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This is discussed from time to time on the groups.io (formerly Yahoo) TekScopes forum. | This is discussed from time to time on the groups.io (formerly Yahoo) TekScopes forum. | ||
Bernie Schroder has found that the HV transformer degeneration can be slowed considerably by keeping the HV module cool and dry. The mod he was taught by Jim Willams involves replacing the tube rectifiers with NTE517 diodes. However when the tube HV rectifiers are | Bernie Schroder has found that the HV transformer degeneration can be slowed considerably by keeping the HV module cool and dry. | ||
The mod he was taught by Jim Willams involves replacing the tube rectifiers with NTE517 diodes. | |||
However, when the tube HV rectifiers are replaced with semiconductor types, the regulator circuit has about 1 Watt of load removed due to the lack of tube heater filaments. | |||
This forces the regulator past its design limit and can result in lack of regulation until the transformer heats up and more power is required due to losses. | |||
After changing to semiconductor diodes, it will be seen that grid pin 7 of V814 will be about 0 volts (fully on, with no swing to regulate) | |||
in an attempt to force its anode (pin 6) to about +55 V, which is the screen input of the 6AU5 (pin 8). | |||
One way to remedy this is to replace the anode resistor coming off pin 6 of V814 (R803) from 56 kΩ to 82 kΩ. | |||
Doing so changes the grid voltage on pin 7 of V814 to a comfortable -0.8 V at switch-on. | |||
Again, this becomes more negative as the losses inside the transformer increase, but not by much if the transformer is healthy. | |||
===Vertical Amplifier=== | ===Vertical Amplifier=== |