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Old 6th Dec 2017, 9:03 pm   #1
dtvmcdonald
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Location: Champaign, Illinois, USA.
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Default Racal RA17C12 IF response

I've been measuring the IF response of my boatanchors. All meet specs
except my Racal on the 8 and 13 kHz settings.

Measured by tuning my synthesizer fed into the antenna input on the set's
RF meter I find them much too narrow. There are small bumps in the response,
about 10 dB down from the peak in the center, right where I expect, b ut
the overall curves are too narrow, see picture in red.

https://www.vintage-radio.net/forum/...1&d=1512589439

I have checked the alignment as described in the manual and the followed
the instructions exactly. The 1 and 3 kHz curves are quite as described. The
response through the 2 100kHz IF stages appears not right. There is no
picture of it, but the description of having two peaks 9 kHz apart
cannot be achieved. Its clear that a right broad curve there with real peaks
rather than shoulders as I observe would make the overall response right.
I checked all the resistors and capacitors in the IF cans with two good
meters, and all are now spot on. Several were wrong but replacement did not
fix the problem. There are of course no coil adjustments in the two IF
stages and L81, just caps.

Does anybody have any ideas?

Now, all this matters not at all for actual use of course, as listening on our
broadcast band in the US this response, at 13k setting is just perfect to
undo our theoretical 75 kHz preemphasis (same as FM), and on SW ... who
cares? There are no music stations with 20 kHz channel width on SW. I do
have two BCB music stations,one Spanish and one "Gospel Rock". The Spanish
one is quite listenable, while the other one is set for one of the most hideous
of Mr. Orban's creations.
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Old 6th Dec 2017, 10:37 pm   #2
dtvmcdonald
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Default Re: Racal RA17C12 IF response

One additional measurement: At the 100 kHZ IF output jacks on the back
the response looks exactly like my red lines if you relabel "-10dB" on the graph to "-6 dB".
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Old 6th Dec 2017, 11:12 pm   #3
Radio Wrangler
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Default Re: Racal RA17C12 IF response

Something looks very badly wrong with the red plots, those square corners would represent very high Qs if they were created in the filters... too high to be believable.

Political matters are best avoided. Who knows, someone with a strongly held diametrically opposite view may happen to have the information wanted and be keeping quiet?

David
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Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
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Old 7th Dec 2017, 12:43 am   #4
dtvmcdonald
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Default Re: Racal RA17C12 IF response

The square corners are not real ... the red curves were drawn a short straight line segments. It actually as smooth as the lumpy black 6.5 kHz curve.

As to politics, its Optimod/Optimud name calling. While there are
several settings for Optimud there is at least one nice one for Optimod.

Last edited by dtvmcdonald; 7th Dec 2017 at 12:56 am.
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Old 15th Dec 2017, 7:00 pm   #5
dtvmcdonald
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Default Re: Racal RA17C12 IF response

The real measured curves (at 100 kHz rear output) are here:
http://antiqueradios.com/forums/view...p?f=5&t=332214

Actually not bad. If the final IF can had a bigger dip they would be perfect.
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