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DEC H724 power supply fan removal

T

thunter0512

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I am trying to remove the fan & heat sink assembly from the H724 power supply in my new PDP-8/e.

The entire assembly becomes loose when I remove the 8 screws holding the two fan grills.

It appears that it should be possible to swing the assembly out towards the right but there is a 10 pin Molex? connector at the top.
The 10 pin Molex? connector won't come off despite pressing the two locking "hooks" on either side of the connector while pulling the connector upwards.
What is the best way to loosen the connector? Contact spray?

Also on the right hand side of the assembly there are 4 diodes (rectifiers) in DO-4 and/or DO-4 package with rather thick cables soldered on which disappear somewhere under the transformer.
The cables soldered to the 4 diodes (rectifiers) are thick and stiff and I am not sure if I can rotate the entire fan/heat-sink assembly with these cables attached.

What is the correct way to remove these power supply fans?

Doug you wrote earlier this year that you have replaced fans on your and Vince's H724.

Thanks for any advice.

Tom Hunter
 
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If this is the first time the fan assembly has been removed the orange wire from the fans will dissapear into the caps on the left. There are a few loops of orange wire on the side of the caps next to the circuit breaker and outlet cable tied out of the way. If you cut the cable tie it makes everything easier giving plenty of slack on the orange wire. If this is not the first time I can't predict what was done before.

You need to get that connecter loose. The two I have done this was not a problem so I can't offer any useful suggestions.

The thick wire/cable bundle on the right going into the transformer is not really an issue if you get everything else loose.

Vince's had never been taken apart so I know what it is supposed to look like. The way it is all bolted together is:
  1. Screw through lock washer.
  2. Screw through fan shroud.
  3. Screw through side of case.
  4. Screw through the fan mounting lug.
  5. Threaded standoff.
On the heat sink side:
  1. Screw through lock washer.
  2. Screw through heat sink sheet metal.
  3. Screw through fan mounting lug.
  4. Screw through washer. This washer is a spacer to hold the threaded standoff in place between the fan mounting holes.
  5. Screw through threaded standoff.
That is the way 6 of the 8 screws are mounted. For the center two on the bottom, instead of threaded standoff's with a washer DEC made a metal block the correct size to fit between the fan lugs so no washer was used. This block ties the fans together into a single unit making assembly easier.

The bad news is unless you use new in box stock from the 70's that match the original fans in the power supply the inside spacing on the fan mounting lugs is not the same so the threaded standoffs and washer trick DEC used won't work. I was able to source longer standoffs from DigiKey. I also had to change the length of the screws going into the machined block as I was not about to spend the time to make a new block to fit the modern fans. The two from the outside needed to be longer and the two on the inside needed to be shorter.

You bolt it all together from the heatsink side and then wiggle the whole assembly into place finally adding the screws from the outside of the case.

Mine was missing several parts, standoffs, spacer washers, lock washers from the previous time it has been molested.

If I was going to do it again I would not use the reverse airflow fans that match what DEC used. This gives a larger selection of fans. But you do need to also replace the wire fan shrouds since the ones in the machine don't protect the spinning center since you still want the airflow to go the same direction. I would choose a lower airflow fan which will be significantly quieter. The original fans were way overkill on airflow. It might have been necessary on a hot day with the cover off and a board in every slot. On the other side, Vince's machine ran for years with only one working fan and the cover off. I was pretty surprised the power supply hadn't blown up.

The other thing I never considered was pulling the original fans apart and trying to repair them. Part of the reason for this is that new fans are somewhere between 15 and 20 dollars. (I did find fans for over $100 but couldn't figure out why they cost so much.) You can blow many hours of time trying to repair something that can just be replaced inexpensively.

I hope you get your connector loose. Good luck on your repair.
 
Thank you Doug.

So you left the thick wires from the 4 diodes in place and after unscrewing the 8 outer screws, unplugging the 10 pin connector and cutting the cable tie around the orange wire loop you could pull out the fan & heat-sink assembly far enough to get out the fans (after removing the inner 8 screws)?

If only I could get the 10 pin (5 x 2) Mate-N-Lok connector unplugged. I tried to gently lever with a flat blade screw driver while pressing in the two locking "hooks", but that just lifted the socket portion higher.

Thanks again.

Tom
 
thunter0512 said:
So you left the thick wires from the 4 diodes in place and after unscrewing the 8 outer screws, unplugging the 10 pin connector and cutting the cable tie around the orange wire loop you could pull out the fan & heat-sink assembly far enough to get out the fans (after removing the inner 8 screws)?
Click to expand...

Essentially yes. You only need to remove 6 of the screws from the heat sink side. The two easy to remove ones in the bottom center just tie the fans together with the metal block DEC made. You can leave those in place on the heat sink side. The fans will stay together until you are ready to replace one or both of them.

I thought about your difficult connector issue and wonder if you have considered a little heat. Not a lot, just warm it with a heat gun. This might cause the outer housing to expand and let you get it loose. Maybe use a hair dryer rather than a heat gun to prevent an accident if you have access to one.
 
I have successfully removed the fan & heat-sink assembly. I had to also loosen the transformer and the bracket which holds C300 & C301 to be able to free the 4 cables going to the D04/D05 diodes (CR200, CR201, CR300 andCR301).

To get access to the two inner fan screws in the center top, I had to remove 6 screws holding holding 3 of the inner heat-sink blades (two blades on the left and one on the right) and also push down the Mate-N-Lok socket below the frame holding the heat-sink blades and the fans.

The length (or shortness) of the orange and white wires supplying 115 VAC to the fans is also challenging even with the cable tie cut.

Easy fan replacement was clearly not a design objective for DEC engineers. :(

Thank you Doug for your help with this puzzle.

Tom
 
You mentioned the top center screws. Taking them out was not a problem. I was able to work them loose at an angle. Putting them back was more difficult. I taped them to the Phillips screwdriver with black electrical tape and was able to feed them through the wires. A single wrap around the screw head was enough to hold them in place.

You are welcome!
 
bobaboba said:
This was my brief write up from when I came to replace a faulty 724 fan. Irritatingly after all the bother the fan was actually OK - one of the (solid core) wires to it had corroded through!
Click to expand...
I had apparently looked at your page at some point as some of the photo links were highlighted. Thanks for the writeup!

Your supply was missing the custom made metal block connecting the fans together on the bottom center. It looks like the spire nuts were used instead of the metal block. Also, one fan was mounted upside down which is not like any I have seen before. I never considered rotating a fan so I wouldn't have to replace a wire between the fans. On the supplies I have seen both wires were the standard Teflon insulated high strand count wire. I wonder if DEC changed the fan mounting at some point or if yours had been previously molested. The other thing I thought was odd was the use of yellow heat shrink. I suppose that might have been a change you made when putting it back together. Or was that white heat shrink that had turned yellow with time?
 
DougIngraham said:
You mentioned the top center screws. Taking them out was not a problem. I was able to work them loose at an angle. Putting them back was more difficult. I taped them to the Phillips screwdriver with black electrical tape and was able to feed them through the wires. A single wrap around the screw head was enough to hold them in place.

You are welcome!
Click to expand...
The trick with the top center screws is to remove the 6 wood screws holding 3 of the center fins of the heat sink assembly. When viewing with the transformer at the right and the Mate-N-Lok at the top, you want to remove the 4 screws on the two center left fins and the 2 screws on the one center right fin. The fins drop down. Finally unlock the two hooks from the Mate-N-Lok socket just above the fins and push it down towards the lose fins and it nicely exposes the two top-center screws.

Trying to work at an angle means you will butcher the Philips head screw heads and won't be able to fasten it well. You are also straining the cables which are in the way.

I have now cleaned the fans which were coated in a thick residue of decayed gooey black foam and dust. After more cleaning and replacement of the two mains filter caps with Malcolm's filter PCB and fitting of a new power cable, and reforming all capacitors I have now reassembled the power supply.

Pleasingly on power up all supply rails are at roughly at there nominal value and stable.

I hope that the rest of the PDP-8/e comes together as nicely as the power supply. I had more trouble with the LAB-8/e just to get stable power.
 
Perhaps factory changes? I used it in the lab from about 1974-1980 and I don’t remember the psu ever being repaired/swapped but I expect there would be a flow of final test failures and reworks cycling through the repair facility.
 
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