I have an interesting version of the Dynaco ST70 on the bench for a service. I took some photos:
The unit had four KT77's in the output stage. Two of them were faulty, so I temporarily fitted a pair of used Shuggies while we wait for the delivery of the new KT77's. On the photo I have fitted a new set of RCA's with their earth tag between them.
Underneath the wiring looked questionable:
The original binding posts were replaced with massive copper types that turned out very hard to solder onto. The negative feedback wires were replaced with massive speaker wire of which the cores were twisted together. The rationale for this mod escapes me.
Massive russian NOS caps were fitted parallel to the original coupling caps. These were duly removed too.
Next I found that one output tube has a positive voltage on G1. On inspection it turn out to be a open circuit 270k grid leak resistor. Here you see the replaced in place:
Next I found the common cathode resistor were damaged:
I replaced it with a equivalent resistor.
I also noticed a few questionable solder joints on the nine pin sockets:
I removed the old solder, and re-soldered allowing the solder to flow properly.
The bulky copper binding posts were removed, and similar types to the original were fitted.
Bias was adjust to 50% max dissipation for the 410V on the plates. I noticed the LEDS on the front panel would bias the tubes very hot if used as indicated in the manual. After consultation with the client, we agreed that I would set the bias at the shop to be left as is.
The unit had four KT77's in the output stage. Two of them were faulty, so I temporarily fitted a pair of used Shuggies while we wait for the delivery of the new KT77's. On the photo I have fitted a new set of RCA's with their earth tag between them.
Underneath the wiring looked questionable:
The original binding posts were replaced with massive copper types that turned out very hard to solder onto. The negative feedback wires were replaced with massive speaker wire of which the cores were twisted together. The rationale for this mod escapes me.
Massive russian NOS caps were fitted parallel to the original coupling caps. These were duly removed too.
Next I found that one output tube has a positive voltage on G1. On inspection it turn out to be a open circuit 270k grid leak resistor. Here you see the replaced in place:
Next I found the common cathode resistor were damaged:
I replaced it with a equivalent resistor.
I also noticed a few questionable solder joints on the nine pin sockets:
I removed the old solder, and re-soldered allowing the solder to flow properly.
The bulky copper binding posts were removed, and similar types to the original were fitted.
Bias was adjust to 50% max dissipation for the 410V on the plates. I noticed the LEDS on the front panel would bias the tubes very hot if used as indicated in the manual. After consultation with the client, we agreed that I would set the bias at the shop to be left as is.
