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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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    Troubleshooting VTA-70, First Time

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    bbrater

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    Post by bbrater Wed Oct 06, 2021 3:30 pm

    Hello Dynaco Tube Audio Forum!'

    I recently received a partial kit from an acquaintance and these are my attempts to fire it up for the first time. This is my first foray into tube amps, so please let me know if I am making any truly rookie mistakes.

    I'm following the initial startup procedures as outlined by Bob, and the driver board tubes light up with no issues.

    I placed the rectifier tube and two output tubes into the left side, and the rectifier tube arced and the 3A fuse blew.

    I checked all of my connections for a short to ground, and tried again with a new rectifier tube. No rectifier arcing or blown fuse, but the quad cap smoked. At this point the power transformer seemed to have a markedly increased hum.

    Shut down, replaced quad cap and re-did all connections for the sockets, rectifier tube, and quad cap.

    Tried again, no arcing rectifier, no blown fuse, but the 120uf caps on the board smoked.

    Picture is the amp in its current state, any recommendations would be welcome from here.

    Troubleshooting VTA-70, First Time Vta-7010
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    bbrater

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    Post by bbrater Wed Oct 06, 2021 7:23 pm

    Additional note, the 2.2k resistor that goes between the 20 and 30 tab on the quad cap is also being roasted (visible in the picture above).
    peterh
    peterh

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    Post by peterh Thu Oct 07, 2021 2:12 am

    A good way to identift the problem is to use an external adjustable DC source connected at the filament lugs of the 5AR4. Starting with a low voltage and examine the voltage among the B+ chain will
    pinpoint the problematic area.
    Without an adjustable DC source a 12V source could do. Small enough to prevent damage but
    large enough to see the voltage

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    bbrater

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    Post by bbrater Thu Oct 07, 2021 12:25 pm

    peterh wrote:A good way to identift the problem is to use an external adjustable DC source connected at the filament lugs of the 5AR4.  Starting with a low voltage and examine the voltage among the B+ chain will
    pinpoint the problematic area.
    Without an adjustable DC  source a 12V source could do. Small enough to prevent damage but
    large enough to see the voltage

    Thanks for the reply peterh. I have access to an AC Variac, would that be acceptable?

    Apologies for the naïve question, but what should I be paying attention to in the B+ chain and any values in particular?
    peterh
    peterh

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    Post by peterh Thu Oct 07, 2021 1:05 pm

    bbrater wrote:
    peterh wrote:A good way to identift the problem is to use an external adjustable DC source connected at the filament lugs of the 5AR4.  Starting with a low voltage and examine the voltage among the B+ chain will
    pinpoint the problematic area.
    Without an adjustable DC  source a 12V source could do. Small enough to prevent damage but
    large enough to see the voltage

    Thanks for the reply peterh. I have access to an AC Variac, would that be acceptable?

    Apologies for the naïve question, but what should I be paying attention to in the B+ chain and any values in particular?
    A variac is not enough, it delivers AC you need DC . AC will destroy electrolytic caps.

    If you can connect +12Volt at the 5AR4 cathode and the - on the chassies then you can measure
    the B+ chain starting with the 5AR4 cathode ( where the 12V is injected), it should be 12Volt.
    Then continue on the other side of the choke and further until the last point.
    If you have zero volt you know that the problem is right there, If you on the other hand find
    12 volt at the B+ to the cuírcuit bord you have a problem that is voltage depending. An
    adjustable DC source is needed then.

    And your variac finally is not of any use it can even be dangerous as it is not isolated from the mains.

    ttocs
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    Post by ttocs Fri Oct 08, 2021 3:43 pm

    You may want to check some of the solder blobs on the tube sockets to verify that some aren't making contact with its neighbor. Maybe it's just the photo, but it looks like two or three might be a bit too much blobby, if that's a word.

    I also can't see well enough in the photo to verify that the wiring from tube to tube twins is all "like for like".
    tubes4hifi
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    Post by tubes4hifi Fri Oct 08, 2021 9:06 pm

    if that 2.2K resistor on the quad cap is burned out, then obviously something on the PCB is shorted out, as that's the only place that voltage goes.
    Is the board mounted with spacers so it doesn't touch the chassis?? If not, well, that's the problem (at least one of them).
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    bbrater

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    Post by bbrater Sun Oct 10, 2021 2:12 pm

    tubes4hifi wrote:if that 2.2K resistor on the quad cap is burned out, then obviously something on the PCB is shorted out, as that's the only place that voltage goes.
    Is the board mounted with spacers so it doesn't touch the chassis??  If not, well, that's the problem (at least one of them).

    I do have spacers between the board and the chassis, I'm confident that is not my issue.

    Taking a second look, I had the ground (green) going to the right lug of my terminal strip and not the center, so the .02 Mfd capacitors weren't doing anything. I have new 120uF caps on order and will report back. Troubleshooting VTA-70, First Time Unname10
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    bbrater

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    Post by bbrater Sat Oct 16, 2021 2:40 am

    Update: changed the ground connection on my lug, replaced the burned components (120uf caps and 2.2k resistor on the quad cap).

    Ran through start up procedure, this time the 7.5k resistor in R15 & R16 burned themselves, no issues with any other components.

    What would be causing those particular resistors to burn?
    tubes4hifi
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    Post by tubes4hifi Sat Oct 16, 2021 1:18 pm

    something on your driver board is definitely shorted to ground, so again, check that none of the chassis or the front octal sockets are touching the PCB.
    If you don't have the original documentation from 12 years ago, send me your email and I can send it to you.
    It might be EZer to replace the board than spend an hour or two trying to figure out what part is shorted.
    Check for solder splashes or anything that looks like it may be a ??? connection.
    Since both channels burned out (R15 is on one side, R16 on the other) that points to a common short somewhere.
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    bbrater

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    Post by bbrater Sat Oct 16, 2021 3:57 pm

    tubes4hifi wrote:something on your driver board is definitely shorted to ground, so again, check that none of the chassis or the front octal sockets are touching the PCB.
    If you don't have the original documentation from 12 years ago, send me your email and I can send it to you.
    It might be EZer to replace the board than spend an hour or two trying to figure out what part is shorted.
    Check for solder splashes or anything that looks like it may be a ??? connection.
    Since both channels burned out (R15 is on one side, R16 on the other) that points to a common short somewhere.

    Thank you for the information, I'll give it a through once-over for a short. I was thinking the same thing about just replacing the entire board, likely will end up going that route if nothing is clear.

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