KY11-LB Programmers Panel - Part 2

I may have got to the bottom of the front panel problem.
Initially I thought that it was related to the scanning technique that is used to read the keypad and update the 7 segment displays given the keypad gave the wrong values and sometimes the display was missing segments.
After further research it appears not to be the case (which is good).
The scanning signals all look good on the scope.
The insight came when I looked at which keys on the number pad worked and which didn't.  I came up with the following table
Pressed = Displayed
1 = 1
2 = 0
3 = 1
4 = 4
5 = 5
6 = 4
7 = 5
0 = 0
If you convert this to binary it becomes pretty obvious where the problem lies.  The invalid display is for the items marked with * and all have the second bit set.  If you mask out the second bit you get the displayed value.
0 = 0000
1 = 0001
2 = 0010 *
3 = 0011 *
4 = 0100
5 = 0101
6 = 0110 *
7 = 0111 *
The front panel uses a 7447 BCD to 7 Segment decoder to drive the LED modules. If pin 1 (B) on this is not being set when required then the behaviour will be what I am seeing..... it isn't that the keypad is getting it wrong... but the display is showing the wrong value due to a missing bit.
Need to do a bit more digging here but given that this line is pulled to +5V via a resistor on the board (the controller board in the backplane pulls this to ground) I am hoping that the problem is either a connection problem on the front panel board with the line not being pulled up or the 7447 needs to be replaced.  If there was a break in the ribbon table to the controller board then the bit would always be set and a different set of incorrect values would be displayed.
Hopefully there isn't a fault on the controller board that is pulling the pin to ground.
The other odd display problems with odd segments missing on the 7 segment displays are due to them being socketed (clearly there was some concern about reliability in 1979) with dirty connections.  I need to pull each 7 segment display and clean the pins.  Pressing on a display that is missing segments will cause then to appear.... might need to solder joins touched up too.

So that is all very positive about the front panel and it's control board.
I still have the problem with the front panel Halt button causing a BUS Error.  The PDP11 unibus uses an interesting Bus Grant interrupt system that is chained through the 9 pin backplane.  Plugin boards can intercept one of the interrupt lines or must pass it through (two pins per Bus Grant per backplane socket).  This means empty sockets must include a bus grant card to join the pins.  All of these are in place and I have checked the continuity of the Bus Grant lines.  My suspicion is that the CPU is not actually running.  More to look at...

KY11-LB Programmers Panel - Part 1

The power supply modules are all back in the mounting box with the transformer and this is reattached to the chassis.

I powered up the machine from the front panel on the weekend.  This machine is equipped with a KY11-LB programmers console which connects to an M7859 Quad board in the backplane (interestingly this board is run by an Intel 8008 microprocessor).

When powered up the programmers console display came up with 000000 and the run light was lit.  Some of the keypad buttons are stuck or give the wrong value when pressed so I need to pull that apart and find out what is going on there.  Hopefully just something stuck behind the panel.  Using the keys that do work I was able to load an address into the Address register on the board and read it back which suggests the M7859 is generally working.

At the moment I am stuck with a Bus Error when I try to halt the CPU.

This generally means the Grant cards are not correctly installed but these are OK so it may be time to get the logic analyser out and start looking at the backplane.  Before I leap into that I will look further at the front panel.  It might also be the front panel buttons mean something other than HALT is being pressed.


PDP-11/04 - Power Supply Part 7

I am continuing to make progress with the power supply modules for the PDP 11/04.

This weekend I got a chance to test the modules.

One of the H7441 +5V supplies worked fine.

The output is a little high at 5.5v but I understand that these do not have great regulation when not under load so this will likely come down with 10 PC boards to power.

The other +5V supply didn't run.

The output went up to just over 1v as the AC comes up and then settles back to 0.5v.  The DC voltage after the rectifier and across the main input capacitor looks OK on the meter.  

After thinking about it overnight I took a small 25VAC plugpack out of the junk box to make comparative testing of the two 7441 supplies easier (i.e from the comfort of the house rather than the workshop with the 1 degree overnight temperatures we have been getting here).

When I tested the known working 7441 it worked fine with the plugpack.

When I tested the "faulty" 7441 it also worked fine.

Reconnecting the faulty 7441 to the main transformer and bringing the voltage up with the variac and it didn't work.  The problem is the rate of voltage increase. If I bring up the variac output slowly (say 0-240v over 5 seconds) then the 7441 will hang with the output under 1 volt.  If I increase the voltage quickly (within 1-2 seconds) then it powers up fine and shows a good 5v on the output.

I have had some discussions on this and it appears that this can be a common problem with switching supplies.

It was a case of being too conservative.

The H745 -15V supply works fine although the Power OK Bulb (not LED like the 7441) on the back has failed.

(The folded plastic bags between the connector pins are to provide some insulation during testing.  The -15V module requires both 20VAC and 15VDC to operate so there are two jumper cables bring power to the module).

I will replace the Bulb with an LED + resistor like the H7441. These LED's are visible at the back of the power supply box so failed supply modules can be easily identified.

I think it is time to put the regulators back in the chassis and try powering up the back plane.

Next weekends project.


PDP-11/04 - Power Supply Part 6

The failed capacitors are replaced so the regulators are ready to test on the upcoming weekend.

The 11 has three regulator modules.



2 x H7441 5V regulators are switching supplies that use the 28V from the main transformer. The main 22000uf capacitor in one of these was faulty. Replaced it with a 20000uf modern one (notice the size comparison between the two pictures). If there is still ripple on the output then another one can be paralleled up. The other 5V regulator looked like the 22000uf capacitor had been replaced sometime at some point and that tested out OK. In both 5V regulators I replaced with small 560uf and 1200uf capacitors in the CLC filter that cleans the switching noise from the output as they looked to be leaking.

5VDC Power Supply

5VDC Power Supply

1 x H745 15V regulator which is an older linear design. This had a massive 33000uf capacitor but it was also faulty. The smaller 4700uf capacitors on the output where fine. 33000uf/50v capacitors similar to the original are not available at any reasonable price so I replaced it with 3 x 10000uf capacitors in parallel. The three caps stack well within the existing case. The packing shown in the picture ensures they don't short together. That can be replaced once it is tested.

-15VDC Power Supply


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