[R-390] LONG: Alignment, Overload and Distortion

Bill Smith [email protected]
Mon, 11 Mar 2002 09:40:31 -0800


Hi, Bill

Now that sounds like a tough one!  I'd look at the mixer stages,
particularly oscillator injection voltage, but that is merely a guess.
Ideas?  You might have two problems, and see one, then the other with
different signal levels.  Also, check resistor values throughout the set,
some resistor may have opened up somewhere (esp. B+ dropping or decoupling
resistor) and stages are over-reacting somehow to make up the difference.

From what you describe, it sounds like the set is operating normally when
there is little or no AVC voltage, and it sounds like the set may not be
operating normally with a strong signal, but the signal is getting through
anyway.  In between you hear distortion, where some stage is affected by AVC
but can not operate normally.

Sounds like a "fun" problem to troubleshoot.  You'll feel great when you
find it!  :-)

73 de Bill, AB6MT
[email protected]



----- Original Message -----
From: "Bill Cotter" <[email protected]>
To: <[email protected]>
Sent: Sunday, March 10, 2002 6:25 AM
Subject: [R-390] LONG: Alignment, Overload and Distortion


> Hi Bill, et al:
>
> I read with interest the problems you have been dealing with, and
> they seem to be one of the tougher R-390A repair issues. I have
> been struggling with a very similar, and possibly related set of
> problems. I have an EAC that behaves in the most peculiar way and
> has given me fits in trying to correct the problem. This receiver
> is a '67 EAC sn#3669, and has been recapped, rebuilt  and aligned.
> Here's what's going on:
>
> Pick a strong station on any band, say 17mHz (problem occurs on all
> bands). Tuning into the station and the carrier level is around
> 90-100dB. With the RF gain at 10 and the AVC on, filters in the
> 8kHz position the station sounds fine as long as there is no
> selective fading. When fading occurs and the signal dips down to
> say 70-80dB, distortion begins. When the level falls to 30-40dB,
> sever distortion appears until the signal rises above 70dB or more.
> Tune in a weak station around zero to 10dB and it is fine until it
> gets stronger, upwards of 30-40dB.
>
> Ok, figure it's the AVC, right?
>
> Go to MVC, and tune back in on the strong station. Now the signal
> jumps way up and I need to drop off the AF gain dramatically. With
> the RF gain full open, the signal is fine until fading occurs,
> signal level drops, and the distortion creeps in once more. While
> looking for that RF Gain sweet spot, I notice that as I advance the
> RFG from say 5 towards 10, an odd thing occurs. The volume through
> the speaker increases as I approach 10. Right around 8 the volume
> suddenly drops 20db or so (gain-compression??), and distortion
> begins. Increasing the control further the volume increases along
> with the distortion. The distortion is worse when the carrier meter
> is in the 10-30dB range.
>
> Now, I haul it up on the bench.
> ---------------------------------------------
> TEST 1 - IF Chain and AVC
>
> Connect the HP-8640B to the IF deck J-518, HP-427C HiZ VM to the
> AVC line, Fluke DMM to the RF gain line, and a HP-332A Distortion
> analyzer to the Diode Load line. Dial in 455kHz +/- to peak with
> the filter at 100Hz, return selectivity to 8kHz, and 30% modulation
> at 1,000Hz. Controls AGC Fast, RFG wide open, and the following
> results (all AC Vrms):
>
> AVC ON
> Vinput Vdload   Vavc    Distortion
> ---------------------------------
> 10uV    0.75v   -0.6v   18.0%
> 100uV   1.00v   -3.5v   12.5%
> 1mV     1.5v    -8.4v   13.0%
> 10mV    2.0v    -14.9v  13.5%
> 100mV   2.75v   -20.6v  14%
> 1V      3.5     -27.0   14%
>
> All looks fine from the AVC operational point of view.
>
> Now to test the influence of RF Bias on the IF-AVC system, in the
> MVC mode. The RFG control is set at different positions and RF bias
> is measured at the RFG point on the rear panel. Two different input
> levels (100uV and 1000uV) are tested. The results:
>
> AVC OFF - MVC
> Vinput Vdload   Vrfbias Distortion
> ---------------------------------
> 100uV   10.0v   1.0v    14.0%
> 100uV   6.0v    2.0v    14.5%
> 100uV   3.8v    3.0v    14.0%
> 100uV   2.5v    4.0v    15.0%
> 100uV   1.9v    5.0v    14.0%
> 100uV   1.5v    6.0v    14.0%
> 100uV   1.1v    7.0v    14.5%
>
> AVC OFF - MVC
> Vinput Vdload   Vrfbias Distortion
> ---------------------------------
> 1000uV  16.5v   4.0v    35.0%
> 1000uV  17.0v   5.0v    22.0%
> 1000uV  14.5v   6.0v    18.0%
> 1000uV  12.0v   7.0v    16.0%
> 1000uV  9.5v    8.0v    15.0%
> 1000uV  6.0v    10.0v   14.5%
>
> Conclusion, the IF deck seems to be operating fine below 1000uV
> input at 6vdc and higher bias. However, as the bias decreases (RF
> gain control advanced towards 10) the distortion increases. Only if
> the input was held below 1000uV, will the IF deck function
> normally. This 'sweet spot' would be found by increasing the
> generator level with the RFG wide open until the distortion sharply
> rises above the 14% average.
>
> Does anyone know offhand the signal level upper-limit entering the
> IF deck??
>
> ---------------------------------------------
>
> TEST 2 - Overall receiver test
>
> Connect the HP-8640B to the antenna C-connector, HP-427C HiZ VM to
> the AVC line, Fluke DMM to the RF gain line, and a HP-332A
> Distortion analyzer to the Diode Load line. Dial in 17.5mHz, and
> 30% modulation at 1,000Hz. Controls AGC Fast, RFG wide open, and
> the following results (all AC Vrms):
>
> AVC ON
> Vinput  Vavc    Dist'n  Meter
> -------------------------------------
> 1uV     -2.2v   12.0%   15
> 3uV     -3.3v   5.5%    22
> 10uV    -4.5v   4.0%    35
> 30uV    -5.7v   3.5%    42
> 100uV   -6.8v   4.0%    52
> 300uV   -7.6v   4.0%    60
> 1mV     -8.4v   4.5%    70
> 3mV     -9.2v   4.5%    78
> 10mV    -9.8v   6.2%    80
> 16mV    -10.5v  25.0%   81
> 30mV    -10.6v  45.0%   82
> 40mV    -10.7v  35.0%   85
> 50mV    -10.8v  40.0%   89
> 66mV    -11.2v  22.0%   88
> 100mV   -11.6v  8.5%    90
> 300mV   -14.9v  14.0%   100+
> 500mV   -17.9v  ****%   100+
> 800mV   -21.3v  9.0%    100+
> 1V      -23.0v  10%     100+
>
> As you can immediately see from the results above there is a spot
> in the input-range where the distortion rises dramatically, then
> falls off. The AGC action and the carrier meter both follow the
> received signal flawlessly through out the range.
>
> ---------------------------------------------
>
> I have done all the usual first steps: tighten connectors, DeOxit
> sockets, swapped tubes with known good tubes, etc. I also swapped
> in a second identical EAC IF deck and had the same listening
> results. What I am wondering about now is the possibility of
> parasitics in the RF AMP, or a breakdown of a component (ie: mica
> cap, etc).
>
> Any Suggestions would be appreciated. All results will be posted
> for the benefit of the group.
>
> Thanks es 73 bill n4alg
>
>
>
>
>
>
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