The choke assembly's fit is checked in the Sibling's chassis and the goals of secure mounting and reversibility are met. Next step is to begin the Sibling rebuild and incorporate the choke mod.
Drilled and countersunk 1/8" thick aluminum mounting plates made from bar stock from the hardware store.
The choke, a Hammond 159Q would replace R28 in the schematic. C12 was also replaced with a 4 section can cap, so C12A is now 20uF. Since the choke's internal resistance was 100 ohms, close to the 150 ohm value of R28, and since the smaller C12A would drop the input voltage a little bit, I did not add the difference of 50 ohms back in to the circuit. So it looked like electrically it would work and there appeared to be enough room to make it fit, so I needed to figure out a way to make it solid mechanically, yet reversible. The answer was to fabricate an aluminum bracket that would fasten to the transformer lugs and to which the choke could be fastened.
This is what the Sibling's chassis looked like underneath before work began. You can see that there is a bunch of room below the power transformer on the right of this photo.
After working on a nice pair of MacKit 30s for the owner of a local audio store, I picked up another McIntosh MC30, I call it the Sibling, from Joe Auwen in Prescott, AZ, to go along with a well preserved sample that I've owned for about 10 years, but only displayed. The new unit's chassis was in pretty rugged shape, with corrosion and downright rust on the top of the chrome chassis, but no apparent dents or gouges in the transformers. Joe also has tons of parts for the MC30s including these beautiful reproduction chassis, so since the old chassis was a real basket case, I picked up a new one while I was there. One thing I've been thinking about since the MacKit rebuild was the possibility of adding a power supply choke. A friend had asked me to add a choke to his Yaqin MC-84L amp, which turned out to be quite successful, sound wise, so since many of the MC30's cousins have chokes, I wondered what it would sound like with the chokes. Fortunately, there is enough room underneath the chassis to install a 7H 150mA choke without permanently modding the chassis itself. I wanted to be able to remove the choke and return the unit to stock in the future if it didn't work out.