December 2003
The three Boxer 110V fans had to be replaced: several used fans were obtained
from on a wrecked PDP-11.
There was no key to action the key switch on the front pane: a new main power
switch has been installed at the rear of the main box.
The upper cover was missing on both the main box and the memory expansion box:
a new cover was cut out from a gold colored aluminum plate fairly similar to
that of the original cover.
July 2004 : the power supply has been fixed (rectifier bridge replaced). Furthermore, the 6 main register Flipcards (M220) are now repaired as well as the M700 manual function generator (using a logic probe with memory, a logic pulser and a HP Logic comparator).. But the only function working so far is loading an address in the main registers. Address incrementing has also been fixed. The number of TTL IC's changed so far exceeds 40 !
November 2004 : using a PC fitted with a digital input-output card, all the following Flipchips have been tested and repaired : G221, M310, M160, M617, M111, M113, M115, M117, M119, M216.
January 2005 : the core memory is now functionning ! It is possible to load and to examine data at any address in the memory. However, running a program has not yet been possible. Several fast switching diodes (DEC672, equivalent to 1N3653) as well as many 7400 and 7440 TTL's had to be replaced on the eight G221 memory selector Flipchips. Five DEC672 diodes also proved faulty on the two G611 diode boards. The front panel has been completely restored, with original "blue base" bulbs and 2N3009 transistors.
January 23, 2005 : The computer is now functionning. Instead of the Teletype, a modern computer and Windows XP is used. The computer is hooked on the DP-8/L Teletype connexion Flipchip using a Black Box RS232 <-> current loop converter CL1090A-M. The sofware used to communicate is the standard Microsoft HyperTerminal with settings : COM1, TTY terminal emulation, 7 bit ASCII, 110 bits/second, no parity, 2 stop bits, no flow control. The RIM progam is loaded but does not work. The clock speed is wrong and must be adjusted using an oscilloscope. Several problem appear on the main registers, although about one third of all chips in the registers have already been replaced. Finally, the decision is taken to replace systematically all the 18 TTLs in all 6 M220 main registers (92 TTLs).
May 2005 : The RIM progam is loaded and works. RIM is used to enter the MAINDEC-08-D1B2-PM program, which tests the memory address. The program can be found on the Internet in RIM format (and not, as usual, in BIN tape format). It loads itself in the high portion of the memory. After several successful loops (a "11" prints everytime a loop is completed, every 28 seconds), the computer breaks down. Several problems appeared with the ISZ and JMP instructions.
July 9, 2005 : The problem with the ISZ and JMP instructions is solved by replacing three TTLs (7460s and 7453) on two Flipchips. The binary loader DEC-08-LBAA-D (in RIM tape format) is successfully loaded using RIM. The the MAINDEC function test program #1 is then entered and the AND instruction turned to faulty for one bit. One last 7460 TTL is replaced on the corresponding main register Flipchip. The test program now cycles without any stop on error.The FOCAL program (in BIN tape format) is then entered and started successfully. THE PDP-8/L CAN NOW BE CONSIDERED AS COMPLETELY REPAIRED !
July 30 : The ASR33 Teletype is now modified according to the DEC specifications to be attached on a PDP-8. This implies some internal rewiring, the removal of several mecanical parts and the addition of a relay.
July 19 : The PDP-8/L is now completely repaired,
after more than two years of work: the RIM loader, the BIN loader, the Instruction
test program #1, the Memory address test program and, eventually, the FOCAL
program can be entered, started and work without any problem.
Work has started on the ASR33 Teletype (which must be modified according to
the DEC specifications to work properly with a PDP-8/L). No repair work has
still been done on the 4K BA08A memory extension and on the fast puncher-reader.
