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Re: [Amps] 87A Display Issue

To: N7SC <>
Subject: Re: [Amps] 87A Display Issue
From: Dick Green <>
Date: Fri, 9 Aug 2019 13:34:22 -0400
List-post: <>
Long time ago I had a similar problem with the Ip display where the lowest LED 
would be on all the time at idle (should be off). Gradually more LEDs lit up. 
Turned out to be a bad op amp. 

Have you checked the output of U12A? If the output is maxed there, could be 
that half of U12 or maybe zener D10. If the voltage is good there, but maxed 
coming out of the mux, then likely it’s the mux input pin for HV. I could be 
wrong, but it looks like the HV signal goes through the U15 analog mix, not 
U13. Was that a typo? If not, check the other mux.

Let us know what you find.

73, Dick WC1M

73, Dick WC1M
> On Aug 8, 2019, at 8:40 PM, N7SC <> wrote:
> Hello All...
>   I'm working on an issue with the Tune-IP-HV LED display.  It works 
> correctly on the tune and IP function,  however it always shows full scale on 
> the HV function.
> The analog voltages from the HV board up to the analog multiplexer (U13) on 
> the microprocessor board look ok.  However, the multi sig voltage going from 
> the A/D IC (U11)  on the processor board to the LM3914 display always shows 
> +5v only on the HV function.
> Could this possibly be a programming issue ?? Has anyone ever seen this ? I'm 
> at a total loss. Hi Hi....Any help would would be appreciated !!
> 73,
> Steve/N7SC
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