by EWBrown » Tue Dec 04, 2007 6:44 am
Depending on your local AC line voltage, the B+ at the OPT Center taps in the SCA-35 can vary between 365 to 380VDC. Estimate a 6V IR drop through the OPT primary, and approx 14 - 15V through the individual cathode resistors (390 to 430 ohms @ 35 mA). So we're looking at between 344 to 360V Plate to cathode for each EL84, which is within its plate dissipation ratings.
I'd suggest inserting a CL-90 Inrush Current Limiter in the primary AC side of the power trannie, in any SCA-35 or ST-35 rebuilds, this will eliminate that hard power-on "slam" and reduce the B+ voltages slightly, by a matter of just a few volts. THis is even more important with the increased capacitance in the PSU.
The signal path wiring behind the front panel is a veritable Minotaur's Maze of unshielded wiring and PECs, ceramic caps and other things that go bump in teh night whilst tormenting the sound quality, and picking up every stray EM signal in the area... I'd ditch the tone controls, filters, and perhaps keep the original volume and balance pots, if they are in good condition. If the phono or tape head stage isn't required, just ditch it and stick with the basic power amp section.
In my first SCA-35 redemption of a "ridden hard and put-up wet" carcass, found on E-bay before the prices went up on these, I ended up using only the original iron, and chassis, and replaced the crispy toasted SCA35 PCBs with aftermarket ST-35 boards (using 7247 / 12DW7) and beefed up the power supply to be more or less equal to that on the DIY35 boards.
The preamp board and selector swith was also excised, and I made up a plate to carry the extra electrolytic caps, and mounted a C354 choke on top of the chassis, close to the original preamp board's location. Circuit topology is more or less identical to that on the DIY-35's PSU section.
I started out with the original volume and balance controls, but later replaced them with one of the Hong Kong 100K stereo stepped attenuators. The original controls were just a tad scratchy and intermittent, and a few attempts at using TV tuner cleaner only gave temporary results.
I didn't bother with multiple input selectors, and added a separate pair of isolated RCA jacks on the back panel (someone had previously very conveniently drilled two 3/8 in diameter holes in just the right location), and ran shielded cable from those to the step attenuator and more shielded wire to the inputs of the PRCs. The unit still has the original phenolic speaker terminals, I may eventually replace those with binding posts, as I typically only use the 8 ohm outputs.
I filled the now-empty holes in the front panel with some Dialco LED pilot lamps, one gives a power-on indication, another samples the DC voltage across the shared cathode resistor in order to show thatit is fully warmed up. (I still have to replace that with individual 390 -430 ohm resistors and 330 uF 35V caps, and then each cathode can have an LED "monitor").
The other SCA-35 I have from a flea market find, was factory built and in such pristine condition that I left it intact.
HTH
/ed B in NH
Real Radios Glow in the Dark