[Premium-Rx] Arg! Help with Alignment and test equipment set up

[John]

Hi Paul,

Geez, I hate stepping in piles of stuff like this-well you've done it 
now...there is no hope-I can only offer you $20 for the rig.

Or, you can try to get the slugs threaded back into the cans...maybe not 
that bad, but I would guess you'll have trouble getting back into the 
ballpark when looking with finely tuned gear.  I've done similar stuff and 
was able to recover by roughing it in and then trying the fine tweak 
again...invariably, I've done a wrong test point, overloaded the test point 
or been on the wrong adjustment...once stuff changes and you're looking 
someplace else (as in "no change"), you can get well beyond any meaningful 
range.  I'm assuming that at this point, you can retrace your steps and that 
the rx is pretty well thrashed...that is, can't get much worse...just don't 
tweak any new adjustments.

But then I expect you knew that.  This will be tedious and you need luck--if 
you find the results drifting away from you, stop and back up.

Here's what I've gotten away with in the past:  Preset any controls that 
call for a preset (as in "set vr5 to midrange").  Set the rx to am wide 
filter and put a voltmeter on the agc-set fast agc...put a sig gen on the 
input (do this at each alignment frequency) and push as high as 0dbm to see 
if you can get a reaction on the voltmeter...if so, you're on your way.  If 
not, see next paragraph.  You can take the slight reading you've got and 
tweak the cans to get more reaction on the voltmeter...keep backing off the 
sig gen level as the receiver becomes more sensitive.  Keep cycling through 
the frequencies and the adjustments until you reach a point of diminishing 
returns.  Now transition over to the proper alignment setup and continue as 
intended...hopefully finishing with a proper alignment.

If you cannot get a reaction on the voltmeter, you will have to preset the 
slugs (keep notes).  At some point, you'll get some reaction--then milk that 
signal up out of the noise and proceed as above.

If you're lucky, the tuneups will be fairly low Q, wide peak--if you're 
unlucky, the sweet spot will be a narrow peak that's hard to hit even if 
you're not having this problem.

The sort of mistakes I have made include:

setting a null for maximum

setting the wrong control for the wrong observation

kept turning even if I do not see the adjustment happening

presetting the receiver to the wrong mode or bandwidth

and stuff like that there

Just go slow and steady, and keep notes...it really sucks if you lose your 
place.

good luck

John Collins
__________________________________
----- Original Message ----- 
From: "Paul" <radioboy at telus.net>
To: <premium-rx at ml.skirrow.org>
Sent: Thursday, December 01, 2005 12:57 PM
Subject: [Premium-Rx] Arg! Help with Alignment and test equipment set up


> Boy oh boy!
>
> Ya think ya know it all and have good test equipment and then go to use it
> and don't get the expected results. I am thinking I may not have a real
> problem with the Racal 6790, I think I may have just mucked it up when I
> really did have a 15 error last month. 15 came up after a random BITE I 
> did,
> and found that the group of coils in A2 were a little off so I did an
> alignment on A1 input filter and A2 1st mixer. That fixed the 15 error but
> now that I have my 6793 installed, I see there is a definite lack if
> sensitivity in the 6790.
>
> Here is my situation. The test equipment I have to play with is an 
> IFR-1500,
> HP 8901B mod analyzer, 8640B generator, 475B scope, Booton RF 
> millivoltmeter
> with RF probe, Millivac RF meter with RF probe, lot's of 50ohm cables with
> SMA, SMB, N etc connectors. Probably some other stuff too but this is what
> is currently setup on the bench. I used the IFR-1500 in Duplex mode, which
> has a spectrum analyzer and sweep function in it, to just tune up the 
> input
> filter, the round cylinder with 4 or 5 slugs in it. Thanks to Francis et 
> al
> who sent me the alignment instructions. Following the instructions, I am
> unable to see any change when I spin the L1 slug around at 30MHz. Nada. 
> Zip,
> no diff. When I use the Boonton with the probe, measuring the open ended
> output of the filter, unterminated, I can see a change but no peak, it 
> just
> keeps rising until I run out of slug. I think I am doing this properly?
>
> I would really like to get the front of this receiver back in alignment 
> and
> get it back in the rack. This should be a simple procedure but it's 
> tunring
> into a multi day project.
>
> I have been working with RF for many years and just can't figure this out.
>
> Any help, suggestions, a shotgun, would be appreciated.
>
> Kind regards,
>
> Paul
> VE7KHz
>
>
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