It is not at all unusual for electrolytic capacitors to decrease in
value as they age. Thus an original 10mfd, after a few, years may
effectively be a 1mfd capacitor. Although I don't care for the
"shotgun" approach, this usually saves time and money.
Keeps life simple and less frustrating.
73
Bob, K4TAX
On 2/26/2021 10:33 AM, Mike Bryce wrote:
I’ve never done a shotgun approach when it comes to replacing electrolytic
capacitors, but in the case of this particular display unit, that was alway the first
thing I did.
Then, and it’s been years ago, some smaller electrolytic caps (1mf) and .001
disc took care of it. Nothing I did eliminated it 100%
Mike Bryce
prosolar@sssnet.com
"The eye is always caught by the light, but shadows have more to say"
On Feb 26, 2021, at 11:20 AM, Paul Christensen <w9ac@arrl.net> wrote:
Mike,
Interesting. If the 544 schematic is correct, the Display Board has no common
+Vcc. +Vcc is supplied individually to each 7-segment LED module via Q14-Q19
(PNP) on the Logic Board. +Vcc on the Logic board is RF bypassed with a
0.047uF cap at C6. +Vcc on the Logic Board is also connected to IC-1 with
ample decoupling, consisting of a 22 ohm resistor and a 0.1 uF cap at C7. The
22 ohm resistor is mainly there to regulate current into a Zener diode that
feeds +Vcc to IC-1 but it also functions to decouple.
To me it looks like Ten Tec adequately decoupled one part of the Logic Board,
but not the other going into the display board. The only problem with
duplicating R/C de-coupling on the +Vcc bus feeding Q14-Q19 is that any series
resistance will make the display slightly dimmer -- and the green LED segments
are already way too dim due to green-wavelength filtering through the smoked
Plexiglass lens. So, that leaves adding an additional RF bypass across C6
(0.047 uF). Possibly something like a 4.7 uF 'lytic cap in parallel with a 0.22
uF film bypass will do the trick.
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