• Please review our updated Terms and Rules here

Reviving the PDP-12 at the RICM

We found a loose faston connector on TB2-5 that connects the POWER AMP DR signal wire from the servo pre-amp to the servo power amp. Wiggling and reseating the faston seems to have fixed the servo problem. OS/8 CCL actually works now, so maybe that was a seek error that was not handled by OS/8. The serial-disk worked OK when we first powered the system on, but stopped working after several hours. Maybe the baud rate generator is not thermally stable? The OS/8 RESOURC program worked OK in the morning, but now hangs the processor with the "B" data-break light on, so something else is thermally unstable. I can punch a paper tape on the teletype and read it on the high-speed reader. I forgot to bring SSSD floppies to try. I used PIP to copy a text file to the "TV" VR14 device and it actually worked!
 
We only had a little PDP-12 time yesterday because of an ex-DEC employee visit. I booted it from the RK05 and made an RX01 configured for the PDP-12. I made the normal rookie mistakes like forgetting to copy DIRECT.SV and FOTP.SV onto the diskette. The RX01 boots OS/8 and it runs OK.

I tried the MUSIC.SV program, but could not pickup any music with an AM radio near the front of the machine. I will try it again next week and search for a signal around the back of the machine. The program works on my 8/e, but I don't know if it will work on other PDP-8 systems. The PDP-12 has a speaker connected to bit-0 of the AC, so maybe someone wrote a music program specifically for the PDP-12.

We are looking into ways to load LINC demo programs like SKYLARK and KALEID from the diskette. This would make demonstrating the capabilities of the system easy.

The servo on the RK05 died after working nicely for several hours. I wiggled the gray wire on TB2-5 and it works again. I will treat it with DEOXIT next week for a more permanent fix.

Almost time to start working on the TC12/TU56 again.
 
The RK05 on the PDP-12 worked OK all day. We ran BASIC and FORTRAN programs. FOCAL71 or FOCAL69 won't run from disk or paper tape. It ends up in the non-existent field 2 and runs 7777 instructions. FOCAL is usually a better test than the diagnostics, so we probably still have some hardware problems.

The modified serialdisk for non-8/e systems doesn't work reliably. We connected a 'scope to the transmit and receive signals to make sure that the baud rate was OK. It was, and the signal rise times and voltages look OK. I will connect the laptop that I am using as a server to my 8/e to verify that the server is working OK.

Charles is working on modifying LAPS-DIAL demonstration programs so that they will run from OS/8. Some of the challenge is finding a PAL compiler that understands the LINC instructions. Several versions of PAL12 exist, but we need to find one that actualy works. The address space for the LINC instructions starts at 4000, so we need to add a few instructions at 0200 to change to LINC mode and then jump into the demo program at 4020. We can put the programs on an RX01 to make it easy to demonstrate the programs.

Warren will visit next weekend so we can work on the LINCtape controller.
 
Warren and I spent two days working on the TC12 LINCtape controller. It passes the controller-only diagnostics, so lots of the controller are OK.

We still see noise on the data from the data tracks. The noise corrupts the data, so the TC12 can't locate blocks.

Warren made test harnesses so we could bypass the wiring in the backplane that the diagnostics can't test. Earlier we tried a TU55 and I/O cable in place of the TU56. We have eliminated everything as a cause of the noise, but still have not eliminated the noise. I took a vacation day so I could work debug one more day before Warren goes home. Wish us luck.
 
The D0(0) coil in the left tape head is open. So, both heads in this TU56 have failed with open coils. We now need three TU56 heads to put everything back together.

We found a bad or very out of adjustment M302 in the TU56 that was causing braking and turnaround problems with the right drive. We swapped it with a M302 from another TU56 and it is behaving much better. We were actually able to boot OS/8 from the RK05 and write files onto the TU56. This is major progress.

OS/8 and LAPS would not boot from tape, but tried very hard. Hopefully it is just adjustments to get the drive working correctly now.

Next weekend I will run through the TU56 adjustments, including what is mentioned in the Tech Notes.

Warren just left for home. He will pick up an RX02 on the way to go with his new PDP-8/I.
 
If I understand this correctly...

The problem we see is that the signal voltage is low because of only a single coil. When the signal goes through the negative to positive level converter the pulses are shortened and are not sampled by the LINCtape controller. Warren suggested that we would also have write problems because each of the coils handles one direction of the magnetic transition, so a missing coil means half of the transition will not be written.

The design is supposed to be redundant, but I don't see how that actually works.
 
Hi All;

M-Thompson, Maybe You can pick up the Lost Art of Rewinding Head Coils ??
You have plenty of them to practice on/with..

THANK YOU Marty
 
Marty,

Google shows 19,000 responses for "Magnetic Tape Head Repair" so I should be able to find some experts.

The problem is likely solder joints on the tiny wires that come out of the coils. Unfortunately the tiny wires are potted, and makes it extremely difficult to repair.
 
Hi All;

M-Thompson, PERFECT !!!!
My guess You got Your X-Ray machine at the same surplus place that I was thinking might have that Electron Microscope..
Great Job.. The Pictures Look Great..

THANK YOU Marty
 
You can thank Bob Vines for the X-Rays.

It was the MT(0) signal that was open. In the top picture you can see a loose wire at the top left. That is one of the MT wires. So, my guess is that the MT that has the problem is the second head coil from the left. The problem is the connection from the MT(0) coil to the common MT(G) wire. I sure that with enough studying you could find the (G) wires that have two head coil wires connected.

There is a whole lot of potting compound on top of some of those connections.
 
jackrubin said:
Scary stuff - not the kind of place I'd want to live too close to!
Click to expand...

Al Kossow said:
and hope it doesn't eat the insulation off the heads or whatever is holding the head stack together
Click to expand...

Well, I was just trying to be helpful.
I've deleted my post to prevent being responsible for any damage to the environment or someone's computer.
I'm sure the 1971 stuff would be a much better choice...:rolleyes:
Have a nice day.
 
jackrubin said:
Here's a Navy report on depotting F-4 components from roughly the same time period. They describe a solvent that seems pretty effective, though I wonder if any of the solvents are still available under today's HazMat regulations -
http://www.dtic.mil/dtic/tr/fulltext/u2/725493.pdf

Jack
Click to expand...

N-methyl-2-pyrrolidone has generally been replaced by 1-methyl-2-pyrrolidone and can be purchased for $148 per Liter. Benzyltrimethylammonium hydroxide is available for $92 for 0.5 Liters. Of course Methanol is available, lab grade is $60.30 per Liter

The NAVY report also says that it smells bad.:D
 
You must log in or register to reply here.
Back
Top