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SCA-35 loses channel

R

Riverstix

New Member
Hi,
I have a Dynaco SCA-35 that drives me crazy. It works fine apart from one thing: occasionally the left channel goes quit. Turning the input selector back and forth a few times "solves" the problem. Until the next time it appears.

I've tried the following, with no success:

Using another input.
Switching to mono.
I've sprayed every pot and switch with Deoxit D5.

The problem seems not related to the input selector, as moving it never causes any harm.
However, when flicking the filter switch, it seems to me that the left channel quite often drops out.

Anyone here who has seen this too, or might have a clue what lies behind?
 
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Use a multimeter always to check for continuity or resistance on your pots / switches as part of your diagnosis.

It sounds like one of the switches in your signal path needs cleaning, and contact cleaner like deoxit likely won't help. You'll need to desolder and take apart whatever is causing the issue. Like lots of vintage equipment, the SCA-35's pots / switches / leads suffer from black corrosion. It needs to be scraped away with something like a fiberglass pen (found on amazon or digikey for example). This is standard kit for working on old amps. I recommend the Excelta 264. Be very careful with these, don't poke your finger or you can get a thread lodged in your skin.

I'm guessing based upon your diagnosis that at least the filter switch needs to be removed and taken apart. Once apart, you'll see the switch contacts are corroded and need to be delicately wiped with the fiberglass pen.

Lastly, in case you've not done it before all switches and pots can easily be disassembled, cleaned, and put together rather quickly so it's not a tough job.
 
D5 will remove oxidation. You need to go through the circuit with a scope to find out where the signal is being lost or simply check continuity with a meter. Once you have localized the problem, if it's the selector switch the contact may need to be gently tightened with a dental pick. Sometimes you have to clean more than once with D5. If the unit was used by a smoker it probably isn't oxidation but crud in the contacts and a non residue cleaner like Caig Contact Cleaner Wash may be needed, then follow with D5 as a preservative. Also don't discount the possibility of a dirty tube socket. Try moving the tubes in and out slightly and see if that helps. Do this with the power off.
 
You do have to be very careful taking switche's and pots apart. And when using a contact cleaner work the selector switches and pots back and forth about 30 times. If it turns out to be the filter switch dropping out, you can always bypass it. Personally I never use them, unless it happens to be subsonic filter and I'm playing records.
 
Clean the switch contacts with a pencil eraser and potentially some deoxit.
Check tension on the pins and be very careful.
Selector switches can be a nightmare to get cleaned and ive got such animosity towards them and their open contacts that I build all of my own stuff using sealed relays so that nobody has to clean a Selector switch or source a new one decades after the original was produced.

Look around at the various hot rod dynaco vendors and see if anybody offeres a replacement for the Selector that uses sealed relays instead of open air switch contacts. (I honestly have no clue)
Relays are the way forward in my opinion.
Good luck with it!
 
Other sources could be the volume pot , or possibly "cold solders".
Try to move the volume slightly next time to see if that restores sound. The selector is better then it's reputation and in addition it shorts unused
inputs to prevent "bleeding" unselected source into the selected source.
 
Thank you all for good advice! I'll start with bypassing the filter switch and keep you posted. If this helps, I'll get the recommended fiber glass pen and do a more proper overhaul.

The volume potentiometer works fine, never had any issues there.
I've previously tried refitting the tubes, but it didn't make any difference.
 
I did not doubt that the volume pot work. My suggestion was to sligtly turn it next time a channel dies to confirm or rule out it as the source of bad contact.
 
peterh said:
I did not doubt that the volume pot work. My suggestion was to sligtly turn it next time a channel dies to confirm or rule out it as the source of bad contact.
Click to expand...
Thanks for the advice. And by the pot is OK, I meant that turning it makes no difference to the problem.
 
Riverstix said:
I am happy to write that the problem seems to be gone. Bypassing the filter switch did the trick.
Until I have time to give the amplifier a complete overhaul, this will be fine.
Thank you all for taking your time to help me out!
Click to expand...
You bypassed the issue, you did not fix it. Yes I'm a cheeky bastard
 
Riverstix said:
I am happy to write that the problem seems to be gone. Bypassing the filter switch did the trick.
Until I have time to give the amplifier a complete overhaul, this will be fine.
Thank you all for taking your time to help me out!
Click to expand...
You can probably find a new filter switch from Dynakit if tou wanted to replace the old one at some point. They sell a lot of the replacement bits for the Dynaco equipment.
 
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