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Reviving the PDP-12 at the RICM

Qbus said:
What’s the RL pack cleaner look like?
Click to expand...

The DIGITAL Data Cartridge cleaner,RL01K-AA , is a thermally molded plastic assembly about 2 feed square that has a spindle that probably came from an RL01 drive. There is an assembly that is like the head carriage from an RL01 drive, but cleaning pads go where the heads would go. I will take some better pictures of it.

RL01K-AA.jpg
 
Yesterday we continued debugging the RK8F data-break problem revealed when running MAINDEC-08-DHRKA-B RK8E Diskless Control Test. We ran the "B" revision of this diag because it was the newest version that we had both a BIN image and a PDF file. We found "C" and "D" versions of the BIN file but no PDF, and an "E" version of the PDF but no BIN.

Last week's failure showed that we could data-break successfully to address 0000 but not to 7777. We suspected a problem in the 74161 or 8881 chips that make up the CA register and Omnibus drivers. From the prints we found that the IOT decoding is done on the M7104 board and the CA is on the M7105. We pulled the M7106 so we could get to the ICs on the M7105. It would have been easier to put the M7105 on an extender, but we don't have extender cables for the over-the-top connectors. We could see the data on the input pins of the 74161 chips, but no output. The LD signal that comes from the 6RK4 signal (IOT 6744) was active but the CLK signal was always high. The IOT 6744 is ANDed with the IDLE signal and TP3. The IDLE signal was at 2V, so probably a bad 7474 flip-flop. Ah, the IDLE signal comes from the M7106 that we removed. We grounded the signal and could see the Omnibus MA signals go active. Unfortunately the upper four bits of the CA also come from the M7106 that we removed. We put the M7106 back in, put the diag in a scope loop, and looked at the Omnibus on the bottom of the wire-wrap backplane in the DW8E. All 12 bits of the CA were active, so the RK8F is probably OK.

One thing that we noticed on the RK8F is that the MA signals on the 8881 chips are not gated like they are on an RK8E. Since there is on processor in the Omnibus in the DW8E this works OK.

We looked at the MA signals on the Posibus and found that one was inactive. The M7102 boards that drive the Omnibus MA signals onto the Posibus cables were previously tested in Warren's flip-chip tester, and one was repaired. We put the M7102 in slot AB20 on an extender and then all of the MA signals went active. The DHRKA diag also passed, so everything in the RK8F and DW8E is now working. We cleaned the gold contacts on all of the DW8E boards with a white eraser and 91% alcohol, and the DHRKA diag still worked OK.

We bought a new NiCd battery pack and weather strip for the RK05, so that will be next week's project. There seems to be a power supply problem in the RK05 because the relay K2 that activates the blower and voice coil in the RK05 drive chatters after the drive is powered on for about 10 minutes.
 
We replaced the seals for the blower and for the cartridge air duct. We cleaned the very dirty blower and ducts. This is a older design of drive that has the rectangular HEPA filter without the built in ducts.

We were not able to adjust the +15V supply. After it has been powered on for a minutes the +15 varied from +14 to +16. I have some spare RK05 drives in my personal collection. I will see if one of the 8-to-20 regulators is OK. If I can find a good one I will put it in the PDP-12's RK05.
 
The Servo Amp board in the RK05 was bad. It was oscillating when the heads were in the HOME position and pulling the +15V down. The +15V is used to activate the relays, so when the +15V went low the relay for the blower started chattering.

We cleaned the disk pack without disassembling it. It just had a little dust on the surfaces. We installed the pack, but the drive would not spin up. After chasing all of the signals going into the M7701 board we found that the interlock signal did not go low. Eventually we found that the microswitch that detects the door lock was not working. A shot of contact cleaner and operating the switch a few hundred times got it working reliably. The disk would still not spin up. After quite a bit of debugging we determined that having the PDP-12 not powered and cabled to the RK05 was keeping the DC LOW L signal low. Powering up the PDP-12 fixed that and then the disk spun up.

We deactivated the servo, manually loaded the heads, and moved them across the disk surface. We did't hear any bad noises so we put the heads back in the home position and activated the servo. The heads went to track 0 and the RDY and ON CYL lights came on.

We tried the two instruction OS/8 RK8E boot, but nothing happened. Next time we will run the combination controller and disk diags.
 
Just because the drive would go RDY we thought it was working. Well, that was wishful thinking. The drive will recalibrate, but if you tell it to seek to any cylinder other than 0, it will slowly seek past cylinder 203 and recalibrate. The M7702 board does the comparison between the current cylinder and the new cylinder, decides on the direction of the seek, and if the difference is more than 32 cylinders it does high-speed seek. I have another M7702 board from a spare drive, so I can give it a try. I will also try the M77?? boards from the PDP-12's disk in the now working RK05 on my PDP-8/e.
 
There's a switch just for this purpose which disables the servo. I presume that Michael just switched this off then pushed the carriage out of its detent and gently across the disk listening for any signs of head contact. I generally do the same when debugging just to try to minimise any damage if there's a problem. I usually find there's some soft 'singing' noises in some positions but I think I'd recognise an unusual noise.
 
That is what I did, deactivated the servo. The RK05 has a very high head flight height so it is very tolerant of head crashes. Manually moving the heads across the pack is the recommended procedure when debugging a drive. If you hear head crash noises you can clean the heads and try again.

I used the same procedure with my RP06, but with 20 heads it a little frightening. If you hear head crash noises with an RP06 you need a new pack and some heads.
 
We spent some more time working on the RK05 disk drive. Last week we installed jumpers on the RK05 backplane to cause it to seek between cylinder 0 to cylinder 2. The heads would slowly move towards the pack hub and slowly retract back to track 0. We speculated that the M7702 module was not correctly calculating the number of cylinders to move so the seek was always in slow speed mode. During the week we received suggestions that the position transducer was not working correctly so the contents of the Current Address register were incorrect. The signals from the position transducer looked fine, but the output on pin 6 of the Comparator E3 on the G938 module was missing even though the input signal on pin 2 looked OK. The output signal from E4 looked OK. The spare G938 had the same failure mode. We will replace E3 (LM301A) and E1 (SN7404) on both G938 modules.
 
More work on the RK05 this week. We replaced E3 (LM301A) and E1 (SN7404) on the G938 module. The COUNT PULSE FWD H and the COUNT PULSE REV H signals from the G938 module are both working now. We pulled the drive out of the rack so we could get to the trimpots on the G938 module. We went through the servo adjustments in section 5.4.2 of the DEC-00-HRK05-C-D RK05 Disk Drive Maintenance Manual Nov74.pdf document. Fortunately my 'scope has math functions to measure peak-to-peak, max, and min signal values so adjusting the amplitude and offset was not difficult. The drive will now seek correctly using the jumpers described in the Maintenance Manual or using seek-only instructions from the RK8-F controller.

We tried the OS/8 and LAPS-DIAL bootstraps, but nothing happened. The first page of core that was read from the pack contained a repeating sequence of 2525-5252. Either the pack was used as a data-only pack, or diagnostics were run on the pack. this week I will use SIMH to make a PDP-12 bootable OS/8 RK05 image. Next week we will use dumprest to make an image of the disk pack, and then write OS/8 to the pack.

The RK05 that I am using on my PDP-8/e includes the Serialdisk handlers so I can use a SIMH disk image at the same time as a real RK05 disk. That makes it very easy to move files onto the real disk. We should be able to do the same on the PDP-12.
 
We ran MAINDEC-08-DHRKB-G-D RK8E Drive Control Test on the PDP-12 using one of my disk packs. It failed with: DISK DATA ERROR PC:3571 GD:4156 ST:4001 CM:0001 DA:4156 CA:7177 AD:7200 DT:4155. Singe the pack might have been formatted sometime in the last 20 years, and we have no idea what the head alignment on this drive is like, we reformatted the pack with MAINDEC-08-DHRKD-D-D RK8E-RK8L Disk Formatter Program. We then reran MAINDEC-08-DHRKB-G-D RK8E Drive Control Test and this time we saw: RK8E DRIVE CONTROL TEST PASS COMPLETE. So I guess that the DW8E, RK8-F, and RK05 are working OK.

We tried using the dumprest programs to make an image of the RK05 pack that came with the PDP-12, but it died complaining about an unexpected interrupt. Probably the combination of the 8/I processor, DW8E, and RK8-F work a little differently than the 8/e. I will make an image of the PDP-12 pack, and then make a bootable OS/8 pack using my 8/e. Hopefully this will boot on the PDP-12.
 
I made an image of the PDP-12 pack on my 8/e. Unfortunately there was nothing on it but the 2525-5252 pattern that we had seen earlier.

I booted the 8/e using serialdisk, and wrote a bootable OS/8 on the PDP-12 pack. I now need to run BUILD to tailor OS/8 to the PDP-12 configuration and limit it to just 8k. If I don't configure the LINC tapes it should still run on my 8/e. Once it boots on the PDP-12 I can run build again and add the LINC tape support.

Maybe this will even work!
 
I used Bobaboba's RK05 pack images and modified serialdisk server to make an OS/8 pack for the PDP-12.
The pack and serialdisk handler works OK on my PDP-8/e.
The pack booted on the PDP-12, but it says that it is missing CCL.
The CCL.SV file is on the pack.

The serial disks don't work when booting, or when accessed from OS/8.
 
Sounds as though the 'missing' CCL.SV is a pointer to a compatibility problem. Does the image work OK otherwise on the pdp-12 (ie by using non-CCL commands)?

The modded serialdisk handler uses Rotates instead of BSWs, no other changes for the system handler. Definitely no BSWs. Not tested on anything other than an 8/e of course, but you've had it working on yours.
 
Hi All;

M-Thompson, Have You read or do You have a copy of "Computing in the middle Ages" by Severo M. Ornstein..
In it He talks about the Inception of the LINC, Origionally a-LINC, and some on the LINC-8 and PDP 12..
It is one of the first mass produced Personal Computer's and it was designed in the early sixties..
I thought You might find it an interesting read, with Your Work on Your PDP 12..

THANK YOU Marty
 
bobaboba said:
Sounds as though the 'missing' CCL.SV is a pointer to a compatibility problem. Does the image work OK otherwise on the pdp-12 (ie by using non-CCL commands)?

The modded serialdisk handler uses Rotates instead of BSWs, no other changes for the system handler. Definitely no BSWs. Not tested on anything other than an 8/e of course, but you've had it working on yours.
Click to expand...

I made the pack on my 8/e, and it worked OK. Since the head alignment might be a little off between the 8/e and the PDP-12 I formatted the pack on the PDP-12 and will recreate it on my 8/e.
 
After some more debugging work the servo in the RK05 drive died. Power cycling the system made it work for a while, and then it died again. With switch S1 on the Servo Power Amp on or off I can easily move the heads. The voltage at pin BU2 of the G938 Head Position Servo Preamp is always about 15mV. The COUNT PULSE FWD H and COUNT PULSE REV H signals look OK. The ON CYL light is always on, even if I move the heads.

Our current guess is that there is a problem with the M7702 PACk CYL ADDR AND DIFF module and the CURRENT ADDRESS REGISTER always contains zero.
 
The PDP-12 disk decided to work this morning. I will run it until the drive dies again.

OS/8 booted OK, but CCL is not working. Commands like DIR and COPY don't work, but DIRECT and PIP work OK.
It periodically hangs with the processor running, but nothing displayed in the AC and MB lights.
Maybe a data-break problem?
 
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