K1 Front Panel resistance checks

[egernet]

Before mounting the LCD on the K1 FP board, I am checking the
resistances of adjacent pins on the U1 socket.

Most of them come out unmeasurably high >1M, but 30 and 31 (and 11
& 12) on the U1 socket come out as 480 ohm which is too low. If I
reverse the leads on my DMM, I get 1.65k. Confusing.

I had a look at the schematic and the pins connect to Vss and Vdd,
where the 6 volt regulator U4 is connected.

My question is: Is this correct?

It sounds like I am just measuring the 78L06 and getting funny but
benign readings.


73, Chris

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[Don Wilhelm-4]

Chris,

It might be OK (and most likely is).  Pins 12 and 30 are connected to
ground, so between pins 11 and 12 and also between 30 and 31, you are
reading the resistance of the 6 volt rail to ground - you are measuring
not only the 78L06, but also U2, U3 and U5 which are also connected to
this same voltage rail.

It may be too that your DMM is using a higher than normal voltage to
measure resistance (which can turn on active devices), or that the
polarity of your leads is different than most DMMs which explains why
the reding is OK when the leads are reversed.  Try a different DMM if
you can.

73,
Don W3FPR

[hidden email] wrote:

> Before mounting the LCD on the K1 FP board, I am checking the
> resistances of adjacent pins on the U1 socket.
>
> Most of them come out unmeasurably high >1M, but 30 and 31 (and 11
> & 12) on the U1 socket come out as 480 ohm which is too low. If I
> reverse the leads on my DMM, I get 1.65k. Confusing.
>
> I had a look at the schematic and the pins connect to Vss and Vdd,
> where the 6 volt regulator U4 is connected.
>
>  
>

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[egernet]

Don,

Thanks for your reply.

>It may be too that your DMM is using a higher than normal voltage
to
>measure resistance (which can turn on active devices),

As I don't have another DMM at hand, I did something different. I
found a junk box 680 ohm resistor, measured it (682) and measured
the pins with the 680 ohm resistor in series. Now I consistently
get 4.8 kohms. Surely, that proves your point, Don. I was measuring
some active devices.

>or that the
>polarity of your leads is different than most DMMs which explains
>why the reding is OK when the leads are reversed.

I don't understand the above statement but that doesn't matter. I
think your advice resolved the problem.


73, Chris

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