The Dynaco Tube Audio Forum

Would you like to react to this message? Create an account in a few clicks or log in to continue.
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


4 posters

    Help with a markIII - very low output

    avatar
    EWALAA

    Posts : 14
    Join date : 2021-02-08
    Location : Va

    Help with a markIII - very low output Empty Help with a markIII - very low output

    Post by EWALAA Sat Aug 28, 2021 5:39 pm

    I have a pair of rebuilt MarkIIIs and one has developed a problem. The amps use the VTA octal driver board and the SDS cap board and were working well and sounded great.

    One amp has lost almost all output. At full input gain, sound can be slightly heard from the speaker. Another amp works in the system, so it's nothing upstream or the speaker. I swapped all tubes between the two amps with no change. The tubes from the "bad" amp function in the "good" amp, so it seems like an issue within the amp.

    When the problem began, I had been listening to the system and with nothing playing I heard a very soft thump or pop from the speaker connected to the "bad" amp. Very reduced output from that time forward either in the system or on the bench.


    With tubes in place and a line voltage of 114vAC, I measure voltages as:

    B+: 438vDC

    Bias Inlet: 52.8vAC

    Rectifier:
    Pin 2 507vDC
    Pin 4 398vAC
    Pin 6 398vAC
    Pin 8 504vDC
    Pin 2 - 8 4.9vAC

    Output tube:
    Pin 3 496vDC
    Pin 4 498vDC
    Pin 5 -62.8vDC
    Pin 6 -62.9vDC
    Pin 8 biased at .5v
    Pin 2 - 7 6.2vAC

    My uneducated guess is that there is an issue with a component on the VTA driver board. The resistors which can be measured in place measure to spec.. Since I had some on hand, I replaced the C6 and C8 .22 400v coupling caps and there was no change.

    Can someone suggest how or what to check next?

    Thanks for any suggestions,
    E.


    bubbasweet
    bubbasweet

    Posts : 87
    Join date : 2016-03-14

    Help with a markIII - very low output Empty Re: Help with a markIII - very low output

    Post by bubbasweet Mon Aug 30, 2021 7:25 pm

    Has the ground come off the RCA Socket where it meets the board? Believe it or not when I bought my MK4s this happened on one side. Mine was scratchy and weak but the ground was just resting against the circuit board.
    WLT
    WLT

    Posts : 151
    Join date : 2013-07-13
    Location : Rochester NY

    Help with a markIII - very low output Empty Re: Help with a markIII - very low output

    Post by WLT Mon Aug 30, 2021 10:27 pm

    Your voltages look fine. You say you have a bench and you have taken DC voltage readings so you are somewhat proficient with testing. If you have an O scope then test both amps on the bench at the same time. Same test signal to both with each having a dummy load. Put a signal to the input and measure it. Maybe .1 -.3 VAC. Test back and forth between amps as you proceed thru the schematic from input to output. If that is not something you have or can do then at least put the bad amp in place and try a few of the next steps. You have a volt meter in lieu of the scope. It will work on volts AC..

    Put a signal to the input and measure it. Maybe .1 -.3 VAC. If not a signal generator then a tuner or CD player playing at that level. Then follow thru to the input tube grid pin. Same VAC? It should be. Then check at the output plate of that section and move on to the next tube. The AC signal should get larger with each section. Somewhere long the path it will not be there or be vary small. When that happens the problem is in or near there.

    Sorry – you do have the correct schematic. Right? The original circuit was very simple but the VTA and SDS boards complicate matters. You have to have the correct schematic to follow this thru. Still not hard but adds a little more to the effort.
    avatar
    EWALAA

    Posts : 14
    Join date : 2021-02-08
    Location : Va

    Help with a markIII - very low output Empty Re: Help with a markIII - very low output

    Post by EWALAA Tue Aug 31, 2021 7:14 am

    Thanks. I don't have a scope, but I do have all the correct schematics. I'll check signal path voltages.

    E.
    tubes4hifi
    tubes4hifi
    Admin

    Posts : 1699
    Join date : 2008-11-30

    Help with a markIII - very low output Empty Re: Help with a markIII - very low output

    Post by tubes4hifi Wed Sep 01, 2021 6:30 pm

    95% chance a bad soldering connection somewhere.
    avatar
    EWALAA

    Posts : 14
    Join date : 2021-02-08
    Location : Va

    Help with a markIII - very low output Empty Update - Problem solved

    Post by EWALAA Thu Sep 09, 2021 9:21 pm

    The amp is fixed. It was the CCS LM334.

    Thanks everyone.

    Sponsored content

    Help with a markIII - very low output Empty Re: Help with a markIII - very low output

    Post by Sponsored content


      Current date/time is Thu Sep 16, 2021 10:08 pm