Leakage problem in Railpower circuit

I have just built the RailPower Mk3 from the October, November and December 1999 issues and it works fine except for one problem. When completed, the maximum voltage could only be adjusted up to about 7V. Changing the 4.7MΩ resistor to 10MΩ allowed the maximum voltage setting to be adjusted from between approximately 6.5V and 15.5V.

The problem is that the 2.2μF capacitor in the voltage hold circuit connecting to pin 5 of the TL072 immediately drains, reducing the voltage from 10.5V to 0V in under one minute. I replaced the TL072 and double-checked the PC board for shorts and found none. Can you help? (T. D., Robina, Qld).

There appears to be leakage on the PC board adjacent to the capacitor. Change the capacitor and clean the PC board with methylated spirits and a toothbrush.

RF modulator for a DVD player

Have you ever done a project for an AV-to-RF modulator? With all these new cheap DVD players coming out, they don’t have an RF output; only AV which doesn’t suit the older TVs with only an RF input. (K. R., via email).

Making your own modulator is not worthwhile since you can buy the finished product so cheaply; eg, Jaycar Cat. LM-3872. Alternatively, you can connect the A/V signals from your DVD to a VCR and thence to your TV.