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Dynaco SCA 35, garbage find

ferran39

ferran39

New Member
Hello guys,
My very first post here in AK, some few weeks ago I found a Dynaco SCA-35 in the garbage (no joke), physically it looks quite ok for its age, I found that the volume pot was a bit stiff and the balance, bass and treble pots were completely stiff and not turning at all.
I have to say, Im a complete noobie, this is a hobby for me, so Im reading on internet and watching youtube to see if I can learn a bit electronics repair.

After inspection and cleaning the boards I run the dim bulb test sucessfully no fireworks neither smoke, the bulb lighted bright first then dimmed out and it got brighter as the amp got warmer, sound tested with my test speakers and got sound on one channel, hmm the balance pot was totally stiffed, could that be the problem? yep, after spraying with contact spray and carefully using pliers I was able to turn the balance pot, sound tested again and voila both channels working properly and the sound coming out from this little amp is really nice!
cheers
Victor

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the inside was covered in dust, which I already have cleaned, see before/after pics

DynacoSCA35_04a.jpg

DynacoSCA35_04b.jpg

under the power amp boards in some areas there was this strange kind of grease, now it is cleaned using isopropanol, unsure if that grease could turn corrosive or conductive.

DynacoSCA35_05.jpg

DynacoSCA35_06.jpg

the rubber feet were completely dried out and in need of replacements, new generic rubber feets

DynacoSCA35_07a.jpg

DynacoSCA35_07b.jpg
 
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ferran39 said:
My very first post here in AK, some few weeks ago I found a Dynaco SCA-35 in the garbage (no joke)
I have to say, Im a complete noobie, this is a hobby for me,
Victor
Click to expand...

First of all welcome to AK! and here's hoping you enjoy your time here.

There are a LOT of Dynaco enthusiasts here and I'm sure they'll be by to offer up assistance, suggestions, do's & dont's

Congrats on your find, it cleaned up really nicely
 
That was one heck of a find! Get the schematic and replace all the coupling capacitors (the blue ones) with polyester film caps. You can buy them at Digikey. Same value capacitance and same or higher voltage rating. If the originals fail they can blow the output transformers. To make it reliable you should replace the power supply filter caps also. There are boards you can buy to replace the caps with modern parts. Very easy to install.
 
Welcome to AK and that is a nice find. For the stuck potentiometers, heat and penetrating oil like Kroil or even WD40 will usually loosen them up. Of course, you don't want to blast them with a Bernz-O-Matic torch (unless you are very careful). As for one channel low, it is often tube sockets and/or the selector switch. If you can clean the tube sockets and tube pins that may be the problem.
 
Welcome to AK.
I had a SCA35 for a while, after multiple attempts to fix the controls, which all failed, I gave up and decided to use the output transformers in a stand alone power amplifier project. I basically made my own Dynaco st-35 on a custom chassis. It turned out great, and is very useful, and sounds about as good as anything freshly rebuilt. It was a good thing to do to put to use the best parts of the unit when the controls couldn't be salvaged.
Best of luck.
 
The grease used on the shafts of those controls dry out over time.
I've even bought brand new old stock Dynaco control pots that were frozen stiff. I drop or two of Break Free or similar should loosen them up.
Just let them sit for a couple days and they should free up.
At the very leased replace the parts already suggested above.
There are some great threads on these here on AK. Everything from stock restores to modifications like replacing or removing the tone controls to turning them into power amplifiers like the Dynaco ST-35.
You don't want to damage those output transformers. They aren't cheap to replace and they are nice sounding transformers.
And remember, your dealing with high voltages here. The power transformers for tube gear are step up transformers producing hundreds of volts.
 
Welcome to AK - that's in beautiful condition! Wish I had bought one or two before prices went crazy. I see it's setup for 220V/50Hz. Are you in Australia?

Jack
 
Hey, thanks everyone for your kind comments & tips...
Before I started this thread I have already read most AKs threads about the SCA-35, so I understood that replacing the coupling caps is a must, so the caps (orange drops) were already ordered and arrived before I posted.
As I understand those caps have no polarity, my question is: how important is the orientation of outer foil when installing them? should I find the outer foil in order to install them in the correct orientation?
thanks

DynacoSCA35_09.jpg
 
The orientation of outer foil will make no difference. In fact, many caps now are made in such a way that there actually is no outer foil. In any event, when someone tells you this is important, remember that the purpose of a coupling cap is to pass the signal from one end to the other with as little loss as possible. This means at line frequency (50/60Hz), the cap is a virtual short. Whatever energy is picked up by the outer foil (which is an exceedingly small antenna anyway) will be present on both ends of the cap. Only when the outer foil is grounded (such as a cap used for decoupling) can the outer foil shield the "hot" end.

Jack
 
TriodeLuvr said:
The orientation of outer foil will make no difference. In fact, many caps now are made in such a way that there actually is no outer foil. In any event, when someone tells you this is important, remember that the purpose of a coupling cap is to pass the signal from one end to the other with as little loss as possible. This means at line frequency (50/60Hz), the cap is a virtual short. Whatever energy is picked up by the outer foil (which is an exceedingly small antenna anyway) will be present on both ends of the cap. Only when the outer foil is grounded (such as a cap used for decoupling) can the outer foil shield the "hot" end.

Jack
Click to expand...
thanks Jack! I learned something new today and now I feel confident to replace those caps without having them with wrong orientation...
I have as well lifted one end of the resistors and checked resistance, I found some few out of specification as stated on the service manual, marked in the pic with red those to be replaced

DynacoSCA35_08.jpg
 
Great integrated in my opinion. Check Dave Gillespie for his power supply board. Also my coupling caps are original and working well. Ymmv. You will probably want really good low noise 12ax7s if you use the phono pre. Gillespie's mod of the heater circuit works well for phono noise. Congratulations!17173453397242461054732160542398.jpg
 
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Bicyclist said:
Great integrated in my opinion. Check Dave Gillespie for his power supply board. Also my coupling caps are original and working well. Ymmv. You will probably want really good low noise 12ax7s if you use the phono pre. Gillespie's mod of the heater circuit works well for phono noise. Congratulations!
Click to expand...
Thank you Bicyclist! yes the tubes in the pre amp are 12ax7s, and it has low noise when I tested the phono section, I first will recap the coupling caps and then wait to decide if I get som of Gillespies mod, by the way what speakers do you use with your SCA-35?
cheers
Victor
 
ferran39 said:
Thank you Bicyclist! yes the tubes in the pre amp are 12ax7s, and it has low noise when I tested the phono section, I first will recap the coupling caps and then wait to decide if I get som of Gillespies mod, by the way what speakers do you use with your SCA-35?
cheers
Victor
Click to expand...
I originally used dynaco a35s but am currently using Heathkit as-101s. These are actually Altec Valencias in 8ohm. Very efficient so any 60cycle hum is quite audible. With Gillespie's help the hum is very low. At listening distance it's inaudible.
 
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ferran39 said:
Thank you Bicyclist! yes the tubes in the pre amp are 12ax7s, and it has low noise when I tested the phono section, I first will recap the coupling caps and then wait to decide if I get som of Gillespies mod, by the way what speakers do you use with your SCA-35?
cheers
Victor
Click to expand...
Congratulations for a nice find, it seems to be a factory made one.

BUT
Leave the soldering tool for a while. You have already started to loosen the pots, be careful with them. The balance pot is
unique dont destroy it!. The volume pot might need replacement, i have suitable replacements located within EU
You EL84 is most likely shot, you need a matched quad EL84 in this amp.

And i repeat, you don't need to replace caps on the boards, you _might_ need to replace out-of-tolerance resistors but
i suggest you wait with this until you have a working amp that delivers full power.
As for the electrolytics , install EFB instead, there is ready made kits for this.
 
peterh said:
Congratulations for a nice find, it seems to be a factory made one.

BUT
Leave the soldering tool for a while. You have already started to loosen the pots, be careful with them. The balance pot is
unique dont destroy it!. The volume pot might need replacement, i have suitable replacements located within EU
You EL84 is most likely shot, you need a matched quad EL84 in this amp.

And i repeat, you don't need to replace caps on the boards, you _might_ need to replace out-of-tolerance resistors but
i suggest you wait with this until you have a working amp that delivers full power.
As for the electrolytics , install EFB instead, there is ready made kits for this.
Click to expand...
Great advice, I still have my original coupling caps, the Gillespie power supply board is quite nice as well as his other small noise reduction mods.
 
Bicyclist said:
Great integrated in my opinion. Check Dave Gillespie for his power supply board. Also my coupling caps are original and working well. Ymmv. You will probably want really good low noise 12ax7s if you use the phono pre. Gillespie's mod of the heater circuit works well for phono noise. Congratulations!
Click to expand...
Is that a Dual 1219 in the photo? It looks right at home next to the Dynacos!
 
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