:
Dave did the calculations about 5 years ago when .068uf and .082uf were becoming a common value in caps and that a .071 or .072 was as close as you could get to the original time constant with the 330K/.047uf. Seeing as those values aren't being made his recommendation was/is 220K/.068uf or 220K/.082uf as these are more common now. The original 220K/.1uf was made when the only commonly available caps were .047uf and .1uf.
Click to expand...

I had not found that info - thank you! Sounds like 200K/.1 can't be too far off....
 
Fur Face said:
Maybe use a wall wart to keep the voltage low also to compare.
Click to expand...

I thought about that - not sure if the transformer maintains it's efficiency at that low of a voltage, but I'll know if my known value OTs test right.
 
You can use a low voltage source of say 10 Volts or less on the Secondary side, then measure the Primary side. Do the calculations the same way.
 
OK - testing completed.

First I tested a Voice Of Music #VM12611 OT. The schematic shows it to have a primary impedance of 10K and single secondary of 4ohms. 7.67vac applied to the secondary produced 324.5vac on primary. This calculates to a primary impedance of 7.15K. Not what I was expecting.

Second I tested an Altec-Lansing Peerless #16458 OT from a Heathkit W-5M. I always thought this to have 10K primaries, but I don't have documentation. 7.72vac applied to the 16ohm secondary produced 180.6vac. This calculates to a primary impedance of 8.75K. Again, not what I was expecting.

Finally I tested the left OT of my 400. 7.76vac applied to the 16ohm secondary produced 193.6vac. This calculates to a primary impedance of 9.96K. Pretty close to what I was expecting. I disconnected the primary center tap and also the NFB circuit from the secondary to ensure it did not skew the results.

All were lower than expected. Is it possible the transformers lost some efficiency operating "backwards" for lack of a better term?

Either way, I think it's safe to say I have the later model OT's and can install the EFB circuit as drawn?

Tom Kuhn
 
I don't pretend to understand even a fraction of this thread. But, it sure looks like poetry to me. Thanks.
 
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