Catch-Up – 16th May

Good news!! The ‘Mode Change not Allowed’ problem has been fixed

One fault and one adjustment were the cause, firstly the Bank A Reload/Available switch was U/S and the second, excessive voltage drop on the +5V to the Digital Input box.

Everything looked OK after the switch replacement, the Argus was good, LEDs on the Digital inputs were correct, power supplies were good so what could it be? This morning I had a flashback to the early days and a voltage drop problem between a +5V power supply and the CHARGE box. A quick check showed we now had the same problem with the +5V on the Digital Input box. Briefly, the power supply was set to +5.25V (so you would think that was good enough), the distribution rail was on the computer rack was the same but when I checked the +5V on the Input Box backplane it was +4.1V! Cranked up the power supply even more, restarted everything and bingo, we are now back in business. I suspect the circuit breakers on the back of the I/O boxes may be partly to blame due to their age.

After going around the houses with the ‘Mode Change’ problem I’ll write up some fault finding notes which should help if this happens again. I also spent a couple of hours today checking all the input cards and output cards while I was at it. What I was looking for was reasons for the +5V loading, again experience tells us that the glass encapsulated AVX decoupling capacitors can fail and reduce the resistive load on the +5V, down to a few Ohms in a worst case. I eventually found three Digital input cards and two PeriBus converters with very low resistance between +5 and 0V on the card. I now hope to replace all the AVX capacitors on these cards this week.

A bit more on voltage drops between the computer rack and the I/O boxes. I have a couple of photos of the Swiss computer racks which show the Sense feature on the Farnell’s being used for all the +5V supplies going to the I/O boxes where our power supplies to the I/O rack have no sense wires fitted. Perhaps there’s some history here, I’m all for modifying our LCP by putting sense wires on the power supplies that have none. I have quite a bit of that Ferranti black and white twisted wire somewhere courtesy of recovery from derelict LCP …….

I hope I’m not speaking too soon but while we have had this ‘Mode’ problem it has exposed other vulnerabilities that can be rectified and once rectified there’s no reason why the simulator shouldn’t be a lot more reliable. We even have a complete set of spare Argus boards in the LCP and they include the GX, GL and ME147. After all the hassle it’s time to move forward as  the T86 aerials need painting, if ever it warms up a bit.

Back at “3rd Line” I tested our spare CRU2 (used in the display console). One relay was U/S and has been replaced which was not an easy task as shown in the next two photos.

We now have a spare CRU1 and CRU2, both ex LCP 1006. Both need a final check in situ.

And finally, I set up a scope today just in case I needed to see what was happening on the PeriBus. A contrast to what was used when the LCP was operational.

The setup includes a PicoScope USB oscilloscope connected to a laptop which also holds PDF’s of any AP’s needed. Digital voltmeter, and a can of the wonderful DeOxit gold for rejuvenating the gold plated connectors. Note: the use of erasers in banned. I remember working on the Filter Bank on the T86 and the card edge connectors were copper, the gold plating being removed by regular use of an eraser !

Pete H

PS – That’s me caught up at last so I will be back with our regular Workday blogs – Mike Strange

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