Retro Tech & Electronics | The Dynaco Dynakit ST70 Stereo Amplifier Restoration Project Part 2 @retrotechandelectronics | Uploaded May 2019 | Updated October 2024, 1 day ago.
Now in Part 2 of the Dynakit ST70 restoration project, we will begin with the choke as the first component for installation, though I thought it interesting to compare both on the Genrad 1650A impedance bridge to see that the impedance and inductance is.
SPEC
62ohms 1.75h
OLD CHOKE
63ohms 1.50h
NEW CHOKE
64ohms 1.35h
All within acceptable range, the new choke is installed into the chassis, though, the old one is definitely a keeper for other projects. All work is done with the intent on maintaining a scratch free project and with extreme care. The tightening of all screws in this project has the slot of the screws pointing in the same direction.
The two switches followed the choke and interestingly, they were not threaded like the original ones. This was helpful though and allowed the screw heads to maintain a parallel appearance. Following the switches was the assembly and installation of the gold plated input jack board.
Returning to the old chassis, the selenium rectifier, power cable, fuse and stand-off board was removed for transfer. It became apparent that a custom screw for the rectifier would need to be fabricated. As these would be put into the new chassis it was also a good time to introduce the bias pots as well. Tightening in such a manner that the flats would be parallel to the font of the chassis.
The rear jack kit was assembled at this point. This kit did not include the 16 ohm option, instead an internal standoff for 16ohm serving as a tie-off for the feedback connection. These are much better than the screws in the original kit. Upon assembly, these too are mounted. along with the end stand-offs.
Returning from Lowes with a 6/32 screw the fuse socket was installed and the screw was cut as a rod in support of the selenium rectifier, using an acorn nut as the top cap. The standoff board was installed and then I'd assembled some hardware to test the rectifier install. It looked good and was done so that the flat again was parallel to the chassis.
The tube sockets were a little tricky because they are a bit smaller than the hole. So I had to shim it with two turns of paper to center the socket whilst tightening. It seemed to work good though. Mundane is having to repeat this several times. Moving to the power transformer was welcome.
Its challenging though, guiding this heavy thing into the chassis without scratching anything with all of these wires everywhere. Somehow I managed to pull it off. Dampeners and all installed. Immediately I install for regular nuts on the bottom to hold the transformer in for safety. One benefit of the transformer is the ability to flip it to the service position.
Power transformer wiring begins with the EL34 filaments, BROWN and GREEN. This included their center taps to the double capacitor to ground. Following this will be the connections to the rectifier. Then the primary black wires off of the power transformer to the fuse and switch. Following this, is the single cable to the selenium rectifier.
At this point I go back to the old chassis to remove the star ground hardware and install it in the new chassis. The first connection to it would be the ground off of the power transformer. At this point a 3 prong power cable is introduced and wired in. The ground of this cable also meets the star ground.
With the power testing is conducted with the Heathkit IM-11 VTVM using a fraction of the voltage. Switches are tested and then the ratio's determine that the windings are ok. The center-taps to heaters were measured by resistance across the coil.
Having completed this testing part 2, has come to an end. #dynaco #st70 #tubeamplifier #antique #restoration
Now in Part 2 of the Dynakit ST70 restoration project, we will begin with the choke as the first component for installation, though I thought it interesting to compare both on the Genrad 1650A impedance bridge to see that the impedance and inductance is.
SPEC
62ohms 1.75h
OLD CHOKE
63ohms 1.50h
NEW CHOKE
64ohms 1.35h
All within acceptable range, the new choke is installed into the chassis, though, the old one is definitely a keeper for other projects. All work is done with the intent on maintaining a scratch free project and with extreme care. The tightening of all screws in this project has the slot of the screws pointing in the same direction.
The two switches followed the choke and interestingly, they were not threaded like the original ones. This was helpful though and allowed the screw heads to maintain a parallel appearance. Following the switches was the assembly and installation of the gold plated input jack board.
Returning to the old chassis, the selenium rectifier, power cable, fuse and stand-off board was removed for transfer. It became apparent that a custom screw for the rectifier would need to be fabricated. As these would be put into the new chassis it was also a good time to introduce the bias pots as well. Tightening in such a manner that the flats would be parallel to the font of the chassis.
The rear jack kit was assembled at this point. This kit did not include the 16 ohm option, instead an internal standoff for 16ohm serving as a tie-off for the feedback connection. These are much better than the screws in the original kit. Upon assembly, these too are mounted. along with the end stand-offs.
Returning from Lowes with a 6/32 screw the fuse socket was installed and the screw was cut as a rod in support of the selenium rectifier, using an acorn nut as the top cap. The standoff board was installed and then I'd assembled some hardware to test the rectifier install. It looked good and was done so that the flat again was parallel to the chassis.
The tube sockets were a little tricky because they are a bit smaller than the hole. So I had to shim it with two turns of paper to center the socket whilst tightening. It seemed to work good though. Mundane is having to repeat this several times. Moving to the power transformer was welcome.
Its challenging though, guiding this heavy thing into the chassis without scratching anything with all of these wires everywhere. Somehow I managed to pull it off. Dampeners and all installed. Immediately I install for regular nuts on the bottom to hold the transformer in for safety. One benefit of the transformer is the ability to flip it to the service position.
Power transformer wiring begins with the EL34 filaments, BROWN and GREEN. This included their center taps to the double capacitor to ground. Following this will be the connections to the rectifier. Then the primary black wires off of the power transformer to the fuse and switch. Following this, is the single cable to the selenium rectifier.
At this point I go back to the old chassis to remove the star ground hardware and install it in the new chassis. The first connection to it would be the ground off of the power transformer. At this point a 3 prong power cable is introduced and wired in. The ground of this cable also meets the star ground.
With the power testing is conducted with the Heathkit IM-11 VTVM using a fraction of the voltage. Switches are tested and then the ratio's determine that the windings are ok. The center-taps to heaters were measured by resistance across the coil.
Having completed this testing part 2, has come to an end. #dynaco #st70 #tubeamplifier #antique #restoration
