Woz’s take on the Apple 1’s noisy -5 volt supply

There was a recent thread about the Apple 1’s noisy -5 volt supply on Applefritter, so I asked Woz the following:

One thing a number of us have noticed while working with reproductions of the Apple 1, is the amount of noise that exists on the -5 volt supply. The stabiliy of the -5 volt rail appears to be affected by the edges of the -12 volt clock that is used to control the video shift registers. Usually it’s not a problem, but I’ve run across a few cases where it affected DRAM reliability. Adding additional decoupling to -5 near the shift registers and the -5 regulator seems to clean things up considerably.

I was wondering if any of you remember noticing that noise on the -5 volt supply and if that ever was an issue back “in the day”.

I recieved the following reply from Woz:

I am sure that what you describe is valid, although I personally wasn’t aware of it.

I did the prototype of the Apple I and debugged the PC board version but didn’t look into such aspects. I’m sure you are quite correct. We knew that this was a low volume product since we were demonstrating the Apple II before shipping the first Apple I. Hence, we did not have many to become aware of issues like this. Part of the problem was that my time was being spent on the Apple II completion.

The ‘productizing’ of the Apple I came under Steve Jobs. I always optimized my prototypes for short distance wiring, but the PC board introduced longer power traces. Please forgive me. I never looked closely at this aspect. I certainly over-minimized in bypassing decoupling capacitors throughout the chips and RAM. I did worse too that was probably copied over to the PC board, like not having pullup resistors on unused TTL inputs. Still, had we at Apple been aware of such an issue while selling maybe 150 Apple I’s, we could and would easily and quickly have rectified it. But we didn’t test fully a product that was a temporary place-holder before the big product. We did try to buy back every Apple I in exchange for Apple II’s.

We had more luck than anyone deserves with things working out just enough to suffice and do what we did.

I will tell you that I and others did observe the power lines and did not notice noise or spiking. And, as I said, it was never a problem that was called to our attention, or at least to my attention. We could have put out an errata sheet for owners to fix the problem themselves, since this was very much a maker product (local stores could modify things to use 16K RAM’s, for example, and they did.

I am totally interested in hearing such things even after all these decades. I awoke one night in Quito, Ecuador, this year and came up with a way to save a chip or two from the Apple II, and a trivial way to have the 2 grays of the Apple II be different (light gray and dark gray) but it’s 38 years too late. It did give me a good smile, since I know how hard it is to improve on that design.


4 Responses to “Woz’s take on the Apple 1’s noisy -5 volt supply”

  1. DF says:

    Are you going to follow up with him on the Apple II improvements?

  2. Mike says:


    I did ask. Regarding the two shades of Grey, they are represented in memory by either A or 5 which in both cases turns into a 50% duty cycle square wave going to the video mixer (those three resistors on the right side of the board). Changing the duty cycle of the signal (percent hi versus low) will change the shade of grey, as well as affect other colors. Woz suggested that it could be done with a resistor, capacitor and diode, though he didn’t specify the exact circuit.

    I did some experiments over the weekend and found I could easily get an effect just by adding a 47pF capacitor to the output of the 74LS74 at B10, pin 5 (rev 0 schematics). This gave slightly different shade of gray, but didn’t affect the other colors very much. However that capacitor and some other things I tried, really messes with the integrity of the signal, so I’m looking for a “cleaner” solution. Maybe I’ll try using a 74LS123 one shot, though it kind of violates Woz’s idea that it could be done without adding chips. Maybe the integrity of the signal doesn’t matter that much and the simple capacitor solution would be fine. You could use a variable cap and make the effect adjustable.

    This flip flop is clocked by the system 14MHz clock, so you can’t do it with normal digital logic. Also, fooling with the clock input to that flip flop would likely upset timing for the entire system.

    He didn’t say what he had in mind for eliminating a chip or two.

    Mike W.

  3. […] detail comes from a bit of communication between Woz and a deeply devoted Apple I reproduction expert, Mike […]

  4. JD says:

    Most of this was WAY over my head, as I am not a EE, but I found this to be a fascinating read, and that he actually cared still – which just goes to show that all the good visionaries are rooted in passion and obsession.