Yet another charming rookie question....................................

Hi Guys

I'm new here and hail from Cape Town. I play vinyl and love vintage gears (Garrard/Micro Seiki/Leak/Nagatron/Quad & Scott).

Over last weekend, a Dynaco SCA-35 came my way and it was basically just dirty and dusty and after a thorough vacccing, dusting with a small paintbrush and some very careful almost dry cleaning with a 'green' cleaner of tubes and faceplate, I completed stage one of preparing the Dyna for connecting to our 220V ac.

I don't have a variac but do own Leak tube gear so know a little about the dangers of connecting vintage stuff to straight mains.

I have made a little fused box to take a 40W,60W and 100W lightbulbs so this is what I used throughout last Saturday and through the night to gently step up the ac to the equipment. During this procedure, I also hooked up a pair of 8 ohm fused speakers and all seemed safe and OK.

Next up, I throughly cleaned all corroded input jacks, fitted a new fuse holder (the old's screw cap was gone) together with a 1A slo-blo fuse, as well as cleaning all controls and switches with Kontakt contact cleaner

I also fitted 4 new, slightly taller rubber chassis feet as the old items were too compressed.

So, on Sunday morning, I hooked-up a tuner and let it run throughout the day, without any troubles (apart from running a lot hotter than my Leak Stereo 20).

Yesterday, I tried the phono (LO) input and let it run throughout the day.

Apart from sounding a bit gutless, which is completely understandable due to its age and long slumber, I picked up a problem with the balance control, which seems to have no left and right stops or centre detent. It still works correctly if I play a test record with left and right channel ID, but it appears someone has forced the ganged pots beyond its stops??
Do you think this control should be replaced or do you have any cure for this problem?

Also, I am aware of the dangers of ac and tube equipment. The SCA-35 is fitted with a fused twincord mains lead. Will it be possible to fit a 3-core, fused earth lead, with the earth tagged to the chassis?

Thanks in advance for your kind replies, advice and recommendations.

Dauphine

RE: Yet another charming rookie question...................................., posted on October 28, 2009 at 07:59:56

GP49
Audiophile

Posts: 111
Location: No Cal
Joined: April 21, 2009

The mechanical stops aren't important to the operation of the amplifier and there was never a center detent. If you can't find a replacement control, which could be tricky, I'd leave it alone since it's functioning correctly in-circuit. A balance control scrapped from something else...except a PAS preamp...probably would not work and/or fit properly.

Some people, more brave than I, might pry the control apart in an attempt to restore the original stops. Like I said, though: more brave than I!

Unfortunately, Dynakit Parts at www.dynakitparts.com doesn't sell any controls other than bias pots.

You're fine on the three-wire replacement AC cord, though it may be necessary to enlarge the hole in the rear panel and use a larger strain relief to safely install it.
GP49

RE: Yet another charming rookie question...................................., posted on October 28, 2009 at 16:17:43

Dauphine
Audiophile

Posts: 5
Location: Cape Town
Joined: October 28, 2009

Thanks, GP49

I may just get lucky in finding a new or used replacement part for that balance control. The particular flea market I frequent is a wonderful place where all kinds of unexpected delights turn up.

Thanks too for the news about the 3-core. I feel a bit 'insecure' about using tube amps with 2-core ac and will be doing this mod over the next few days.

Do you think other SCA-35 users have done this to their Dyna integrateds? I did quite a bit of reading up about this amp but couldn't find any user doing this kind of mod.

Regards

Dauphine

I have said this many times in the past, but let's try again., posted on October 28, 2009 at 17:38:18

Bill Thomas
Industry Professional

Posts: 1330
Location: Southern USA
Joined: January 3, 2004

Do NOT replace the 2-wire cord with a 3-wire cord. If you DO replace it, chances are you will have MULTIPLE ground loops in your system and SERIOUS hum problems. To solve THOSE problems, you will wind up having to "lift" the third (grounding) wire.

While it is not *impossible* for the chassis to become electrified, the ONLY way that can happen is if the Power Transformer shorts the Primary to the frame. I have NEVER seen this happen in over FIFTY years of my "Dynaco Addiction."

This "modification" goes under the heading of "Well, it *seemed* like a good idea at the time!" Have others performed this "mod" and "gotten away with it?" Sure. But it is FAR more likely that you will introduce ground loops that will increase the hum and noise levels in the system. Your system will ALSO be MUCH more susceptible to RF pickup. If you live near an AM radio transmitter, you *might* wind up having your audio "overridden" by the AM signal.

The problem is: The chassis is used as the audio ground return. When you ALSO ground that chassis to the "Grounding Conductor" (the third wire) you add an entire HOUSE full of "circulating ground currents." Ground potentials may be up to several Volts different at different outlets in your home. If you introduce "several Volts" of AC to the chassis of your amplifier, you will "modulate" the audio you are TRYING to amplify. That audio is generally only about a Volt or so.

If it were SO important to have a third grounding wire on electronic gear, cable TV boxes would also have a 3-wire cord. They don't! That's because the RF Input cable is ALSO grounded where the cable enters the house. (At least, it is *supposed* to be grounded there.) The effect of a 3-wire Power Cord would be to introduce a "hum bar" into the picture. (By the way, one of the PRIME sources for "hum introduction" into an audio system is due to the cable box audio feed!)

If the amplifier is the ONLY item with a 3 wire cord, you have a better chance of "getting away with it" because the amplifier will be referenced to the Service Entrance Power Ground and all other components will be referenced to the amplifier's ground point. Add just ONE more unit with a 3-wire cord and you have a DANDY ground loop.

The choice is yours, but *I* wouldn't do it.

I hope this explanation makes sense to you.

Bill Thomas

P.S. The balance control is listed as a 750K dual potentiometer, but it is NOT a standard pot by ANY means. The *actual* value of each section of the potentiometer is around 500K. This is because one half of the resistance element is coated with a conductive surface. When the balance control is "in the middle" the wiper makes contact with both of those conductive surfaces. That means the actual control contains a log taper in one section, and and anti-log taper in the other section - topped off with the conductive coating. Needless to say, these controls are now made of "unobtanium." Unless the control is not functioning correctly, leave it alone. The ONLY way you will find another one is to rob a "parts unit." If the two sections have some "slop" because the two sections are not firmly joined, I would try to center the two sections and then solder a wire to the front and back cases of the individual sections so they no longer shift when the stops are reached. The other method is to dis-assemble the potentiometer, re-crimp the mounting tabs INSIDE the front section and re-assemble the control. If the wiper contact is "sloppy on the common shaft" this won't help at all. In THAT case, you will need to find a way to "re-swedge" the wiper. Good luck with THAT!

I would file this under "minor annoyances" rather than "major problems."

Hope this all helps.

Bill

RE: I have said this many times in the past, but let's try again., posted on October 29, 2009 at 02:39:21

Dauphine
Audiophile

Posts: 5
Location: Cape Town
Joined: October 28, 2009

Dear Bill

Thank you so much for your very kind reply and explanation which makes absolute sense. I will therefore leave things as they are. I have several audio systems around the house. In one room where I test phono cartridges for example, I have a Leak Stereo 20 (grounded), a Quad 405 (grounded) and a Scott 410A (no ground), the latter used to amplify my computer's sound. The Leak and Quad amps are used one at a time, each with their own pre-amplifiers - mostly with turntables and tuners and I have absolutely no hum problems, ever. The same goes for my lounge/listening room where there are two more amps (an Arcam integrated and another Quad and these are both grounded - the way they were made).

I'm not sure yet where I'm going to use the SCA-35, but I'm now listening to it and looking forward to getting acquainted more closely with its charms and will probably renew some bits over the following few weeks when time permits.

Regarding the balance control, this sure is an unusual arrangement. I think I should have sufficient skills to repair it but for now, I used a test record on one of my turntables with left and right channel ID to determine equal channel balance.

A friend sent me a link to your incredible re-build article on (audiocircle?)of your SCA-35 and I spent an enjoyable hour reading through it until it came to an abrupt end. Did the rest of it appear somewhere else?

Thanks again for your valuable time and good advices.

Best regards

Dauphine

I just checked the article. Did you view all three pages?, posted on October 29, 2009 at 15:03:23

Bill Thomas
Industry Professional

Posts: 1330
Location: Southern USA
Joined: January 3, 2004

It *appears* the article is complete. There are a total of three pages devoted to it. The final "beauty shot" of the amplifier is shown above Thank you for the VERY kind words. These rebuilds are a "labor of love" for me.

Sincerely,

Bill

RE: I just checked the article. Did you view all three pages?, posted on October 29, 2009 at 17:02:53
Dauphine Audiophile Posts: 5 Location: Cape Town Joined: October 28, 2009	Ah, Bill I somehow missed this. Too beautiful. As mentioned, I spent an entire hour reading through your restoration. Here's mine, after some initial cleanup procedures. For now, I have just fitted slightly taller feet underneath, a new fuse holder as the old missed its screw cap, and last night - cleaned all the tube pins and sockets. Regards Dauphine

That is *truly* BEAUTIFUL!!!, posted on October 29, 2009 at 17:47:30

Bill Thomas
Industry Professional

Posts: 1330
Location: Southern USA
Joined: January 3, 2004

WELL DONE!!! I have a special "love" for the SCA-35. It was my very first Dynaco kit, WAAAYY back in 1965! Mine served me VERY well until it was lost in a warehouse fire in the early 1980's.

Some folks think the SCA-35 is a bit "light" on the bottom end, but with appropriate high-efficiency speakers, I haven't found this to be the case. Back "in the day" I paired mine with some AR-4x speakers (hey, that was all I could afford back in High School!) Of course, acoustic suspension speakers are ANYTHING but efficient, but they sure do have some NICE bottom end performance! As long as I didn't crank it up TOO loud, it was pretty darned nice!

Yours looks truly beautiful! I hope you enjoy it as much as *I* did back in the 60's!

Sincerely,

Bill Thomas

RE: That is *truly* BEAUTIFUL!!!, posted on October 30, 2009 at 05:38:19

Dauphine
Audiophile

Posts: 5
Location: Cape Town
Joined: October 28, 2009

Hi again, Bill

Ummm, I think if they leave the world's management to vintage audio blokes, it will be a much better place.

I must admit at this time that my knowledge and restorations skills with amplification are few, apart from knowing how to keep live and neutral apart! I do however have reverent respect for the way gear was built back in the day and I do have an almost 'fanatical' compulsion to restore as close as possible to what the original designer intended.

Where I'm on firmer ground has to do with turntables. My last restore was a Micro MR-711 direct drive and there's lots of before and after images. But that's another story and OT here.

Regards

Dauphine

RE: Yet another charming rookie question...................................., posted on October 25, 2009 at 21:15:47
GP49 Audiophile Posts: 111 Location: No Cal Joined: April 21, 2009	Yes. Connect them in parallel and their capacitances add. GP49

Thanks so much! It is greatly appreciated. nt, posted on October 26, 2009 at 08:26:32
viridian Audiophile Posts: 779 Location: Portland, Oregon Joined: May 26, 2005	nt

RE: Thanks so much! It is greatly appreciated. nt, posted on October 26, 2009 at 08:48:39
GP49 Audiophile Posts: 111 Location: No Cal Joined: April 21, 2009	Always glad!... GP49

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Yet another charming rookie question....................................

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