LOL, we could! 😂

I understand Colin, You made me doubt my wiring. So I went and triple checked it. I can’t assure you that not a single wire is crossed. Surely enough not A1-B1 and A13-B13.... Each wire goes to

There exists a Tektronix plug-in for testing TM500/TM5000-series mainframes. It does all sort of things that could be useful, including checking the phase of the two 25 VAC windings with respect to

There is nothing to forgive.

Oh ok. I know what you mean then. I thought you were referring to something else more obscure (to me). I do have some mid level electronics training, although from 40 years ago. Forgive my lack of

No, you’re on the right track. Something about the extender connector vs. the plug-in connector is different.

In order to have a “circuit” there has to be a loop that includes all components. I the case of the DC-503, there is a signal that goes “to” the plug-in which carries current. That current

Colin, Although I didn’t fully understand all the theory behind your post, I do get the general idea. I’m not sure I feel confident enough to do that. But at any rate, “if” the windings were

I just right this very moment received a DC505A. And guess what, It displays the exact same behavior!!!! Only one digit randomly comes up when inside the TM506, but all are present when connected to

I don’t understand what the “return path” is. I have a very basic training in electronics.

I have a DM 502 that would work in all of my TM5xx mainframes except one. This puzzled me until I read the section of the FG 504 manual on transformer phasing and decided to investigate. In the DM

I’ve been fooliwing this thread but not really paying close attention. I assume that you’ve tried the DC-503 in all mainframe slots?

You probably also need to check the return path in the same manner.

I have -20 V grid bias and the bias pot has no effect either on the grid voltage or brightness of the trace so I will check components around it. Simon

I did not write that. Please be careful with your attributions.

I will certainly do that and report back. Thank for the suggestion, Dave.

I would make six measurements with an oscilloscope: - Measure the signal of pin 6A at the backplane where the connector is soldered to the motherboard with the extender disconnected - measure the

Some possibilties for the very bright trace: 1. One pole of the beam finder (S450B on schematic <12>) is stuck 2. The grid bias R1375 might need adjustment, possibly someone tried to compensate

Yes, I can see how that could be a problem. It’s not the case here, but I was thinking that maybe inserting the plugin repeatedly, may have, over time, worn out the copper pads to the point where

Some progress, I tested the PDA voltage with an ignition lead and spark plug. It produced a good spark that would certainly start my lawnmower, and I put the CRT back. The time base is working and I