Archive for April, 2010

3 hours of effort and

Tuesday, April 27th, 2010

this is the result

consider it a very very rough draft… let me know if you are interested in me pursuing this further

Get your Mimeo 1 or A2 rev 0 replica, signed by WOZ

Tuesday, April 27th, 2010

Cameron Cooper had his Mimeo 1 signed by WOZ. I had another Mimeo 1 builder mention that he was going to have it done, as well. This second builder pointed out the web site that offers the mail in service.

On the site, there happens to be a picture of WOZ signing Cameron’s Mimeo 1!

Note that Cameron was told that this was the first replica Apple 1 that WOZ has seen. It seems that it will not be the last.

I’m tempted to send in a batch of bare boards, but I’ll leave it up to the replica builder to decide whether it is worth the trouble or not. I’ve already spent enough money on this project.

Limited quantities of Mimeo Kits now available

Friday, April 23rd, 2010

I have finally finished working through the wait list and now have a very limited quantity of Mimeo 1 kits available. Shipment should be within a few days of ordering, though it could take a few weeks, if I run short of any specific component. Send me an email, and I’ll let you know what you should expect.

I have an ample supply of PS/2 keyboard adapters in stock.

Finally, if you can’t find the parts you need to complete your Apple-1 clone, send me an email, and I’ll send you a price list. Just a word of warning – don’t expect distributer type pricing.

Updating Mimeo web pages and the enclosure is started

Saturday, April 17th, 2010

Revamped my Apple 1 web pages – basically splitting information into separate pages that should be easier to browse:

I also have started work on my Mimeo 1 enclosure. I’ll document work on this web page:

My “process page” now includes a close up side by side detail of a small part of the original and Mineo PCBs.

Apple Cassette Interface Revisited

Sunday, April 11th, 2010

Yesterday, I spent the good part of the day with the ACI and finally have a good fix that makes it far more robust without changing the board layout. In the past I had discovered that the Apple II cassette interface is far more reliable than the Apple 1 was. I did spice simulations of both and quite a bit of fooling around with the actual ACI in order to see if I could figure out the problem. The best improvement I could come up with, was to add a low pass filter on the input. This was done by adding a .01uF cap between the input side of the comparator and ground.

In revisiting the ACI yesterday, I finally discovered the real difference in the design that makes the Apple II so much more reliable. The Apple II uses a .1uF capacitor for input coupling versus a .01uF on the ACI. When I looked at this in the past, I must have forgotten to make this change when evaluating the designs with spice. This change provides a lot more noise immunity in the comparator inputs.

Here is the ACI input signal with the stock .01uF input coupling capacitor. The source of the signal is the 1HZ tone (all ones) at the front of Wendell Sanders Hammerabi clip being played from an iPod with the equalizer set to “treble reducer”. All signals are displayed at .5 volts/division on this page.

In the image, above, the input is the bottom signal and top signal is how it is presented to the comparator input after passing through the capacitor. Note how the signal spikes and then returns to the value set by the 10K resistors.

Above is the same input and output signals with a .1uF capacitor. The signal follows the shape of the input waveform much more closely. The bottom trace is input signal and is the same as in the first image. The top trace is input to comparator after passing through cap. Note how the extra capacitance prevents the resistors from quickly returning to the base value set by the resistors.

This image shows both inputs to the comparator with the stock .01uF capacitor. Spiky looking trace is input signal to comparator (same signal as seen in first image). Other trace is reference signal with hysterisis. The reference signal is pulled up and down slightly as the comparator switches state to prevent instability. This results in the square wave.

In the picture above, the output switches state when input signal (more spiky looking signal) passes the level of the reference signal as can be plainly seen in this image. The problem with this design is the limited amount of room between the input and the reference, any input noise with an amplitude of around .2 volts or more may be enough to trigger a switch in output levels.

Below are the same two inputs to the comparator with the .1uF cap. This wave form increases noise immunity to almost a volt, about 5 times improvement over the apple 1 stock implementation.

Unless you are interested in maintaining complete commonality with the original Apple 1, I highly recommend using a .1uF capacitor in your replica cassette interface instead of the .01uF used in the original design. Keep in mind that the folks at Apple recognized this improvement before coming out with the Apple II, as that design includes .1uF cap for this application.

Since I was using a clone ACI, built with components based on the schematics, I wondered if Apple actually shipped with .1uF caps on the ACI. I exchanged a couple of emails with Wendell Sander. He confirmed that the schematics are correct and the Apple ACI did indeed ship with a .01uF caps. He also confirmed that he had independently come to the same conclusion regarding the reliability improvement that could be attained with a .1uF cap.

Replacement Mimeo Boards Arrive

Thursday, April 8th, 2010

They look great – just like the first batch with the silk screen printed right on the copper in the power supply section. – I’m working through the wait list now, notifying people that I have units ready to ship. By next week I hope to be completely caught up.

I’m a little short of two part types. I only have a few 74161s which is needed for location D-8 instead of 74161As and I may have to ship a few kits with the A parts, but I’ll ship the 74161s for location D-8 to anyone who ends up in that category, once the backordered parts arrive. See this post for info on the problem with the 74161 parts.

The other part is the P-8667 transformer, but I think I have enough of those in stock to satisfy immediate demand. In any case, the transformer is a current production part, so if I run short, I can refill stocks quickly.

PS/2 RS232 to ASCII Keyboard Adapters Now Available

Wednesday, April 7th, 2010

Go to this web page for details:

In my opinion, building one of these adapters is not very difficult. I have been able build and test one of these adapters in about an hour.

If you can get a piece of Apple 1 software into Apple Monitor Command format, I believe that this RS232 interface is one of the most straight-forward ways to download software from a PC to your Apple 1.

Silk Screen Problem Resolution

Tuesday, April 6th, 2010

The last post mentioned a problem with the silk screen with my new batch of boards. Well, I contacted the fab house, Advanced Circuits, and explained how critical the silk screen is to the people I share the boards with. They are going to rerun the boards- hopefully I’ll have the new boards in my hands at the end of the week.

One more thing – Advanced also explained why they like to mask off the silk screen from the copper. If you decide to flow solder your board, keep in mind that the flow soldering process may cause the silk screen that is located directly on the copper layer to lift off.

In case you have any thoughts of getting a deal on the boards with the bad silk screen, Advanced asked me to send them back, so it isn’t going to happen.

Boards are in

Tuesday, April 6th, 2010

The keyboard adapters are sweet looking boards if you ask me. It took me about an hour to build up one keyboard adapters, that is after the three hours it took to remember how to program the AVR correctly.. So far it looks fine, but I have a little more testing to do. Here is the front and back of the PCB and the board I just built.

The second Mimeo 1 boards are also in. One little hitch though. This time, the fab house decided to mask the silk screen with the solder mask, against my explicit instructions. The only place where you can tell the difference is in the power supply section as shown here. I didn’t notice it at all until the third time I was reviewing the board. The rest of the board looks exactly like the first batch, as it should, since the same CAD files were used.

I’m going to complain to the fab guys tomorrow and see what they will do about it.

6502 Memory Test Posted

Saturday, April 3rd, 2010

I put together a 6502 memory test that can be loaded through the monitor. This is to help A2 and A1 kit builders test out their boards. Here is the link:

Next batch of Mimeo’s is due to arrive Monday along with the first batch of PS/2-RS232 ASCII keyboard adaptors.