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Fisher X-100 5AR4 screw-up

Discussion in 'Tube Audio' started by ronton3, Oct 13, 2018.

  1. ronton3

    ronton3 ronton3

    Messages:
    134
    Location:
    Macon Mo
    This amp was in good working order. I had borrowed the 5AR4, for another amp, when I wanted to use the X-100 again I inadvertently, put a bad 7591 in that place( I have not forgiven myself yet). After the sparks and realizing what I had done, I put a 5AR4 in and hoped things would be okay---they aren't. For some reason 12AX7s V1,6,7and 2 do not light up, the others and the power tubes do. I believe the brown and white wires supply the AC for these filaments. How can I check to see what may be wrong. Is that symbol #47 a lamp if it was burned out could that be the problem. Thanks for any advice.
     

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  2. primosounds

    primosounds SE KT120 w/ 6J5G drivers. Subscriber

    Messages:
    2,646
    Location:
    Terra, 21st century CE
    The tube filaments are hooked up in series which means if one of the heaters opened ahead of the others no heater voltage will go past it. Hopefully you just need to change out the bad tube. To be safe you should do some other voltage checks to verify that there is no other problems.
     
  3. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,583
    Location:
    Southern NJ
    Also the heaters act as a cathode resistor, so if there is no plate voltage going to the output tubes, there will be no current flow to light the 12ax7 tubes. Check the power supply voltages and see whats going on.

    The possible problem is the connection between pin 4 and 8 on the 7591. The 5AR4 has the heater on 2 and 8, and high voltage on 4 and 6. DC output is from pin 8. Connecting 4 to 8 would put high voltage AC to the first filter cap. If you're lucky, the 250 ohm resistors took the abuse and died. If you're not lucky, the power transformer might have lost the high voltage winding. Time to get out the voltmeter.
     
  4. ronton3

    ronton3 ronton3

    Messages:
    134
    Location:
    Macon Mo
    There are 4 12AX7s, 2 7247s, 4 6BQ5s and the 5AR4, I just put in 4 known good 12AX7s, and still no light on the 12AX7s the 7247s and the 6BQ5s do light up. I will check the power supply voltages. What does the #47 symbol hooked up between X @ Y on the schematic I posted stand for please? The little light on the front panel does not light up, could that be the problem, I know a light bulb on my Hickock 539c acts as a fuse. Thanks ron
     
    Last edited: Oct 14, 2018
  5. knockbill

    knockbill Addicted Member

    Messages:
    7,702
    Location:
    SE PA
    #47 is the bulb...
     
  6. ronton3

    ronton3 ronton3

    Messages:
    134
    Location:
    Macon Mo
    Pins 4 & 6 of the 5AR4 show 381 VAC pins 2 & 8 show 374 DC, I am sending a photo of the Fluke because I do not understand the -0.427 reading on the C1 cap C2 is the same kind of reading. Does the bulb being out make any difference? Thanks ron
     

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    Last edited: Oct 14, 2018

     

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  7. century tek

    century tek Super Member

    Messages:
    3,281
    Location:
    The Evergreen State
    If your 12AX7 tubes are not lighting at all, check the heater circuit with a volt meter. Are you not getting any sound? Usually the 12AX7's in this circuit are for the phono preamp and output biasing. Have you tried to play any music yet?

    Might want to check that 120Ω sand brick resistor.
     
  8. ronton3

    ronton3 ronton3

    Messages:
    134
    Location:
    Macon Mo
    The 120 watt resistor shows OL on all ohm settings on the Fluke, so I believe it is bad. I believe the schematic in the first post shows 2, 7 watt 250 ohm resistors in parallel, should I try to do that or is the 120/25 watt the same. What would work best here, I will probably have to order something. Thanks every one, maybe it is not the Trf, which I have feared.
     
    Last edited: Oct 14, 2018
  9. cademan

    cademan Addicted Member

    Messages:
    8,992
    Yeah, that brick resistor probably took the brunt of the extra current and opened up when you accidentally used the wrong tube. On your meter with it set to DC volts and with the black probe on the chassis, and the red probe on the other side of the resistor going to the main filter capacitor, you should see at least 300 volts DC. If not, the resistor is definitely bad.

    Probably the 120Ω, 25 watt resistor is the only one you need to replace but you can be proof positive if it is bad by testing it with your volt/ohm meter.
     
  10. ronton3

    ronton3 ronton3

    Messages:
    134
    Location:
    Macon Mo
    I found a Dale 150/25 watt, with it in place, I now have on C1 and C2 400,339--247,283,37 volts. I am listening to to the distortion free phono section, the aux also works, but both are only on one channel, and volume is low. At least I am grateful that what I have thought of as my most stupid, screwup has been I have not used this for years so it needs a deoxting. Now I just have to hope one of the outputs did not suffer, Channel A plays, Channel B is silent. I should mention that there is no hum, I replaced C1&2 with LCRs 20 years ago. Thanks guys for bailing me out again. Any advice on only one channel playing, or how to test the channel A trf would be appreciated.
    Edited the silent channel is B, A plays.
     
    Last edited: Oct 16, 2018 at 4:53 PM
  11. knockbill

    knockbill Addicted Member

    Messages:
    7,702
    Location:
    SE PA
    You can compare voltage readings from the good channel to the bad...
     

     

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  12. cademan

    cademan Addicted Member

    Messages:
    8,992
    You should probably try and find a 120Ω. The 150Ω could be why you have low volume. 30 ohms difference really can make a difference although its probably within the engineers spec.
     
    Last edited: Oct 14, 2018
  13. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,583
    Location:
    Southern NJ
    yeah the voltages will be low. With low plate voltage, the preamp tubes will have low voltage as well and it could be enough to cause this problem. At least you have signs of life and the power transformer didn't roast. Everything else is fairly easy to replace.
     
  14. ronton3

    ronton3 ronton3

    Messages:
    134
    Location:
    Macon Mo
    I believe I have found the problem with only one channel playing, which I hope someone will verify. This X-100 #35497 is not the one in the Sams, it has a 3 part can cap, I replaced 20 years ago with an LCR, and used a Nichicon for the 3rd part A which was 100MF/150V-I used a Nichicon 100/350. The whitish wire attached to the negative end leads to the V2 pin 4 which reads 0, Pin 4 is 8.5 VDC, reading from ground. I reflowed the connections, and I have deoxited all sockets. I believe the wire should be on the positive end, if that is correct I will redo that mess, and do another voltage chart later and we should know what is really going on. Thank you for all the help. On the voltage chart, I did some more reading and work on the Fisher Schematic for this unit, the 5AR4 is 743 VAC pin 4-6. V3-8-4-5-9-10 are all 6.46 VAC instead of 6.3. I bias adjusted to 34 VDC.

    Update: according to the Sams pin 4 of V2 is supposed to be on ground so I guess I have it right after all. Still only 1 channel, the selector switch has mono, reverse. stereo, A, B they all play the same on one channel.
     

    Attached Files:

    Last edited: Oct 16, 2018 at 12:11 AM
  15. ronton3

    ronton3 ronton3

    Messages:
    134
    Location:
    Macon Mo
    Can anyone comment on the voltages I have posted, would they have any influence on one channel not playing. I have ordered, 120 ohm 7 watt resistors to replace the 150 temporarily installed. Thanks ron
     
  16. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,583
    Location:
    Southern NJ
    The small signal tubes have DC on the heaters, not AC. To ground, you'll find that they add as you get "closer" to the output tubes. I can't read the schematic well enough to see what order it runs in, but you should see about 12vdc from pin 4 to pin 5 on each tube if thats where the heater supply is connected. If pin 4 and 5 connect together and heater supply runs to pin 9 on one side and 4+5 on the other, you'll see about 6vdc across each tube. It seems like you've got 4 tubes on DC heat, and 6 on AC. Output tubes should be on AC, and I'm guessing the two phase inverters as well but I can't read the numbers. The tubes with AC should see ~6.3 vac from pin 9 to pins 4+5.
     

     

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  17. ronton3

    ronton3 ronton3

    Messages:
    134
    Location:
    Macon Mo
    Thanks for responding, the 2 7247s each have, 6.54 VAC, V1 first in line with from the power tubes is 7.9 dc, then V6, 8.7, V7,7.9 and V2 8.7. They are oddly numbered. I had replaced the fixed caps around the drivers 20 years ago with 716P orange drops, the only fixed caps left are around V2 and V7, would they be the likely reason that that channel B does not have a signal, or should I look elsewhere. Thanks ron
     
  18. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,583
    Location:
    Southern NJ
    If you're getting less than 9v across each tube on the DC string, thats probably not enough. I pulled up a schematic for the X100 and you should have 44vdc at pin 3 on the output tubes. Adjust R38 until you get there and it may take care of the problem.

    Unequal voltage drop on a 12ax7 string isn't too uncommon, especially if they aren't 12ax7a's. Also reduced heater voltage can make them act strange. As long as it nets out so one isn't way over voltage, its probably OK but it wouldn't hurt to use all 12AX7A tubes in here since those are designed for series string use.
     
  19. ronton3

    ronton3 ronton3

    Messages:
    134
    Location:
    Macon Mo
    You may be looking at the schematic from Sams, I was confused about this until I found the Fisher schematic for the up to 39,000 unit mark which mine falls into (37,457) it says 34 volts for the bias point. I will look at adjusting the resistor late, right now I am just trying to get channel B to pass a signal.
     
  20. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,583
    Location:
    Southern NJ
    This is a Fisher X100 schematic for 30001 to 39999

    https://elektrotanya.com/fisher_x-100_parts_sch.pdf/download.html

    it definitely says 44v at pin 3 on the output tubes, which would give about 11v per tube. My point is that if the preamp tubes are not hot enough, they may not pass a signal. Get the heater voltage reasonably close to ballpark and see if it starts working before getting too far down a rabbithole.
     

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