Re: TM500 Plug-in Extention

Colin Herbert

I have got around to operating my PS 503A on a known-good TM504 Mainframe and it works fine. I also checked it on a known-good TM503 mainframe using my Dan Meeks extender; again fine. I was looking at the manual for the PS 503A and it makes your head spin. The 5 VDC supply is referred to ground, which is the white banana-socket next to the red 5 VDC socket. The two 0-20 VDC supplies are not referenced to ground, but to a common return, which is the black banana-socket on the front. These two supplies are floating from ground, but are related so that the can be added together using the green and red banana-sockets on the front panel and ignoring the black "common" socket. Checking for a voltage from the green or the red 0-20 VDC sockets to ground will show no voltage unless one of these sockets is deliberately connected to chassis ground. When I was looking at the PS 503A manual last night, I think I noticed that there were several chassis grounds - I suppose this is to ensure that the supplies are floating from true chassis ground, but it did make my head spin.

I think that you may be considering the Power-Supply TM500 plug-ins to be "normal" when this isn't really the case because of the floating supplies.

If the "Option" added to the mainframe was causing your problems, then it is a good idea to remove any wire interconnections. My TM 504 has option 1, which adds a 50-pin connector and 4 BNC sockets to the rear. None of these are wired to anything else, but originally, wiring kits were available to make connections. This option was designed for the ease of "building a system", as it is mentioned in the manual, with interconnecting modules and external items. If you have no need of such interconnections, then remove them. Some may be soldered on, but as originally imagined, the connections would have been square-pin push-on connectors so that soldering and un-soldering and any subsequent possible damage was minimised. Perhaps you could tell us what the "Option" included and what, if any, connections had been made.

-----Original Message-----
From: [] On Behalf Of Stephen
Sent: 13 August 2020 08:17
Subject: Re: [TekScopes] TM500 Plug-in Extention

On Wed, Aug 12, 2020 at 02:32 PM, teamlarryohio wrote:

...and the Common for the variable supplies is tied to 21A, 21B, 34A, and 34B
if the schematic is accurate.
You probably mean 24A/A

Hint: The 5V ground is tied to the chassis of the PS503A. See if that is useful.
Yes, this is exactly what I have.

However, on my extender, none of these are connected. Only every pin, but 6B, from 1 to 13 are.
And, according to another post, that seems to make this unit behave “normally”...
That doesn’t make any sense...
None of this makes any sense. How can things work as intended when on the extender, but not when directly inside the mainframe? This is mind boggling....

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