7 Jun
2010
7 Jun
'10
2:40 a.m.
Chris Elmquist wrote:
Have you tried the simplest bridge rectifier test of all - simply hook
your voltmeter to the outputs of the bridge and then using a single
diode of sufficient voltage and amperage you then jumper each internal
diode with the external one. If/when the voltage jumps up, then you have
proven the bridge rectifier is defective. Static tests (power off -
voltage drop across the bridge) doesn't catch bridges that fail under load.
Further to that - if you are not clear about what I mean by jumpering -
you take your external diode and bend the leads so you can place one
lead on an AC terminal and the other to either the '+' or '-' terminals
of the bridge. Taking EXTREME care to always have the banded end of the
external diode connecting to the more positive side of the bridge under
test!!!
I hope that is clear enough to enable you to check your bridge
recitifier which I (and others here) believe is the actual problem, not
the transformer!
I completely agree... although, I am now prepared to replace the transformer
so once I have desoldered the original I suspect I'll be putting back the
new ones.
Please don't do this until you have tested the bridge. If the bridge is
OK, then the next suspect is the primary filter capacitor for your 8VDC
system. Again, bridge it with a known to be good capacitor or similar or
less capacity and watch to see if the voltage goes up...
I agree with this too absolutely.
I have another homebrew S100 machine from the same era and it too has
a marginal power supply... but I won't think twice about rebuilding
that one with a set of switchers. It has no pedigree or authentity to
be preserved... other than it is built in a wooden box with a bright
orange Formica front panel... a piece that was left over from my mom's
new kitchen in 1979 ;-)
Chris
Switchers are great for protecting against power supply failures,
however some reset circuits assume linear supplies and don't work
reliably with switching supplies unless you modify them so the voltage
rails go up as the system expects.
If you decide against switches get TRANSORPS for the critical voltage
lines to clamp the voltage (and blow fuses) if the linear supply voltage
rises due to failure.
John :-#)#
PS - my experience is in servicing tube and solid state equipment since
the 1960s - since the mid 70s as a living (flippers.com ;-).
On Sunday (06/06/2010 at 06:54PM +0100), Tony Duell
wrote:
Yes... this is a fuse but it has not blown... ever. The machine got
pretty extensive use from early 1976 until 1984 or so and even with the
low 8V rail, it did heroic efforts for me.
Transformers are extremely simple beasts - they either work or they
don't. When they fail you really know it - they blow fuses and/or start
smoking with goop leaking out and making a general smelly mess or a
winding opens and you get no output at all. A transformer does NOT get
weak.
That
sounds very low indeed. I wouldn't expect a transformer secondary
to drop below rated voltage with only half load on it! Are you sure you
haven't got a shorted turn somewhere?
My experience is that a single shorted turn in a mains transformer will
cause enough primary-side current to be drawn to blow the fuse. Of course
that's assuming there is a primary-side fuse -- but if there isn't there
darn well should be :-)
Still, it can't hurt to measure the primary
current with no load on the
secondaaries. If you have a wattmeter, masure the power consumed by the
trransformer with no secodnary load (note that this is _not_ the product
of the primary corrent and the mains voltage, they will not be in phase).
If there is significant power drawn on no-load, then suyspect a shorted turn.
Yup. Have a wattmeter and can make this measurement... however only
after desoldering all the connections from the transformer to the
chassis mounted bridge and the bridge mounted on a PCB since none of
this is connectorized.
I agree with Tony that you should do some more tests
to see if there's
something wrong with the transformer or rectifier. For a start, look at
the secondary voltages and current drawn from the supply with all the
secondaries open circuit. Excessive no-load current is a reasonable
indicator of a shorted turn.
I should have read on before writing the above :-)
If you eventually do need to upgrade the power supply,
could you fit
just one switcher in the cabinet with that transformer? Use it for the
8V line, and keep the old PSU for the rest?
My personal view (if I had such a machine) is that as the originial PSU
wasn't a switcher, you shouldn't use a switcher to do any form of upgrade
or repair. But it's really up to you.