[Premium-Rx] Alignment test points in Racal 6790

[Karl-Arne Markström]

I would recommend the "divide and conquer" method.
Start from the "rear" of the receiver, and inject signals wiht the proper frequencies,
progressing towards the front end and the antenna input.

When you reach a stage where the required signal level is very different from the level expected,
you have found the culprit. In this context, don't overlook oscillator injection levels.
Some mixers are very sensitive for proper injection.

This is actually easier to do than to describe, and it forms the logic behind almost all 
"module-level" diagnostics, and is also a major part of  "component-level" trouble-shooting.

GL and 73/

Karl-Arne
SM0AOM

----- Original Message ----- 
From: "Paul" <radioboy at telus.net>
To: <premium-rx at ml.skirrow.org>
Sent: Tuesday, December 06, 2005 4:38 PM
Subject: [Premium-Rx] Alignment test points in Racal 6790


> Hi Group,
> 
> I think I have determined that the RA6790 first mixer FETS are actually ok.
> I turned on the radio the other day and had a good listen to it and it was
> right in there with the 6793. Maybe only 1/2 and LCD segment lower than the
> 6793 for a given station. I don't know why it was way down 2 weeks ago. Mood
> swings? 
> 
> Anyway, I would like to do a proper alignment on the A1 filter and A2 mixer
> as I have played with the slugs. Question is, am I correct in assuming when
> testing the levels in the various modules, I should be using a high
> impedance RF probe when on the internal test points, and a terminated 50 ohm
> RF probe when measuring at the SMB output connectors like J2 etc.?
> Also when measuring relative levels as in figure 5-2 in the receiver block
> diagram in the manual, would those levels indicated, like -21dBm out of A2,
> assume using a 50 terminated RF probe as well?
> 
> Finally, when I put in -50dBm, which I think is about 300uV, out of my
> IFR-1500 or 8640B into the antenna input of the RX, I don't see any LCD
> segments lit up on the 6790, but I see about 5 or 6 on the 6793. Then as
> mentioned above, if I connect to an antenna and listen to a station, they
> actually read the same RF level. Is there an alignment for the D/A driver
> for the LCD? I didn't see anything in the schematics or alignment manual.
> 
> Thanks again boys for your help.
> 
> Kind regards
> 
> Paul
> VE7KHz
> 
> 
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