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 boards, 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.
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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?
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 🙂
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?
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 🙂
Hi, if I want to do this mod, which 4.7 nF capacitor would do? They are offered with different voltage indications (50V, 1000V???). I am a noob when it comes to this, so which capacitor should I buy?
Cheers, Andreas
Hi Andreas, caps rated above about 25V should work fine. Thus, a cap rated at 50V will be perfect for the job 🙂
Hello, I have short board, is this relevant for me?
Hi Rafal, the issue was fixed in the later short boards (Assy 250469) 🙂
I have 250407 rev C board. During recapping I looked at C38 position and was surprised to see a resistor not a capacitor there. Was it normal for some boards? This one seemed pristine (i.e. not touched by anyone before), so I do not think it was a later addition.
Hi there. C38 is a capacitor in a resistor case. It is usually green. So no worries – it’s all good 🙂
That would be very reassuring but… this element has a typical color banding of a resistor, measures 1.5MOhm with a meter, and shows no capacitance with a capacitance meter
Hi Roman, according to the C64 service manual C38 should be a 51pF, 50V, 5% ceramic capacitor. The color code should be green-red/brown-black and look like this: link. The high resistance you are measuring may be due to the capacitor being in circuit. That is probably also why you don’t measure any capacitance with your meter 🙂
Thank you! This explains it perfectly. The capacitor looks exactly as the one in the link. I really appreciate you guiding me through it and sorry for being a pain.
Happy to help 🙂
I just performed this mod on a PAL ASSY NO. 250407, and it fixed that stubborn RESTORE key. Keep up the good work!
Hi MtnBuffalo, I have done this mod on three breadbin 64s and each have worked beautifully. I remember as a child having to thwack the restore key which always annoyed me to the point I was worried I would break the keyboard. I am so pleased having gotten into retro gaming my C64 Restore key does not have to be bashed now.
Thank you for posting this mod I am very grateful.
Hi Ian, Happy to hear that you’ve got the mod working 🙂
I just did this MOD on a 250466 and was surprised as I unsoldered the original axial capacitor (in a green resistor casing) to find a second connection to one side of the cap, with the same spacing as the diodes we see a bit higher on the board. Makes adding the ceramic cap easier. Made me wonder if by 250466 they were planing to change the cap… Other axial caps on this board also have multiple solder points, so it must have only be about versatility.
Hi all. I got this board 250407 no.251137 REV B. I need to replace the MOS 7707 , MOS 7712 and MC3456P. Reading some forum and looking at the photos of my identical board, I have seen (I think) that MOS 7707 is equivalent at the 74LS06. MC3456P is equivalent at the NE556N. Correct ? Do you know if I can change the MOS 7712 with an equivalent? Thanks a lot to all