PSU7 - intermittent operation in Commodore VIC20

[sdf3sdf]

I’m trying to use a PSU7 in place of an LM323K in an original Commodore VIC20 (two-prong power socket). Yeah, not sure why I got a bunch of PSU7s instead of PSU5s, but not sure if that would make a difference.

The VIC20 receives a nominal 9VAC (around 10.2VAC when measured) which is then rectified (about 11.5VDC) before being sent to the LM323/PSU7.

The computer will start-up fine the first time - your PSU7 outputting 5VDC.

However if I switch the VIC20 off and then on again, even if I pause for a few minutes before powering on again, the PSU7 refuses to operate. Waiting some time, the computer will then work again. It’s definitely not due to heat or over-temp.

Measuring the voltages again, with the PSU7 outputting 0V, shows about 10.6VAC incoming and about 13.2VDC from the rectifier … but this is still well-within the PSU7’s tolerances, but strangely a few volts higher than a cold boot.

What I just noticed last night is that the input to the PSU7 remains at around 6V after power-off and slowly drops … likely because of the large 4700µF (blue) capacitor that is connected to the PSU7 input pin.

So I wanted to ask your advice - even though the PSU7 should work because the input voltage is okay, is the PSU7 deliberately protecting itself from too large inrush current or something due to the discharging capacitor?

I really want to keep the PSU7 in there (the reason why I bought a bunch from you), but I’m thinking that I might have to revert back to an LM323K and all the associated heat sinks.

Thanks!
Brett.

[sdf3sdf]

Discussed with Daniel via email:

This is a "feature" of the PSU7. The reason is that the PSU7 want's to see the input and output at 0V (approximately) before it will start up normally. The PSU7 is designed for 24V operation and can withstand 28V if you are careful about spikes on the supplies.

Some regulators are OK starting up into an existing voltage on the output (prebias) because they are designed for that, some are not. The PSU7 does not like prebias, the control loop becomes unstable. Because of that it has a holdoff circuit to prevent it from starting into an existing output. When the input voltage takes a long time to go to zero the holdoff circuit does not reset. I forget the voltage where it resets (its not specified) but its around 2V.

A few solutions are possible. Add a resistor to the cap to drain it.

[sdf3sdf]

I added a push button switch that grounds the rectifier output via a 100Ω resistor - fixed!

Now when I have to power-off/power-on (i.e. changing cartridge) I just hold the push button down for a few seconds after power-off.

The VIC20, and PSU7, will then power-on again ... no more having to wait a few hours for the capacitor to slowly discharge.

I found this calculator at Digikey to be useful - I could've used a 56Ω resistor to discharge it faster, but 100Ω was fast enough: 4700µF capacitor, initial voltage 13V (when VIC20 is first powered off), safe voltage 2V (when PSU7 hold-off circuit resets)

Capacitor Safety Discharge Calculator