Switching PSU Theory of Operation / Repair Tips

From: jpero_at_sympatico.ca <(jpero_at_sympatico.ca)>
Date: Sat Mar 16 17:57:18 2002

> From: "John Salmon" <reversebias_at_talk21.com>
> To: <classiccmp_at_classiccmp.org>
> Subject: Re: Switching PSU Theory of Operation / Repair Tips
> Date: Sun, 17 Mar 2002 03:38:27 -0800
> Reply-to: classiccmp_at_classiccmp.org

> Re Where does the 600v come from?
>
> The SMPSU in question is probably of the 'Flyback' or of the 'Forward Converter' type. In either case the converter O/P Tx primary winding is connected between the +300vdc rail and the collector of t>
> During the conducting period of the switching transistor (lets call it TR4 - it often is) the potential across TR4 is very low, a few volts. Hence virtually all of the 300vdc appears across the prima>
> When TR4 is driven into its 'off' period Lenz's Law causes the primary inductance of Tx1 to try to maintain the primary current. The resultant induced emf (due to collapse of primary flux) now produ>
> Therefore the collector of Tr4 is now 600v +tve to the 0v (-tve) rail ofv the PSU. Don't touch its Heatsink while the PSU is switched on. Also allow 4 or 5 mins for the voltages to bleed down after s>
> Hope this is clear and useful - it is 3.30am in the morning here in UK.
> Regards

Lot of comments on mine: :-) boy that small one snowballed into big
paper. :-P

That helped! Also I have comments on capacitor bleed down after
unplugged from outlet. Some didn't have bleed down resistor(s),
required by safety in peecee PSUs I think but some did have
bleed down resistor fail or simply didn't exist. How I know? I got
bit by charged main filter capacitor in servicing modern JVC TV. Btw
that one didn't have that bleed down resistor even JVC is decent
brand! Now I test for that for any reason and flatten it off with
15k ohms resistor. :-P

My story:
One hand rests on certain area circuit board - BUZZ buzz buzz!
AAAAGGGGGGH! In loud, shrill voice to make singers proud. DC felt
like buzz like I get with raw AC from that outlets. That capacitor
didn't discharge at all even after a week. That's very good quality
capcitor, JVC! :-)

This reminds me on this high voltages in both monitors and TVs.
Give BIG due respect to high voltage in any monitors and tv, biggest
ones is around 35,000 or so volts. Enough energy on HV to kick like
mule and hurts like hell but not kill you. The danger is your hand
or part of your body fly away and hit something else (injury or get
cut.)

High voltage is used to impart focused electrons emitted from
electron guns to high speed against phosrphous (sp?) that glows when
hit. Tube bell is made of painted graphite (that characteric dark
grey) and glue binder on two parallel plates construction with glass
as dielectric basically tiny capacitor to smooth HV pulses (more of
sprikes from the flyback coil, inner surface has HV, outside surface
on that tube is at ground potiential for HV circuit at few volts in
respect to true frame ground especially in TVs because it does not
have true ground design because of two prong plug while mesuring
voltages on that circuit board, for that reason, I always use
circuit board ground not that grounding wires across tube
bell.

In compact Macs, tech sheets warns DO not to use dead short to
discharge tube! Properly done requires homemade tool or proper tool
that has resistor to bleed out tube's stored HV gradually. Shorting
out that tube with a noisy bang using piece of wire, this burns out
the chips on that logic board, seen this happen before. Also this is
preferred on everything else since the anode well on the inside
conductive coating connection may burn open by high
current then tube becomes junk. $$$.

When off or unplugged still have HV charge present for few
days to a week before that tube becomes discharged.

Freshly charged tube has enough punch to jump a inch or two gap -
LOUD SNAP. And, one more thing, after manual
discharged, tubes always regains some charge after few minutes, don't
be surprised by a tiny zap or see tiny spark. :-)

Cheers,

Wizard

PS: Even good monitor or tv, sometimes HV leak out and bit you if
you're careless. I saw this happen when tech rested that grounded
screwdriver gently on glass near edge of suction cup - CRACK! Tech
then looked surprised turned to me and I said oooh.

PSS: Snapping, crackling, hissing noise is sign of HV leak or
overvoltage fault this one must be serviced NOW because of
HV had skyrocketed due to component failure somewhere and excessive
X-ray emssions limiter or trip had failed also.
 
> John Salmon
> Q&R Engineer
>
Received on Sat Mar 16 2002 - 17:57:18 GMT