Tekrtonix Type 503 -- Power Supply Issue


Dear Collective Wisdom,

I was given a Tektronix Type 503 oscilloscope recently. It wasn't showing a trace. I checked it out and realized that a bad diode coming off of the the second power supply transformer was the culprit. No power was getting to the heaters of the nuvistors in the vertical and horizontal amplifiers. Once the diode was replaced the scope began working again. Hurray! After running it for a while, I decided to recheck the voltages to see if it was all good, accident I nicked the main transformer with the lead from the multimeter. It sparked and the fuse blew.

When I replaced the fuse and started it up again the scope was kaput. The problem is in the power supply. The first transformer (T601) is putting out specified voltages. But I'm getting absolutely no voltage readings from the the second transformer (T620), which--if I understand it--is supposed to be oscillating. To be more precise, none of the various taps on the secondary of T620 are showing any voltages--all are at 0V.

The voltages are now off around the 6DQ6 tube (V620) that connects to the primary of T620. I tested the tube and it tested fine. I also stuck in a replacement, just to be sure, and no still no dice. It looks like I'm getting just under 500V across the primary winding of T620 and to the plate of the 6DQ6 tube. But I'm only getting 40V on Pin 4 of the tube, and 0V on pin 5.

I'm worried that the transformer got ruined when the thing sparked. Is that a possibility?

So here is where I need your collective wisdom. What's the best way to proceed? How can I check that the second transformer (T620) is still functional?

Here's what I've done so far:
1. I've checked resistances on the second transformer (T620) and all the resistance readings seem to check out with ones I can find on line. (Here's the link:
2. However, I am getting ~600K ohms between the primary of T620 and ground. It that normal or does it indicate that the transformer is ruined?

Thanks ahead of time,