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    Dynaco mkiv problem

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    Trilogy 901

    Posts : 3
    Join date : 2015-05-26

    Dynaco mkiv problem

    Post by Trilogy 901 on Tue May 26, 2015 12:20 pm

    Hallo, one my mk iv I hissed when intermittent and hot , what can be

    Bob Latino
    Admin

    Posts : 2377
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Dynaco mkiv problem

    Post by Bob Latino on Tue May 26, 2015 3:04 pm

    Trilogy 901 wrote:Hallo, one my mk iv I hissed when intermittent and hot , what can be

    Sometimes this can be a bad driver tube .. Swap the driver tubes between the two amps and see if the hiss moves over to the other amp.

    Bob

    Trilogy 901

    Posts : 3
    Join date : 2015-05-26

    Re: Dynaco mkiv problem

    Post by Trilogy 901 on Wed May 27, 2015 11:11 am

    changed the driver , still remains a slight hiss in the same hand, it could be the choke I've never changed .

    Bob Latino
    Admin

    Posts : 2377
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Dynaco mkiv problem

    Post by Bob Latino on Wed May 27, 2015 11:24 am

    Trilogy 901 wrote:changed the driver , still remains a slight hiss in the same hand, it could be the choke I've never changed .

    If you can measure about 50 to 65 ohms across the choke, then the choke is probably OK. If you have the original driver board in that amp, you should consider replacing the driver board. The original Dynaco amps used fairly cheap driver boards with open solder traces. The fact that it hisses when hot could also be a sign of a bad solder trace on the bottom of the board OR a bad part on the board.

    Another idea to trace down the problem would be to do a voltage check on the good amp and then do the same voltage check on the amp that hisses. Compare them and see if there are any major differences.

    Personally, if you are going to use these amps on a daily basis AND they have the original driver boards, I would just replace the driver boards on BOTH amps. The Mark IV came out in 1960, the year after the ST-70. Some of these amps are now 50+ years old and who knows how many thousands of hours the amps had on them before you acquired them ? Parts don't last forever ...

    Bob

    Trilogy 901

    Posts : 3
    Join date : 2015-05-26

    Re: Dynaco mkiv problem

    Post by Trilogy 901 on Wed May 27, 2015 11:56 am

    thanks , in wekend do the tests then the lighthouse know thank you very much courtesy.

    Cristiano

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