Modifying the Fisher 400 with EFB

Discussion in 'Fisher' started by dcgillespie, Mar 16, 2013.

  1. eduarsan1

    eduarsan1 Luis Eduardo Sanchez Subscriber

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    Hello

    Do you have another Mosfet to test it in the board.??? Probably that one is gone.

    NOTE: Beware with the static!!!

    BTW good design :)
     

     

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  2. Fur Face

    Fur Face New Member

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    Thank you eduarsan1 for the compliment and your suggestion. I do have a extra Mosfet and I will retest the one in circuit.
     
  3. Fur Face

    Fur Face New Member

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    Test picture of the MOSFET with the gate and drain unhooked from circuit.
    Also in all of the test pictures the red test clip is number 1 green number 2 and black number 3.

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    Last edited: Feb 24, 2018
  4. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Fur -- Very nice work! With regards to the problem your having, it appears that you likely have a 270Ω resistor installed from the Mosfet Gate to ground. This resistor should in fact be a 270K 2 Watt resistor. If this is the case, that would certainly account why you have no screen grid voltage!!

    Let us know, and I hope this helps!

    Dave
     
  5. Fur Face

    Fur Face New Member

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    Thank you Mr Gillespie for your complament and leading me straight to my problem. And that’s Why I should be measuring every resistor instead of half of them or of course learning the code better.

    Having 118v ac going to amplifier I’m getting 394v B+. My AC and DC filament voltage is 6.34 and 11.04. Useing the pot on the EFB I have a ajustment from -12.40 to -15.54 with -56 coming from the bridge rectifier and happy now 286v going to screen grids.

    The power tubes readings.
    Pin 3. Pin 2/6.
    .21 -12.17
    .37 -13.55
    .21 -12.30
    .19 -13.

    I also hooked up a scope to see what was going on and the right channel is not so happy.

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  6. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Fur -- The ultimate output tube voltage to check is the cathode voltage (pin 3) at each output tube socket. These pins were originally all connected straight to ground, but each should now have a precision 10Ω resistor connecting it to ground. This allows you then to measure the voltage across these resistors, and determine current flow through each output tube. You should set the EFB bias control so that the voltage reading at all four tubes averages 0.21-0.22 vdc.

    The right channel looks like it is displaying instability. Check that the capacitor across the NFB resistor is the correct value and in place, and also check that the 100 pF/27K resistor network is properly connected to the 12AX7 driver tube for the right channel.

    Dave
     

     

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  7. Fur Face

    Fur Face New Member

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    Thank you Mr Gillespie for your quick response and your guidance to helping me resolve my issues. But I will be detained for a week or so before I can devote some more time to working on the amplifier. So thank you again and I shall return hopefully with some great news.
     
  8. Fur Face

    Fur Face New Member

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    Okay adding the 4 - 10 ohm resistors I have ether 0.21 or 0.22 volts on the power tubes cathodes. But have not been able to find the instability on the right channel.

    Have disconnected the parts that make up the feedback loop to check value and they check good. Also checked that the 100 pF/27K resistor were also the correct value and that the network was properly connected to the 12AX7. In all the scope pictures the top is the right channel.

    Will add some underside pictures in next post.

    The top picture is attached at the volume control and the bottom is attached at the 4 ohm tap.
    BC31F8E2-A9D1-4388-8B52-38212F38CC23.jpeg
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    The first two pictures are on pin 1 on the 12AX7, pin 2/6 in the middle and pin 8 on the bottom.
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    Two pictures at volume pot with one probe removed.
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    Two pictures also with one probe at pin 3.
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  9. Fur Face

    Fur Face New Member

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    Location:
    60404 IL
    Some underside pictures of both right and left side of the 12AX7. So in the past I have missed the obvious so please if you have any suggestions they will be welcomed. Thanks to all in advance.

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  10. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Fur - Two observations:

    1. The sign wave signal appearing at the top of the volume control in the first pic looks very compromised, with the bottom crest of each wave looking very bloated, while the tops appear rather normal. I would connect your scope directly to the source you are using to verify if this is a source problem, or an additional problem in the Fisher.

    2. The signal at the output terminals from both channels appears to be compromised. Granted, the right more than the left, but both compromised above and beyond the potential source issue noted above. With both channels showing the same issues then, that usually speaks of power supply problems, since that is the only thing in common with both channels, other than the source. Either that, or the same mistake has been made in both channels.

    3. With no signal applied, do the outputs show a flat line? Or is there garbage present with no signal? Advance your scope sensitivity to check.

    4. Have you verified that your OPTs are the later version units?.

    5. Remember that with the modified amplifier, the NFB is taken from the 4Ω tap -- NOT the 16Ω tap as was done with the original design. Make sure you moved the NFB lead from the 16Ω tap to the 4Ω tap. Failure to do this will result in twice the amount of NFB applied to the amplifier, and unstable operation as well.

    Let us know!

    Dave
     
  11. Fur Face

    Fur Face New Member

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    60404 IL
    My apologies for not verifying some pertinent information. The OPT are T1020-116-1AX and T1020-116-2AX. And did use the 4 ohm tap with the new feedback. Also other than then the mod from everything around the 12AX7 (phase inverter) tube I rebuilt the B+ and - power supply so far. I have two CL80 at AC input because AC runs at 125 and up to 129 at times. B+ at controlled 117AC are.

    402
    355
    329
    292
    205
    200
    192
    184
    AC and DC filament V are 6.37 - 11.32

    12AX7 tube voltage are.
    Pin 1. 222
    Pin 2/6. 99
    Pin 3. 100
    Pin 8. 0.65

    The sine wave is 400Hz. This first picture is with the amp off.
    F71DC8C9-09A6-44E1-B113-625516239933.jpeg

    At volume pot.
    D77A5EB0-E384-4701-B7FB-8A52A65AF82F.jpeg

    At 4ohm tap and just the left channel on pic 3.
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  12. Fur Face

    Fur Face New Member

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    Mr. Gillespie your suspicion that the sine wave source may have been bad was wight on. I downloaded a sound generator app and the single looks much better on the scope.

    First one with no power to amplify.


    6B9ECB36-E06F-4B29-8037-5DBAC2FD1FD0.jpeg
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  13. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Fur - OK. I need more information --

    1. What points do the B+ voltages represent? I can guess the most of them, but there appears to be more voltages than B+ points...........

    2. What power level do these wave forms represent?

    3. The top trace almost represents a sort of sync issue with the scope, and yet I know it displays other dual traces just fine. In any event, since it appears that we need to concentrate on the right channel, why don't you go to single trace mode for that channel, so that no sync issues can interfere with the presentation. Or, if you are syncing from the left channel, try syncing from the right.

    4. What does a full power 1 kHz sign wave look like, and how much power is produced at the onset of clipping? Use a 16Ω load on the 16Ω tap if possible for the test.

    Dave
     
  14. Fur Face

    Fur Face New Member

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    60404 IL
    Thank you Mr. Gillespie for your remote help to assist me in finding my blunder or blunders. Also I will try not to assume that you can read my mind. So if I may I will reference that alsome schematic that you and Luis put together and l believe it to be the last version. I will put the voltage on the schematic next to part number and then my actual number from my amplifier.

    You were also wondering what the power level of those wave forms represent and sorry I do not.

    CR-1 bridge rectifier supplys V8-V11 filament voltage for the 4 12AX7 tubes and after rebuild I have -23v at the same yellow wire hookup point as before.

    C52
    A-210v............207v
    B-310v............293v
    C-355v...........334v
    D-390v...........360v

    C-56-435v.....406v

    C-57
    A-166v............186v
    B-182v............194v
    C-198v............202v

    Phase inverter 12AX7
    Pin
    1-260v............222v
    2/6-90v..........99v
    3-92v.............100v
    8-0.8v............0.65 v

    The setting on the scope is .5 on the sweep and with 10x probe the division is at 5. I got 11.54 rms on my meter taken across load when I backed off the gain,so if I did the math wright were looking at 8.3 watts, ouch. This was at a 1 kHz signal into a 16 ohm load.
    BFEF4B54-5B6C-4B50-BEA2-C635F7759A6F.jpeg
    I tried the left channel at the same settings and came up with 7.6 watts.
    540D1DF0-E575-4B9F-974C-B20E22CD4FD0.jpeg
    And one with both channels. 29144FAA-6C7B-44C7-AE1B-2C50367A5741.jpeg
    One thing I noticed also was the left 16 ohm load got hotter then the right side. I assembled them together with 4 4 ohm 25 watt resistors.
    0C2D8CB8-5BD2-4507-B81F-8EC6B92A752B.jpeg
     
  15. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Fur -- Your scope not syncing in the next to the last pic, but is in the last pic. In the last pic, which channel is providing the sync signal?

    Also -- what brand of output tubes are you using, and what is their condition?

    The last pic is showing notable cross over distortion. This could be from the lower screen voltage your unit is deriving from the lower main B+ voltage. Note that my unit produces a main B+ voltage 435 vdc. Later units have been noted to produce less B+ voltage -- more on the order of what yours in producing. Try biasing the output tubes to 27 mA and see if that doesn't clean up the crossover distortion.

    Dave
     
  16. Fur Face

    Fur Face New Member

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    60404 IL
    Far as scope not syncing we can call it operator error, all left channel pictures will have trigger INT set to B.

    Tubes are all used pulls, but the ones that are in the amplifier now have a range from 15 to 29 mA with the EFB pot.

    With 27 mA and the C-56 B+ at 406v the output watts went from 8.3 to 9.2 watts on the right channel and on the left channel it went from 7.6 to 8.8 watts. I did back the gain off before I took my rms reading to figure watts with.
    6ADEB238-3008-4A28-B7E6-F838A86BB257.jpeg
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    I then turned up the B+ and below are the new voltages. The AC and DC filament voltages are 6.7 and 12.3 with the new B+.

    C52
    A-210v............218v
    B-310v............311v
    C-355v...........352v
    D-390v...........381v

    C-56-435v.....432v

    C-57
    A-166v............198v
    B-182v............206v
    C-198v............215v

    Phase inverter 12AX7
    Pin
    1-260v............242v
    2/6-90v..........102v
    3-92v.............104v
    8-0.8v............0.73v

    With the new B+ I took readings with 23 and 27 mA on the power tubes with a difference of .2 and .4 of a watt between the two. So with 27 mA I got 12.7 Watts on the right channel and 11.9 on the left channel. Also 316v on pin one of power tube.
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  17. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    If those power readings are at the onset of clipping, then the output stages are only producing about 1/2 of what they are capable of. If that's the case, then something is very wrong. The B+ voltages look good, but it may be time to measure the impedance of the OPTs to ensure that you do have the correct transformers.

    Also, what are the OPT CT B+ and screen grid voltage readings when full power is reached?

    Dave
     
  18. Fur Face

    Fur Face New Member

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    Sorry, yes the readings are at the onset of clipping.

    With the tubes bias set at 27 mA about 4 minutes warm up time the B+ is at a resting 419v and also 419v at OPT CT and the screens are at 304v.

    With the power turned up to onset of clipping the OPT CT is at 402v and the screens are at 293v.

    For what it is worth the outside covers do have 1AX and 2AX on them. And yes more than willing to measure the impedance of the OPTs.

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  19. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Question: You are no longer showing any weird clipping, instability, or what appeared to be scope sync problems with your scope pics. Why? What did you do to change this? Specifically, your scope pics in post #51 shows all kinds of issues and then some, which no longer seem to be present.

    I would go ahead and measure the impedance of the transformers. I remember one unit documented here that had transformers that included the AX suffix, but measured like non-AX transformers.

    Dave
     
  20. Fur Face

    Fur Face New Member

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    Good question about the weird clipping, instability so I went to see if I could replicate it. Pictures below.

    About measuring the impedance of the transformers I measured the ohms between the blue and the blue with the tracer and then from the center tap to each blue wire.

    Left side is.
    273 then 130 and 142

    Right side is.
    279 then 136 and 143

    And for comparison I have a transformer out of a fisher KX 200 # T-908-116-1 and the reading is taken the same way.

    104 then 55 and 49

    If I should be performing some other impedance testing please send me the procedure thanks.

    On the pictures the scope trigger is set on .5 ms and using 10x probe. The volume control is set at 2 for the first 2 pictures. First picture is 50m per division then 20m per division. In conclusion with more volume gain and higher division settings the instability of the signal gets under control.

    1EC8BD54-FD6C-488D-B19C-5C2D9F5A1E06.jpeg

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    This picture has volume at 2 1/2 and .1 division.

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    This picture has volume at 3 and division is set at .2

    FC187F4B-1E0F-4A63-BD55-A69C4EBC90CF.jpeg
     

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