apollo psa test point adaptor

Curious Marc curiousmarc3 at gmail.com
Wed May 22 15:27:27 CDT 2019

It’s hard to find documentation for the ground test equipment apparently. The flyable hardware is very well documented, and Mike Stewart and co. are in the process of scanning it all at NARA. I can glean a few things from the markings on your picture. The PSA, or Power and Servo Assembly, had all the analog electronics for the Inertial Measurement Units and the star alignment optics, namely the scanning telescope and the sextant and the 3 pendulum accelerometers or PIPAs. It was a very large beast having maybe 100 modules, all the DC, 800Hz and 3200 Hz power supplies for the gyros and resolvers, the servo amplifiers for the 3 axes of the Inertial Measurement Unit, and the various temperature controllers. The test points give you access to internal key test points in the PSA to such things as the inertial measurement unit 3 gimbal error signals, the Scanning Telescope and Sextant optics trunnion and shaft error, corresponding servo test inputs (so you could rotate the gimbals or the optics), all the temperature monitors (there are many). 


There are two types of PSAs, one for the CM and one from the LM. Yours mentions SCT (Scanning Optical Telescope) and SXT (Sextant), and only the Command Module had a sextant, so we can pretty much tell this is a command module PSA test unit. There are also two very different revisions of the PSAs, Block I and Block II. Looking at the Block II PSA description (find it in the ND-1021043 manual), it appeared to have two test connectors, J1 and J2, 61 pins each. But your tester breaks out 195 test points. The Block I seems to have more test pins, but I haven’t yet found the exact description. So my guess is that it is a Block I tester. 


Reading from the top: TB1 to TB5 – my guess is that this is a breakout of 5 test connectors at the back of the PSA. A guess only, I don’t have the doc to confirm it. You put a voltmeter or a signal in these pins to measure your test point of put a signal into it. Pulse Probe, Direct Probe, Buffered Probe: I don’t know. The lowest left corner seems to deal with testing the servos, 3 at a time. If you are in the ISS (inertial sensing system) position, you’d probably move the 3 gimbal servos of the IMU. If you have it in the OPT position, you’d move the 3 axes of the optics which you can see on the other buttons, the SCT trunnion and the SXT trunnion and shaft. Apparently each of them has a slow or fast setting (that’s how I read 1:1 and 1:10 markings). The large commutator in the middle marked IRIG S.F. may be referring to the Integrating Inertial Reference Integrating Gyro Scale Factor monitoring. These are resistors networks that contained the calibration of the individual gyros. Problem is, there were 3 gyros and the knob has 5 positions, so that does not make a whole lot of sense to me.  


If it’s indeed a Block I tester, then I have a one of the Block I PSA trays it connects to. I was planning to used with my IRIG gyro, so the tester could potentially be useful to the project, if you were to lend it to us before you modify it. But probably not essential, as we can always break out the connectors ourselves (although it would be way less cool). And playing with the PSA is probably not going to happen for a while. It would be interesting to open it up and see if we could glean more insight from looking at the innards, and thoroughly document it before you modify it, so we can at least reproduce it.





From: cctalk <cctalk-bounces at classiccmp.org> on behalf of "cctalk at classiccmp.org" <cctalk at classiccmp.org>
Reply-To: Jon Elson <elson at pico-systems.com>, "cctalk at classiccmp.org" <cctalk at classiccmp.org>
Date: Sunday, May 19, 2019 at 7:46 AM
To: Adrian Stoness <tdk.knight at gmail.com>, <General at ezwind.net>, "cctalk at classiccmp.org" <cctalk at classiccmp.org>
Subject: Re: apollo psa test point adaptor


On 05/18/2019 10:08 PM, Adrian Stoness via cctalk wrote:

anyone know where i could find manual or drawings on this im up in northern

manitoba canada picked it up at a rr auction to experiment with as a audio

interface not sure if the jacks on the side are the weird pins nasa had or

another standard i can find?




There's a switch labeled "IRIG" which stands for Inter Range 

Instrumentation Group, and refers to a standard for 

telemetry encoding.  There is a standard for time code, a 

standard for modulating analog signas onto a bunch of FM 

carriers, and a standard for multiplexing several analog 

signals onto one FM carrier.


Apollo documents are probably VERY hard to come by these days.




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