Continuing with part 3 of the Dynakit ST70 restoration project, we will get right into the output transformers, though given the mass, and the nice shiny chassis, I cover everything first with masking tape in case of a mishap. All of the wires need be carefully negotiated through and the screws and nuts placed in and gently tightened just enough so the the transformer doesn't shift. Once this is accomplished, there must be an accurate account taken of the location from the end so that it remains parallel before tightening, as well as allowing the other side to be symmetrical. In the desired location, it is torques down. Wash rinse, repeat. Though the second one has the burden of matching the measurements of the first.
Sadly, having caught one wire under the chassis, I got to remove and install the same transformer, twice. I was then able to solder the newly connected transformers to the output jacks and then the connections to the EL34 sockets. The only outstanding transformer connections at this time are the red center taps to the electrolytic capacitor, not yet installed.
Having established a colour code for the circuits within the amp, I had a go first a snipped wires already in the amp from swapped components, this was predominantly the bias circuit, for which yellow was chosen. Green was chosen for Ground. Black for anything after Labor day. Also the grounds off of the output jacks were installed to the star ground.
This is a great time to pull the electrolytic cap from the old chassis, which is new, by the way. and prepare it for the new chassis. This means polish it so it doesn't look so industrial. I just ripped off the sticker and stuck it to the bottom of the power transformer. This Fiasco started at 800 and worked up to 1000 and then to 2000, and the chrome polish sealed the deal. Shiny and new, the capacitor is affixed to the new chassis, taps twisted for a tight and permanent fit.
The cap is immediately grounded to the star ground. and its terminals cleaned up, and its resistors tested. Wiring to the capacitor then begins starting from the rectifier, the HT colour will be Red. This is followed by the coil C-354. The resistors are left on the capacitor, though there are further connections being added later once the circuit board is introduced.
Smoke test time, round two. This time well take several measurements, at full voltage governed by five volts at the tube rectifier filament. This allows the testing of the bias circuit, onward through the individual pots. Also, a further inspection of the AC ripple before and after filtering shows the filter to be working just fine. A final check of the AC output on the tube rectifier socket shows an unladen value of 800 volts. Incredible.
So dropping a 5AR4 in will allow some DC measurement . The unladen rectifier output is 540VDC. This seems like a good stopping point for this Part of the project.... #dynaco #st70 #tubeamplifier #antique #restoration
5 окт 2024
