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Fisher X-100-C Output Transformer needed

Wharf_Creek

Member
2007-11-27 1:07 am
Hello DIY Group,

First....if I'm violating some kind of rule here....please forgive me. That is NOT my intention....and hopefully I'm not doing so. I did note a similar thread to this, so I'm going on the thought that this is OK!

I just finished 3 days of diagnosis on a problem with my X-100-C Fisher that has seemingly all boiled down to a bad output transformer. I had one of the 4 output tubes that was pulsating internally as the music changed in amplitude. And, the louder I played it...the more the tube pulsated. I was told this is called 'Red-plating'.....although I noted that the tube seemingly 'dimmed' to the beat....but also seemed to be getting hotter and hotter as time passed. Anyway, I changed out nearly the complete phase inverter....cleaned tube sockets and tube pins, change out some bias resistors, and swapped tubes, all without changing a thing! I 'finally' decided to check the primary windings for continuity, and found that the right channel...the good one...had 3K 'in-circuit' across the primary, and was 1.5K to each tube from the center tap. The Left channel was open across the primary....but did have 1.5K from the center tap to the one properly functioning tube. Needless to say...the bad tube side of the transformer was connected to the problematic tube.

So......now it appears I'm in need of a Fisher X-100-C transformer. This thing has a part number of T1114-116 - 1 or 2 (for right or left..either will work)

If anyone has any ideas of where to obtain one....if you know of a salvage unit, or a suitable replacement for use with these 7868 tubes, please let me know. Also, I'd mention that Fisher used a weird feedback loop in this amp, where the 'C' terminal was NOT grounded but did feed the phase inverter, and the 4 ohm speaker output WAS connected to ground......so....go figure? It appears that there were really two 4 ohm sections, which when added together became the 8 ohm section...but the "C" was NOT a chassis ground as is often associated with similar designations. This being true, I don't know if a standard transformer with 4, 8, and 16 ohm speaker taps and a 'G' as is normally attached to ground would actually work in this topography. Perhaps with some other mods that someone may know about???

So.....that's it! My name is Tom, my email address is:

Wharf_Creek at Hotmail Dot Com

Any help would be GREATLY appreciated!!

Kindly, Tom D.
 
http://www.diyaudio.com/forums/tubes-valves/177359-fisher-output-transformer-id.html

Also I hold in my hand right next to my Fisher X-100-3 an Edcor CXPP10-MS-5K.
Not a listed CXPP model, but also not a custom. Didn't cost me any design fees.
I bought it for something else entirely, but couldn't help noticing the similarity.

I'm not saying its the same transformer, only saying its the exact same physical
dimensions and bolt pattern. OK maybe 2 or 3 lams thinner, but not by much...
The end bells are identical in every way except Edcor blue color, I suspect you
could bolt on Fisher end bells with no problem.

I do not know the Fisher's primary impedance, and the fisher is rated at 17 WPC.
But at least you know Edcorusa has all the right sized pieces if you asked them
to make one.

I took a photo, but got no data cable for my phone right now. Only wall charger...

Wait, you wrote -C not -3? I'm now unsure anything I've said is even relevant...

Grounding the 4 ohm tap was also a modification that ARC did to the ST-70-C3.
ARC successfully abused standard Dynaco iron, don't see any reason the same
trick woud have caused Fisher to go winding a custom? Its probably standard
Fisher iron, but just guessing...
 
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Wharf_Creek

Member
2007-11-27 1:07 am
Hello Ken,

I do hope you plan to clarify this note for me! Starting from the 'Also' in the beginning, to the discussion on ARC and the ST-70. I'm pretty sure an ST-70 OT will not work in this Fisher. I'm told that any 7591 amp OT might work....as long as it had both 4 and 8 ohm speaker taps. I might lose my headphone and center speaker availability, but, so much for sweet surrender! Better that, than the complete loss of the amp. However, I'd still like to find an OE Fisher Output transformer for the 7868 tubes. If you've got any other ideas, please let me know. Thanks, Tom D.
 
"Also" after reading posted link to another thread relevant to Fisher iron.
Top of my Post #2 in blue, will turn red as you mouse over. Yes, click it...

Of course Dynaco iron will not fit, that wasn't the point. The point was
to show an example rewiring the usual 4 ohm tap as C, not so unusual.
And did not require a specially wound transformer in that other instance.

I have not yet seen a Fisher C3 schematic to confirm or deny it abuses
ordinary iron and the same center tapped output trick as ARC's modified
ST70... After mod, the ST70's usual C and 16 were fed back to cathodes.

Can you feed a signal into the speaker terminal of the good channel and
measure the plate swing (with the amp turned off)? That way we could
hazard a guess at the primary impedance and/or winding ratio?

Now please glance at the schamatic at the bottom of the following link:
audioresearchst-70-c3amplifier - mpbarneyamps
Nevermind the pre-amp section, just ask if the strange output transformer
rewiring looks like your Fisher X-100-C or not?

Meanwhile, I will try to dig up a schematic for the -C and see if we are on
the same page or not? I only know my -3, and its clearly not the same...
 

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Wharf_Creek

Member
2007-11-27 1:07 am
Hey Ken,

YES....that diagram looks very similar. However, I note at least one difference, that being that the 4 ohm tap on the ST-70 Mod is NOT grounded....where it IS grounded on the Fisher. And, like you said...the driver circuits are different once beyond the OTs, so I didn't go much further. I DID manage to extract the subject failed OT from my Fisher this evening.....and it is indeed 'Half-Dead'! Its funny that it's also half good, which disguised it's failed nature. But.....I'm pretty confident that once I find a suitable replacement, and do some basic 're-capping'.....I'll have a pretty nice amp to listen to for a long time to come. I wish I could just go buy one of these things some place....although I think the price tag might be about a week's pay for me! Anyway, I'll keep looking....and to anyone else who happens to be reading this thread.....if you can help me find an OT for this Fisher X-100-C......I'll be REALLY grateful!! Many Thanks Ken. BTW....the primary on the Good Transformer....tubes removed so no load or connections of any kind: 300 ohms...or actually about 295. One side measures about 150 ohms and the other is about 145. And, the 'good' side of the 'BAD' OT measures 149 ohms......so.....I guess that's a starting point! Thanks again, Regards, Tom
 
ARC's 4 is definately grounded, I cropped the schematic is all.

If it is the same size transformer as mine, Edcor can replace it for 40~50 bucks.
But you need to know the primary impedance and doublecheck the physical dims.
Mine just happens to be the same size and endbells as CXPP10W, maybe yours
is different. Bust out a ruler, and measure it.

EDCOR Electronics Corporation. CXPP Series Output Transformers

Expect 3 to 6 weeks if they have to wind something not in stock.
Many more designs than listed on the websight are in the system,
and not considered "custom". You just have to get on the phone
with them and ask.
 
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I see from your post on anther sight that you are confused by your measurement
of primary ohms. This is only resistance to DC, not impedance. You need to put
it to an LCR meter with a huge inductive scale. Or pump a known signal through
the good one backwards and measure the voltage swing on the plates. You do
not even have to take it out of circuit to do so, just be sure to turn the amp off.

Measure us the speaker terminal AC voltage and the plate AC voltage, and we
can figure the rest. Any test tone of consistant volume from another audio amp
will do. Don't use actual music, as the level might change.
 
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Wharf_Creek

Member
2007-11-27 1:07 am
Hi Ken,

Well......'yes'.....I did go back to the Schematic on that ARC amp and I saw where the 4 ohm tap was, indeed', 'Grounded' on the 'star-ground'....back after that string of diodes! So, I guess that setup is nearly exactly the same.....as far as I can see.

OK......regarding the measuring of the Primary impedance......I'm still a bit unclear on a few things. I have a number of LCD meters......just VOMs of varying quality. So, I'm a bit unsure of what an LCR meter is.....so that's a starting problem for me. Next, as for pumping a known signal through 'backwards'......I can do this on the 'good' channel, as that OT is still in the amp. The BAD OT has been extracted already. So, if I were to use a signal generator on another amp....do I come out of the speaker outputs on that amp and just go directly into the speaker outputs of this amp? And, at what frequency and amplitude level do I start? Trying to be as simple and basic as I can with this, could I hook up a mono-block amp.....like an old Dynakit Mark III......and just connect up the two speaker outputs to each other.....'C' to 'G'....and '8' to '8'......no speaker in either amp? And, as my signal generator has a variable output, I don't really need a pre-amp, so I could just plug into the MkIII's input, fire up the Signal Generator....and then what? Without a speaker in line....how do I know at what level I'm driving this set-up...and is there any risk to the OT of the Dynaco? Finally....how do I measure this 'swing' on the plates you reference? Where do I put the VOM if it will work? Do I use an AC VOM, an RMS VOM.....or what? As you can see.......I'm still very confused!! So, I hope you can help walk me through all this.....as I think the education will do me a LOT of good!

Thanks Ken!!

Tom
 
I see GNF, not the same thing at all.
Not sure why they ground 4 rather
than reverse phase of transformer?
Something to do with center channel?
Whatever...

Service manual suggests testing with
25W dummy load, so guestimate 25W?
Also figure about +/-300V plate swing,
what impedance ratio is this? I gots to
go git my Jethro Bodine hat an' cipher...
 

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I'm a bit unsure of what an LCR meter is?
LCR = Inductor(L) Capacitor(C) Resistor(R)

Few meters measure inductance, and even fewer HUGE inductance
of a tube OPT primary... Mine peaks at 20H, so not terribly useful.

The way you need to do it is with the backward signal, forget I
even mentioned the exotic piece of kit, its not something you
gonna find at Frys or Ratshack...
 
and just connect up the two speaker outputs to each other.....'C' to 'G'....and '8' to '8'......no speaker in either amp?

If the other amp doing the driving is Tube, you need either a speaker or
a beefy power resistor of 4~16 ohms.... Won't interfere with what we
need to measure. And keep the volume low, its not gonna need a lot of
excess volts to measure the in to out ratio.

You probably don't want to use an amp with higher rated power than the
Fisher to do this, as you could accidentally cause primary voltage excess
of the design. If there is a pop or click or some noise unexpected...

----

To be clear, the driving amp needs a speaker or load on BOTH channels
regardless we are only abusing one side for this test. Operating any tube
amp without a load invites arcs that can open or short a primary winding.
I don't need to tell you what sort of grief that can be...

And pull the plug and all other cables on the Fisher, just to make sure the
ground floats and don't damage the other amp.
 
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