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One of our members, who found my post on installing a brass static shield on the Standby Switch on the SVT-VR contacted me and then brought over his newly acquired SVT Heritage 50thAnniversary Amp head. Looks just like an SVT-VR at a glance. When we connected it here in my shop, using the Ampeg SVT 115HLR bass cabinet I have in the shop, we didn't hear the usual static buzz on Ch 2 that I always hear on SVT-VR's. That had led to cutting and folding up a partially open shielded box to place the Standby Switch into, as the switch is running on 120VAC, and sits right next to the Ch 2 input stage preamp.
I found in my first scribed and folding/cutting process of my 3.15" x 1.95" 5-mil brass blank, that I had cut up four pieces, taped together squarely and punched the 1/2" hole for the Standby Switch to fit thru, that my drawing from Sept 2019 didn't quite match the photos I had taken previously. So, had some fumbling to do to correct that yesterday morning. Got the part correct on the fourth pass, soldered the folded pieces together, then applied 3M Type 56 Yellow insulated tape over all the surfaces that insulated it from the preamp circuitry it sat above and adjacent to.
Now that I had a shield ready to install, I removed the rear fan panel, turned the cabinet around to remove the front grille, had to fumble with the Fan connector that appears to be in a less accessible position where human hands can't remove it from!! Got that removed, unplugged the two preamp connectors, and hoped the M3 threaded stud used to secure the Ground wire from the Preamp chassis wasn't instead a screw that could slip thru. Then, I proceeded to remove the four M5 chassis mounting screws. The first one, upon pulling the screw out, I heard a fastener drop and land on the chassis below, or inside the preamp. Happened again on the same side with a second screw upon removal. The other two came out without hearing further surprises.
It was that cheap Chinese M5 Cage nuts that self-destructed during removal, where their 'wings' that hold them into place in the square opening of the chassis, one had broken off, allowing the cage nut and the little piece of metal that broke off to drop into the chassis, or onto the top of the power amp chassis!! How delightful. I see nothing has changed with Ampeg's purchasing dept, buying cheap with no regards to quality.
I also discovered while installing the insulated brass shield I fabricated onto the Standby Switch, that they've selected a different brand of switch, it having much coarser machine threads, and they did NOT use a nut at the rear of the switch. Just the mtg nut up front, with a washer, so if you tighten those switches up good and snug, you can break the switch, as you're pulling the threaded bushing right out of the power and standby switches themselves!! I found a couple mating nuts to this odd thread size, threaded them into place, added fine pitch ITL washer on the rear and installed the shield, as well as the same on the power switch.
So, knowing these Chinese Cage Nuts are also present in the power amp chassis, I didn't take the chance of removing the chassis mounting screws that lack lock washers under the screw heads. That adventure into the power amp chassis will happen another day, where more production follies remain to be found.
Nice sounding amp. On the silkscreen of the Preamp there's a label 1969 under Channel One, and 1975 under Channel Two to represent the Preamp circuits as designed for the SVT amp in those years.
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