Re: Tek 545A Scope


Jonathan Pyle
 

When I restored my 1963 rackmount 545A (RM45A) I replaced all the electrolytic and "PTM" (paper tubular molded) capacitors. Before replacing the capacitors, I had ripple and the supply rails were far off, and replacing the capacitors cured all that. I don't know whether the capacitors in the parts list that are marked as "PTM" were actually paper in my instrument; maybe they were actually plastic and didn't need replacement, but I didn't want to take any chances.

My long-shot, non-expert, probably bad ideas about the disappearing trace: 1) High voltages in this part of the circuit go directly to the potentiometers for intensity, focus, and astigmatism, so maybe scratchy pots could cause the trace to go away (another deoxit fix?); 2) If V814A or V814B stopped working after heating up to a certain point, maybe that would explain the fact that the trace disappears after a while. To test that, you could temporarily swap them out with 12AU7s from elsewhere in the instrument to see if the problem goes away. (V800 is also important but it's the only 6AU5 in the instrument so that debugging technique is not available.) I don't have any problems with the trace blooming and disappearing but I often see vertical instability that seems to be related to thermal changes. It will be working fine and then the trace will wobble vertically or even shoot off the screen. Putting the scope back in its case seems to prevent this; I assume it works best if the temperature is warmer and the airflow is more consistent. So it is conceivable to me that if you got the regular fan working and closed up the instrument, maybe it would work more reliably.

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