> -----Original Message-----
> From: Peter Coghlan via cctalk <[email protected]>
> Sent: 09 May 2023 17:58
> To: 'General Discussion: On-Topic and Off-Topic Posts'
> <[email protected]>
> Cc: Peter Coghlan <[email protected]>
> Subject: [cctalk] Re: Rainbow H7842 PSU Fault
>
> >
> > I am going to read your answer more carefully later. But I wanted to
> > check one thing. I measured the base-emitter voltage as negative in
> > both cases, and yet the TIP121 appears to be conducting on the bad
> > PSU. Surely that means that the TIP121 is not working correctly?
> >
>
> I'd be very reluctant to draw any conclusions from measurements made
> when the device is not biased correctly. As Brent says, there could be
> current
> flowing through the presumably forward biased base-collector junction.
> Also, this device is a not just a straighforward transistor. It's data sheet
> says it
> is a package containing two transistors and some other components,
> including a normally reverse biased diode between the collector and emitter
> terminals. A positive voltage of sufficient magnitude on it's emitter could
> be
> forward biasing this diode resulting in current flow through it and therefore
> also through the 20R resistors causing a voltage to be developed across
> them.
>
> I think a better way to determine if the TIP121 is causing excess current draw
> on the -12V line is to make the voltage at it's emitter approximately correct
> and see if it then draws enough current through itself to cause the excess
> current trip to operate. This current can then be measured by observing the
> voltage across the 20R resistors (or the smoke coming from them if they are
> not of sufficient power rating...). It should also be possible to observe the
> conditions around the zener diode and the MPSA55 and see if these
> components are behaving reasonably.
I will do all the suggested checks, but I won't be able to do this for a number
of days. However, I wanted to understand something in the meantime. The
conditions I am applying are (I think!) what would happen during startup, and
during startup the control board has to make the PWM run, otherwise the main
switching transistor won't operate and the transformer won't operate to produce
the -12V in the first place. So surely in the startup condition I am applying
(which is to supply Vstart from a bench PSU) is valid? Where is the flaw in my
reasoning here?
>
> Regards,
> Peter.
