• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

SCA-35 Project

albusark said:
I've never seen any discussion on Packaged Electronic Circuits, the proto IC's that are in various Dynaco amps. They contain capacitors and resistors that I assume set values for the tone controls. Should they be replaced? I always wondered about these but never have seen any focus on them. You can find NOS with PN 555003. Safe to assume that the capacitors inside are some sort of ceramic type build? I ordered some to investigate. Maybe I can pull some values from them and we can experiment with wiring up discrete components instead.

View attachment 3473007
Click to expand...
Ya, they probably should be replaced also.
The ceraminc caps inside are probably ok, but the resistors are all carbon comp. You can make your own using COG capacities and metal film resistors built on small pieces of perforated project board. I replaced the PEC's in my FM3 tuner with some built that way.
You can cover them with large diameter shrink tubing, or dip them.
 
Last edited:
Lavane said:
Ya, they probably should be replaced also.
The cerwminc caps inside are probably ok, bit the resistors are all ca4bon comp. You can make your own using COG capacities and metal film resistors built on small pieces of perforated project board. I replaced the PEC's in my FM3 tuner with some built that way.
You can cover 5hem with large diameter shrink tubing, or dip them.
Click to expand...
This may explain a PAS 3 preamp I have where I could never figure out a slight imbalance in the tone knobs. I measured pots, fully replaced all caps and resistors and the problem never changed. I think these PEC's need to be investigated so that future amp maintainers can fix such issues. I'll look into schematics and see if I can get values.
 
albusark said:
I've never seen any discussion on Packaged Electronic Circuits, the proto IC's that are in various Dynaco amps. They contain capacitors and resistors that I assume set values for the tone controls. Should they be replaced? I always wondered about these but never have seen any focus on them. You can find NOS with PN 555003. Safe to assume that the capacitors inside are some sort of ceramic type build? I ordered some to investigate. Maybe I can pull some values from them and we can experiment with wiring up discrete components instead.

View attachment 3473007
Click to expand...

Analysis of the PEC's is included in my latest SCA-35 Line Stage article, It's comprehensive, not a short read, but includes information you'll want to know, especially when replacing the PEC's with discrete parts.

 
albusark said:
I just happen to have a complete set of Gillespie boards :) Are they really that much better? I've been holding on to them as a collector I guess, wasn't sure I'd ever use them.
Click to expand...
Yes, particularly the EFB power supply boards. They address several design shortcomings that were the result of Dynaco's need to keep a low price point. This is mostly from a beefier power supply, but also from an improved method of maintaining proper bias.
 
albusark said:
This may explain a PAS 3 preamp I have where I could never figure out a slight imbalance in the tone knobs. I measured pots, fully replaced all caps and resistors and the problem never changed. I think these PEC's need to be investigated so that future amp maintainers can fix such issues. I'll look into schematics and see if I can get values.
Click to expand...
Dynaco PAS has no PEC's Are you mixing this up with SCA-35 ?
 
peterh said:
Dynaco PAS has no PEC's Are you mixing this up with SCA-35 ?
Click to expand...
Dangit you're right, thanks Peter. I'll keep troubleshooting. So many variables can cause tonal imbalance between channels, I need to investigate more in another thread and use the oscilloscope.
 
albusark said:
Dangit you're right, thanks Peter. I'll keep troubleshooting. So many variables can cause tonal imbalance between channels, I need to investigate more in another thread and use the oscilloscope.
Click to expand...
Check the Loudness and Filter switches. 50-year-old slide switches can be very troublesome.

Jack
 
albusark said:
Hello again all,

I'm embarking on an SCA-35 rebuild. The base amp is in excellent condition. Coupling capacitors have been replaced with extremely cheap Chinese poly caps. Anyone have a good argument for why I should not toss them?

I'll be replacing filter cans, bias circuit, resistors, and whatever else is needed to make this a rock solid amp for the next 60 years, will document the process.

I also have some fun EL-84 matched tube sets to test, including a nos JAN green letter kit and some Amperex bugle boys. This will be my keeper SCA-35, but will be offering fully restored amplifiers in the marketplace as I consolidate my collection.
Click to expand...
I’m following along!

I made an update to my SCA 35 start up issue and it was 100% the 7199 tubes making the noise on start up. I changed them out with a NOS set of Tung-Sol 6u8a’s with the adapter bases from Tube Depot…fyi, don’t do like I did and get some gucci looking adapter bases off of eBay with ceramic sockets! The tubes didn’t fit in the pins and one got stuck and took very careful prying to get out. Just use the less cool looking ones from TD if you are keeping the original circuit boards.

I’m keeping mine original for trouble shooting so I can get better at that and plan to build another one from scratch with Dave’s boards to comparison the two side by side.
 
Big & Pink said:
I’m following along!

I made an update to my SCA 35 start up issue and it was 100% the 7199 tubes making the noise on start up. I changed them out with a NOS set of Tung-Sol 6u8a’s with the adapter bases from Tube Depot…fyi, don’t do like I did and get some gucci looking adapter bases off of eBay with ceramic sockets! The tubes didn’t fit in the pins and one got stuck and took very careful prying to get out. Just use the less cool looking ones from TD if you are keeping the original circuit boards.

I’m keeping mine original for trouble shooting so I can get better at that and plan to build another one from scratch with Dave’s boards to comparison the two side by side.
Click to expand...

I do have a good quantity of 7199's in my collection, but I actually really like the 6u8a / 6gh8 type tubes and think they sound as good or better. Considering that I was able to buy a lot of RCA nos 6gh8a's for a reasonable price, I'm thinking I'll rewire the socket to simply accept that type instead. My steel ST-70 series II still has Dynaco branded 6gh8a's and they kick butt, that's a beautiful amplifier. I know some resistor value changes are recommended, but I can't seem to find that information at the moment. In using one of those tube adapters in the past I wasn't able to notice any change in the quality of the sound but it did fix the issue of bad 7199's on another SCA-35 I was working on. I think I did order those on tubedepot.

I was able to fix the EQ imbalance of the frequency response of the right channel by simply putting in a new set of balanced tubes. It was after all using the original Dynaco branded tubes. I settled on a quad of Sylvania JAN nos tubes with the boxes showing year of manufacture 1969. The 7199's are also Dynaco branded and seem to be doing well for now.

I did receive 2 brand new PEC's from a distributor that bought out the remaining Dynaco components when they folded and they're quite gorgeous, will be doing testing on their values and work on creating a solution for replacing them in the future.

I have two more SCA-35's to work on, also in great condition, and one of those I'll do the full Gillespie PCB replacement like you said Big & Pink. This one I'm keeping original because it's in such nice condition and I really like simply restoring and enjoying the original Hafler circuits - the Mark 3 being my all time favorite, but in terms of sound quality and enjoyment I think they are very close. The Mark 3 with a set of good KT88's is like a supercharged ST-35/SCA-35, with the gorgeous detail of the EL84 but absolutely incredible bass and volume, they are earth shakers.

I'll take a lot more photos and document the process as I service this particular amp to be rock solid.

1744260100134.png
 
Last edited:
ramjet said:
If your budget allows, replacement of the pc boards with modern fiberglass (and new tube sockets and components (if desired)), will position the amplifier best for the "next 60 years" vs the original phenolic.
Click to expand...
Agree highly, the original phenolic has delicate traces (like most all early PC boards). Replacing the boards with modern fiberglass, and new tube sockets and whatever new passive parts desired, would be future insurance. Just be patient, and do neat work.
 
Unfortunately this is a factory made unit and as a result, EVERYTHING is riveted - including the can caps. I've dealt with this on other amps by violently / carefully drilling the rivets out. Is there any better method? Drilling works fine and in my experience hasn't caused any damage but I really dislike the glowing red metal pieces flying around.
 
use a sharper drill bit. If the metal is glowing red it means you're rubbing through them rather than cutting through. Especially if its brass like many of them are.

or just leave the insulating wafer alone and twist the tabs to un-mount them. If you are careful it won't break the insulator and you don't have to mess with rivets.
 
True, but having pulled a lot of extremely dull drill bits out of other people's sets I can understand why some like it. I'm not a sharpening master or anything but I can usually make one at least cut rather than smoke.
 
You must log in or register to reply here.
Back
Top Bottom