Re: TM500 Plug-in Extention

Colin Herbert

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.

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.


-----Original Message-----
From: [] 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
work well.

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.

Perhaps now?

-Chuck Harris

Harvey White wrote:
interleaved responses

On 8/10/2020 4:13 PM, Stephen wrote:
I figured that the plugins that were not working before but are now were the ones
needing the NPN transistor.
The PS503 probably needs the PNP only.
Actually, looking at the TM503A manual gives you an idea of the reason. The outputs
of the supplies go through the frame transistors. However the drivers for those
transistors also provide a path for the regulated output. It's a very "weak" current
limited output, which would show up with a good meter, but the supply would not
provide anything near full current. The 5 volts uses a 7805 regulator, hence the 1
amp current limit.

Now that all my plugins are powering on, I have to figure out 2 things. Not that
the 1st is a real issue, but I’m just trying to understand the concept.

1- While the unit is inside the 506, and when my DMM ground lead is connected to
ANY of the ground banana plugs can I measure the +/- 20VDC as well as the 5VDC.
But when the plugin is connected To the extender, I have to switch the ground lead
to the appropriate one on the PS to be able to read the voltages
The DC503A 5 volt ground is common to all modules. The common lead for the +/-
supply is floating. I'd measure from the frame to frame of the power supply to see
what's going on there. However, a possible suggestion is to look on the backplane to
see if there any added wires.

2- I still don’t get why the DC503 shows only 1 digit or nothing at all when
plugged in directly into the 506, but is showing every single one, albeit some are
missing a segment, when used with the extender... A mystery to me. But I’ll
follow up on this in the appropriate thread I started on troubleshooting it.
I'd suggest backplane wiring, as in there shouldn't be any. If someone used this
with backplane wiring, then the slots become dedicated to a particular module. You
didn't connect the additional wires to bring out the plugin signals, where in the
frame, they're connected.


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