This DS-5 is from the earliest production, when the transformer was a Stancor TP-2 with a 115 VAC primary winding only, and there were taps for varying the voltage by a few percent, and those extra taps were cut off, some on the primary side and some on the secondary side to only leave the proper primary winding tap and the proper center tapped secondary winding for the bridge rectifier.
In those days grounded plugs were just being introduced in new construction and usually couldn't find safety grounded outlets, especially in small clubs and theater stages, so we stayed with the two-prong plug. Instead of a fuse which didn't offer protection from overload transformer overheating, we used a thermal interrupter which would open at a temperature lower than the rated insulation in the transformer. It was not uncommon in those days of two-prong plugs for a thermal cutout device to be incorporated inside the transformer winding, especially in a transformer that was connected 24/7.
The thermal cutout had to be connected with wire nuts and not soldering, because soldering heat opens open the device with no external change visible. Also note the folded fish paper (rubberized paper) as extra barrier insulation where the power line wiring is housed.
As Jimmy J noted we would never use a carbon composition resistor here, and in addition we don't need a voltage drop here anyway, so if you don't have a replacement thermal protector, connect the line cord wire directly to the primary wire.
The original line cords were Belden HPN type and those had rugged Neoprene synthetic rubber that could stand hard abuse as well as high temperatures (we didn't need that aspect) and were often used for electric heaters, toasters, cooking utensils, etc. and had lots of fine wires for greater flexibility and longer flex life than the usual plastic insulated line cords used in lamps and other such household devices. There were 41 very fine wires instead of the normal strand count of 7 fine wires.
But even with the increased flex life of the finely stranded wires, the first failure often occurs at the Heyco strain relief device probably because when stored inside the case if rotated in one orientation the cord is bent sharply against the wall of the storage box. And this is made harder to diagnose because when intermittent, moving the wire one way will remake the connection and the storage capacitors keep the sound running for several seconds when moved another direction that breaks the power. And there are some failures at the plug end of the cable with the same intermittent result.
Cool that the genuine Heyco tools are still available (although can't get used to the inflated prices these days) and the old tool we have has red handles. But can't let the photo on the Amazon go without comment, as the plastic strain relief device is put in the tool backwards. So you'd have to squeeze the handles really really hard and use extra pliers to make it go through the chassis hole backwards.
-ron (writing on Mica's machine)
