Retro Tech & Electronics | The Dynaco Dynakit ST70 Stereo Amplifier Restoration Project Part 4 @retrotechandelectronics | Uploaded July 2019 | Updated October 2024, 5 hours ago.
Now in part four of the Dynakit ST70 restoration project, I start off by demonstrating how dangerous of a condition i've left this unit in given the fact that nothing is connected to the electrolytic capacitor to drain it off to a load over time. Were talking about well over half an hour and the chassis is still lethal. Food for thought.....
With that out of the way the heaters are brought down from the EL34s to their respective pairs, I choose some nice fabric wire for this task to match the power transformer. After this it was an appropriate time to remove the old PC-3 circuit board from the old chassis by cutting it out and unscrewing it. It really required a cleaning and polishing too, so that was done before continuing. I was careful to leave short ends of the cut wires on that board to keep track of what would be reconnected to this board, I knew with the new bias controls there would be some modifications in the future.
The board was mounted into the new chassis and tightened down. The wiring immediately begins starting from the output transformer for the capacitive feedback. I chose blue for that. Then the bias pots were added to the board. Logically, the bias extended out of the board to the four EL34's though since it goes through 1K resistors on pin5, the cables go to unused pin6 on the EL34 and a new resistor goes to each pin 6 to pin 5.
The high voltage connections come off the electrolytic capacitors in red to the board, also making working on the amp now less dangerous upon shutoff, then the blue wires continue for the capacitive feedback circuit from the board out to one EL34 from each side. After this the main PCB ground is added.
Now is a good time to finally re-add the clamps that hold all of the cables in place by the transformer, so I stop to do this, since work now moved to the front of the chassis. This work now the front input jacks connects the jacks to the mono/stereo switch as well as the circuit board. These signals on input will be White.
The Bias Balance Control Upgrade, BBCU is the next part of the installation, the special kit i've ordered for this amp. I follow the instructions provided in the provided manual to do so. And remove the rest of the cur cables remaining in the PCB at this time. The only modification to this install is that I tie off all of the grounds to the chassis star ground, and I use my own thicker wire. Again, during socket installation I had to shim with paper to center them.
With the new kit installed, the cables are laced in and this portion of the project is now complete. #dynaco #st70 #tubeamplifier #antique #restoration
Now in part four of the Dynakit ST70 restoration project, I start off by demonstrating how dangerous of a condition i've left this unit in given the fact that nothing is connected to the electrolytic capacitor to drain it off to a load over time. Were talking about well over half an hour and the chassis is still lethal. Food for thought.....
With that out of the way the heaters are brought down from the EL34s to their respective pairs, I choose some nice fabric wire for this task to match the power transformer. After this it was an appropriate time to remove the old PC-3 circuit board from the old chassis by cutting it out and unscrewing it. It really required a cleaning and polishing too, so that was done before continuing. I was careful to leave short ends of the cut wires on that board to keep track of what would be reconnected to this board, I knew with the new bias controls there would be some modifications in the future.
The board was mounted into the new chassis and tightened down. The wiring immediately begins starting from the output transformer for the capacitive feedback. I chose blue for that. Then the bias pots were added to the board. Logically, the bias extended out of the board to the four EL34's though since it goes through 1K resistors on pin5, the cables go to unused pin6 on the EL34 and a new resistor goes to each pin 6 to pin 5.
The high voltage connections come off the electrolytic capacitors in red to the board, also making working on the amp now less dangerous upon shutoff, then the blue wires continue for the capacitive feedback circuit from the board out to one EL34 from each side. After this the main PCB ground is added.
Now is a good time to finally re-add the clamps that hold all of the cables in place by the transformer, so I stop to do this, since work now moved to the front of the chassis. This work now the front input jacks connects the jacks to the mono/stereo switch as well as the circuit board. These signals on input will be White.
The Bias Balance Control Upgrade, BBCU is the next part of the installation, the special kit i've ordered for this amp. I follow the instructions provided in the provided manual to do so. And remove the rest of the cur cables remaining in the PCB at this time. The only modification to this install is that I tie off all of the grounds to the chassis star ground, and I use my own thicker wire. Again, during socket installation I had to shim with paper to center them.
With the new kit installed, the cables are laced in and this portion of the project is now complete. #dynaco #st70 #tubeamplifier #antique #restoration
