X-100-B left channel tremolo

Discussion in 'Fisher' started by rufleruf, Nov 5, 2017.

  1. rufleruf

    rufleruf AK Subscriber Subscriber

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    I just finally got around to installing a new output transformer in an X-100-B I've had for a while, and it works now but on the left channel, the sound is modulated by a sort of tremolo effect as though AC is getting into the signal.

    When I first started it up after the work it didn't have this issue, it started after about 45 seconds. I swapped tubes around, but see no difference. Any ideas?
     
  2. larryderouin

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

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    sounds like motorboating or oscillation. Is the transformer one for an X-100-B or something else that's close? You might have to fiddle with the NFB circuit. Start at the transformer and work back thru the NFB circuit for either weak or bad component.
     
  3. rufleruf

    rufleruf AK Subscriber Subscriber

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    It was from a KX100, so it should be compatible.

    Are we thinking capacitor?
     
  4. Tim D

    Tim D AK Subscriber Subscriber

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    KX100 = X100B excepting for some really minor circuit diffs. The trannies will be fine.

    Does it happen on all sources? Phono section? Aux? Etc? Is it on the side where you replaced the trannie? Might be a problem on that side that caused the original trannie to fail?

    Pending that, you might want to try an O-scope to see where the problem starts. Just some in-expert thoughts.
     
  5. rufleruf

    rufleruf AK Subscriber Subscriber

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    I'll do some further tests tomorrow. A lot of work was done to it before I got it, and I haven't gone over it very closely. Was focused on the OPT.

    An oscilloscope is on my with list...
     
  6. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Does the volume control setting produce any change in the effect?

    Dave
     
  7. rufleruf

    rufleruf AK Subscriber Subscriber

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    It tracks with the input - volume turns it up - down along with whatever music is playing.
     
  8. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    I understand. What I'm wondering is this: Does the setting of the volume control change how the effect impacts the sound? Or, is the effect always a constant with the sound, and the volume control merely adjusts how loud or soft the affected sound is?

    Also, is the effect present when playing a source through the Tape Monitor setting?

    Dave
     
  9. gadget73

    gadget73 junk junkie Subscriber

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    you got the colors matched up right? Reversing the leads on the output tubes will make it oscillate. Usually comes out as a high pitch shriek but sometimes its different. Not grounding the appropriate terminal on the secondary can make it do stupid things too, but it will also usually have one loud channel since the feedback isn't working right.

    Whats the electrical condition of the rest of the amp? Sometimes bad power supply caps can make for strange oscillation problems.
     
  10. rufleruf

    rufleruf AK Subscriber Subscriber

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    At very low volume its not there and sound is fairly well balanced, once it gets to around 2 it starts and, turning it up just turns up the effect. Speed and depth of the effect is the same. It's on the opposite channel to the one I replaced the OPT on.

    Something I noticed: The two left output tubes as you look down on it are not glowing evenly. The left most tube has the blue ion haze or whatever it is, but its partner does not, and the blue glow in the left most tube seems to be collapsing in time with the tremolo effect - it's subtle, but noticeable. In swapped tubes around, and it seems to be the socket.

    Overall it seems to be in good shape as can be seen. Not sure why the socket was changed. Tubes are all new EH items that came with it.

    In the top picture the tube on top is the left most tube that has the fluctuating blue glow.

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]
     
    Last edited: Nov 5, 2017
  11. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    If you turn the balance control all the way to the good channel, are you sure there is no effect in the good channel at all?
     
  12. rufleruf

    rufleruf AK Subscriber Subscriber

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    Dave - I'm pretty sure it's not having an effect on the good channel - I had it up to about 3 and the balance all the way over. 5 year old is asleep so i can't go louder. I'll check to make sure at higher volume tomorrow when I get home.
     
  13. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    OK. Next test. One the bad channel, remove the connections to electrical top of the volume control -- the one with a 68 pF and .047 uF cap connected to it. With the balance control centered, now inject a source signal at the top of the volume control -- source ground to the chassis, hot signal to the top of the volume control. Is the effect still present with the source injected at this point?

    Dave
     
  14. Tim D

    Tim D AK Subscriber Subscriber

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    Listen to Dave first, but I've got a few thoughts if you finish with his suggestion - before he gives the next one.

    The difference in the blue glow between the two tubes is interesting. Maybe someone that has experience can say what the blue glow vs no blue glow means, but it makes me wonder if you have voltage differences between the two channels - somewhere.

    Have you checked that the DC values are the same through both channels? I'd be checking for obvious differences between left and right channels. That's how I found out that one of the OP trannies was half open. In fact the open ended OPT had the power supply voltages out of whack. How are the PS voltages? There are at least 4 different DC levels that come out of the PS section. In mine, both channels still worked, but they were drastically uneven. For the channel with the bad trannie it has two OP tubes. One tube in that channel wasn't doing anything because that side of the associated OP trannie was open. It was kind of like rowing a boat with one oar. Still moved around, but it wasn't right at all.

    The diagrams have the expected DC values at lots of points throughout. It is easy to check with a multimeter that goes high enough. Just be careful as the voltages are dangerously high and don't short anything when you are checking. You could wrap most of the shaft of the end of the probe with electrical tape to reduce the chance of accidentally shorting while sticking it in there. [Don't do it unless you are comfortable and safe with working around the high voltages.]
     
  15. larryderouin

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

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    Tim; Blue glow is pretty much normal in the EH7591's. Some glow very little and some like the statue of liberty. Here's an old Sylvania note that details reasons for blue glow. His are new tubes so a gassy tube could very well be ruled out. In his case he's swapped tubes in the pair and the lack of glow stays on the same socket. I'm leaning toward a loose socket among other things. These are way down the list under Dave's impressions.

    Larry

    Reasons-for-blue-glow-in-tubes.jpg
     
  16. Tim D

    Tim D AK Subscriber Subscriber

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    Thanks Larry. I am also wondering why the blue glow didn't move when the tubes were moved around. Seems you might have hit on something with that socket.

    I'm looking at Matt's picture of the sockets for the OP tubes from underneath and I can clearly see the socket (V8) that he says was replaced. The rivets are replaced by nut/bolt and the bakelite is black instead of brown. I see a blue resistor connected to Pin 6 on V8 and that doesn't strike me as original. The value on it looks like 100 ohm (brown-black-brown). The other OP tube sockets seem to have 1,000 ohms (brown-black-orange) on Pin 6 (particularly V10 which is on the other channel and should be identical in every way). I'm not calling foul until I check my KX-100 which is the same unit when I get home tonight, but I will do some digging and let the guys know if this might be something to look closer at.

    I've got the resistors on Pin 6 of V8 and V6 circled in red below. That's what I'll check on tonight. Not saying this is Matt's problem, but the new/blue resistor stood out and caught my eye and now I'm on a mission.

    EDIT: Corrected the brown black orange to say 1,000 ohms.

    EDIT: Corrected the reference to V6 to be V10.

    Resistor.jpg
     
    Last edited: Nov 7, 2017
  17. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Those resistors are 1K grid stopper resistors -- one of which has been replaced by a modern metal film piece that uses the 4 band color code system. It is likely a 1K value as well.

    My last test has Matt injecting a line level signal directly into the power amplifier section, bypassing all line/tone stages. If the problem still exists, then this test proves that it must be due to an issue in the power amplifier section proper. At this point, I'm just trying to get the cause localized.

    Dave
     
  18. gadget73

    gadget73 junk junkie Subscriber

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    Looks like the 1K was replaced with the socket. Probably had to be replaced because the old resistor didn't have enough lead length to remove and re-install.
     
  19. larryderouin

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

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    I blew it up and the bands don't look right. One way looks like (top to bottom)BLK, BRN, BLK, BLK, (Blk or Brn) for 10ohms 1%. The other way blk,blk,blk,brn,blk gives 0ohms 1%. I'm thinking it's a 10 ohm.

    Resistor1.jpg
     
  20. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Could well be Brn - Blk - Blk - Red too, which would be a 1K value.

    Dave
     

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