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Author Topic: Distortion meter for alignment?  (Read 3822 times)
N4NYY
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« on: January 12, 2013, 09:52:14 AM »

Quick question. I came across an alignment procedure and as best as I can recall, this is the first time I have seen such a requirement. Of course, I do not have one. I have a good scope. Is there any way I can adjust it with a scope or other piece of test equipment? I really do not feel like buying another piece of test equipment. And of course ebay has el cheapos like Heathkit, to really expensive ones.
« Last Edit: January 12, 2013, 10:04:06 AM by N4NYY » Logged
W4OP
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« Reply #1 on: January 12, 2013, 09:57:32 AM »

Tell us what it is you are restoring and where in a block diagram  the distortion meter is required.

Dale W4OP
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N4NYY
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« Reply #2 on: January 12, 2013, 10:08:46 AM »

Tell us what it is you are restoring and where in a block diagram  the distortion meter is required.

Dale W4OP

Well, it is not a boatanchor, but I posted hear because this is the best forum for alignments. I am aligning a TRC-422A CB. This is the radio my dad gave to me in 1982. This is my first ever transceiver (that was not a cheap toy). I guess storage has not been good. It it way off and I even had to order new 10.240 crystals. The one that was in there was off by about 2 KHz.

This has all sentimental value. My dad is not doing so great. So I want to show him I still have it, that it works, and that I am going to keep it.

It is a one can adjustment. RF generator at 27.185 MHz, 1 KHz audio at 30%. Adjust for minimum distortion. This is the very next step after tuning the 4 receiver cans.
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AC5UP
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« Reply #3 on: January 12, 2013, 10:14:54 AM »

As is your habit, you have failed to mention what is being aligned and specifically how a distortion meter would be used...

This means I'm forced to guess you're working on a consumer grade stereo receiver with two slug tuned transformers in the detector. One is adjusted for zero volts DC at a test point, the other for minimal distortion at the Tape Out phono jack.

If I didn't have a distortion analyzer I'd likely consider three things and two of them use a 'scope on the Tape Out jack:

*  Fuhgedaboudit. Don't touch the slug that needs a distortion analyzer, even if you have a golden screwdriver.

*  Mark the slug position with a Sharpie. Pump a 400 cycle tone through an FM signal generator @ 10.7 MC's through the IF strip, then watch the audio output on the 'scope as you run through the adjustment range. With a little luck you'll see distortion at both ends of the range. Split the difference.

*  Mark the slug position with a Sharpie. Disconnect the antenna, turn off the muting, then tune to the bottom or top of the band where you hear nothing but noise. While watching the noise on the 'scope, rock the distortion slug to see if there's a null or a peak. If you can see one or the other that's probably where you want it.

Note that any adjustment more than a quarter turn from original is probably wrong, anything over half a turn is most likely bo-gus. That's why you marked the slug before you tweaked it..............
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N4NYY
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« Reply #4 on: January 12, 2013, 10:17:32 AM »

Tell us what it is you are restoring and where in a block diagram  the distortion meter is required.

Dale W4OP

Oh, I can scan and email the block diagram and procedure if needed.
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AC5UP
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« Reply #5 on: January 12, 2013, 10:23:02 AM »

Even though my guess was wrong, I'm not changing it because it was so well written.

Betcha' the 'can' is the detector coil on the last stage of the 455 KC IF. That's a no-brainer usually tweaked by an AC meter across the speaker. Adjust for peak voltage. I usually adjust them by ear and a 'scope. What I'm listening for is center slot on the channel, same as you would when tuning a regular radio. Too high / too low the audio goes tinny, find the center. If the 'scope shows a peak on the waveform that sounds about right that's what I want.

BTW:  I was cleaning out the garage last year and had a stack of HP 334A distortion analyzers until......................
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AC5UP
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« Reply #6 on: January 12, 2013, 10:26:50 AM »

God forbid you should tell us it's T7 on this schematic..........  http://www.cbtricks.com/radios/realistic/trc_422a/index.htm
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N4NYY
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« Reply #7 on: January 12, 2013, 11:29:17 AM »



BTW:  I was cleaning out the garage last year and had a stack of HP 334A distortion analyzers until......................


You SOB  Roll Eyes
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N4NYY
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« Reply #8 on: January 12, 2013, 11:40:21 AM »

Even though my guess was wrong, I'm not changing it because it was so well written.

Betcha' the 'can' is the detector coil on the last stage of the 455 KC IF. That's a no-brainer usually tweaked by an AC meter across the speaker. Adjust for peak voltage. I usually adjust them by ear and a 'scope. What I'm listening for is center slot on the channel, same as you would when tuning a regular radio. Too high / too low the audio goes tinny, find the center. If the 'scope shows a peak on the waveform that sounds about right that's what I want.

I could just connect this to an antenna and ladjust to the best sounding receive audio. This is essentially what you are saying, right?


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AC5UP
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« Reply #9 on: January 12, 2013, 01:24:39 PM »

The alignment instructions you quoted recommend 27.195 as the alignment frequency........ If you have stable signals on Channel 19 you know are on frequency, yeah, you could do that, but the chances of hearing stable and accurate signals on 19 isn't that great.

I mean.........  AREN'T YOU THE GUY WHO PLAYED SIGNAL GENERATOR ROULETTE LAST YEAR ?  You'd think a guy with a rebuilt Iggy-42 could manage to..............

BTW:  Since there is a schematic, what's the problem with sharing the part designator of the 'can' you're talking about?

You do recall that an IF strip is nothing more than a TRF receiver tuned to only one frequency, and alignment is the way you adjust all gain stages to perform best at that frequency. In the case of a radio with a ceramic filter that was on 455 KC's but has drifted to 456 KC the new alignment point is 456 KC. It does you no good to have a 'can' adjusted off center relative to a ceramic filter as the audio will (probably) sound Winchester Cathedral. Since you can't QSY the ceramic filter, QSY the 'can' adjustment to match.
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N4NYY
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« Reply #10 on: January 12, 2013, 02:31:53 PM »

The alignment instructions you quoted recommend 27.195 as the alignment frequency........ If you have stable signals on Channel 19 you know are on frequency, yeah, you could do that, but the chances of hearing stable and accurate signals on 19 isn't that great.

I mean.........  AREN'T YOU THE GUY WHO PLAYED SIGNAL GENERATOR ROULETTE LAST YEAR ?  You'd think a guy with a rebuilt Iggy-42 could manage to..............

BTW:  Since there is a schematic, what's the problem with sharing the part designator of the 'can' you're talking about?

You do recall that an IF strip is nothing more than a TRF receiver tuned to only one frequency, and alignment is the way you adjust all gain stages to perform best at that frequency. In the case of a radio with a ceramic filter that was on 455 KC's but has drifted to 456 KC the new alignment point is 456 KC. It does you no good to have a 'can' adjusted off center relative to a ceramic filter as the audio will (probably) sound Winchester Cathedral. Since you can't QSY the ceramic filter, QSY the 'can' adjustment to match.

You had the right can which is why I did not correct you. The instructions said Ch 19, but I woul have used a local channel as CH 19 here is S5 of noise consistantly
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AC5UP
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« Reply #11 on: January 12, 2013, 03:34:55 PM »

You had the right can which is why I did not correct you.

Yeah, but you didn't confirm it, so now we're able to move forward...

T7 is the detector coil and should be peaked at 455 KC's - if - CF1 (upstream from Q6) is still on frequency. Ceramic filters can drift and that's why I tend to align to wherever the filter wants to be. You could tune channel 19 then peak T6 and T7 for maximum S-Meter, or AC volts across the speaker coil. It's a dual conversion receiver with a crystal and ceramic filter so most of the selectivity is upstream from the second IF.

You should also note that XF, the crystal filter that was originally on 10.7 MC's, may have drifted with age so a little compensation on T4 wouldn't hurt. If you want to try a Q&D alignment, here's the way I'd go:

Peak T1 for maximum signal on Channel 19. Peak T2 for maximum signal on Ch 10 and T3 for maximum signal on Ch 30. The goal is to give the 1st mixer & IF a fairly flat bandpass from Ch 1 through Ch 40. Peak T4, T5, T6 and T7 for best audio with maximum signal on Ch 19. That should get the receiver back in shape. It's also a good idea to rough check the sensitivity on Ch 1 and 40 by switching between the two with the audio turned up and no antenna. If the hiss level is about the same that's good. If not, try re-tuning T2 and T3 with T2 on Ch 30 and T3 on Ch 10. Those two 'cans' are in cascade so a little stagger tuning is probably a good idea.

As for the TX alignment... By the book.
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AC5UP
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« Reply #12 on: January 12, 2013, 04:07:53 PM »

BTW:  I really do have an HP 334A on the bench and although it's quite the piece of test gear, the basic concept is simple.

Imagine a wideband audio amplifier with a really good notch filter in the front end and a precise AC voltmeter on the back end. Feed it a steady tone test signal, notch out the tone, and what's left? Anything that wasn't part of the fundamental audio signal and most likely the products of distortion. To use an RF analogy, if you wanted to check for spurs and harmonics a wideband amplifier with the fundamental carrier notched out would see only the harmonics. Same deal with a distortion analyzer.

The HP has a diode detector good to 65 MC's so it can measure distortion on an AM signal. Like when SportsTalk 1480 runs a periodic proof on their transmitter. I rarely use mine for distortion measurements but the Decibel & AC voltmeter does come in handy.

The HP is no Extech... Which is why I like it.

I suppose the CB alignment instructions calling for a distortion analyzer really meant to say 'adjust for best audio'. Considering it's a CB I wouldn't expect Hi-Fi anyway and IMHO that makes the distortion analyzer concept technical overkill. It's also possible that was one way to idiot proof the factory alignment procedure for a noisy place where hearing 'center slot' isn't that easy to do...........  Tongue
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N4NYY
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« Reply #13 on: January 12, 2013, 04:26:56 PM »

BTW:  I really do have an HP 334A on the bench and although it's quite the piece of test gear, the basic concept is simple.

Imagine a wideband audio amplifier with a really good notch filter in the front end and a precise AC voltmeter on the back end. Feed it a steady tone test signal, notch out the tone, and what's left? Anything that wasn't part of the fundamental audio signal and most likely the products of distortion. To use an RF analogy, if you wanted to check for spurs and harmonics a wideband amplifier with the fundamental carrier notched out would see only the harmonics. Same deal with a distortion analyzer.

The HP has a diode detector good to 65 MC's so it can measure distortion on an AM signal. Like when SportsTalk 1480 runs a periodic proof on their transmitter. I rarely use mine for distortion measurements but the Decibel & AC voltmeter does come in handy.

The HP is no Extech... Which is why I like it.

I suppose the CB alignment instructions calling for a distortion analyzer really meant to say 'adjust for best audio'. Considering it's a CB I wouldn't expect Hi-Fi anyway and IMHO that makes the distortion analyzer concept technical overkill. It's also possible that was one way to idiot proof the factory alignment procedure for a noisy place where hearing 'center slot' isn't that easy to do...........  Tongue

Since this is an AM only rig, the alignment is pretty simple. I can scan an email a copy of you wish.
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AC5UP
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« Reply #14 on: January 12, 2013, 04:33:06 PM »

I can scan an email a copy of you wish.

No point in it. You're the one with the radio that might benefit from an alignment check, and if I needed to I could probably find the instructions for a very similar model on the web. The RatShack CB's were Uniden and not every model had a unique design.
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