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Stromberg Carlson ASR-444 No phono or tape HELP Please!!!

Discussion in 'Tube Audio' started by TubeJunkie, Jan 13, 2018.

  1. TubeJunkie

    TubeJunkie Master of None Subscriber

    Messages:
    333
    Location:
    TN
    Hi. I picked up a rough SC Stereo 60 recently. Powered it up slowly with a variac, and got sound through Aux and Tuner inputs. I didn’t check the phono or tape head inputs. So I then recapped the electrolytics and tubular caps. Restuffed the 20/20/20/20uf can cap. The Aux and Tuner inputs sound great. But the phono and tape head inputs don’t. I can barely hear audio signal coming through one speaker. Tried different 12ax7 tubes. I think I may have a wrong connection on the can cap, even though I took pictures before I started. ... lol. I’m looking for the 75 vdc on R78 & R79, but I’m only getting around 30 vdc.

    I have R80 (150k) connected to the negative can cap. Is this correct?

    I have R70 (3.9k 5watt) connected To a positive can section. Is this correct?
    Any and all help is greatly appreciated. Thanks for reading
     

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  2. BinaryMike

    BinaryMike Pelagic EE Subscriber

  3. TubeJunkie

    TubeJunkie Master of None Subscriber

    Messages:
    333
    Location:
    TN
    Hi BinaryMike Thanks For chiming in. And for the much more ledgable schematic. I pulled R73 (220k) out of circuit to test it. It was spot on perfect. Checked that section of the can cap while it was unsoldered, it was good.

    And speaking of that same section of the can cap, which I think is C48A, I’m only getting 0.648 vdc. See pic below. The other sections are 229 vdc, 372 vdc, 316 vdc. Do you have any other suggestions? And Thank You again for taking the time to help me!
     

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  4. BinaryMike

    BinaryMike Pelagic EE Subscriber

    So you had 30V and now it's only 0.648V at C48C? Something's very wrong here. Maybe the cap is connected backwards and/or failed. Check DC resistance to ground after complete discharge. Can you sub in a replacement with clip leads?
     
  5. gadget73

    gadget73 junk junkie Subscriber

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    Southern NJ
    Not much it can be if you've got basically normal voltage at the 220v supply. Either C48C is shorted, or there is a short somewhere along that supply between the cap and the phono tubes.

    It would be worth confirming all of the supply voltages against the schematic to see if something stands out.
     
    TubeJunkie likes this.
  6. TubeJunkie

    TubeJunkie Master of None Subscriber

    Messages:
    333
    Location:
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    I figured it out fella's. One channel had a disc capacitor lead touching a resistor lead. The other channel had a loose wire and dirty and sloppy 12ax7 socket. Resolved those issues. Both phono channels are working great now. Even though there is a very slight hum in the phono. Sometimes, I don't check the most simple things first. LOL....The tape head input sounds overdriven, which I gather is normal.

    One more question. Could I change the phono input resistors to bring up the phono volume a little bit more?

    Thank You guys very much for all your time, guidance, and wisdom. Hopefully someday I'll have the knowledge and experience to help others too. So, happy to hear this old crusty beast live again!
     

     

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  7. TubeJunkie

    TubeJunkie Master of None Subscriber

    Messages:
    333
    Location:
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    Update on the phono hum. I moved my turntable ground screw away from the AC fuse holder that i installed. Hum is almost non existent. Much, much better. Thanks again guys. Very much appreciated.

    Like I said, this old girl looks rough and rusty, but I love it. One more historic tube piece saved from the scrap yard.
     

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    Last edited: Jan 14, 2018
  8. BinaryMike

    BinaryMike Pelagic EE Subscriber

    You could pick up a little more phono gain by shunting R10/R43. It won't be much. The next step would be to bypass R8/R41 with something like 47uF, but that's likely to affect phono equalization. I would try splitting R8/R43, then bypassing just half of the split resistance. Note that input loading is wrong for most modern MM cartridges --- it should be 47K. Equalization might be partially dependent on this, so you could be fighting an uphill battle to achieve excellent phono preamp performance. It's tempting to consider total circuit replacement to get known-good characteristics here. Do you have audio test gear?
     
  9. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,582
    Location:
    Southern NJ
    If this is like my ASR-433, there is also a 100K at the phono input jack so it ends up being a 50K load. Changing those to 89K will get it more right if you're concerned about it.

    The tape input does not have that, so it runs a 100K load. I usually convert tape head inputs to phono just so they become useful for something. Usually its just moving a wire on the selector switch and adding the proper load resistor at the input jacks to make it all go.
     
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