Re: TM500 Plug-in Extention
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 502, two of the 25 VAC transformer windings are connected in series to a transformer in the plug-in which supplies +9 VDC unregulated and +12 VDC and -12 VDC regulated. If the mainframe transformer windings are out-of-phase, the plugin's transformer will receive nothing, because the windings cancel out. Although the method in the FG 504 manual is supposed to be easy, that nearly always isn't quite the case. I have found what I think is a pretty fool-proof method.toggle quoted messageShow quoted text
My method for checking the phasing of these windings is to gain access to each end of the two 25 VAC windings and each end of the 17.5 VAC winding. These will be 1A & 1B (one 25 VAC winding), 13A & 13B (the other 25 VAC winding) and 5A & 5B (the 17.5 VAC winding). Connect all of the "B" connectors together and take them to earth on a two (or more) channel scope. Connect 5A to one channel and display the sine-wave at an easily-viewable amplitude, TRIGGERING THE TIME-BASE OFF THAT CHANNEL ONLY. Then with the other channels, check connections 1A and 13A, leaving the triggering from the 17.5 VAC. Any out-of-phase-ness will become apparent. It is good to use the 17.5 VAC winding as the benchmark, as that's what Tektronix seem to suggest in their method of checking the phase. Also, the connections 1A, 1B, 13A and 13B on the backplane are easier to access, once the outer casing is removed. You will find that 5A and 5B are more buried and harder to access. If you find that either or both of the 25 VAC windings are out-of-phase with the 17.5 VAC winding, then reverse the connections to the backplane. Check all of the plug-in bays; you just never know what might (or might not) have been going through the head of the person wiring-up your mainframe.
For information, I used my Dan Meeks extender with a JAMMA male connection to access the various backplane connectors. I modified the JAMMA male connector by disconnecting the "twinned" contacts with a Dremel-type tool and soldered short lengths of tinned copper wire to the connectors that I wanted to access. This enabled me to not only do this phase-checking (all of the bays on one mainframe had a 25 VAC winding wired out-of-phase), but also to check the series-pass transistors. I did this latter with one of the cheap Chinese component checkers and it was easy. Incidentally, I wouldn't do any "tinning" of the contact surfaces. I think they may be gold-plated and already therefore have a two-metal contact - copper and gold. Adding a third metal (solder - actually several metals alloyed together) is likely to achieve the possibility of two extra problems - thermocouple effects and generating a contact that will oxidise readily and eventually cause resistance problems. You might find that you can put a little more "spring" back into the backplane contacts with an appropriate tool. I have a set of metal probes, but what you want is some kind of short hook that you can get behind the backplane connector to apply a little pressure to bring the connector back to a better position. Don't overdo it to the extent that you make contact between the contact and its A-or-B relative. This may not matter when a plug-in is in place, but it could cause a disastrous short with no plug-in present. Also, avoid damaging the connector, as replacement looks to be almost an impossible job - at least fiddly and time-consuming. I have a backplane connector which has been damaged and thought to solve the problem by either trying to replace that one connector or the entire backplane connector. Looking at the problem and trying to de-solder the connector was a pain and since the connector was an otherwise unused 22B, I decided to "let sleeping dogs lie".
You should now find that any plug-ins that use any of the transformer AC windings in series will now work correctly and that any excessive ripple which might give FG 504 (and perhaps similar) plug-ins problems will be solved. Easy when you know, or get told, how.
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Chuck Harris
Sent: 10 August 2020 23:28
Subject: Re: [TekScopes] TM500 Plug-in Extention
There is some discussion in the FG504 manual that suggests
that many TM50X frames were built with one of the power
transformer windings out of proper phase for the FG504 to
I would guess that the windings, being AC were just a pair
of unmarked wires, and the assemblers didn't know that they
needed to pay attention to which wires went to which pad.
It wouldn't be a problem most of the time, but in some of
the many possible configurations it could result in issues.
Harvey White wrote: