This is a 50 volt capacitor (at least in the machines I have here)
running reverse biased at 5.333. It wouldn't be surprising if there
was little or no leakage current through such a capacitor.
I tested two. One had no detectable leakage at 5.333 volts - less
than 10 nano-amps. The other had 0.85uA leakage. So it was
behaving like a presumably ~1uF capacitor with a 6.7 meg ohm
resistor across it. This would not upset the circuit in any audible
I have never encountered any problem with this part of the TB-303
- and I have worked on 300 to 400 of them.
I doubt the capacitor would lose its capacitance. At worst, there
would be growing leakage current due to holes in the aluminium
oxide layer, which are not being self-healed by the electrolyte,
because the polarity is the opposite of what it should be.
I have no reason to think that electrolytic or other capacitors from
the early 1980s have anything wrong with them. I don't recall any
failing of their own accord in TB-303s.
I guess a capacitor could leak so much that its voltage would drop
in a manner, such as to 3V or so, where the voltage across R106
(2.2k) is reduced sufficiently that the current through the Q21
pair is reduced so much as to cause clipping.
I think it would have to be a drastic drop to affect the gain of the
Q21 differential amplifier to any noticeable degree. If the
leakage was noisy, or erratic, that would probably cause audible
I think it is quite likely that in all the TB-303s made, few, if any,
of these capacitors have ever failed to the point of audible
changes. Nonetheless, I will replace them in the future, with a
similar capacitor connected the right way.
In the first revision of the boards, there is another reversed
capacitor, again due to a PCB layout slip-up. This is C51, the
2.2uF 50V cap on the input of the headphone amplifier. The first
revision boards had the dot and so the negative terminal on the
R159 pin, while it should be to the IC14 pin 2 pin. Again, the
reverse voltage is small compared to the 50 volt rating of the
capacitor, and it would take a lot of leakage to affect the
behaviour of the headphone amp. This capacitor only has
voltage across it when headphones are plugged in, so for many
TB-303s, it has only rarely and briefly reverse biased.
I can't imagine a good reason for reversing the polarity of C29
on a x0xb0x.
- Robin http://www.firstpr.com.au/rwi/dfish/