X-101-C McShane kit electronic restoration

Discussion in 'Fisher' started by 3l3ctron, Oct 17, 2017.

  1. Rockyhill

    Rockyhill 148 State Street Subscriber

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    I used a kit from Jim for my 500B. And likewise some can caps were replaced with discrete components. That took some time to figure out how to wire but I eventually did. It’s been a couple years and the receiver still sounds great. It looks like you are on the right track here.
     
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  2. Patrice B

    Patrice B AK Subscriber Subscriber

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    Is it an illusion or the bottom resistors leads are a little bit too close to the screw with the star washer?
     
  3. 3l3ctron

    3l3ctron Active Member

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    Hi. There isn’t any contact being made whatsoever by the parallel resistors. It’s placement is identical to the big single glass resistor it replaced - except that it is a bit even more raised up away from the chassis. Thanks.
     
  4. Rockyhill

    Rockyhill 148 State Street Subscriber

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    I did A LOT of tracing and meditating too! Your highlighted schematic looks like what I did. I compared JIm's bill of material with the schematic and once I located each part on the schematic I would highlight it. I would then locate and label the part on the chassis.
     
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  5. 3l3ctron

    3l3ctron Active Member

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    232
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    I connected my minimus 7w 8ohm bench speakers to the appropriate screw terminals in the back, plugged my iPhone into the aux section, plugged the x-101-c into the dim bulb tester with a 120v 150w bulb, and when it starts warming up, the light came on. Dammit. Now what?
     
  6. N8Nagel

    N8Nagel AK Subscriber Subscriber

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    when you say came on, does it come on bright briefly and start dimming, or does it stay bright?
     
  7. 3l3ctron

    3l3ctron Active Member

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    232
    Location:
    West Seattle, Washington.
    Initially it’s off. Then it starts brightening up, and stays illuminated.

    I’ve been poking around with my DMM. And, there seems to be a ground connection being made somewhere between the white wire that connects to the terminal that joins the bias adjust, and is present all of the way through the grid resistors until they reach the pos side of C22.

    Any contact from that string of parts/connections makes a beep on my continuity checker from ground.
     
    Last edited: Oct 25, 2017
  8. 3l3ctron

    3l3ctron Active Member

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    232
    Location:
    West Seattle, Washington.
    So...I’m getting continuity from ground any time I connect R68 (original part) and C22 (it’s one of the new one’s) together back into the circuit from their non-grounded end leads. They do both go to ground on the opposite end.

    But, when I check either of the non-grounded lead ends separately at their terminal with one or the other disconnected, the DMM doesn’t beep continuity.

    How is this happening? It’s passing through these components only when they are back in parallel?

    I’ll start by replacing the resistor. And, I’ll pick up a new C22 too.

    The amp worked fine before I started working on installing the kit.
     
    Last edited: Oct 25, 2017
  9. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    313 -- Most likely what your meter is responding to when testing in circuit is the heaters in the heater series string that connects to the positive side of C22. When checking for any shorts, remove any one of the four 12AX7 tubes V1, V2, V3, or V4 to break the heater series string continuity.

    The bulb being initially dark and then lighting as the unit warms up is not necessarily a bad thing. The fact that it's initially dim indicates that there are no shorts in the power supply, and then as the output tubes warm up and start to draw current, then the current draw through the bulb is enough to cause it to light.

    Dave
     
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  10. 3l3ctron

    3l3ctron Active Member

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    232
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    Thanks for your response, Dave. I just made a video. Maybe it demonstrates what I’m talking about better than my writing.

    When I powered up to test, I let it go for about 5 mins and no sound was ever generated. It never has taken that long before.
     
  11. 3l3ctron

    3l3ctron Active Member

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  12. larryderouin

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

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    3l3; What size bulb are you using? For the 500c 100W to 150W is generally best. If everything is good, when you flip the power switch, the bulb should initially glow very bright with a dimming down almost immediately (within a couple seconds) on wall voltage. If using a variac the same thing should happen only proportionally dimmer. As you increase voltage the bulb will get brighter as the voltage increases. Too small of a bulb and the bulb will stay bright all the time, and too large a bulb and the bulb will not flash initially and dim down (it'll stay dim). If there is a short, then any bulb will glow brightly.
     
  13. 3l3ctron

    3l3ctron Active Member

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    232
    Location:
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    I reconnected everything again. I have a 120v 150w bulb in the tester. Results with no sound being generated after 5 mins.
     
    Last edited: Oct 27, 2017
  14. larryderouin

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

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    Warm up time will be extended as the DBT is being used as a current limiter with subsequent lower voltages to heaters, etc. . From what I can see the DBT is acting normally. Check the REVERB IN AND REVERB OUT JUMPERS and make sure they aren't pushed in to the stops. This will ground them out and cause no sound. Pull them back about 1/8" and check again.

    With the DBT acting the way it is, you can stop using it and keep testing with it on the wall or variac @ 117V.
     
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  15. 3l3ctron

    3l3ctron Active Member

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    Ok. I’ll give it another go plugged straight in.
     
  16. 3l3ctron

    3l3ctron Active Member

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    232
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    And, plugged straight into the wall it warmed up quickly and started playing sweet music nice and healthy. Whew!!
     
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  17. 3l3ctron

    3l3ctron Active Member

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    Jeez I’m a worry wart!

    So, the power supply and cathode resistors section has been completed.

    Now to finish up with the audio section and grid resistors.
     
  18. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    313 -- The beeping of your meter in the video is completely normal in accordance with how theory says your meter should behave in that scenario. With the resistor and capacitor in parallel as they are, the resistor acts to discharge any charge that an external source might try to place across the capacitor. Therefore, when you touch the meter leads to the resistor/cap pair as you were, the capacitor initially looks like a short to the meter, so it beeps. But then the cap charges according to the potential that the meter places across the cap, so that when charged, it no longer looks like a short to the meter, so the beeping stops. So you remove the probe, where upon the resistor then discharges the cap. You touch the probe again, and the process starts all over -- the meter beeps but quits when the cap is charged, you remove the probe, the resistor discharges the cap, etc., etc., etc.

    Glad it's working for you now!

    Dave
     
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  19. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

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    Just curious, but what brand of coupling caps are included in this kit??
    Nice pics btw!
     
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  20. 3l3ctron

    3l3ctron Active Member

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    232
    Location:
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    Me too, buddy! Major relief. I love this piece of equipment. And, I’ve never experienced that behavior before with my multimeter. Since it wasn’t warming up, I went looking for something to be wrong, and I thought that I’d found it. Thanks for watching...I’m only 1/2 way though with this job.
     

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