Orange Terror Bass - SMPS not consistently starting up

[Mars Amp Repair]

Hello folks,
I know there is another post about these Orange SMPS's however this has a specific issue.
the SMPS will not consistently start up. Initially, it wouldn't start up as evidenced by the lack of current being drawn by the unit w/power hitting power switch. With all connected to proper isolation xfrmr I went to scope the oscillator output of IC UF4004 and as soon as I touched it the oscillator started up & will not quit.
I seem to recall on some of these SMPS supplies there is a circuit that serves to kind of kick-start the oscillator.
I'm not that familiar with these in that aspect. Other than just replacing the IC, does anyone see any components in the SMPS that would serve that purpose or do you have any other suggestions?
This SMPS does not fall in the ser# range of the troublesome SMPS's that Orange dealt with origially.
Thanx, glen

[The Dude]

I'd check pin 1 VCC with scope. Make sure it's clean and clean at power up.

[g1]

And make sure R102 on spec. & solder good.

[Mick Bailey]

The bootstrap circuit for the IC consists of D101 and C107. Once running, the IC is self-oscillating. Usually I don't advocate shotgunning parts, but for the the small cost of replacing a diode and maybe a capacitor, I'd change them - starting with D101. Once the circuit is running the diode is no longer needed to supply a charge to the capacitor. Probing the circuit may be introducing noise that causes the oscillator to start (as it's a micropower startup device) - all of this assumes that the connections and supply to the IC are all good.

EDIT: Thinking about this again, C107 is most likely in good order - I think if this was defective the circuit wouldn't oscillate at all. R102 is also part of the bootstrap circuit, (I overlooked this) as already mentioned by G1.

[dmeek]

In SMPS repairs I often find the electrolytic C1 is bad. Be sure to use low ESR caps for any replacements.