H724 Power Harness

[vrs42]

I thought I might make this it's own thread, so it is a little easier to find the information.

I've recently built a replacement power harness for the H724 in my PDP-8/E, and I plan to describe the process, hopefully in a series of posts with enough information to allow others to succeed, without too much trouble.

I found the old post which has the part numbers for the pins and housings for the power supply end. Here it is again:

TE Connectivity Commercial MATE-N-LOK

Power supply side:
Shell 1-171196-0
Pins 60619-1

Cable harness side:
Shell 1-163035-0 or 1-480276-0
Pins 60620-1

I used these, with the a Twippo SN-48B crimper which I acquired on eBay, and a selection of 14 gauge automotive wire I bought on Amazon. The wire insulation isn't the original teflon, which I think was probably overkill, and is considered problematic these days. It also isn't the original assortment of wire gauges; essentially I have up-sized everything to the 14 gauge that the red wire requires.

It should be noted that 14 gauge is at the limit for the pins and the crimper, so you'll want to be sure your crimps are turning out well. I found that the Twippo crimper made excellent crimps. Positioning the pin and wire perfectly while engaging the ratchet was the hardest part.

I did find that I had some pins lying around which were compatible with the housing and the socket, but when I tried to use them up I discovered they were too small to accommodate #14 wire.

Next up: Wire colors, placement and lengths

Vince

 

[vrs42]

Here's a table of wire color, length, and maximum gauge:

black	16"	#14
black	16"	#18
red	23"	#14
grey/violet?	18.5"	#18
orange	14"	#18
blue	21"	#14

Oddly, the documentation refers to "violet" and "blue", and I have "grey" and "blue". I'm not sure what's up there.

What I actually did (as mentioned earlier) was to copy the length and color (as I perceived it) from my existing cable, and up-size everything to #14.

For the other end, I used ACE 345-22 #16-#14 insulated blue female terminals and an ACE GD-388 crimping pliers. The GD-388 doesn't ratchet, but it is very sturdy and allows the application of plenty of crimping force.

Placement is fun, but a little scary. The issue is that you get just one shot at each wire, and there's basically no getting it back out. So screwing up the placement of a wire means you get to start over.

If you look into the shroud face that mates with the power supply, you'll see it is basically a rectangular recess. One of the long sides, though, has two keying posts. Hold the shroud withe the recess facing away from you, and it's little keying posts down, so that you are looking at the six rectangular openings for the pins.
Using the chart above, start with the black wire in the upper left, and proceed to fill the holes in the clockwise direction around the perimeter. Each pin should insert until it clicks into place.

Now, there are zip ties on the original at 5", 10", and 12", from the far edge of the housing, which include all six wires. (I used 4" zip ties from ACE hardware.) Looking at the wire side of the shroud with the black and red wires at the right, form a bend so that the wire bundle is heading down toward the back-plane before tightening the first zip tie. The bundle continues down through the ties at 10" and 12", as well.

Leave the orange wire out, and install a zip tie at 13.5", but form another bend so that the bundle heads away from the shroud a little above the back-plane.
Leave the two black wires out and install a zip tie at 16".
Leave the grey wire out and install a zip tie at 18.5".

Each wire will make another downward turn and fit over a spade lug. If we have done our job right, the lugs will be near the natural position for the wire. Once the spade lugs are attached, the M935 bus jumpers can be installed to cover the lugs and wires. Finally, the shroud end can be mated with the power supply. That connector isn't rated for a lot of insertions and removals, so you might want to check everything and just mate it once. It has also been suggested that a DeOxit protectant spray might reduce air contact and help preserve the contact surface.

It seems likely I've managed to leave something out or made some factual mistake. Let everyone here know how it goes!

Vince

 

[Henk]

I do not have a crimper.
It is a bit more work, but would it be OK to solder the wire to the pin?
Anybody with experience soldering the wire to the pin?

 

[daver2]

I will tell you shortly...

I have a power cable to make up for a little trainer board I have using a 4*1 MOLEX power connector. I need to solder the cables - not having a crimper myself. However, I have an electrician friend who should have some crimpers, so I may ask him first.

You will still need to make a good mechanical connection between the pin and the copper cables though.

My trainer only consumes an Amp or so of current at +5V.

The 8/e consumes way more current (up to 20A at +5V) - so would a solder connection be good enough for that I wonder - or will it introduce a high-resistance joint that would drop a voltage across it and lead to a temperature increase?

Dave

 

[vrs42]

I do not have a crimper.
It is a bit more work, but would it be OK to solder the wire to the pin?
Anybody with experience soldering the wire to the pin?

My understanding is that soldered joints are universally avoided in situations which require high reliability, particularly if vibration is present.

As I understand the issue, the point where the hardened solder meets the wire becomes a fulcrum, absorbing the vibrational energy and fatiguing the metal. I have indeed often noticed that when a joint fails, it is always right behind the solder. Also, as I understand it, this is why the insulation bit of the crimp exists -- to form an additional strain relief.

That's why I included part numbers and sources for the crimpers.

Also, in a sort of superstitious way (since I don't fully understand harsh environments) why I chose to use automotive wire; if it's good enough for a car, hopefully it will be adequate here. It seemed likely that it was a good combination of quality with low price (mostly because it's currently produced and widely available).

 

[daver2]

Yes, solder is a very poor second to a sound mechanical connection.

This is why wire-wrap (cold welding) is a preferred method of termination on our plant signals.

However, a poorly made crimp is worse than a soldered connection - and I have found a few of those in my life!

Dave

 

[DougIngraham]

I do not have a crimper.
It is a bit more work, but would it be OK to solder the wire to the pin?
Anybody with experience soldering the wire to the pin?

There are a number of issues with soldering in this situation. The pins were not designed to be soldered. They are rated at approximately 15 amps. They are also rated for a number of insertion cycles which is extremely low in the 15 to 20 cycles range. The heat of soldering will affect the temper of the materials. It will affect the surface finish and conductivity due to oxidation and deposition of flux on the surface. The temper of the materials will change making it either more or less ductile, I am not certain which way it will go here. Solder will wick up the wire inside the insulation. At the point where the solder ends there will be a stress riser which in an environment where there is vibration or motion will eventually break. This is a minor concern in this situation since the wires don't move once installed. The crimp on these pins has two functions. One is electrical, and one is strain relief.

Will soldering work? Yes, you will be able to connect the wire to the pin. If you do a good job of it the pin will still fit in the housing. If you accidently get any solder on the mating surface it is not practical to remove it and you get to start over. If you clean off the flux from the contact surface you will probably be able to get a reasonable electrical connection. The temper of the material is less important on the male pin. The female half is the springy side and will be harder to clean and will be degraded more if soldered. The net result is that you will most likely increase the contact resistance lowering the rated current. You will also reduce the number of insertion cycles. The failure mode will be excessive heating and voltage drop in the contact. There are photos of burned/melted housings caused by this.

The crimping tool Vince used is not expensive ($28) and it does a good job. I am almost certain you could order something like it for less off of Ali. Or ask around. Do you know anyone who has built a 3D printer or someone into Ham radio or any of the sub categories of Radio Control modeling. They may already have a set of crimpers you could borrow. If you have a makerspace, it is possible they would have a tool you could borrow.

I don't recommend soldering in this case.

 

[daver2]

>>> Oddly, the documentation refers to "violet" and "blue", and I have "grey" and "blue". I'm not sure what's up there.

The documentation I have refers to grey for the POWER OK signal on OMNIBUS pin BV2 (not violet). Perhaps someone was colour blind? I am (green/blue) so was the person that painted our first house when we were married!

I have just realised that it is simple spade connectors that are on the OMNIBUS backplanes. I am going to purchase some (industrial) switch-mode supplies initially rather than use the H724A that came with the chassis. That solves my issue regarding the MATE-N-LOCK connectors. But I will still order the parts and get them made up, because I would still like to get the rather heavy original PSU running.

Is your J5 wired up? This carries the 14V AC signal for the 50/60 Hz clock. I am just hunting out the specifications for the PSU items I will need to purchase. Just hunting around the schematics etc. but I can't find where J5 is wired to (if at all).

Dave

 

[vrs42]

Is your J5 wired up? This carries the 14V AC signal for the 50/60 Hz clock.

IIRC, I have a couple of the 60 Hz clock cards. They have a cable attached to the board that runs over to the power supply (presumably J5). Haven't used those in a while.

 

[daver2]

If it requires an option card that I haven't got, I shall ignore it.

Thanks.

EDIT: Found it, the DK8-EA.

Dave