Source of Sn72741 op-amps

July 16th, 2016

The SCELBI cassette and oscilloscope interfaces both use a rather unusual variation of the jelly bean 741 op-amp. This is the TI SN72741, which puts this op-amp in a 14 pin DIP package. Though they can be had on e-bay, I recently discovered that has them for $5.50 each. They can be found at this link:

SCELBI Power Supply Internals

July 13th, 2016

This SCELBI 8B power supply is built in a shadow box type chassis that is approximately 10″x5″x6″.


The internals are simply two Power-One linear Supplies, a switch, a fuse, 3 binding post type connectors, an amphenol 78S4 type connector and a fan. Here is an image of the interior. The fan is connected to the top, so isn’t visible in this view. The 5 volt supply is a C5-6 and the -9 is a modified B15-1.5. Be aware that the -9 supply is not powerful enough for a fully loaded 8H chassis.

SCELBI Power Supply Internals

SCELBI Power Supply Internals

Repro SCELBI Keyboard Interface In House

July 4th, 2016
SCELBI Keyboard Interface

SCELBI Keyboard Interface

Looks like I have enough parts on hand to build one of these babies up. It expects a decoded but unlatched keyboard. I don’t have one of these, so I’ll modify the firmware on one of my PS/2 adapters to act like one. With that I should be able to hack together a simple 8008 driver that will eventually work with MEA on the SCELBI.

SCELBI 8B with Power Supply

July 3rd, 2016
SCELBI 8B Complete

SCELBI 8B Complete

Here is a complete, original, SCELBI 8B system that I was recently able to examine and operate. In the past, I already had examined the TTY and Cassette interfaces. This time, I was able to examine the power supply and will put up some posts in the near feature, including interior shots.

SCELBI Oscilloscope Digital Board Rework Instructions.

June 6th, 2016

This document provides rework instructions that will bring the PCB up to the revision seen in the schematics. I now have a pretty good understanding of how this interfaces works and will provide more details in a future post.

Oscilloscope Rework Instructions

Where Did All The Moths Come From?

June 1st, 2016

Over the last few years, we have had a plague of small moths, especially in the spring time. There are so many, that the larva from these moths defoliate certain types of trees in the spring. So far, these trees have managed to survive, but I’m wondering how many more years that they will be able to tolerate this. The moths are attacted to the lights inside the house at night. There are so many moths, that they get into the house almost every time you open a door at night.

While discussing the disapperance of the Bats with a neighbor the other day, I think I came up with the likely reason that the moth population has run amok. Bats eat moths and other similar sized flying insects, such as dragonflys. With the bats out of the picture, the moths have had little to curb their population, except the available food supply.

Note that bats typically do not eat mosquitos, since mosquitos are too small too provide much nourishment. Dragonflys do eat mosquitos, so with the bats gone, there may also be a reduction in the mosquito population as one consumer of the dragonfly population has been taken out of the picture. Come to think of it, the mosquito population does seems to be low for this time of year.

The balance of nature sure is sensitive and it’s surprising to see it out of whack in our own backyard.

SCELBI O-scope Layout Nears Completion

May 30th, 2016
Back of SCELBI O-Scope Digital Board with rework highlighted

Back of SCELBI O-Scope Digital Board with rework highlighted

This is an image of the current state of the back sidelayout of the long awaited (by a few) digital board overlaid over an image of the actual board. Base image is curtesy of Jack Rubin, the only person I know, that has seen an SCELBI oscilloscope up close and personal in recent memory. This one is in the Computer History Museum’s off site storage facility.

I haven’t worked on any of the lettering, but I’ve gone through multiple passes of the rest of the board. Based on file size, this board is actually more complex than the SCELBI CPU board.

One other thing that I’m going to do is more study of the rework on the only known Oscilloscope digital board. The arrows in the image point to areas where rework was applied. Initial review seems to indicate that the schematics found at match the board after rework was applied. There is also a bit of rework on the front side.

The analog and keyboard layouts are ready to go, though I’ll probably give each of those one more quick design review pass before ordering a batch.

SCELBI App Now Available On Apple’s App Store

March 30th, 2016

Link to SCELBI’s app on Apple’s App Store This is the same version (2.7) as available for download from my SCELBI web page. If you have used this version it would be great if you would review this Application on Apple’s App Store.

Brain Boards – batch 2 now available.

March 5th, 2016

I now have a batch of Brain Board kits ready to go.

For kits sent to the US – send $59 per kit to my Paypal account (at end of this email)

For kits sent internationally – send $59 per kit, plus $10 postage (combined shipping for all kits) to my Paypal address (at end of this email)

For those that want to build and test the kits add $25 per kit built and tested.

Shipment should occur within a few days, except for those that want me to build and test – expect a week or so before shipment.

My PayPal address is: (

Make sure you include your shipping address with Payment.

thanks and best regards,
Mike Willegal

PCB and kit stocking status

February 27th, 2016

I now have everything that I normally stock on hand – except SCELBI front panels.

I was a bit behind on things, but today I shipped a few items that I owed people, so am caught up, with the exception of those SCELBI front panels.

In addition, due to popular demand, I made a new run of Brain Board kits. I tested an example earlier in the week, and except for a bad 74LS74 IC, I found they work fine. I’ll have to go through my stock of 74LS74’s and test them before finishing putting together kits. With luck, I’ll have kits ready to ship by next weekend. Watch for an update in the next few days before sending money.