K2 AGC problem?

[Matt Osborn]

Someone, Don I think, posted a message this week suggesting that the
background noise with no antenna connected should be the same with AGC
either on or off.  I checked mine out of curiosity and discovered that
mine was quieter with AGC on.

Before making adjustments to R1, I thought I'd run through the AGC
voltage checks in the manual to see what they looked like.

With RF gain at maximum, pin 5 of U2 = 3.8vdc

I turned to Troubleshooting/Signal Tracing (Appendix E, page11) and
made the following measurements on the control board:

Using the XG2 signal generator, 7040khz @ 50uv

Test 1, XG2 off, RF Gain MAX
U2 pin 1 = 3.41vdc - expected 3.6

Test 2, XG2 off, RF Gain MIN
U2 pin 1 = 4.50vdc - expected 4.6

Test 3, XG2 on, RF Gain MIN
U2 pin 1 = 4.50vdc - expected 6.9 (VFO adjusted for max)
U2 pin 5 = 4.89vdc - expected 5.0

In test 3, U2 pin 1 is 2.5vdc too low.

Just for grins, I compared the voltages on U2 pins 1 & 3 at various RF
Gain settings and I found the same voltages on both pins; pin 1 seems
to be tracking pin 3 accurately; I think this is OK.

Using an RF probe, I measured the anode of D1 and found 65mv with XG2
off and 110mv with XG2 on, a 45mv swing.  Using an DMM, I measured the
cathode of D1 and found 4.73vdc with XG2 off and 4.86vdc with XG2 on.

Q12 measures:
C = 7.9vdc
E = 4.5vdc
B = 4.5vdc

I'm inclined to believe Q12 may be defective. Before I order parts,
though, thought I'd check with you folks and see if I'm even in the
ball park.
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[Don Wilhelm-3]

Matt,

At this point, I don't think anything is wrong with your AGC circuit.

Re-check the TEST 3 readings with the RF Gain set to MAX and I believe you
will see readings a bit closer to the Expected values (I think a step is
missing here to tell you to set the RF Gain to max).  The 'key' is that the
bold print line says that you are testing the AGC at saturation - and with
the RF Gain set at minimum, there is no way the AGC will saturate unless you
have a VERY strong signal - even the 50 uV from the XG1 is not adequate to
saturate the AGC (saturation is S-9 + 40 dB or more - XG1 is only S-9), so
do expect a voltage somewhat lower than 6.9 volts.

Your results are consistent with what I would expect with the RF Gain at
minimum.  Your 8 volt supply seems to be actually at 7.9 volts, so you can
expect your measurements to be a bit lower than those shown in the manual
(they are based on an 8 volt supply level).

If you have reasonable 'action' on your S-meter, the AGC circuits are
working.

Yes, I was the one suggesting the AGC Threshold optimization.

73,
Don W3FPR

> -----Original Message-----
>
> Someone, Don I think, posted a message this week suggesting that the
> background noise with no antenna connected should be the same with AGC
> either on or off.  I checked mine out of curiosity and discovered that
> mine was quieter with AGC on.
>
> Before making adjustments to R1, I thought I'd run through the AGC
> voltage checks in the manual to see what they looked like.
>
> With RF gain at maximum, pin 5 of U2 = 3.8vdc
>
> I turned to Troubleshooting/Signal Tracing (Appendix E, page11) and
> made the following measurements on the control board:
>
> Using the XG2 signal generator, 7040khz @ 50uv
>
> Test 1, XG2 off, RF Gain MAX
> U2 pin 1 = 3.41vdc - expected 3.6
>
> Test 2, XG2 off, RF Gain MIN
> U2 pin 1 = 4.50vdc - expected 4.6
>
> Test 3, XG2 on, RF Gain MIN
> U2 pin 1 = 4.50vdc - expected 6.9 (VFO adjusted for max)
> U2 pin 5 = 4.89vdc - expected 5.0
>
> In test 3, U2 pin 1 is 2.5vdc too low.
>
> Just for grins, I compared the voltages on U2 pins 1 & 3 at various RF
> Gain settings and I found the same voltages on both pins; pin 1 seems
> to be tracking pin 3 accurately; I think this is OK.
>
> Using an RF probe, I measured the anode of D1 and found 65mv with XG2
> off and 110mv with XG2 on, a 45mv swing.  Using an DMM, I measured the
> cathode of D1 and found 4.73vdc with XG2 off and 4.86vdc with XG2 on.
>
> Q12 measures:
> C = 7.9vdc
> E = 4.5vdc
> B = 4.5vdc
>
> I'm inclined to believe Q12 may be defective. Before I order parts,
> though, thought I'd check with you folks and see if I'm even in the
> ball park.
>

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