Help with amp repair, need some resistors.

Discussion in 'Tube Audio' started by Ken Boyd, Dec 16, 2018.

  1. Ken Boyd

    Ken Boyd AK Subscriber Subscriber

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    Hello, I have an amp that I am attempting to repair, an AQ1002 Antique Sound Lab. Seems like a simple straight forward amp, and I have pulled it apart. Someones already been in their replacing some burnt resistors. They just jumped the foil on the board and have used two resistors tied together. I have cleaned up the board some and want to use the correct resistor values. I have the schematic and it gives me the resistance value but it doesn't give me the wattage. The resistors I want to change are ones right off the main output tubes, they use 6550 tubes or KT88, and all the resistors I want to change are listed on the schematic as either 10 or 100. Is a 1 watt resistor ok to use in this application? Thanks for any help on this. I already have one amp just like it and want to fix this so I can bi amp my speakers.

    I want to replace resistors R15,R25,R16,R26,R37,R39,R38,R40

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    Last edited: Dec 16, 2018

     

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  2. LinderK

    LinderK New Member

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    Hi Ken,
    A 1 watt resistor would be fine. However, in those positions, a 1/2 watt or even a 1/4 watt may be preferable as they can act as fuse for the screen (100), and the cathode (10) If there is a fault in either location.
     
  3. gadget73

    gadget73 junk junkie Subscriber

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    agreed on the relatively low watt parts. The stuff on the board looks like its probably not more than 1/2. That board does look a bit torched too, I'm guessing something rather unfortunate happened?
     
  4. Ken Boyd

    Ken Boyd AK Subscriber Subscriber

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    I
    Thanks, for the info guys, I am sourcing parts now, trying to line up the parts so I can give this a shot. Yes, not really sure the history of this amp, as a member here gave it to me the way it is. I had put a WTB ad on Barter Town for this model amp and a member here had this one as a project that he had never got around to and gave it to me, all I had to do was pay the shipping..
     
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  5. Ken Boyd

    Ken Boyd AK Subscriber Subscriber

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    Replaced the resistors that someone had replaced, where the burnt spots were. One channel seemed to perform ok, but the other channel did not have much output, some hum and I was checking all the tube bias one tube, output, I could not get the correct bias voltage, it was to high. Instead of the .5 voltage DC, It was over 30 and I couldn't get it to come down, and it blew R39 resistor again. That is the left channel looks like that resistor is on pin 8 which I think is one off the cathode I think. I am new to this but would love to learn more.

    Anyone know why I would have to much voltage on the cathode at pin 8. Is it save to pull the output tubes out and power it up to check all the voltages out of the transformrs. I am just guessing but thinking some other resistors down the line are bad which would allow to high a voltage to be present at that pin. Anyone have any ideas on where I start?
     
    Last edited: Jan 12, 2019 at 9:02 PM
  6. gadget73

    gadget73 junk junkie Subscriber

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    wild guess, bad coupling caps. try removing them and see if the tubes bias normally. Should be those white ones on the bottom side of the board.

    You should also be able to measure negative voltage at pin 5 if things are right. I'd do this with the output tubes removed. If you get no voltage or positive voltage, suspect the coupling caps or something else in the bias circuit.

    pin 4 is the screen. An internal tube failure could do this, or just the tube being seriously over-current.
     
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  7. Ken Boyd

    Ken Boyd AK Subscriber Subscriber

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    I have another identical amp to this one, and just got new tubes for it, so I used the tubes I took out of it to test this amp, they worked fine in the other amp. I replaced the caps, I used these



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  8. primosounds

    primosounds Powered with pure tube sounds. Subscriber

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    Audio Cap is a very fine cap and you should be able to get excellent performance with them. But they are somewhat of an audiophile type as can be determined by the "hype". I prefer paper in oil type like vitaminQ or K40y-9.
    BTW, whenever you get a basket case amp, you should have the seller guarantee the transformers so you don't have an amp that is going to need new trannys.
     
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  9. gadget73

    gadget73 junk junkie Subscriber

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    fair point, if the transformers haven't been ohm checked at least, thats worth doing. If the plate lead on one end is open its going to cause all sorts of trouble. A quick and dirty ohm check from pin 3 to pin 3 on the 6550 sockets in the channel that isn't working right will tell you. I'd expect under 500 ohms.
     
  10. Ken Boyd

    Ken Boyd AK Subscriber Subscriber

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    It was given to me along with good tubes I just paid shipping so even if it was not repairable I figured the tubes and parts was worth the shipping expenses, if I couldn't fix it or find someone to fix it for me.

    That was what I thought I should check as to why my question if it was safe to power up without the tubes. I was going thinking I should check the output voltage of the transformers. That one red lead coming down looks like it is supposed to be 483 volts, the other lead coming into R 31 and R32 should be 352 volts, and the other lead coming into R28 and R29 should test at 288 volts if I am reading things right. I don't know what the resistance should be, but I guess since the Right channel is working I will check the resistance on those taps and compare them to the ones on the left channel. Thanks for the help on this, if the tranny is bad finding another one might be challenge unless I can find another one of these that can be purchased as a donner.
     
  11. Ken Boyd

    Ken Boyd AK Subscriber Subscriber

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    I have tested all the leads on both output transformers and they both measure pretty close to the same resistance to each other. I think the they are ok.
     

     

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  12. Ken Boyd

    Ken Boyd AK Subscriber Subscriber

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    I will check that out. But I need to replace the resistor that blew again. I am making an order up for a pump to remover the old solder and a flux pen. I want to remove all this old solder and give all these points some fresh solder. and then check using my meter check all the foils are intact.
     

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