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    Panor / Dynaco ST80 - distortion one channel at higher volumes

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    boeingpilot


    Posts : 34
    Join date : 2015-06-20
    Location : Central PA

    Panor / Dynaco ST80 - distortion one channel at higher volumes Empty Panor / Dynaco ST80 - distortion one channel at higher volumes

    Post by boeingpilot Fri Oct 29, 2021 3:26 pm

    Opening a new thread seeing as I'm seeking advice.  (Been posting to the ST80 / ST160 thread)

    Just purchased a ST80.  As I discussed in the other thread, it had issues with the bias adjust display, which turned out to be a burned PC trace.  This was on the right channel.  I repaired the damage to that, as well as some collateral damage to bias adjust board.  Noticed last night that it had some distortion I couldn't put a finger on.  

    Let the unit burn-in today.  Noticed that the left channel (not associated with my repairs), is distorting when increasing volume.  Did the usual to try to isolate: swapped power tubes and input tubes.  Problem stays with the left channel.  Looking at the LEDs for the bias while it playing, the  two LEDs for the left channel bias, start to flicker red as the distortion increases, indicating that the voltage (current) on those tubes are getting driven high.  

    Unfortunately, there are no test points for the bias, so I can't readily track the bias voltage.  (For that matter, running this thing without the bottom cover is going to be a real issue, so trying to get ideas up front).

    Can I get a 'reality check' on my trouble shooting plan?  Going to get a set of tube test adapters so I can probe the tube pins while it's operating.  I suspect first thing to do would be to check the high voltage supplies.  Each driver tube has it's own dedicated solid state HV supply (+475V) and a dedicated +480V B+ for the outputs.  

    As the issue is only on one channel, if this is power supply related, I'd assume (looking at the schematic), that the .68uf / 630V caps are fine as they are common to all supplies.  

    Now where I need a little guidance is where / how to check the amplification, if the power supplies check.  As I said, tubes were swapped channel to channel with no change, so it's not a tube issue.  I'm thinking if I can put my hands on a scope, check the output of the first stage of the 12AT7 going into the 6DJ8.  I guess I should be looking at pin 2 and 7 of the 6DJ8?  I suppose I can compare results with the right channel.  

    Assuming inputs are distorted there, I would guess I need to check the passives (R7, R8, R9 R10,R11)  If those check then C1 and C2 would be suspect?  If there's no distortion there, then I guess look at pin1 and pin 8 of the 6DJ8?  If that's distorted,thoughts?  I'd assume I start looking at C3 and C4.  If it looks good there, then I guess C5A / C5B?

    Having not done this before, can check the driver stage without the power tubes in place? Or do I need them to load everything down?   

    Sorry to be wordy, but it helps if I talk out my logic, and I'm trying to see from the collective group if I'm proceeding reasonably?  (of course this all assumes I can get my hands on a scope).  

    I've attached the schematics in case anyone wants to play along.

    Panor / Dynaco ST80 - distortion one channel at higher volumes Screen10
    Panor / Dynaco ST80 - distortion one channel at higher volumes Screen11
    Panor / Dynaco ST80 - distortion one channel at higher volumes Screen12
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    Seamus


    Posts : 8
    Join date : 2020-03-17

    Panor / Dynaco ST80 - distortion one channel at higher volumes Empty Re: Panor / Dynaco ST80 - distortion one channel at higher volumes

    Post by Seamus Fri Nov 12, 2021 12:43 pm

    boeingpilot wrote: (of course this all assumes I can get my hands on a scope).  

    PC based USB scopes, some of which work with a smartphone, are pretty inexpensive.
    https://loto.0sc.dev/
    http://www.loto-ins.com/en/

    Downside is limited input range, so you need 10x and 20x attenuators to go along with probe 10x to work on tube gear.

    Be doubly careful as you can easily fry a computer or smartphone with the wrong connection.

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    tubes4hifi
    tubes4hifi
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    Panor / Dynaco ST80 - distortion one channel at higher volumes Empty Re: Panor / Dynaco ST80 - distortion one channel at higher volumes

    Post by tubes4hifi Fri Nov 12, 2021 6:50 pm

    should be fairly EZ to find the problem since you have two identical channels and one is good.
    Start by checking the plate and cathode voltages on each tube and compare to the good side.

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    Tuninfork


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    Panor / Dynaco ST80 - distortion one channel at higher volumes Empty Re: Panor / Dynaco ST80 - distortion one channel at higher volumes

    Post by Tuninfork Sat Nov 13, 2021 7:15 pm

    I agree, easiest thing to do is compare voltages of the good and bad channel. Start with input tubes and work your way through. Any major differences isolate where the issue is. Leaky coupling caps can cause exactly the problem you describe. Measure voltage in input and output side of each cap. C103 and C 104 would be the ones to check. Remember you have bias voltage on the outputs of both of them, Bias could be the problem. You should have around 475 volts on the input side of them and whatever the bias voltage is on the other side. If you have over the bias voltage by even a few volts you may have a leaky coupling cap. Then start checking parts, making sure no resistors have drifted.
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    Seamus


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    Join date : 2020-03-17

    Panor / Dynaco ST80 - distortion one channel at higher volumes Empty VTA Debug Workbook

    Post by Seamus Sun Nov 14, 2021 2:08 pm

    Found this workbook online for debugging M-125 build.
    The M-125 is basically half of a Tubes4HiFi Stereo xx or any number of typical two tube input amps like PrimaLuna, Cayin, Dynaco, etc.

    It's pretty useful for channel comparison to find swapped resistors, bad solder joints, confirm transformer health, etc.

    https://docs.google.com/spreadsheets/d/1lotw6IwJmBPydUpCq7_JiYpKtGpgh-dJFlRcJSZ5UdM/edit?usp=sharing

    It's been updated a couple of times since I found it a while back.

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    boeingpilot


    Posts : 34
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    Location : Central PA

    Panor / Dynaco ST80 - distortion one channel at higher volumes Empty Still no idea....

    Post by boeingpilot Tue Nov 30, 2021 1:11 pm

    Ok - so, as much as I hate to just 'shotgun' things, the design of this amp is really hard to have it running and disassembled at the same time (disadvantage of a huge PCB with the power supply and amplification all on one). Plus I'm traveling a lot, so don't have too much free time to spend on it.

    (as a review, problem is with left channel). Replaced C3, C4, C2 and C1. (as an aside, and I doubt it made a difference, the 12AT7 and 6DJ8 were replaced as the 12AT7's that came with it tested weak, and I didn't really trust the russian 6H1Ns that were in it. As expected, same issue). No change, same issues. Taking Tuninfork's suggestions, I guess when I get the time, next drill will be to check each resistor. I was only able to find on octal tube test adapter, really need to find a couple more so I can run the amp and take measurements at the tube pins. Guess I'm going to be search for them.

    Thank you for all the good suggestions. I just need to find a free evening to really analyze.

    Just to throw a stupid question out there.... I replaced the ultralinear / triode relay on the right channel because of the PCB damage. Mmmm, seeing as the entire output signal go through there, wonder if the contacts could be bad on the one I left in there. (Thoughts?)

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