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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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    Pas 2 Problem water running sound one side.

    bubbasweet
    bubbasweet


    Posts : 87
    Join date : 2016-03-14

    Pas 2 Problem water running sound one side. Empty Pas 2 Problem water running sound one side.

    Post by bubbasweet Tue Jul 06, 2021 10:46 am

    I have a Pas2 Pre and seems one channel has a sound in the audio kind of like faint water running through a pipe. Lower dull but also high frequency. Gets worse when the bass is up. Not a hum but as I described. when I turn the switch to tape it is not there. I guess it is coming from one side of the phono input. It is present on Special Phono etc.

    Did the DC measurements and nothing that odd.. I think it is the left channel of the PC6. I swapped the 12ax7 tubes and problem stays on same side.

    Any ideas? I do not have a scope.

    I cannot find any loose connections. Jiggles Loudness and all other switches. Could be a cap or resistor but not sure how to efficiently check this without lifting all of them. Should I just do the black caps and see? I hate replacing stuff when it is not defective.


    I sort of sounds like faint water running under the audio. Any help appreciated thx.

    Also when I put my hand near the 12AX7 near back of the chassis on the pc-6 phono board it hums.


    Last edited by bubbasweet on Tue Jul 06, 2021 12:26 pm; edited 1 time in total (Reason for editing : additinal info)
    Bob Latino
    Bob Latino
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    Post by Bob Latino Tue Jul 06, 2021 6:24 pm

    bubbasweet wrote:I have a Pas2 Pre and seems one channel has a sound in the audio kind of like faint water running through a pipe. Lower dull but also high frequency. Gets worse when the bass is up. Not a hum but as I described. when I turn the switch to tape it is not there. I guess it is coming from one side of the phono input. It is present on Special Phono etc.

    Did the DC measurements and nothing that odd.. I think it is the left channel of the PC6. I swapped the 12ax7 tubes and problem stays on same side.

    Any ideas? I do not have a scope.

    I cannot find any loose connections. Jiggles Loudness and all other switches. Could be a cap or resistor but not sure how to efficiently check this without lifting all of them. Should I just do the black caps and see? I hate replacing stuff when it is not defective.


    I sort of sounds like faint water running under the audio. Any help appreciated thx.

    Also when I put my hand near the 12AX7 near back of the chassis on the pc-6 phono board it hums.

    Sometimes a sound like that (a repetitive bubbling sound) can also be categorized as "motorboating". Motorboating sounds are usually related to a leaking capacitor. Rather than go searching for which cap is causing the issue, you would be well served to just replace the PC-5 line stage board and the PC-6 phono board. The PAS-2 and PAS-3 came out in the early 1960's and many of these units are about 60 years old. Parts don't last forever and many of these parts that are still working on the boards have probably drifted somewhat from their original value. You could also try replacing the quad cap on your preamp. You could have a leaky section on that quad cap.

    Bob

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    bubbasweet
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    Post by bubbasweet Wed Jul 07, 2021 2:00 am

    THX Bob great advice. It sort of sounds like water running. but faint and under the audio almost like white noise but a smoother hiss. Really just hear it when you lift the needle up on the record player.
    peterh
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    Post by peterh Wed Jul 07, 2021 2:56 am

    The plate resistors are of a type that might develop noice as you describe.
    Replace them with filmtype ( 470k and 100k resistors) That might fix your problem
    for a small amount. If not, replaceing whole boards as bob suggest might be a logical step.

    bubbasweet
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    Post by bubbasweet Sat Jul 10, 2021 9:56 pm

    If I connect a CD player to the spare input does that isolate or remove the phono board and only use the PC-5 board? Therefore if sound issue is gone it would mean likely the Phono board has an issue on left side? As the issue is not on both channels.. ?
    peterh
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    Post by peterh Sun Jul 11, 2021 1:42 am

    bubbasweet wrote:If I connect a CD player to the spare input does that isolate or remove the phono board and only use the PC-5 board? Therefore if sound  issue is gone it would mean likely the Phono board has an issue on left side? As the issue is not on both channels.. ?

    Yes, you seem to have isolated the noise to the phono board.

    Then by the same logic there is one 150k and one 100k resistor at the plates that could
    be exchanged as a first attempt.

    ( carbon comp resistors as used in the 60 is noisy and may degrade to be very noisy )
    bubbasweet
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    Post by bubbasweet Sun Jul 11, 2021 10:27 am

    A couple things.. I did measure all the plate resistors can they do this even if they measure the correct resistance? If so I will swap em out. Also that water running sound is gone but one channel sounds more dull or thinner than the other. lastly all the black caps do seem to be doing what they are supposed to which is block DC. Even when the sound was there. It is a tuff problem and nothing measures "off".. I see why Bob recommend just swapping board. THX for you input tho. I will try the plate resistors if the hiss can still be there even if the resistance is correct.
    peterh
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    Post by peterh Sun Jul 11, 2021 10:36 am

    bubbasweet wrote:A couple things.. I did measure all the plate resistors can they do this even if they measure the correct resistance? If so I will swap em out.  Also that water running sound is gone but one channel sounds more dull or thinner than the other. lastly all the black caps do seem to be doing what they are supposed to which is block DC. Even when the sound was there. It is a tuff problem and nothing measures "off".. I see why Bob recommend just swapping board. THX for you input tho. I will try the plate resistors if the hiss can still be there even if the resistance is correct.
    Yes a noisy resistor might still measure ok. Just replace them !

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    bubbasweet
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    Post by bubbasweet Sun Jul 11, 2021 11:41 am

    Will Do THX Peter.
    bubbasweet
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    Post by bubbasweet Sun Jul 11, 2021 8:22 pm

    Peter, I have tried it and with a CD player plugged into the Spare input the sound is the same so I guess it is coming from the SB-5 not the 6 as the Spare input does not use the SB-6 correct?. But I assume same thing still replace Plate Resistors on that board.
    peterh
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    Post by peterh Mon Jul 12, 2021 2:45 am

    bubbasweet wrote:Peter, I have tried it and with a CD player plugged into the Spare input the sound is the same so I guess it is coming from the SB-5 not the 6 as the Spare input does not use the SB-6 correct?. But I assume same thing still replace Plate Resistors on that board.

    plate resistors has the largest probability to cause noise. Other non-bypassed resistors that can get noisy is cathode resistors 47k and 1k and to a lesser extent the volume pot, the volume pot may be eliminated as cause by temporarily shorting eyelet 1-2-9 if noise disappears the volume pot needs replacement.
    bubbasweet
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    Post by bubbasweet Mon Jul 12, 2021 1:54 pm

    To be sure, IF I am using spare input the Phono board is not in picture correct?
    peterh
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    Post by peterh Mon Jul 12, 2021 2:49 pm

    bubbasweet wrote:To be sure, IF I am using spare input the Phono board is not in picture correct?

    "Spare" is one of 4 high level inputs.

    The ones passing pc6 is phono NAB and "special".
    bubbasweet
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    Post by bubbasweet Mon Jul 12, 2021 3:04 pm

    OK I was just confirming the Spare input only uses PC-5 and does not use or go through PC-6.

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