7 Jun
2010
7 Jun
'10
10:58 a.m.
On Sunday (06/06/2010 at 07:40PM -0700), John Robertson wrote:
Capacitor has been replaced with no improvement. The original transformer
with a new bridge and new filter cap are unable to keep 6VDC on the
DC side with 5A load. That's half the rating of the supply.
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.
I completely understand and I never believed it "got weak". I believe
it has been defective from the beginning. Probably not a failure but
a manufacturing defect or was just plain too small for the job.
I am not the first person to find the 8V rail being low on a SWTPC 6800
system.
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.
Yup. I've done the even simpler test which is to replace the entire bridge.
I have been using a dummy load to test the supply and in one of the tests
I used the original transformer and filter cap but substituted a new bridge.
Same inability to source 5A.
I then also swapped the filter capacitor... same inability to source 5A.
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!
An abundance of clearness. Thank you. I too have been doing this
stuff for 35+ years so the actual tests, circuit modifications, etc are
not a problem.
My original question was seeking input on how much religion there is
about making modifications to original equipment or not. I had pretty
much already determined that the transformer was the problem.
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... 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.
Understood. This system has a 555 timer that resets it sometime a few
SECONDS after 5V goes valid. I think we'll be OK.
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.
A good plan. Thank you.
PS - my experience is in servicing tube and solid
state equipment since
the 1960s - since the mid 70s as a living (flippers.com ;-).
Great. I appreciate the input but I think unfortunately, in the case
of my SWTPC 6800, it does need a new transformer (or two).
Chris
--
Chris Elmquist