The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all Tubes4hifi.com products and all Dynakitparts.com products


    Dynaco Sca 80

    Share

    Tjonmcd

    Posts : 5
    Join date : 2013-04-21

    Dynaco Sca 80

    Post by Tjonmcd on Sun Apr 21, 2013 6:03 pm

    Okay I know this isn't a tube dynaco but I purchased this amp recently for fairly cheap it's in mint condition aside from the fact that the left channel seems to get distorted and crackle. At lower volumes its fine but once it gets over half maximum volume it starts to sound as if the speaker is blown (which it isn't, I've tried switching speakers, always the left channel) does this sound like it could be cured with some contact spray or am I looking at something more serious?

    Thanks in advance

    peterh

    Posts : 642
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Dynaco Sca 80

    Post by peterh on Mon Apr 22, 2013 2:46 am

    Yes. Broken transistor, probably not the power ones but still broken.

    Tjonmcd

    Posts : 5
    Join date : 2013-04-21

    Re: Dynaco Sca 80

    Post by Tjonmcd on Mon Apr 22, 2013 3:03 am

    Would this be an expensive fix? I can solder and build circuits but I'm not familiar with troubleshooting or diagnosing problems.

    peterh

    Posts : 642
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Dynaco Sca 80

    Post by peterh on Mon Apr 22, 2013 4:41 am

    Tjonmcd wrote:Would this be an expensive fix? I can solder and build circuits but I'm not familiar with troubleshooting or diagnosing problems.
    It's an easy fix ( providing you can obtain the transistor) BUT
    finding which one without damaging everything else is the tricky part.
    Schematics a signal generator and a scope is i think the minimum needed
    to pinpoint the culprit.

    Don't bother unless you find a technician that has experience with this amp,
    if you cannot find a suitable resource, keep it in storage until you find
    ( or someone who needs what you have: a partly working sca80 )

    Tjonmcd

    Posts : 5
    Join date : 2013-04-21

    Re: Dynaco Sca 80

    Post by Tjonmcd on Mon Apr 22, 2013 11:30 am

    Thanks for the help. There are no substitutes for these transistors? I've found a website that seems to be selling a few if these transistors online.

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: Dynaco Sca 80

    Post by tubes4hifi on Mon Apr 22, 2013 12:03 pm

    it could be a dirty volume control, or it could be a bad transistor.
    I see working SCA-80s on ebay from $50-150. Someone sells a complete set of all transistors for $29.

    hawaii.ken

    Posts : 157
    Join date : 2012-01-31

    Re: Dynaco Sca 80

    Post by hawaii.ken on Mon Apr 22, 2013 12:05 pm

    Before I suspect bad transistors I would suspect the output capacitors (the ones with the coil wrapped around them).

    Tjonmcd

    Posts : 5
    Join date : 2013-04-21

    Re: Dynaco Sca 80

    Post by Tjonmcd on Mon Apr 22, 2013 12:23 pm

    I heard the capacitors go bad in these things quite often. I know you can get all of the replacemrbt capacitors for 30 on eBay. I'm sure I can find them at a local place as well. Thanks for the help everyone. Any added info would be appreciated

    hawaii.ken

    Posts : 157
    Join date : 2012-01-31

    Re: Dynaco Sca 80

    Post by hawaii.ken on Mon Apr 22, 2013 12:32 pm

    It's not just replacing the capacitor, it's a capacitor & coil set.

    DYNACO ST-80 SCA-80Q 4700 MFD OUTPUT CAP AND COIL MATCHED SET

    GP49

    Posts : 718
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Dynaco Sca 80

    Post by GP49 on Mon Apr 22, 2013 12:58 pm

    Before you do ANYTHING, download the SCA-80Q assembly manual at the link so generously provided by Bob:

    Dynaco SCA-80Q assembly manual

    It includes the circuitry for the amplifier, printed circuit board diagrams, a parts list, and troubleshooting instructions (including "SERVICE INFORMATION FOR THE TECHNICIAN, for qualified personnel only). If you are new at this, I would recommend reading especially Pages 28-30 thoroughly before proceeding, before ordering ANY parts of any kind, and before starting to poke around at the amplifer...particularly with the power ON!

    The manual is for the SCA-80Q; the SCA-80 is electrically identical. Only the hookup for the optional Dyna derived-four-channel speaker hookup is different, and you can ignore that.

    Yes, the output coupling capacitors (the big cans with wires wound around them) do go bad in the SCA-80 and the Stereo 80 and Stereo 120, which are basically the same amplifier; in the Heathkit AR-15, Pioneer SX-1000, and others...any transistor amplifier with a quasi-complimentary symmetry output stage. IF you have an electrolytic capacitor of anything between 1000µF and 6000µF, and a MINIMUM working voltage 80 volts, you can check them by temporarily connecting your test capacitor to the terminals on the ones in the amplifier. OBSERVE POLARITY! There is a positive terminal and a negative one. If that clears up the problem immediately, you need to replace the output coupling capacitor with one of correct or near-correct capacitance, 5000µF, but you MUST retain the coil of wire around the existing capacitor, or use something functionally equivalent, as it is an inductor to roll off high frequency response over 20KHz and suppress oscillation (that's why the "capacitor & coil set" mentioned above as being offered on EBay).

    If you have a voltmeter, check the DC voltage at the POSITIVE terminal of the output coupling capacitors. It should be in the 30-40 volt range (technically, if the circuit is operating perfectly, that voltage will be ONE-HALF of the power supply voltage for the amplifier boards). It should be close to the same on both channels. If not, one or more transistors has failed. The assembly manual has instructions on how to check them, but it takes some expertise (do you know the difference between the Base and Collector on a transistor?).

    Before retiring from audio repair, I fixed many of the SCA-80 and similar Dynaco amplifiers. They are not that hard to fix, though many were passed on to me by other techs who either gave up or were scared of them. They do have more different common failure modes than do the common Japanese mid-fi transistor amplifiers. The easiest one, in fact, was a Stereo 120 that had your exact symptom, with NO voltage anomalies, and good output coupling capacitors. It had an bad input transistor...and that was all. But DO NOT count on this being the problem on yours!


    Last edited by GP49 on Mon Apr 22, 2013 2:01 pm; edited 1 time in total

    Tjonmcd

    Posts : 5
    Join date : 2013-04-21

    Re: Dynaco Sca 80

    Post by Tjonmcd on Mon Apr 22, 2013 1:56 pm

    Thank you so much. My friends father is an electronic engineering professor (or something of that nature) I usually bring my problems to him as I buy a lot of old guitar amplifiers but he doesn't charge me and I feel sort of bad bringing stuff over for him to fix. If its something I might figure out I'd like to give it a month or so to figure out on my own. Thanks again

    Sponsored content

    Re: Dynaco Sca 80

    Post by Sponsored content Today at 6:08 am


      Current date/time is Thu Sep 29, 2016 6:08 am