Scope Chassis ready for wiring

October 11th, 2016
Inside Repro Oscope Chassis

Inside Repro Oscope Chassis

I bought several versions of terminal strips only to use a 4 postion version from Antique Electric Supply. I ended up cutting them down to make the three position set.

I’m going to wire up the power supply first, then wire in the edge connectors. Note that one side of the transformer shares a mounting screw with a terminal strip. Also, on the original unit, the edge connectors are mounted on a panel and secured to the mounting brackets. At the moment, I don’t have enough stock to make a full panel, so I’ve made a mini panel, and I’ll retrofit a full panel later on.

SCELBI O-scope Opamp Power Supply

October 10th, 2016

Here is a view of my version of the SCELBI O-scope Opamp Power Supply test setup. AC power is not connected in this photo, but would be connected to the transformer during the actual test.

Opamp Power Supply

Opamp Power Supply

I’m using a Triad VPL28-180 transformer and 1N4002 diodes. I found the diodes in my spare parts stash. The first smoothing capacitor is rated at 100uf/50 Volts. The +18 and -18 supplies have 470uF/25 volt smoothing capacitors. The latter are probably overkill, but it’s what I could find in my spare parts stash. The resistors are rated at 120 ohm, 1/4 watts. The zener diodes are the BZX79C18 that I mentioned in a previous post.

After hooking up to the analog board, the voltages are within a volt of +/- 18, with no measurable ripple. Transformer output is a little higher than with the original transformer that I tested with, but not enough to be of concern. Zener shunt current is OK, so I’ll just move this set up into the enclosure, without changing any components.

SCELBI O-scope Chassis Progress

October 8th, 2016

Well there is a reason why I changed majors from mechanical engineering to computer science.

SCELBI Oscope Front Panel

SCELBI Oscope Front Panel

A few imperfections in execution, but it will look fine when the connectors and switches and such are added. This is a Bud AC413 chassis, the original chassis was constructed a bit differently, and 1/2 inch higher. I didn’t notice the Z and G when making out the rub-on artwork, but fortunately I included some extra text including an extra “GND”. The N turned sideways makes a decent Z.

I have a few more coats of clear lacquer to spray before moving onto the next stage.

For Comparison, the following is an image of an original SCELBI O-scope Chassis taken by Jack Rubin. It has been slightly photo-shopped to fix the perspective.

SCELBI Oscope Original Front Panel

SCELBI Oscope Original Front Panel

SCELBI Keyboard Interface All Hooked Up and Running

October 1st, 2016
SCELBI Keyboard Interface All Hooked Up and Running

SCELBI Keyboard Interface All Hooked Up and Running

For now, I’m using one of my PS/2 adapters (on top of the enclosure) to connect to a PS/2 keyboard. I was going to cobble together a cable for my reproduction Datanetics keyboard, but it requires -12 volts, which would have to be generated separately just for this keyboard.

Now I can focus on the Oscilloscope Interface.

Wiring the Keyboard Interface Enclosure

September 30th, 2016
Wiring the Keyboard Enclosure

Wiring the Keyboard Enclosure

At this point, I decided to call it a day and said to myself, my next project isn’t going to involve so much point to point wiring. Counting both chassis, the peripherals and cabling, I must have soldered about 500 point to point wires during the course of the SCELBI project.

By the way, I think the best process for constructing one of these enclosures is as follows.

  • cut and drill the holes
  • add the rub on lettering
  • clear coat the lettering with lacquer
  • add the connectors
  • wire it
  • There are 12 wires left to connect on this chassis.

    For the oscilloscope interface checkout, I think I’m just going to build it up in the chassis, rather than test it and then make the chassis.

    SCELBI Keyboard Peripheral Enclosure nearly done

    September 28th, 2016
    Peripheral Enclosures

    Peripheral Enclosures

    Internal wiring is left to be done, but the mechanical part is ready for wiring. Note the front panel layout is reversed from the cassette interface. It turns out that I have seen the cassette interface made both ways. Stuff like this happens when your products are hand made.

    SCELBI – Oscope Digital Board

    September 20th, 2016
    OSCOPE DIGITAL PCB - front - with rework

    OSCOPE DIGITAL PCB – front – with rework

    SCELBI OSCOPE DIGITAL PCB - back - with rework

    OSCOPE DIGITAL PCB – back – with rework

    Notice that I have also recreated the rework. Note that the proper way to cut a trace on a PCB, is to use a sharp hobby knife to make two parallel cuts in the trace an 1/16″ or so apart. Then you remove the small piece of copper between the cuts.

    The Analog board has also been built. What’s left is wiring up the board edge connectors to power and amphenol connectors.

    Scope Analog Power Supply Part II

    September 11th, 2016

    Before I finalize on values for the SCELBI scope analog supply components, I needed to know how much power the board consumes. To determine this, I set up two bench supplies to power the board and measure current consumption.

    Scope Analog Board Current Test

    Scope Analog Board Current Test

    Current consumption on both positive and negative rails measures at only about 25 mA. It’s possible current could vary a bit when hooked up to scope output and the digital board for input, but looking at the schematic makes me think that it’s not going to change very much.

    The way this zener based regulator works, is that if the voltage is over 18 volts, the zener shunts current to ground causing a voltage drop over the series resistor. The zener selected has to have enough current sinking capability to shunt the excess current to control the voltage. The resistor has to have enough current capacity to sustain the current for the entire circuit.

    With the 28 volt transformer I found in my stash, the rectified DC voltage with no load is about 22 volts. The formula for finding the amount of current that the series resistor must handle is simply ohms law. Here is the formula for the 150 ohm resistor found on the original power supply.

    I=V/R
    V= 22-18
    R=150
    I= (22-18)/150
    I = 4/150
    I = .026 AMP
    I = 26 mA

    Here’s the tricky part. For a zener to regulate correctly, it must pass a minimum amount of current. The current, the series resistor passes, will be split between the zener and the board. If the board consumes 25 mA, then almost the entire drop of 26 mA over the series resistor is due to the board, not the zener and the zener will only be passing 1 mA. I checked the data sheet of a typical zener, the Fairchild BZX79C18 and it is specified at 5 mA. 1 mA may not be enough to regulate the voltage well, at least for that part. Either a larger input voltage coming from the transformer or smaller series resister will be required, if I was to use a BZX79C18.

    If I switch to a 120 ohm series resistor. The formula looks a little more promising.

    I = (22-18)/120 = .033 AMP
    I = 33 mA

    With the board consuming 25 mA, this leaves 8 mA for the zener to shunt, which should be enough, at least for the BZX79C18.

    I don’t have the specification for the original power supply’s transformer, which may account for the slightly different series resistor value of 150 ohms that is in the original device.

    The wattage capacity of the series resistor and the zener is also important. The formula for watts is simple W = I x V. First for the resistor.

    W = I x V
    W = .033 x 4
    W = .132 watts
    W = 132 mW

    Assuming my other calculations and measurements are correct, even small .25 watt resistors should be sufficient for this application.

    Then for the zener

    I = .033 – .025 (the board consumes .025 Amps, which is not shunted through the zener)
    I = .008
    W = .008 x 18
    W = .144 watts
    W = 144 mW

    The BZX79C18 is rated at 500 mW, so it also should be fine for this application.

    With the low current requirement for this power supply, I’m going to pick up a smaller transformer, as the 1 AMP transformer I had in my parts stash, is clearly overkill and will not fit in the oscilloscope interface enclosure.

    Analog Supply Reverse Engineered.

    September 10th, 2016

    Thanks to Jack Rubin, I have two pictures of the inside of the only known SCELBI Oscilloscope Display interface. Here is one of them.

    SCELBI Oscope Internal

    SCELBI Oscope Internal

    To me, the surprise was the small power supply located in this chassis. Further research indicates that it was used to power the analog board, which contains 4 SN72741 op amps. The data sheet of these op-amps indicate that they take a +18/-18 volt split power supply. This also can be seen in the SCELBI schematics. Since I had only this and one other picture to go on, at first I wondered whether I could figure out how this supply was constructed. However, it didn’t take too long to come up with the following schematic.

    SCELBI Scope power supply

    SCELBI Scope power supply

    Ignore part numbers in this schematic. The resitors appear to be 150 ohms, probably 1/2 watt. The smoothing caps, C2 and C3 on the +18 and -18 supplies appear to be 100uFD, rated at 10 volts. 10 volts seems to be under rated for an 18 volt supply. I don’t know what value the first smoothing cap C1 is, but it should be rated for around 50 volts. I also don’t know exactly which rectifier diodes, D1-D4 are used. The zenor diodes, D5 and D6 should be 18 volt devices and are used to set the output voltage.

    In order to confirm my schematic, I rigged up a test power supply, using some parts that I had on hand.

    Scope Analog Supply Prototype

    Scope Analog Supply Prototype

    This test jig lacks the zenor diodes, and uses an 28 volt transformer. Output is around +/-20 volts, which should be about right. Next, I’ll have to find some 18 volt zeners and see if I can power the analog board with it.

    SCELBI Oscilloscope PCBs Arrived

    August 19th, 2016

    Sometime in 2011, Cameron Cooper planted the idea of reproducing a SCELBI computer into my head. Once he had me convinced, I knew that I wanted to reproduce the entire line of SCELBI computers and I/O peripherals. SCELBI produced 16 different PCBs over the short lived life of their computer line, so this was going to be no small task. I knew that it would take a long time, and I was never sure that I would be able to maintain focus long enough to do them all.

    At last, after 5 years, the last reproduction SCELBI PCBs have been made! Though I still have to build them up, write some software, and test them, the hard and most expensive part of making the PCBs has been done.

    SCELBI Oscope PCBs

    SCELBI Oscope PCBs


    The digital board is the big one and the analog board is the small one. The digital board takes 16 bits of input from the computer for each character and coverts it into 4 bit digital X and Y vectors and the blanking information that make up a single character. The analog board coverts the XY vectors into analog voltages suitable for oscilloscope input. The analog card also will control horizontal and vertical positioning of each succeeding character and line.

    I still have some parts to acquire, but I should have the missing parts and be able to begin assembly in about a week or two. Meanwhile, I can do some work on the software.

    Here is the list of all 16 SCELBI PCBs
    Main System Cards
    1100 CPU – 8H/8B
    1101 Data bus buffer – 8H/8B
    1102 Input – 8H/8B
    1103 Backplane – 8H
    1104 Front Panel – 8H/8B
    1105 1K SRAM – 8H
    1106 Memory Expansion – 8B
    1107 4K SRAM – 8B
    1108 Backplane – 8B
    1109 PROM – 8B

    Peripheral Cards
    2100 Oscilloscope digital
    2101 Oscilloscope analog
    2102 Audio Tape output
    2103 Audio Tape read
    2104 Teletype interface
    2105 Keyboard