I was wanting a vacuum column tape drive to go with my DECtape and managed to acquire one though non operating hence this repair page. Now all I need is a DEC TU60 DECcassette drive and I will have the complete set of PDP-8 tape drives. I traded a PDP-8 floppy system for the TU10 in 2007 and had been slowly working on it until I decided to bring it to VCF East 2009. That made me work hard at getting it ready. I have mostly succeeded (assuming it is still working at VCF). When I got the drive it had not been used in about 25 years and had some obvious problems but was complete and in reasonable shape for the age. One issue was that it was used on a PDP-10 which only uses slave drives. The PDP-8 controller requires a master drive. While pondering making the required cards to convert it a complete TU10 backplane appeared on eBay and I got it. That saved a lot of work.
Next obvious problem was the rubber on the capstan had deteriorated. Since
the capstan is critical for drive operation I decided not to try any repair
myself and had it recovered by
Terry's Rubber Rollers. The repaired part
seems to work fine. I did find after a while it seemed to slip during
rewind making it unstable until the drive faulted. Cleaning didn't
help but rubber renew made it happy again. The black rubber probably
isn't the ideal material. Here is the bad and repaired capstan.
Next I checked the vacuum switches. They were all bad. Digikey had 4 in
stock so I ordered them with some other stuff I needed. Digikey no longer
carries them but another distributor does. The other 4 I repaired by replacing
the diaphragm. Here is the bad switch and making a new diaphragm.
Next the blower sounded noisy when I turned it by hand. I replaced the
bearings in it and removed bits of tape stuck in it. Here is the blower
shaft and blower opened with some of the tape found in the blower.
Next the lower reel motor didn't turn freely by hand. I was hoping it was the
brake but it was the motor bearing. One of the bearings had locked up so the
whole bearing assembly was turning in the motor. When I opened the motor
it was full of brush and commutator dust where they had worn down. That
motor had enough brush left I just cleaned it and replaced the bearings.
The second motor the brushes had worn down to the shunt wire so I had to
smooth the commutator and put new brushes in along with new bearings.
The manual says the brushes will be worn out after 10,000 hours. In 10,000
hours of continuous tape motion the drive would have processed 25,000 miles of
tape.
Commutator with brush path worn in and smoothed with insulation cut down.
Bad brushes and my setup to turn the commutator. I used hand files and commutator stones.
The motor filters also looked dirty so I replaced them. The exact size
filter isn't available but a similar size one was available that I could take
the element out of and put in the old housingthat .
Last some tape guides also didn't turn smoothly so they got new bearings
also.
Now with the drive back together I could try operating it. No go, the
tape just piled up in the vacuum column. I tracked it down to a bad motor
driver card. With that replaced I was able to get the tape to load and
move forward and backward under manual control.
Next I installed the 4 card TM8E controller in my PDP-8/E and made a set of
interface cables. The cables are standard 40 pin ribbon cables with M954 cards
on the end to plug into the backplane. Since I didn't have any M954
I made some equivalent cards using Douglas Electronics prototype cards
they still sell.
After installing them it sort of worked. I was able sometimes to read and write OK, other times it acted strange. After it failed the controller only test after passing it I pulled the controller boards and cleaned the edge fingers better and removed some flux residue from repairs and it seems to be working better. The drive itself occasionally also acts up. The latest was it wouldn't move forward from beginning of tape. I tracked that down to a problem in a delay circuit but it went away before I could figure out the root cause. Intermittent problems are really annoying to track down.
The drive will need some more work to make it reliable and more preventive maintenance such as gettin the bearings that currently seem OK greased before they fail and performing more of the alignment procedure. I don't have a skew tape so can't perform the full procedure unless I can acquire one.
This writeup only covers the major items. It also required general cleaning, and various minor repairs like replacing missing screws etc.
Feel free to contact me, David Gesswein djg@pdp8online.com with any questions, comments on the web site, or if you have related equipment, documentation, software etc. you are willing to part with. I am interested in anything PDP-8 related, computers, peripherals used with them, DEC or third party, or documentation.
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