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Reviving the PDP-12 at the RICM

We received more diagnostic documentation from the donor last week so we were able to run the processor LINC instruction tests. The new docs are already scanned and on Bitsavers. The LINC diags failed, but we quickly found the bad flip-chip and now both LINC diags run OK.

More debugging on the "LGP GP=GPC PRESET" problem in the TC12 LINCtape controller in the PDP-12. After LOTS of signal chasing we eventually found that the timing track signals in the right TU56 tape head are open. I had the same problem with the TU56 in my PDP-8/e and the tape head is probably impossible to fix.
 
That's sad. I remember that you tried (unsuccessfully?) to fix the one on your 8/e.

I wonder why the windings would be open? It's not as if they have to carry huge currents normally. (Especially the TT which is very seldom written to). I have been luckier so far - lots of faults but none of them irreparable!
 
DEC has a similar problem with core memory. When they assembled the core stack they sometimes broke the wire. The normal procedure was to solder another wire onto the end of the break and keep going. We have several core stacks where the solder joints have opened. If you look between the core stacks you can usually see the solder joints sticking up from the core plane. They are very difficult, but not impossible to repair.

It is possible that the TU56 tape heads have the same manufacturing issue with poor solder joints. Add potting epoxy to the mix and it might make a poor solder joint come apart.
 
We borrowed a TU56 tape head from the TU56 in the warehouse and replaced the broken right head. We reran ran MAINDEC-12-D3AE-PB PDP-12 TAPE CONTROL TEST, PART 1 OF 2. The diag runs OK, so at least the timing track in the borrowed tape head is OK.

We reran MAINDEC-12-D3FB-D Tape Data Test. This diag searches for the block number, but rocks the tape back and forth looking for the block. If you put the tape a long way from the beginning of the tape the diag will move the tape in the correct direction. We connected Warren's logic analyzer to the data signals that from the tape heads and the register outputs that have the current block number.

It looks like there are glitches on Data Track 1. We swapped the G882 modules in F07 and F09 for tracks D1 and D3. D1 looks much better. Swapped the G882 modules back to the original slots and both D1 and D3 look OK. Maybe just a bad connection between the G882 module's gold fingers and the backplane's tin connector contacts. Now it looks like the current block number in the tape controller logic is reasonable, and it looks like it can find blocks on the tape.

The Tape Data Testdiag still does not work, so next week we debug the write & read logic in the LINCtape controller.
 
Yesterday we watched what the TC12 was doing when it searched for blocks on the LINCtape with Warren's logic analyzer. Sometimes the D1 track had glitches so the block numbers were wrong. Swapping the G882 card for that track didn't make a difference. Using the right drive, the one with the borrowed tape head, worked better. When the D3FB Tape Data Test diag looked for block 374 to do the write/read test it got lost. We tried a different LINCtape and it worked OK until the diag looked at all of the block numbers and could not find block 5.

Now we think that the two tapes that we are using for the diags both need to be reformatted. Dave Gesswein was able to extract the MARK12 file from a LAPS/DIAL LINCtape image, convert it to a paper tape image and send it to us. We will use it to reformat the tapes that we have been using for diags. Hopefully that will show that the TC12 is working OK, and we can start debugging other parts of the system.
 
We were able to remove the implosion shield from the VR14 CRT with very little work. The decomposed PVA was only stuck to the glass in a few places so it was easy to remove.

Some restorers just use double-sided tape to hold the implosion shield in place. Since the PDP-12 is in a public space we planned to sandwich a sheet of LEXAN between the CRT and the implosion shield to improve safety.

We put the sheet of 0.093" LEXAN on top of the back side of the implosion shield and put it in an oven. We slowly increased the temperature and when it hit 325F the LEXAN sagged and conformed to the implosion shield. We trimmed the excess LEXAN with a radial arm saw, router, and belt sander.

During reassembly we found that if the LEXAN sheet touched the face of the CRT you could see diffraction patterns that almost looked like oil or water between the two parts. We made a spacer out of two layers of 1/4" wide double-sided foam tape to hold the LEXAN away from the CRT. Then we glued the CRT, LEXAN, and implosion shield together with silicone, and after it cured we glued the metal bezel on with silicone.

The leaking decomposed PVA from the CRT ate the paint from the steel shield for the CRT. We repainted it with satin finish black and it looks like new.

Today we will reassemble the VR14 and start debugging and testing.
 
Nice work!

Do you have pictures of the LEXAN with tape applied? Did you "float" it between the implosion shield and CRT with the two layers of foam tape or does it touch the shield? I also wonder about the longevity of the foam tape - foam and adhesives seem to be less than reliable when dealing with "permanent" repairs. If nothing else, replacement will certainly be easier in the future.

Jack
 
We used the double-sided tape to space the Lexan away from the CRT. The Lexan is molded to fit the implosion shield, so it sits against it. We coated the outside of the CRT/Lexan/shield with silicone to hold it together. We put a layer of silicone on the inside front of the metal bezel to glue it to the shield. We also put a lot of silicone along the sides of the bezel where it fits to the CRT. The metal bezel will hold everything in place for a long time.

We found a shorted diode block for the +/-40VDC power supply. That was why the main fuse was blowing. We received a spare with the PDP-12 so it was a quick repair. We should be able to reinstall it in the PDP-12 tomorrow and see if it works.
 
The odds of the tube giving out are extremely unlikely. Even so, the shield is pressing up against the display bezel so it's not going to be flying out in the extremely unlikely event it does go. The addition of lexan and silicon seems like a lot of extra work. Should the face ever need to be worked on again I feel sorry for whoever has to do it. :/
 
Most restorers just use a few bits of double-sided foam tape to hold the implosion shield to the CRT. If the PDP-12 was not in a public space I would probably just skip the Lexan.

Disassembly of this tube will be LOTS easier because there is no PVA to deal with. Other restorers have damaged tubes by heating them to remove the PVA. We were really luck that the PVA was very degraded and came off easily.
 
The VR14 display works! We found a bad trimpot for the vertical gain flip-chip and replaced it with a fixed resistor for now. The display when running the diags looks very nice and crisp.

We booted LAP6-DIAL and could display a listing of the files on the tape on the monitor. After about 20 minutes of running nicely, the TC12 went back into the mode where it could not find blocks. Oh well, more debugging to do.
 
We need to get a copy of Spacewar on LINCtape so we can show moving images. Everything from the diagnostics and the LAP6-DIAL console is static.
 
We did a lot more debugging on the TC12 LINCtape controller. At this point we don't think that we are working with bad tapes, but the problem might be in either the TU56 tape drive, or the TC12 LINCtape controller. We see bad behavior in both devices so we will do as Charles Lasner suggested and swap a TU55 and TU56 between the PDP-12 and the PDP-8/I. This will let us test the TU56 with a known good TC01 LINCtape controller, and test a known good TU55 with a questionable TC12 LINCtape controller.

We ran the A-to-D converter test and were rewarded with a display on the VR14 that showed correct operation of the knobs and A-to-D converters.
 
The design of LINCtapes/DECtapes make them extremely rugged and reliable. They are not prone to oxide shedding like other tapes because both sides are laminated.
 
NeXT,

The myth that DECtapes and LINCtapes are laminated on both sides of the oxide has been reposted for years. Two weeks ago we cut a few feet off a LINCtape to removed a damaged part. We wiped the cut off piece of tape with alcohol and the oxide came right off. The tape was a single layer of Mylar with oxide on the inside surface, just like any other magnetic tape.
 
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That goes against what's been observed with my tapes, plus the tapes at the LCM. Perhaps you have some sort of early variant of the tape design?
 
NeXT said:
That goes against what's been observed with my tapes, plus the tapes at the LCM. Perhaps you have some sort of early variant of the tape design?
Click to expand...

Anything is possible with LINC and DEC tapes. There were a lot of manufacturers that made the tapes.
 
m_thompson said:
NeXT,

The myth that DECtapes and LINCtapes are laminated on both sides of the oxide has been reposted for years. Two weeks ago we cut a few feet off a LINCtape to removed a damaged part. We wiped the cut off piece of tape with alcohol and the oxide came right off. The tape was a single layer of Mylar with oxide on the inside surface, just like any other magnetic tape.
Click to expand...

Just like it says on the spec sheet at bitsavers.
http://bitsavers.org/pdf/dec/dectape/3M_DECtape_Spec_Nov66.pdf

and AFAIK, 3M was the only manufacturer of the tape.

I also mentioned to someone else that the head stack may have been used in instrumentation recorders. Considering the origins of the design through Lincoln Labs
and the target for a low cost system it seems really unlikely they would have picked a redundant head stack for the LINC if it wasn't already available.
 
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