Re: Help restoring a 547 scope


Albert Otten
 

Hi Felix,
Today and maybe also tomorrow I can't check my 547. I think such a check would be wise because it's easy to make mistakes in following the circuit description only.
The hold-off circuit around V145 appears to be nicely in the resting state. The cathode of V145A is at +1.5 V, slightly below 2 V in the manual. Hence from there slightly less bias current is added to D85 in your case. But since there is no way to adjust the D85 bias current I think this is not critical; the switching D45 will provide a high enough current pulse to D85 to force switching of D85. It could be that D85 is dead (shortcut I think).
From your present remarks I conclude that the B sweep does nothing when you select AUTO mode. You should test this without input signal and then measure the voltage (or waveform) at the collector of Q75. That voltage should increase to about +20 V and then the zener D76 should start to conduct and supply extra bias current to D85. That can be as much as 3 mA and should definitively be enough to switch D85 to its high state. That situation can only be reset after the end-of-sweep when the hold-off time starts. I think this measurement would be "of diagnostic value".

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