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SM for Philips 7951 receiver?...

J

John-Del

Guest
Came in \"dead\", but only had really really bad tape monitor/source switch contacts. Cleaned all controls and switches and runs fine, except the FM seems to have weak sensitivity. AM seems OK. Might be normal but I\'d like to eyeball the FM section anyway.
 
D

Dave Platt

Guest
Came in \"dead\", but only had really really bad tape monitor/source
switch contacts. Cleaned all controls and switches and runs fine,
except the FM seems to have weak sensitivity. AM seems OK. Might be
normal but I\'d like to eyeball the FM section anyway.
One net article suggests that the 7951 is a cosmetic (black) variation
of the 795.

HiFiEngine.com doesn\'t have pages for the AH795/AH7951, but has a page
for the AH794, and the service-manual for that model implies that it
might apply to the 90, 92, 95, and 99 models as well. Philips seems to
have produced families of receivers which varied cosmetically and in
their audio power outputs, but it seems plausible that the basic
schematic may be the same (or at least quite similar) across the
family.

So, go register at HiFiEngine.com (it\'s free, and I\'ve not gotten any
spam or other nonsense from them after registering years ago) and
download the AH794 service manual. I\'d guess it\'ll be close enough to
let you debug the FM section.

If it is, I don\'t think you should expect miracles. If I\'m reading the
schematic for U1001 (the FM tuner board) rightly... well, it appears
to have _one_ varactor-tuned RF amplifier transistor (a BF324 run
common-base) and the varactor-tuned local oscillator, feeding a
one-transistor BF494 mixer/amp, which then feeds a 10.7 MHz crystal
filter, a BF494 IF amplifier, and another crystal filter. There\'s
more IF gain in the TBA570A AM/FM chip. There\'s a double-tuned
transformer in front of all of this, which probably gives it some
selectivity against out-of-band signals, but that\'s it.

So, its selectivity isn\'t going to be wonderful. Sensitivity might be
something you can address, perhaps... RF and IF alignment, maybe
changing out those BF324/BF494 transistors with something a bit less
Neolithic :)
 
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