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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