Re: Noob needs help with a 453 no sweep

Albert Otten

Hi Brian,

You "complained " that there were no comments on your measurements:
The only thing that seemed off was the collector of Q524 was +6v when the schematic has +10.5v but I don't know if that is a project or not.
I am seeing a -.5vdc on the cathode side of D501 but only +.030vdc on the anode side.
Right next to that is TP505 which is also +.030vdc and should be -.046vdc.
... is TP504 and I am supposed to have -.3vdc there but have -3.7vdc so that is also way off.
Let's give it a try. I only have the early 453 manual and a (paper) 454 manual.
First of all the DC voltages in the manual are average values while the circuits are running in AUTO mode. Your's are most likely true DC in some kind of rest state. Then together with your other findings my impression is that the Sweep Reset multivib does not return to its rest state after the end off the Hold Off period. Your TD voltage is slightly positive, indicating that the TD is not armed to produce a trigger event; the voltage should be slightly negative. (In the paper 454 manual this is also visible in the waveform which shows a slightly positive value during Hold Off and a slightly negative voltage during the waiting time for the next trigger arrival.) The TD is likely blocked because Q575 is conducting and Q585 is cut-off. It should be visa versa at the end of the Hold Off period. This fault can happen when C550 doesn't get charged (positive) enough to reset the multivib, so for instance when the switch above R551 is bad. But your measurements seem to show that Q514 might still conduct somewhat after end-of-sweep and during Hold Off. Then Q514 prevents the voltage at C550 to increase far enough. The culprit I think will be in the circuit with Q504, Q514 and Q524.
When in Single Sweep mode the Hold Off circuit is not active at all. Pressing the Reset button forces a rest of the multivib with a very strong positive pulse from Q564.
I suggest you do also measurements at Q514-collector, Q524-base (should be the indicated value) and TP585 (Normal mode, no signal). TP585 is there to monitor the state of the multivib. Should be high in rest state and low after end off trace and during Hold Off.


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