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    Starting a rebuild (ST-70)

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    ray.moats

    Posts : 2
    Join date : 2014-10-23

    Starting a rebuild (ST-70)

    Post by ray.moats on Thu Oct 23, 2014 11:37 pm

    I recieved an ST-70 as a gift a while back. It ran fine for a while, but I ran into some troubles and put it away for a while. I decided to get started on a rebuild this winter and was looking for some advice and maybe a finger in the right direction at times.

    I have a few pictures that I cannot post until a week from now.

    The ST-70 never sounded perfect, it sounded good, but there was always a hum or a hiss or a crackle. It wasn't taken care of and I have it now.

    The amp turns on, the tubes warm up and glow their normal glow, but I have no audio output. I've tried different sources. It does not make a "thump" sound like mentioned in the manual (It used to). So far, all the tubes other than the 7199's have been replaced (I had a rectifier go a few months ago). A new capacitor was just installed today under the advisement of the person that gifted me the amp. I'm probably sure he's a member of this forum.

    I'm not sure how much it would cost to do a rebuild, but I have some amazing Klipsch loudspeakers to use with this thing, and I'd love to get some information on what I can do to get this going under a constrained budget.

    Thank you!

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Starting a rebuild (ST-70)

    Post by Bob Latino on Fri Oct 24, 2014 8:41 am

    ray.moats wrote:I recieved an ST-70 as a gift a while back. It ran fine for a while, but I ran into some troubles and put it away for a while. I decided to get started on a rebuild this winter and was looking for some advice and maybe a finger in the right direction at times.

    I have a few pictures that I cannot post until a week from now.

    The ST-70 never sounded perfect, it sounded good, but there was always a hum or a hiss or a crackle. It wasn't taken care of and I have it now.

    The amp turns on, the tubes warm up and glow their normal glow, but I have no audio output. I've tried different sources. It does not make a "thump" sound like mentioned in the manual (It used to). So far, all the tubes other than the 7199's have been replaced (I had a rectifier go a few months ago). A new capacitor was just installed today under the advisement of the person that gifted me the amp. I'm probably sure he's a member of this forum.

    I'm not sure how much it would cost to do a rebuild, but I have some amazing Klipsch loudspeakers to use with this thing, and I'd love to get some information on what I can do to get this going under a constrained budget.

    Thank you!

    Hi, and welcome to the Dynaco Tube Audio Forum .. The most common cause of "all the tubes light up but no sound" is a bad rectifier tube. Just because a tube lights up does not mean that the tube is good. If you also measure no bias voltage on the bias measuring point (for each channel) then you can be almost certain that the rectifier is no good. Buy a new 5AR4 rectifier tube and try the new one in place of the one that you have now.

    As for the hum, hiss, crackle .. A new quad cap will usually stop hum in an old Dynaco tube amp. I am assuming that this is the cap that you mentioned that you replaced. As for hiss and crackle > The original Dynaco ST-70 driver board had open solder traces that sometimes lift and/or form hairline cracks. Advice > Replace the entire driver board with one of the VTA upgraded driver boards. The VTA ST-70 driver board has no open solder traces and is much more reliable (and better sounding) than the original Dynaco driver board. Yes - The older driver board used 7199 tubes. 7199's are not made anymore and the price of those still in existence keeps going up and up. The VTA driver board uses more common 12AU7 type tubes which are lower in price and are still being made ..

    Bob

    ray.moats

    Posts : 2
    Join date : 2014-10-23

    Re: Starting a rebuild (ST-70)

    Post by ray.moats on Fri Oct 24, 2014 11:08 am

    Thanks so much for your reply. This definitely gets me started in the right direction. I'm doing a full rebuild, and disassembled her last night, so much oxidation around mounting points of the transformer as well. I didn't know about the 7199s being rare either, thank you so much.

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