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Author Topic: Distortion meter for alignment?  (Read 4509 times)
WB2WIK
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« Reply #15 on: January 12, 2013, 04:57:48 PM »

A distortion analyzer is a nice thing to have, and I have an old HP-331A for that.  It's from the 1970s but still works fine.

However, I doubt it's needed.  I'd tune for a clean signal on the scope.  If you introduce a sine wave, the signal should look like one.

BTW, the old 331A is available surplus all over the place for probably under $100 (simply due to age, not lack of performance).  I've had mine since 1980 and it was much newer then, and still only about $250 back then.
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N4NYY
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« Reply #16 on: January 12, 2013, 06:42:28 PM »

Thanks!
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G3RZP
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« Reply #17 on: January 12, 2013, 10:08:31 PM »

I'd use a sig gen, a counter and a 'scope and measure the centre frequency of each filter. T 7 should be aligned for best audio at the frequency which is the centre of the 455 filter - as the man says, after all these years, it may well not be 455 any more. While you're at it, check the shape of the 455 filter - it may need replacing.
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AC2EU
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« Reply #18 on: January 12, 2013, 10:35:16 PM »

Well, let me wrap this up with an analogy.

Using a distortion meter for this type of alignment is like killing a fruit fly with a sledge hammer!
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AC5UP
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« Reply #19 on: January 13, 2013, 05:47:16 AM »

Yeah, but when it's done, the fruit fly is really, really, really, really dead.................................   Grin
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KE3WD
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« Reply #20 on: January 13, 2013, 09:49:21 AM »

If its on the receiver side, it likely can simply be adjusted into the ballpark by ear. 

If Transmitter section, it likely was used as a way to keep Type Acceptance within the rather strict (at the time) regulations regarding the CB band equipments. 

Use of Distortion Analyzer was likely in a lot of the older CB service manuals due to the FCC regulations regarding Modulation.  Might be one of those overkill CYA situations on the part of the mfrs and design engineers.  Might have been added after the fact by someone in the Factory Service Dept., as prodded by the advice of legal. 

These days, I doubt if any amount of distortion on 11 meters would be noticed as a detriment, consider that it is now considered a feature to overmodulate the AM on the darn things, I exhibit all the webpages where "clipping out the Modulation Limiter" is a much sought after "mod"...


73
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KD0REQ
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« Reply #21 on: January 14, 2013, 10:29:01 AM »

since you mentioned scopes... use a dual-trace, feed A from the sig gen, feed B from the IF strip, overlay the traces and tune for identical.  or invert B and tune for flatline.

but a communications device meant to not have a wide bandwidth, I always just tuned for maximum output, and received attaboys.  you want wide bandwidth, you have to stagger tune the coils.
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N4NYY
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« Reply #22 on: February 01, 2013, 02:23:34 PM »

Quick question. I got this working and am aligning the RX.

The first part of the RX procedure says to adjust local osc to max output. The adjustment is T9, and says reference 20 mV. However, it says nothing about where to put the test leads. Should I put them at the base of the Q3 mixer? Should I use either a scope or DMM?
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AC5UP
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« Reply #23 on: February 01, 2013, 06:39:40 PM »

Yeah, base of Q3 or the same side of R80 if that's more convenient. Best bet is the use a 10:1 probe on your 'scope.

As for the values of C63 and C65, fuhgedaboudit. They're DC blocking condensers on the output end of IC3, the audio power amp. Based on my experience they're too large, but for all I know the chip likes 'em that way.
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N4NYY
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« Reply #24 on: February 01, 2013, 07:22:19 PM »

Quote
As for the values of C63 and C65, fuhgedaboudit. They're DC blocking condensers on the output end of IC3, the audio power amp. Based on my experience they're too large, but for all I know the chip likes 'em that way.

What was on the PCB was a 330 at C65 and a 100 at C63. The schematic showed a 470uf at C65 and a 47 at C63. Should I go back and just put the original values in? I thought it may have been tone or such, but could not tell by ear. I could not find a good datasheet on this chip.
« Last Edit: February 01, 2013, 07:27:03 PM by N4NYY » Logged
AC5UP
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« Reply #25 on: February 01, 2013, 07:56:19 PM »

If it works don't fix it.

What you have is either a parts sub or a running revision. For all you know the original design would motorboat at low battery voltage.
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N4NYY
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« Reply #26 on: February 03, 2013, 09:44:42 AM »

Worked like a charm. I aligned it and got great audio report. It is stock except for one mod. I replaced the stock detector and installed a 1N5711 Schottky. I think I read here how they make good detectors. I am going to stick this in my shack and can now say that I have my first ever transceiver in my shack.
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