Archive for the ‘8008’ Category

SCELBI Cassette and TTY Enclosures

Wednesday, September 30th, 2015

Bud Industries CU-3009 A enclosures seem to be an exact match for the enclosures used to build the originals. I hope to find time in the coming weeks to post details on external and internal modifications needed to turn the enclosure into an accurate reproduction SCELBI peripheral chassis.

I’ve also been working on adding cassette support to my OS/X SCELBI emulator application. The other day, I was able to read an SCELBI formated AIFF file for the first time using the cassette driver normally included with MEA. While working on this, I think I may have discovered a bug in the cassette read driver. More about that in a future blog post, once I do a bit more analysis.

Also, I discovered the diodes on the cassette read and write boards may actually be 1n4148 types, not 1n914s.

SCELBI Audio (AIFF format) to Intel Hex Converter Tool

Thursday, September 24th, 2015

This program converts SCELBI Audio files that have been digitized into AIFF format into Intel hex files. My 8008 emulators use Intel hex files, so I know that this will be an efficient way to move files back and forth between the two worlds. It works well enough that I was able to recover a 40 year old original SCELBI tape that my reproduction SCELBI couldn’t completely handle.

The program is a work in progress, and there is some ugliness in the commenting, coding style, etc, but it works well enough, that I figured I’d share it, rather than wait to find the time to perfect it. I put the source in the retro-restore newsgroup files section under SCELBI. Be aware that you need to download an AIFF “C” language library from source forge to build it.

It should be possible to use the program as a starting point for other audio format conversions that use FSK or similar encoding methods. At some point I’ll write a program that goes the other way from Intel hex to audio, like I have done for the Apple 1 and II.

Mike Willegal

SCELBI Write Card Update

Tuesday, September 22nd, 2015

In a previous post, I described how I discovered that the SCELBI cassette write card didn’t work with a standard 7493 in position Z4 without changing the value of the capacitor located between Z13 and Z14. I went ahead and ordered some 7493A parts, which arrived yesterday. Last night, I installed one in position Z4 of the cassette write card. As expected, with the “A” version part, the SCELBI cassette write card works with the standard .02uF capacitor.

Also, I have posted a new version of the Cassette BOM V2.1.


Sunday, September 20th, 2015

My last update was in December of 2014 and I’ve made a bit of progress on a couple of fronts.


I’ve done a substantial amount of work on completely OCRing the original 8B hardware manual. This is the first step of making a new manual for people interested in reproductions. I plan on going through the manual and adding all the information that I’ve accumulated that is relevant to people building reproductions. Much of this information is scattered about over several years of blog posts, emails and in some cases, web pages.

I just started work on putting together a build guide for the cassette interface boards. It will have the look and feel of the other SCELBI construction guides, but as we haven’t found original build instructions, it is new work. Though build instructions are missing, some original documentation on the cassette interface has turned up and that will be OCR’d, as well.


I have built reproductions of the 8H using a BUD AC-413 chassis and the 8B using a custom chassis that Corey had fabricated. I still need to put together reproduction chassis for the peripherals and the power supplies. For those, I’m using off the shelf enclosures of approximate the same dimensions of each type of original enclosure. At one point I was looking into making my own reproduction enclosures, but I haven’t found a good deal on a box and pan brake that is sufficiently capable to handle these jobs, so that idea is on indefinite hold. Screen printing of the 8B front panels should happen in the next week or so.


My estimate of getting the cassette boards done in the first half of 2015 was off by a few months, but it did happen in 2015. I do feel a bit bad that the people working on reproduction 8B projects had to wait so long for those cards.

My estimate of getting keyboard and oscilloscope interfaces done in 2015 is not likely to happen, though I expect the fairly simple keyboard card should be done in early 2016 and I hope the oscilloscope cards are done by end of 2016. Both of these cards will take some software work, as no one has come up with original drivers for either.

In all, there were 16 different PCBs made by SCELBI for the 8H and 8B computers and associated peripherals. For quite a while, I’ve dreamed of having reproduced the entire set. It’s hard to believe, but at this point, I’m only three PCBs away from having completed that task. For those that are into numbers, that means that I’m over 81% of the way there!

The current status of my reproduction efforts of each of those cards follows.

Main System Cards – working reproductions of all 10 cards types have been completed.

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 – 3 reproduced, 1 in progress, 2 haven’t been started.

2102 Audio Tape output
2103 Audio Tape read
2104 Teletype interface*

In progress
2105 Keyboard

Not started
2100 Oscilloscope digital**
2101 Oscilloscope analog

* not as high as normal vector boards
** double width vector board

SCELBI 8B with MEA and SCELBAL Video

Saturday, September 19th, 2015

I just put a video demo of the SCELBI 8B with MEA and SCELBAL. Like usual, with my videos, the quality is substandard, but at least you’ll get to see a glimpse what I have working. The truth is creating quality videos is extremely time consuming and I’d rather spend my spare time (and money) on the vintage computers, themselves, rather than movies about them.

podcast feed is on iTunes store under “Mike’s Hobby Pages”
or direct feed at
or direct link to movie at

Likely Changes to Cassette Board BOMs

Tuesday, September 15th, 2015

I posted a preliminary copy of the cassette read and write board BOMs back in this posting.

Further investigation into the problem with the cassette write board reveals that the original SCELBI’s used revision A of the 7493 counter ICs, while I had purchased the original version. There seems to be significant differences in the specs, so I ordered a few 7493As to try. I’m fully expecting that the write board will work fine with the A revision parts, without having to change the capacitor. I have had very few problems with SCELBI designs in the past. I think that the inclusion of 7493As in production boards instead of plain vanilla 7493s was probably intentional, possibly to avoid the problem that I encountered.

The other change that is likely, is do to another image of the schematics that were sent to me. This version of the schematics have parts numbers labeled on the transistors. The image I received wasn’t all that clear, but the PNP types appear to be 2N2907 and the NPN types 2N2222. I doubt that the use of 2N3906 and 2N3904 on my reproduction in their place makes any difference, but I intend on updating the BOM.

SCELBI Cassette PCB Pricing

Monday, September 14th, 2015

I just added two items to my SCELBI PCB price list.

A set of SCELBI cassette interface boards is $50.

A complete set of all 13 boards needed to make up a full, 12K SRAM, 4K PROM, SCELBI 8B, plus cassette and TTY interfaces is available for $475.

8B Front Panels are not ready yet, as I have been focussed on the cassette interface for the past several weeks.

Send an email with your address and and questions for a shipping quote and ordering information.

Using an Apple Powerbook as a Cassette Tape Recorder

Sunday, September 13th, 2015

During my debug of the SCELBI cassette tape interface, I decided I needed to get better visibility to the actual output of the cassette interface. There are a couple of ways to do this. One way was to use a DSO (digital storage oscilloscope) or logic analyzer to capture and analyze the signal. I don’t have either, so I had to resort to a second method. That is to capture the cassette output as an audio clip using an audio recorder and editor on a personal computer. One such application is Audacity, which is a free download.

The only trouble with using a modern personal computer is getting the audio clip into the PC. Apple Powerbooks include a headphone port that includes speaker out and microphone in. Wendell Sander has an Apple 1 site that includes directions on connecting an Apple 1 to an iPod’s headphone port. I did a bit of research and determined that the Powerbook headphone port is basically the same.

I constructed a dongle based on Wendells directions and hooked it up to an Apple IIe for testing. Unfortunately, I found that the Powerbook didn’t recognize the microphone input. After some more web searches I found a couple of sites that indicated that a 1600 ohm resistor between microphone in and ground would cause the microphone to be recognized. I hooked up a 1500 ohm resistor and found that it worked most of the time. The next larger size resistor in my stash was 2K ohms, so I switched to that and found that it worked reliably. I tested with the Apple IIe and found I could read and write reliably. I also decided to add a .1 uF capacitor on the microphone input line to decouple it.

I next switched to the SCELBI to capture the output of the SCELBI write card. This proved very helpful and with the help of the Audacity audio editor, I was able to zero in on the problem I was having. During the examination of the SCELBI cassette signal I noticed that the high frequency tone was greatly attenuated. This was due to the .33uF capacitor suggested in Wendell’s notes. The SCELBI uses much higher frequencies than the Apple. I needed to change the capacitor to .01uF so it didn’t attenuate the 2700 Hz high frequency signal used in the SCELBI cassette interface. The resulting circuit is depicted below.

powerbook cassette adapter circuit

powerbook cassette adapter circuit

Here is a picture of the dongle. Note that I took a 4 conductor patch cord and cut it in two for the plug end. The reason is that the small clearances in 4 conductor plugs are difficult to solder cleanly. It was much easier to cut the wire and solder the wires and components together.

Powerbook Cassette Interface Dongle

Powerbook Cassette Interface Dongle

And here is a screen shot of the Audacity application showing a piece of SCELBI cassette audio out. The higher frequency portions represent ones, and the lower frequency are zeros. It takes 2 cycles of low frequency or 4 cycles of high frequency to represent a single bit. This encoding method is called frequency shift keying or FSK.

SCELBI Cassette Clip

SCELBI Cassette Clip

SCELBI Cassette Interface Now Working!

Saturday, September 12th, 2015

Yeah!! Finally a SCELBI 8B is working like it was meant to be with TTY and cassette interface. This may be the only working fully configured SCELBI 8B in the world at this moment.

After much investigation, which I’ll document more fully in later posts, I finally figured out the problem with the SCELBI cassette interface. Turns out that the problem was on the write board. The .02 uF capacitor on the inputs to Z14 (pins 3 and 4) was creating problems with the counter at Z4, which uses the same clock. Apparently due to the altered shape of the clock, the counter was not counting properly. This created chaos with the written data. I changed the capacitor to .004 uF and everything started working perfectly.

Once I made this change, I just slightly tweaked cassette player volume and I was able to write and read a small buffer of all zeros and another of all ones. I was also able to write a 2K buffer of random data and read it back without any parity errors. I need to do some more testing and tuning, but based on this initial experience, I’m expecting that this interface will work pretty well in practice.

I’ll need to take a breath and figure out board prices, which will be similar or less than the TTY board and then, at last, I’ll be able to start shipping cassette PCBs.

Here’s a picture of my current SCELBI 8B setup, with TTY card, Cassette read and write cards and RS-232 dongle all attached. The TTY and Cassette cards will eventually be moved to enclosures with female 78S11 Amphenol connectors. These will be connected with standard SCELBI 11(really 12) conductor cable with male 86CP11 Amphenol connectors on both ends to the chassis.

SCELBI loaded up with I/O

SCELBI loaded up with I/O

SCELBI Cassette Update

Friday, September 4th, 2015

First, my previous post had a mistake. Input is low(zero) on high frequency and high(one) on low frequency. I forgot to account for the inverter that does the final TTL normalization.

The other good news is that I think I’ve been able to get a partial read of one of Mark Arnold’s recordings. It’s not surprising to me that I can’t get an entire read of a copy of a 40 year old recording. This is a move in the right direction, because it gives me a good look at how the read card recovers a proper SCELBI recording. In fact, the TTL normalized signal looks different than what I get from a recording coming out of my system. I need to look closer at my write card operation.