thunter0512
Veteran Member
I recently rescued a PDP-11/05 in a low-profile chassis. The poor machine was exposed to extreme moisture levels, dust and other filth and was infested by millipedes and spiders. I took it home and started the journey to clean, repair and restore it.
The key was missing, so I immediately placed an order on Ebay for a replacement key for A$29.95 (about US$20.00): https://www.ebay.com/itm/324778106688
Some ham-fisted individual has been in the machine before me and broke one of the sockets on the top of the backplane. There were a few chassis bolts missing, some were the wrong length and other were completely loose.
After opening the machine, I pulled all the backpanel PCBs:
I then removed the front panel assembly noting that the plastic decal was mostly detached from the assembly held by decayed double sided sticky tape. Luckily it was otherwise undamaged and I managed to gently pull it off the metal body. I also removed the front panel PCB with the switches and LEDs from the assembly. The switches were filthy but cleaned up well using first a moist cloth and then some isopropyl alcohol.
I then used a relatively soft short haired pain brush to gently loosen all dust and other debris from all the PCBs taking particular car with any mod wires. I then used compressed air to blow all the loose stuff off again taking particular care with any components which might be damaged by the compressed air. I was very carefully with the H214 Core Stack keeping the compressed air far away and directing it away from the transparent acrylic sheet covered core matt so as not to damage the fragile core.
I used some isopropyl alcohol to clean all the PCB edge contacts and any areas which had some grime which wasn’t removed by brush and compressed air.
I removed the microswitch assembly from the front panel and cleaned it with moist paper towels.
Next I removed and cleaned the power supply PCB from the U-shaped power supply chassis disconnecting the AC inlet and the DC outlet Mate-N-Lok connectors. By undoing for bolts I could carefully lift out the U-shaped power supply chassis with the attached mains transformer and power supply fan. I had to carefully feed the 9 pin DC supply Mate-N-Lok connector through the arch shaped hole in the power supply chassis after removing the flexible grommet to create a little more space to get the connector fed through.
Using the hot air nozzle of my soldering station set to 100 degrees C I gently warmed the decayed double sided sticky tape left on the plastic front panel decal and rubbed it off without damaging the decal or point. I then carefully tried a solvents in one corner of the decal trying to find one which does not attack the paint. The first one was isopropyl alcohol and after applying some tiny drop and immediately wiping it off I saw a little colour on the white tissue. I then tried the same with kerosene and there was no paint coming off. Encouraged I left a little more kerosene on the paint and after maybe 10 seconds wiped it off without any colour on the tissue. The kerosene nicely dissolved the remaining latex based glue on the decal but did not attack the paint.
The key for the keylock arrived and I removed the keylock and soaked it in kerosene to remove decades old hardened grease. After some soaking I used compressed air to blow out the kerosene and residual grease and finally reoiled the lock.
Using brush and compressed air I cleaned all the power supply components, the main PDP-11/05 chassis and backplane and front panel. I then wiped over all parts with soapy water on a microfiber cloth and subsequently used compressed air to drive any moisture out. One hour in the sun made sure that everything was completely dry.
Now as all the parts were clean I moved everything into my small electronics lab to start the “bring-up” of the system.
After taking photos and carefully marking the 4 large electrolytic capacitors I removed all and slowly reformed them using a lab-supply current limited to 50mA. I slowly stepped up from 1V in 1V increments to the rated voltage of the capacitor. I paused at each step until the current dropped to 1mA or less.
The two large capacitors had some double-sided stick foam between them and the power PCB supply PCB to prevent the capacitors insulation from being punctured by protruding solder pins and shorting. Sadly moisture got into this foam and badly corroded PCB tracks in that area. I cleaned the mess with acetone and then sand papered it so that it would accept fresh solder. I then used copper wires and generous amounts of solder to restore the affected PCB tracks.
I re-fitted the 4 now reformed capacitors to the power supply PCB and with the PCB outside of the chassis, connected the 3 centre-tapped secondary leads coming from the transformer.
The power supply obviously required mains power. At first, I struggled with the mains power to the transformer. It turned out that the mains power is directly switched by two flimsy microswitches mounted to the front panel. One of the key lock operated microswitches did not reliably switch on until I slightly bent the switch lever. On other systems there is often a power relay or a more substantial switch to handle the mains power. Using just a microswitch was a surprise to me.
Once I had reliable mains power, the fans were running and the power supply nicely produced +15V, -15V but the +5V rail was only about +1.2V. With the excellent description of the circuit and trouble shooting chart in “DEC-11-H05AA-B-D PDP-11-05, 11-10 Computer Manual.pdf” from Bitsavers, I eventually identified D9 – a 2.4V Zener as the culprit. After replacing it the +5V rail became +5.25V which was close enough.
I then used very large wire wound 15 Ohm and 5 Ohm resistors on the +15V, -15V and +5V rails directly connected to the 9 pin DC Mate-N-Lok connector to load test the supply. The voltages were stable with or without the resistive load. A multimeter set to AC Voltage showed minimal ripple.
With good stable power I then started reassembly of the machine. Fitting the power supply PCB back into the power supply chassis turned into a very frustrating 2 hour struggle as the stiff cables and Mate-N-Lok connectors interfered with the two smaller electrolytic capacitors on the power supply PCB. I wrote about this and the solution in another thread: PDP-11/05 DC Regulator Module (5409728) cable routing
Briefly, the fix was using spacers to raise the transformer by about 4 – 5 mm so that the primary side Mate-N-Lok connector could be pushed underneath the edge of the transformer.
I reattached the cleaned decal to the metal front panel body using double sided 6 mm wide sticky tape. I then fitted the front panel assembly to the PDP-11/05 chassis.
With none of the PCBs plugged into the backplane I power on the system. Both Fans were blowing and all 16 the front panel LEDs were lit.
I then applied a very thin layer of Deoxit Gold to all PCB connector edges to ease insertion and removal and plugged all 6 PCBs back in. With some trepidation I powered the system on. Luckily there was no smoke or bang. I immediately checked all 3 power rails and there were at their nominal voltage and stable.
Using the front panel switches I deposited data patterns into core memory and was able using the examine switch to confirm that the correct values have been stored and that they remained stored across a power cycle. I finally entered the bootstrap loader as documented in the manual, verified that it was correctly entered and finally run and single stepped it.
It looks like the system is operational at a basic level. The only serious debugging required was the faulty Zener for the +5V rail.
I have not yet attempted to interface to the serial port as apparently that is not operating with RS232 levels, but with TTL levels. I will rig up a little converter circuit to deal with the conversion.
Once I have serial comms working I will be able to run diagnostics.
Now I am trying to find some simple serial output and input programs to toggle in.
Here are some photos (click on the photos to see them full size):
The key was missing, so I immediately placed an order on Ebay for a replacement key for A$29.95 (about US$20.00): https://www.ebay.com/itm/324778106688
Some ham-fisted individual has been in the machine before me and broke one of the sockets on the top of the backplane. There were a few chassis bolts missing, some were the wrong length and other were completely loose.
After opening the machine, I pulled all the backpanel PCBs:
- M7260 – Processor Data Path and UART,
- M7261 – Processor Control Logic and Microprogram,
- G110 – Memory Control,
- G231 – Memory Driver,
- H214 – 8K Core Stack,
- M930 - Terminator
I then removed the front panel assembly noting that the plastic decal was mostly detached from the assembly held by decayed double sided sticky tape. Luckily it was otherwise undamaged and I managed to gently pull it off the metal body. I also removed the front panel PCB with the switches and LEDs from the assembly. The switches were filthy but cleaned up well using first a moist cloth and then some isopropyl alcohol.
I then used a relatively soft short haired pain brush to gently loosen all dust and other debris from all the PCBs taking particular car with any mod wires. I then used compressed air to blow all the loose stuff off again taking particular care with any components which might be damaged by the compressed air. I was very carefully with the H214 Core Stack keeping the compressed air far away and directing it away from the transparent acrylic sheet covered core matt so as not to damage the fragile core.
I used some isopropyl alcohol to clean all the PCB edge contacts and any areas which had some grime which wasn’t removed by brush and compressed air.
I removed the microswitch assembly from the front panel and cleaned it with moist paper towels.
Next I removed and cleaned the power supply PCB from the U-shaped power supply chassis disconnecting the AC inlet and the DC outlet Mate-N-Lok connectors. By undoing for bolts I could carefully lift out the U-shaped power supply chassis with the attached mains transformer and power supply fan. I had to carefully feed the 9 pin DC supply Mate-N-Lok connector through the arch shaped hole in the power supply chassis after removing the flexible grommet to create a little more space to get the connector fed through.
Using the hot air nozzle of my soldering station set to 100 degrees C I gently warmed the decayed double sided sticky tape left on the plastic front panel decal and rubbed it off without damaging the decal or point. I then carefully tried a solvents in one corner of the decal trying to find one which does not attack the paint. The first one was isopropyl alcohol and after applying some tiny drop and immediately wiping it off I saw a little colour on the white tissue. I then tried the same with kerosene and there was no paint coming off. Encouraged I left a little more kerosene on the paint and after maybe 10 seconds wiped it off without any colour on the tissue. The kerosene nicely dissolved the remaining latex based glue on the decal but did not attack the paint.
The key for the keylock arrived and I removed the keylock and soaked it in kerosene to remove decades old hardened grease. After some soaking I used compressed air to blow out the kerosene and residual grease and finally reoiled the lock.
Using brush and compressed air I cleaned all the power supply components, the main PDP-11/05 chassis and backplane and front panel. I then wiped over all parts with soapy water on a microfiber cloth and subsequently used compressed air to drive any moisture out. One hour in the sun made sure that everything was completely dry.
Now as all the parts were clean I moved everything into my small electronics lab to start the “bring-up” of the system.
After taking photos and carefully marking the 4 large electrolytic capacitors I removed all and slowly reformed them using a lab-supply current limited to 50mA. I slowly stepped up from 1V in 1V increments to the rated voltage of the capacitor. I paused at each step until the current dropped to 1mA or less.
The two large capacitors had some double-sided stick foam between them and the power PCB supply PCB to prevent the capacitors insulation from being punctured by protruding solder pins and shorting. Sadly moisture got into this foam and badly corroded PCB tracks in that area. I cleaned the mess with acetone and then sand papered it so that it would accept fresh solder. I then used copper wires and generous amounts of solder to restore the affected PCB tracks.
I re-fitted the 4 now reformed capacitors to the power supply PCB and with the PCB outside of the chassis, connected the 3 centre-tapped secondary leads coming from the transformer.
The power supply obviously required mains power. At first, I struggled with the mains power to the transformer. It turned out that the mains power is directly switched by two flimsy microswitches mounted to the front panel. One of the key lock operated microswitches did not reliably switch on until I slightly bent the switch lever. On other systems there is often a power relay or a more substantial switch to handle the mains power. Using just a microswitch was a surprise to me.
Once I had reliable mains power, the fans were running and the power supply nicely produced +15V, -15V but the +5V rail was only about +1.2V. With the excellent description of the circuit and trouble shooting chart in “DEC-11-H05AA-B-D PDP-11-05, 11-10 Computer Manual.pdf” from Bitsavers, I eventually identified D9 – a 2.4V Zener as the culprit. After replacing it the +5V rail became +5.25V which was close enough.
I then used very large wire wound 15 Ohm and 5 Ohm resistors on the +15V, -15V and +5V rails directly connected to the 9 pin DC Mate-N-Lok connector to load test the supply. The voltages were stable with or without the resistive load. A multimeter set to AC Voltage showed minimal ripple.
With good stable power I then started reassembly of the machine. Fitting the power supply PCB back into the power supply chassis turned into a very frustrating 2 hour struggle as the stiff cables and Mate-N-Lok connectors interfered with the two smaller electrolytic capacitors on the power supply PCB. I wrote about this and the solution in another thread: PDP-11/05 DC Regulator Module (5409728) cable routing
Briefly, the fix was using spacers to raise the transformer by about 4 – 5 mm so that the primary side Mate-N-Lok connector could be pushed underneath the edge of the transformer.
I reattached the cleaned decal to the metal front panel body using double sided 6 mm wide sticky tape. I then fitted the front panel assembly to the PDP-11/05 chassis.
With none of the PCBs plugged into the backplane I power on the system. Both Fans were blowing and all 16 the front panel LEDs were lit.
I then applied a very thin layer of Deoxit Gold to all PCB connector edges to ease insertion and removal and plugged all 6 PCBs back in. With some trepidation I powered the system on. Luckily there was no smoke or bang. I immediately checked all 3 power rails and there were at their nominal voltage and stable.
Using the front panel switches I deposited data patterns into core memory and was able using the examine switch to confirm that the correct values have been stored and that they remained stored across a power cycle. I finally entered the bootstrap loader as documented in the manual, verified that it was correctly entered and finally run and single stepped it.
It looks like the system is operational at a basic level. The only serious debugging required was the faulty Zener for the +5V rail.
I have not yet attempted to interface to the serial port as apparently that is not operating with RS232 levels, but with TTL levels. I will rig up a little converter circuit to deal with the conversion.
Once I have serial comms working I will be able to run diagnostics.
Now I am trying to find some simple serial output and input programs to toggle in.
Here are some photos (click on the photos to see them full size):
Last edited:
- but, your followup definitely points out my misunderstanding - thanks again)
