So I replaced the two capacitors (C1414 and C1442), reviewed the schematic
and made sure that any connected points were still making contact (need to add
a jumper from C1442 to CR1415, due to my removal accident where I damaged a
trace).?
I backed off the +50V adjustment, fired it up, adjusted it back to 50.0V
and now I get the following:
+50=50.0V
+110=91.0V
+15=15.0V
+5=5V
-15=-14.8V
-8=-7.9V
+105=132.0V
UNREG 50=~66V
Now, those all look to be within spec except for +110 (at 91V) and
possibly the 105 (at 132, schematic shows 136). I did some searching/testing
and didn't see anything major wrong, so I tested the resistance values.
TP - expected - measured
110 - 11K - >20K (too high)
50 - 2.7K - 2.7K (perfect)
15 - 63 - 66.3 (within range)
5 - 46 - 49.9 (within range)
-15 - 480 - 12.9K (too high)
-8 - 32 - 31.2 (within range)
UNREG 50 - 14K - >20K (too high)
105 - 12K - >20K (too high)
So my 110 and 105 lines are both showing low voltage, so the higher
resistance made sense to me. But the high resistance on the -15 line seemed
odd. My assumption is that it means the voltage compensation is working
overtime and properly correcting for an issue somewhere. Is that
correct?
In reading the service manual, it states to attempt to test the
rectifier, the current-limiting transistor, and the op amp. I see that the 105
and 110 both get fed from the +50V rectifier as well as the 110 op-amp pulling
from the regulated 50V line.
When I get home from work, I intend to test the 50V supply area more.
What I'm wondering is if anything I've stated throws an "obvious" flag for
anyone with more experience. When I tested the current-limiting transistor for
the 50V section, it had some voltage (the manual states it should be "off",
which I take to mean 0V). I'm just not sure if that's the cause of the issue,
or a result.
Based on some time staring at the manual, I'm assuming the rectifier is
more of an issue, but that's a guess.
Also, as a side note, I can get a line when I press the beam finder
button now (I did not previously). So some progress is being made (aside from
the obvious progress of mostly-proper voltages).
One final thing to mention: This morning I powered it up before heading
to work, just to make sure I had noted the proper values. When I did that, the
voltage readings were better on 110 and 105 (94, 136.9 respectively). But as a
minute or two went by, and things warmed up, the voltages started to drop.
Again, this tends to make sense to me as something is most likely expanding,
providing more resistance, and cutting down the potential. Any
confirmation/correction would be appreciated.
Also, again, those caps are tricky. I've been soldering (hobbiest, not
full-time) for at least 15 years, and I had a rough time replacing the caps.
Everything works and tests out nicely, but it doesn't look so great (good
joints, but heat marks, etc). I'm definitely not used to this older style of
circuit board. Way too easy to lift traces, but that was a good lesson
learned.
As always, thanks for all of the information. It's been greatly useful
for my learning process.
-Josh
On Mon, Jul 16, 2012 at 11:08 AM, David
<davidwhess@...> wrote:
?
On Mon, 16 Jul 2012 10:25:01 -0500, "
fakecrap@..."
<
fakecrap@...>
wrote:
>Great info, thanks. I was under the impression that I
should replace with
>like-sized capacitance and at least similar
voltage (higher being OK as
>well).
In high ripple
current applications, sometimes higher voltage
capacitors than strictly
necessary are used because the larger
physical size can dissipate more
heat so they last longer.
>I assume the capacitance is OK to increase because
it's a power supply and
>gets regulated afterword,
correct?
It is fine within reason. Increasing the capacitance
could increase
the startup current or delay the startup in a switching
power supply.
In this case it is a standard 60Hz rectifier and capacitor
filter so
the surge current will be higher with more capacitance. I would
use
the same value or the next higher standard value if I was
being
conservative.
>Either way, I'll order some of those up and see
what happens. I'm thinking
>I'll leave others alone, but I'm still
internally re-hashing the age-old
>debate of whether or not to just
replace all the big cans while I'm at it.
If the failure is age
related, then I usually replace all of the
similar capacitors.
