X100B receives 400 output transformer swap?

Discussion in 'Fisher' started by rufleruf, Apr 26, 2017.

  1. rufleruf

    rufleruf AK Subscriber Subscriber

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    Yet another transformer question outta me...

    I bought an X100B locally last week, serial number around 14000. One of the output transformers was shorted - unit was inexpensive based on this known issue. I have a 400, serial number about 59000, that is rusty crusty and incomplete to scavenge some output transformers from. Both these units are PP 7868 based, so it should work right? That's what I thought, so I installed one last night and voila! it works.

    I mentioned this in Tim's KX100 thread and Larry mentioned the impedance of the X transformers is higher than the early ones. I started reading Dave's long post on the topic of X vs non-X 400 transformers and now I'm confused.

    What if any circuit modifications do I need to do in order to use these AX type 400 transformers in the X100B. I can see the voltages are bit higher in the 400, but should this matter?

    It reads like I ought to install Dave's EFB(tm) in the X100B to use the AX 400 transformers, or find another to use. I have a pair of 500B units I can use.

    Below in order (in the style of Dave):

    1. X100B 10001 - 19999 output section.
    2. 400 48001 and on output section
    3 - 6. the unit in question. Looks pretty good, but when I got it one of the two piece knobs was destroyed, all brights were off something else entirely that fit poorly, had some sort of toy jewel glued over the light hole -the glue residue from which was all over the face plate and a ton of work to remove, three of the switches were from some other unit - I replaced these the night before last. Just started cleaning up the chassis as can be seen. Lots of caps have been replaced, so it should give years of service once it's together and set up.

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    Last edited: Apr 27, 2017
  2. gadget73

    gadget73 junk junkie Subscriber

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    Without spending a great amount of time looking at the circuit differences, my gut instinct is to simply duplicate the circuit values from the 400. If these are the late 10K PI 400 trafos, use the stuff Dave came up with on his 400 with EFBII project since that will actually make the output stage work properly with those transformers.
     
  3. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    Matt I expanded all of your pics of the schematics. They left out the NFB circuits. So the top of the pair is one side with the NFB circuit and the 2nd is the complete output to P.I. Sections with the NFB.(X-100-B 10001-19999) I repeated with the 400 scats(48001-and up). Also there is a 400 Early Left channel with NFB (10001-19999 manual)
    The 1st numbers after Screenhunter is the photo #. 24 thru 28 in order.

    X-100-B Left side with NFB. (24) 10001-19999 manual
    ScreenHunter_24 Apr. 27 03.23.jpg

    X-100-B complete output section with NFB and Cathode bias. (25)
    ScreenHunter_25 Apr. 27 03.24.jpg

    400 Left output with NFB (26) 48001 and up Manual
    ScreenHunter_26 Apr. 27 03.26.jpg

    400 complete output with NFB (Fixed Bias) (27)
    ScreenHunter_27 Apr. 27 03.27.jpg

    And an EARLY 400 Left side with NFB (28) From the 10001-19999 manual.
    ScreenHunter_28 Apr. 27 03.39.jpg
     
  4. gadget73

    gadget73 junk junkie Subscriber

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    meh, fixed bias vs cathode. Swap the power trafo too and put that part in as well :) You'll have to change the heater supply on the tubes formerly used as a cathode resistor to feed like they do on a 400, but that shouldn't be any big issue either. Essentially if you do this right at the end of the day you'll have a 400 without the tuner. Should make for a nice integrated, and a nice way to make one good out of two bad.
     
  5. rufleruf

    rufleruf AK Subscriber Subscriber

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    Larry,

    What's the N in NFB? I take it you are suggesting to make picture 24 like picture 28?

    Gadget73,

    The 400 power transformer is HUGE compared to the X100B unit.

    Time to read the Dave EFBII thread again I think, and to maybe finish assembling the KM60 tonight so I don't get spread too thin.
     
  6. gadget73

    gadget73 junk junkie Subscriber

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    N = Negative.

    Only reason I'd swap the trafo is to get the bias supply and DC for the tube heaters. Using it in cathode bias will cost you some power and slightly change the operating points needed for the output stage.
     
  7. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    I'm not suggesting anything. I'm just showing the differences in the circuits. Did you ohm out the good X-100b transformer for primary side impedance? If it's a 6.6kohm and the 400 is a 10K, you'll have problems with the NFB as it stands. You'll have to do some changes to the circuitry to get the correct numbers and for it to work without extra distortion. BUT! If it's a 10Kohm transformer then the 400 (AX) transformer should just Plug and Pray. You need to know what to scope out, when and how. I don't know, so my answer is somewhat generalized. But it's not generally advised to install a different impedance transformer with out re-doing the NFB (NEGATIVE FEED-BACK) to optimum levels.
     
  8. rufleruf

    rufleruf AK Subscriber Subscriber

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    Negative indeed! I guess the coffee hadn't reached my brains acronym center yet.

    Plan would be to just use both 400 transformers to avoid visual asymmetry if nothing else, in that case I guess the mismatch between OPTs doesn't matter, but I'd need to get away from cathode bias, so maybe as gadget73 commented, use the 400 power transformer too. I guess it isn't that much bigger:idea:

    Would using the 500B output transformers I have make more sense IF the X100B has 6.6kohm OPTs?

    I was thinking the 500B transformers would go in a KX200 I have that is missing OPT's, but I also have a terrible condition but working X202B I can pull OPTs from, but that would be kind of sad, even though the X202B is so rough.

    When all my transformer swaps are done I hope to have a much better understanding of them!
     
  9. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    Do the measurements on the X100b transformer 1st. Then see what you have that will "match" with theX100b. If the 500b trannies match up impedance wise then 1/2 the battle is won. The change from Cathode to Fixed bias will be approx the same exercise. So be prepared for some painful head follicle removal.

    500b output section Left Channel with NFB (29)
    ScreenHunter_29 Apr. 27 21.45.jpg

    500b Output section (both channels) (30)
    ScreenHunter_30 Apr. 27 21.46.jpg
     
  10. rufleruf

    rufleruf AK Subscriber Subscriber

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    Spent a few more minutes on this tonight after finishing up the KM60.

    I have been reading up on determining the impedance of the output transformer on radioremembered.org. It seems straight forward enough - I'll be careful and set up a test where I put a known AC voltage into the primary and measure the load out of the secondary. But of course I have some basic questions about this. On the site they don't get into what to do about output transformers with lots of other leads, and the output transformers I'm dealing with have a lot of leads! Questions:

    The primary: this is the blue and blue/yellow pair?
    Measuring the secondary, this is the black and green wires? or will any of the three color outputs give the same result? I think the 8 ohm load is the oft cited impedance, so I suspect this is the one I need to measure.

    Dave was kind enough to post this info on my X100 3 thread of a few months ago:

    "The (nearly) standard Fisher color code (particularly in the stereo years) is:

    Black = Secondary Common
    Brown = 4 Ohm
    Green = 8 Ohm
    Yellow = 16 Ohm

    Blue w/Tracer = Plate Lead (in phase polarity with 16 Ohm tap)
    Red = B+
    Blue = Plate Lead"
     
  11. gadget73

    gadget73 junk junkie Subscriber

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    blue/blue white should be primary. They'd be what goes to the output tube plates.

    I usually feed in some known voltage across the primary and measure each of the secondary leads. Measure them from ground and just take notes of voltages. The math you can do from there once you have the winding ratio.

    If it would be of any help, I knocked together a quick and dirty spreadsheet to take care of the math and I can send you a copy when I get home. It just requires putting in some voltages and it works out the primary impedance and winding ratio for you.
     
  12. gadget73

    gadget73 junk junkie Subscriber

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  13. rufleruf

    rufleruf AK Subscriber Subscriber

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    Thanks - that loaded for me - will try and get to it tonight!
     
  14. rufleruf

    rufleruf AK Subscriber Subscriber

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    Okay - so I put 19V in the blue / blue yellow primary of the T1020-116-1AX 352552 late model 400 opt and got:

    4 ohm 0.389V = ~9,550 ohm
    8 ohm 0.571V = ~8,867 ohm
    16 ohm 0.756V = ~10,116 ohm

    for the original X100B transformer with 19V in

    4 ohm 0.461V = ~6,850 ohm
    8 ohm 0.68V = ~6,298 ohm
    16 ohm 0.898V = ~7,223 ohm

    Does this make sense? I'm using a DVM and a variac.
     
    Last edited: May 4, 2017
  15. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Matt -- Normally you only need to measure the 16 Ohm winding, or whatever the full secondary impedance represents, because being the full winding, it absolutely represents the design intent of the transformer. The intermediate secondary taps will then fall in place based on how close the taps could actually be placed. The 4Ω tap will normally measure identically to the 16Ω tap, since that tap can physically be placed quite accurately in most transformers. The 8Ω tap will usually produce a slightly lower reading due to tap placement limitations, but the difference is negligible in normal use. For best accuracy, its usually best to apply a higher voltage across the primary -- this to minimize any resolution error in your meter -- but in any event, your readings are quite close to mine. I typically apply 100 vac across the full primary winding.

    Dave
     
  16. rufleruf

    rufleruf AK Subscriber Subscriber

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    Dave,

    Thanks for the input. I did the the measurements a few times at different voltages, and as the voltage got higher, the readings got more in line with expectations.

    Tonight I'm going to do similar with a power transformer so I am comfortable with transformers in general.

    Now I have to decide whether I want to use the 400 AX type output transformers, with their need for circuit modifications or the 500B output transformers I have, that would probably be plug and play - but I would rather use elsewhere.
     
  17. gadget73

    gadget73 junk junkie Subscriber

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    So it looks like late style 10K PI 400 trafos and the 100B is a 7K.

    Haven't a couple people lately had 100B's with one or more bad transformers?


    Only reason I put the extra calcs into that spreadsheet was to verify taps. I also use it for working out unknown transformers. For transformers with secondary taps at a known point its not strictly necessary, but since all it takes is dropping in a voltage I usually do it anyway.

    I usually adjust the input voltage until I get about 1 volt at the output. I use my variac and an isolation transformer as the power source.
     
  18. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    My understanding is that the 7868 ideally wants a secondary of 6.6K. Unless in AB1 and Cathode bias it wants 10K. If in AB1 G2 is tied to the tap on the plate winding on the output transformer, then in AB1 the 7868 wants 6.6K in Cathode bias. See data sheet below. From RCA RC-23 of 1964. If you don't have the RC-23, download the sheets and bring them up in a photo viewer (irfanview for example).
    7868-1 tube data.jpg

    7868-2 tube data.jpg

    7868-3 tube data.jpg
     
  19. rufleruf

    rufleruf AK Subscriber Subscriber

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    Wow Larry - helpful as always. I did the same tests on a Dynakit Stereo 70 that showed up in the mail today. They must have factory matched those transformer sets - I got 4361.5 and 4341.8 at around 19V! I don't want to make anyone cry, so I wont mention the test readings of the matched quad of Mullard xf2 EL34's. I need to covert away from 7199's - maybe 6U8? I only have a few good 7199s.

    As I mentioned I would, I did the same tests with a power transformer - the ST70 again, and got really reassuring numbers (about 40V in, about 2V at the heaters on each side and about 1.5 at the 5AR4). I feel like I have a new super power! Time to bring the can cap up slowly in it.

    Back to the X100B. I would really like to use the 400 AX transformers - I have 4 spares to find homes for, so I will never miss them. Is there a way to get the bias revised and use the stock power transformer? I think Dave has a thread about going away from cathode bias if I recall - need to find that.
     
  20. gadget73

    gadget73 junk junkie Subscriber

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    Transformers are usually pretty consistent unit to unit if they're a mass produced item. I doubt they were hand matched honestly.

    If the 7868 wants 10K transformers in cathode bias, and the X100B is already cathode bias, that sounds like the ticket. It might actually match up better than the original configuration did.
     

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