Re: Type 106 Saga (Again)

Albert Otten

On Sat, Mar 12, 2022 at 07:17 AM, Stephen wrote:

Taking into account all your recommendations and suggestions, I finally fixed
my Type 106.
While I was about to make more measurements on the A supply, R233 burnt almost
instantly after I turned it on. I checked again that part of the circuit, the
transistor was fine, but I found that D233 was being seen by my component
tester as a resistor instead of a diode. I replaced both the bad resistor and
the diode, and bingo. The unit came back to life.

I was a nice situation that you found the culprit this way, especially because the specified condition in the Power Supply diagram is quite weird. I only noted that because I tried to understand how R233 could draw a lot of current. The condition is Amplitude CCW. But then Q234 is far in saturation and draws about 5 mA base current! Apparently that is because "A" is pulled negative somewhere (even when the tubes are not yet conducting). This is not a condition in which the "A" supply is in regulation and hence not very helpful. When the Amplitude is turned up the base current and the voltage across R233 drop to near zero. From there normal regulation starts and drives the "A" voltage to more negative values.


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