A friend of mine asked me to do something about the sluggish RESTORE key in one of his Breadboxes. It turns out that on the old longboards of the Commodore 64, the RESTORE key needs to be hit hard to function due to a misdimensioned capacitor at C38 also known as the ‘NMI Capacitor’. On the new short board, the problem was fixed. On MJK’s Commodore 64 & LCD Page I found the solution to the problem. If the original capacitor of 51 pF is exchanged by a 4.7 nF one, the RESTORE key functions as well as all the other keys of the keyboard. On all the longboards I’ve seen, the original capacitor has been build into a green resistor case. I ordered 100 new 4.7 nF ceramic capacitors from China for 2-3 $.

In the following images, the ceramic capacitor has already been exchanged. When the component has been swapped, test it by hitting the RUN/STOP + RESTORE key combination. It should now respond with the same amount of pressure as the rest of the keys on the keyboard. The images have been taken from an Assy 250407 Rev. C board. However, on all longboards (from all old Breadbins and some of the early C64C’s with the old boards installed), the location of the capacitor is pretty much the same.

© 2015

4 comments on “The C64 RESTORE Mod

  1. Hi, I tried this mod on my 250407 Rev B, and it rendered run/stop and several keys around it non-functional. Key presses would not register at all. I put the NMI cap back and function returned to normal. Have you ever encountered this?

  2. Hi Kirk, sounds strange. I have done the mod on about 10 machines so far and never experienced the issues you describe. I’ve even done it on a Assy 250407 rev. B just like yours (link). Are you sure that the capacitor you used was the correct size (4.7 nF) or that it was working properly? Other than that I have no idea what could be the reason for the strange behaviour. Hope you get it fixed 🙂

  3. You mention that the key needs to be hit ‘hard’ and that the fix makes it respond to the same amount of ‘pressure’ as the other keys. Is this because of the conductivity of the plunger and the amount of its surface that’s pressing on the contacts? Given its flat shape I wouldn’t expect it to make much difference. Is the issue really the pressure or the time for which it’s held?

  4. Hi Mike, the pressure dependent resistance of the plungers are identical on all keys. However, for triggering the NMI 556 timer the pressure RESTORE key must be hit harder/longer. To overcome this, replacing the capacitor with a bigger one fixes the issue. So I guess you may right that the sluggishness of the RESTORE key is more related to the amount of pressure than the pressure 🙂

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