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Fisher 500c Output Transformers

No -- it would be accurate to say they are somewhat similar electrically (which includes those units using 7868 tubes as well), and even similar enough that an outright swap between similar transformers would likely "work" -- but not optimally. Some transformers are capable of greater power handling and/or greater power response (X202B, X200 and KX-200 which all share the same transformer), while others are more economically designed with less iron, power response, and power handling ability (X-100B, etc.). The 500C/800C, X-101D transformers fall in between these two groups, but much closer to the X-202B transformers, which are really superb transformers.

Dave
 
dcgillespie said:
No -- it would be accurate to say they are somewhat similar electrically (which includes those units using 7868 tubes as well), and even similar enough that an outright swap between similar transformers would likely "work" -- but not optimally. Some transformers are capable of greater power handling and/or greater power response (X202B, X200 and KX-200 which all share the same transformer), while others are more economically designed with less iron, power response, and power handling ability (X-100B, etc.). The 500C/800C, X-101D transformers fall in between these two groups, but much closer to the X-202B transformers, which are really superb transformers.

Dave
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Thanks Dave
You not only answered my question but grouped the transformers with their associated models.
I often wished someone would compile an output transformer list that would help identify an OEM transformer number to its applications.
Thanks again
 
There are others too. For example, the X -101C and the early version 400 receiver share the same output transformer as well.

Dave
 
Isn't there something with the late 400 transformer having a very different impedance than the others ?
 
larryderouin said:
Don't limit your search to just the 500c. The 800c used the exact same transformers (500c is an 800c without the AM Band (or vice versa)). Otherwise Heyboer transformers can probably build you one. They are in Michigan.

The suffix # (-1 or -2) denotes which channel. Wiring lengths are different (slightly) between them. Try to get the matching suffix transformer. If you have it rebuilt or a new one built, ask them to leave the leads from the transformer @ 12" from the hole in the end bell. Adjust length as needed for neat install.
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Good information to know! I have an 800C output transformers former with an open primary. I've visited that site on the net and sent them an email. I do have questions and hope you can answer them.
1) Can they or do they rebuild your old output transformers or do they build new to physical/electrical specs?
2) When you say wiring lengths are different, are you talking windings or leads?
ANY help answering these questions is appreciated.

Gilbert
 
geedaddy45 said:
Good information to know! I have an 800C output transformers former with an open primary. I've visited that site on the net and sent them an email. I do have questions and hope you can answer them.
1) Can they or do they rebuild your old output transformers or do they build new to physical/electrical specs?
2) When you say wiring lengths are different, are you talking windings or leads?
ANY help answering these questions is appreciated.

Gilbert
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He is talking about the leads outside the transformer.
 
1.)They built a new transformer based on the original windings for my MPX-100. Used the original endbells. Imagine they'd use the spec's for your's and build a new one.
2.) leads outside the transformers.
 
Sometimes they go as far as to unwind the original and count the windings so they can remake it exactly as it was. That’s not cheap by any definition! But you do get an exact replacement of the unit. Lots of times there is special sauce in the layering of the windings that go way beyond just the total number of turns.
 
Hello! Can someone explain in detail why the primary "center tap" in the 800C OPT is offset in the winding? I ring out my primary windings from each lead to center tap and get a huge difference(tubes removed, reading through windings only). And, explain how this "offset" interacts with the secondary outputs. Hopefully I'm asking this question clearly. Thank you!
 

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Thats probably more of a drawing error than anything, unless they were being really fussy and trying to indicate the DC resistance from center to each end isn't quite equal. Thats actually not unusual, but it shouldn't be a "huge difference". Exactly what are you getting ?

also have you tried measuring with a different meter? Some digitals get funny when reading through transformer coils. Something about the inductance confuses them. I usually drag out my analog meter for this kind of thing, it doesn't seem to care.
 
A better test is to apply a low AC voltage and measure each side. It should be balanced (same number of turns on each side) but since it is not bifilar (I think McIntosh had the exclusive patent for that), one side of the winding will be underneath the other, resulting in a different total length of wire (and DC resistance) for each half.
 
McIntosh was able to use a bifilar winding since the unity coupled design splits the AC path in two - only half the AC voltage at the "hot" end of each winding. The lower impedance means that the (considerable) inter-winding capacitance has much less effect. They also used wire with a very high voltage rating.

With a full plate-to-plate winding, the peak wire-to-wire voltage would be twice the B+ voltage - better to have some separation than adjacent wires... and parallel wires would have very high capacitance (but very good magnetic coupling!).

Primary halves are seldom equal in resistance, since wire length per turn increases as windings are added. Thus the same number of turns on an "outer" portion have more resistance. Some transformers connect inner and outer windings together for each half of the secondary, which will reduce the difference. The resistance is small compared to the impedance at audio frequencies, so a difference has little effect. For example, I measured a 500C transformer DCR at 117 and 123 Ohms - but inductance of the two halves measured precisely the same. And the 6 Ohm difference is nothing compared to the 6000 Ohm Primary impedance.
 
fred soop said:
A better test is to apply a low AC voltage and measure each side. It should be balanced (same number of turns on each side) but since it is not bifilar (I think McIntosh had the exclusive patent for that), one side of the winding will be underneath the other, resulting in a different total length of wire (and DC resistance) for each half.
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Thanks for responding Fred. I researched testing the OPT online and will introduce a low voltage into the transformer and see what values I get back. In one thread here in AK, it was mentioned that an X-101-D and a T991-116 (Iike I have)OPT are exactly the same electronically. I lucked out and found a new X-101-D OPT(or T991-116) clone online and I'm hoping that purchase works out. I will test it also once it's in my hands and compare values with the good OEM transformer I have. I removed the end bells on my "open primary" transformer and found a nasty burn on the coils, so, that one is toast.
 

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sometimes they are just bad, and thats always unfortunate. I've had a couple different output transformers that had failed in the same way. Some didn't read open circuit though, they just started sparking once enough voltage was applied.
 
Hello AK people! I have several questions about my recently acquired OPT. The manufacturer sold this to me as an X-101-D/T991-116-1 replacement clone. To start off, I compared primary DC ohms of my Fisher OEM OPT and of the clone. The difference was 50 ohms(237 OEM to 287 clone). I have not completed a thorough investigation just yet nor have I ohmed out the secondaries. I'm assuming that both OPT's "should be" relatively close as far as specs go, but, how close? Are we talking about 1-2% close? And IF this OPT was put into service with the other existing OPT, what would happen negatively speaking? This OPT would be installed in an 800C. Tube/circuit fatigue or stress? I am a novice and this is my first tube receiver renovation so any feed back would be greatly appreciated!

Gilbert
 
The resistance of 287 Ohms will result in about 1.4 watt of loss when carrying 30W, slightly more than the lower resistance winding (1.1 watt). Not really significant. What really matters is number of turns and the arrangement of the layers in the windings, which are likely the same. Using a smaller gauge in the same layering arrangement would leave a little more space at the edges of the winding, potentially increasing the voltage breakdown rating.
 
Tom Bavis said:
The resistance of 287 Ohms will result in about 1.4 watt of loss when carrying 30W, slightly more than the lower resistance winding (1.1 watt). Not really significant. What really matters is number of turns and the arrangement of the layers in the windings, which are likely the same. Using a smaller gauge in the same layering arrangement would leave a little more space at the edges of the winding, potentially increasing the voltage breakdown rating.
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Thanks for replying! I performed what I believe is a thorough investigation of both OPT'S, the clone and OEM. I used Uncle Doug's method and sent 6 volts into each of the secondary leads and checked the voltage at the primary. After writing the figures down, I went through a couple more steps to calculate the output values using 7591 tubes. After all calculations, both transformers were very similar and I'm pretty confident they both will perform just fine! Thanks again for your reply!

Gilbert
 
Restarting this old thread. I have a Fisher 800C. Acquired it a few months ago. It worked well for a short time and then one of the output transformers went bad. I don't know if the seller knew there was a problem but I figure I'll have to get it fixed.

My tech guy is waiting to find one at a decent price, which he says is like $200 or so. The ones on ebay are going for way more. Does anyone know if there's a way to find a replacement that doesn't break the bank. I'm already into this thing for $1,100.
 
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