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Old 10-12-2017, 8:41 AM
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Default BC780XLT Dim Display Fix ++

Apologies in advance, the original thread is archived: https://forums.radioreference.com/un...t-display.html

My 780 has been on 24/7 for about 15 years. It was used for a stream until they went to P25. At some point I had done a mod to bring the display contrast back. I had to short out the 8.2K entirely. Now even that isn't enough. Additionally, it wasn't working at all but the zener diode fix (http://forums.radioreference.com/uni...ml#post1750806) took care of that. The schematics I have do not exactly match the radio. The diode was in fact a 10V zener despite the schematic showing a 3V.

Now for the display. The V5OUT from the module is a -V supply. V5 is the contrast input. This input needs a negative voltage with respect to ground. I was able to bring this scanner back to life by using an isolated power supply module in this way (see http://cds.linear.com/docs/en/design-note/dn1021fa.pdf). I actually used one of these because it was small and I had it laying around: http://www.ti.com/lit/ug/sbau256a/sbau256a.pdf.

On the display board all the signals we need to access are brought out to test points, which makes it easy. You need +3V, GND, and V5 (the contrast input). On the isolated supply, you connect the Vout to the GNDin on the input side. Then your GNDout is now -Vout with respect to GNDin. I ran -Vout through a 10k pot and a 10k in series to V5.

The TI eval board is small enough to fit right onto the display board with some stiffened double sided tape (mack tack).

Any isolated supply should work if it can output around -5V with an input of +3V. There is 6V and 12V (main power) available in the radio too so you could use that for the input. Alternatively there are some circuit designs using an oscillator like a 555 which can produce a negative voltage. In many cases you can get an eval board from TI or LinearTech free for the asking.

I'll take a picture tonight and post it to this thread.

Nick
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Old 10-12-2017, 8:38 PM
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Old 10-14-2017, 7:41 PM
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Be careful with switching power supplies near a receiver. It could block out whole frequency ranges if they are not RF compatible.

/Ubbe
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Old 10-14-2017, 9:02 PM
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I know, and I thought about that. It runs at 200k so it would be quite a few harmonics to get to the IF. It seems to be running great at VHF, which is what I wanted it for.

Thanks for the comment, you make a good point and others reading this should be aware of this.
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