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.
