On Fans

Parent Category: Stories Category: My PDP-11/44 Written by Administrator

Only a few minutes after powering my 11/44 on for the first time, I get worried about the noise of the fans. It was very ... eh ... characteristic.

Not only a normal fan air rush, but also screeching, howling ... very powerful but also somehow alarming. Despite the machine stood in the basement and there were three doors between her and the ‘44, my wive complained about the sound in the night.

The first one dies

After a few hours of operation the noise suddently got less annoying, but this wasn’t really a good message: after another few more hours I detected that the most important one of the fans had blocked. It cooled the power supplies big +5V regulator and the cpu-side of the card cage. From the temperature of the processor IC’s and the smell of the power supply I learned, that from now on I had to pay attention on proper cooling.

The fan unit of the 11/44 is very easy to remove. Loosen two tiny screws, disconnect one power connector, and you can pull it off the BA-11 box. Just as if they had to be replaced every few weeks ...

 

pdp1144_fanunit

Replacement strategies

The fans have standard 12 cm dimension, but the ratings get me puzzled: They want to be driven by a 35VAC and 150 Hz signal, which delivers 3.5 Ampere. Just try to generate this on todays standard lab equipment! A web recherche for supplier of those fans showed me, that they are not made anymore. The only thing I could get were complete fan units from other 11/44’s, at high prices and probably as worn out as my own ones ... I was not too excited about this offer.

As first aid, I moved the remaining rotating fans to places were I expected the most heat. The remaining noise was still too loud for long night time operation! If I stood beside the machine, it seemed quite normal, but a few doors away a faint high buzzing sound was still audible.

So there was this decision to made: replace the original fans by good ones or not? This would make the 11/44 less “original”, and the noise of old ‘70s fans is certainly something I wished to preserve. I found 3 options:

  1. replace only the dead fan by a single new one. Cheapest solution, keeps machine as original as possible. On the other hand: I did not even trusted the remaining two fans ... same type, same wear, also too loud ... why should they run much longer than the first one? And I definitively would not risk another dead fan!
  2. replace all 3 fans with new ones. Most impact on machine state, but technically the best solution.
  3. buy another original 11/44 fan unit. Machine state would have been preserved, but probably still noise and untrusty cooling.

Well, I decided for option 2. I finally found 48 VDC fans from SUNON, and converted the 35VAC 150Hz signal to 35 VDC over a rectifier.

 

pdp1144_fans_sunon02

Putting the additional components into the fan unit was not easy, but this story is over now ... and they rotated happily ever after!