9 May
2023
9 May
'23
9:52 p.m.
-----Original Message-----
From: Peter Coghlan via cctalk <cctalk(a)classiccmp.org>
Sent: 09 May 2023 17:58
To: 'General Discussion: On-Topic and Off-Topic Posts'
<cctalk(a)classiccmp.org>
Cc: Peter Coghlan <cctalk(a)beyondthepale.ie>
Subject: [cctalk] Re: Rainbow H7842 PSU Fault
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?
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.
Regards,
Peter.