The University of Iowa's DEC PDP-8Restoration Log
Part of
the UI-8 pages
|
The University of Iowa's DEC PDP-8Restoration Log
Part of
the UI-8 pages
|
Bug 64: In preparation for an open-house, we decided to start a serious attempt at tuning the memory. We began by investigating the behavior of the memory as we varied the slice voltage. For each slice voltage we tested, we stored 32 consecutive 00008 in memory starting at address 60008 and then examined memory to see how many values were still zero, and then we stored we stored 32 consecutive 77778 to see how many values were still all ones.
| Voltage | not 0000 | not 7777 |
|---|---|---|
| 7.7 | 32 | 0 |
| 7.68 | 19 | 0 |
| 7.62 | 17 | 0 |
| 7.6 | 13 | 0 |
| 7.56 | 0 | 0 |
| 7.5 | 0 | 13 |
| 7.54 | 0 | 13 |
| 7.4 | 0 | 15 |
| 7.1 | 0 | 32 |
The slice voltage was adjusted by turning the topmost trimmer on the G008 Master Slice Control board. In adjusting this and other Bourns TrimPot trimmers on DEC flip-chip modules, we have observed that the voltage is quantized, changeing in discrete steps as the trimmer screw is turned. We wonder if the trimmers are wire-wound with a contact brush touches one turn at a time.
The voltage readings here were eyeballed off of an analog meter, so the accuracy is probably 0.02V at best.
What is clear here is that the memory only just barely works. We were hoping to find a range of slice voltages that worked and then set the potentiometer to the middle of the working range.
The working range is probably a 3-dimensional volume, with the dimensions being
The next memory tuning exercise will involve playing with the other two dimensions.
With memory working as best we could adjust it, we loaded demo code in memory for the open house on Jan 20-22:
Some of the above were hand-relocated to run in new memory locations. All were tested and seemed to work.
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| Before and after transformer rewiring | |
|---|---|
Eliminating the bare wire jumper on the transformer reduces the hazards posed
by this power supply, but it would be nice to make a sheet metal cover over
the transformer comparable to the transformer covers on the Type 779
transformers. This will require some sheet metal bending.
Bug 80: During a demo, after turning off the Teletype, when we turned it on again, the motor did not spin. It buzzed, and when you turned the fan, you could feel it vibrating. This implies that there is power to the motor. It is a capacitor start motor with a starting relay. Teletype's schematic FS-10 MOTORS documents this and several other motor options. Any of the following could cause the motor not to spin despite there being power to the motor:
We pulled the quick-connect connectors to the starting relay, allowing us to use an ohm-meter to verify that the primary motor windings is 9Ω and the starting winding is 16.5Ω ruling out a burnt-out motor winding. Measuring between the primary and starting widings, have 25.5Ω, so there are no shorts between the windings.
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| The Teletype motor start relay | |
|---|---|
The relay is a normally open relay with an approximately 0.1Ω coil. During the motor's starting surge, the relay closes to power the starting winding. Once the motor is up to speed, the current through the primary motor winding is too low to hold the relay closed. Aside from evidence of contact arcing, the relay is in good condition. We burnished the contacts and put the relay back. We did not make any changes to the adjusting screw (the screw on the left of the right photo with a red dot of paint on it).
After reassembly, we used an ohm meter to verify that the start relay closes when power is briefly applied to the motor.
The above observations point to the motor start capacitor itself. Teletype's schematic identifies this as C3 88-108, which agrees with the markings on the capacitor, 88-108MFD. This is an 88-108μF capacitor, a very common value of motor start capacitor.
Bug 80: Continuing the diagnostic work from Jan 22, we used an analog ohm meter on the Rx1K scale to test the 88-108μF motor-start capacitor. It conducts very briefly when the meter leads are first connected and then drifts up to infinite resistance. That is how a capacitor ought to behave when measured this way. We compared it with a known good 5μF capacitor, and saw that the response on the ohm meter was far stronger with the latter than with the capacitor from the Teletype. Therefore, while the capacitor still has some capacity, it has much less than 5μF.
So, we ordered a replacement capacitor. The original capacitor was a 1" (26mm) diameter cylinder 2.5" (65mm) long. The smallest replacement we could find was 34mm by 69mm, 8mm larger in diameter. This will be a tight fit in the space in the Teletype. Also, we will need some 4.8mm (0.187") faston connectors to attach the capacitor to the wiring. Teletype corp. seems to have used that size exclusively, while the similar connectors in the PDP-8 are all the more common 1/4" wide.
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| Before and after transformer rewiring | |
|---|---|
We had finished rewiring the transformer primary. Now, we rewired the secondary, moving the fuse to the bridge between the two secondary windings and finishing adding spiral-wrapped insulation to the remaining uninsulated wires.
We still need to install metal shields to protect fingers and make accidental
shorts less likely. This is most important on the 120V primary side of the
transformer, but would also be helpful on the 30V secondary side.