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uniden pc 66 XL no power
- Thread starter kaos513
- Start date
Not so strange.
Since the 66 doesn't have a freq adjust (my PC-66 didn't), I just used the schematic of a 25LTD and added a 20pf compression cap and had to change the value of two fixed disc caps to make it adjustable like the 25 has.
Worked fine for me.
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If you want exact, use / sub in - a trimmer or liv with <1kHz off - to me it's splitting hairs - it's only AM.
IF you need to do anything -
Don't worry - no one using a typical CB can really tell the difference - only radios with NARROW filters like the HR2510 - may take issue with your off-frequency carrier...
You do have a means - install a Varicap - else you can sub in different NPO's until you get it right.
I myself have only changed this part 2 or 3 times - that was due to age.
IF you have a receive issue, take a look at C161 - that one is for the 1st IF - that one is a tricky one to get right.
They install it on the bottom of the board, not a big deal - but I prefer a clean install - they use about 33pF to pare down the output of the IF from L16.
The output of L16 can peak out pretty high, and if you're having issues with IF noise, scope TP3 - and you'll see why they used a padding cap like C161 to smooth off, or trim down - the output level into the C10/R5 (TP3 header wire on R5 1K) so you're not getting more noise into the signal than you should. Can push the FET pretty hard SK192 (Q24).
So if you feel the "RX" isn't quite right - it may not be from the Xtal base frequency and it being off. It may be how the Image is getting sent thru IF. Instead of trying to parse to the "center" frequency by playing with C111 or using a Varicap - check L16 and C161 - you can have some leeway to make the skirt of the RX on the 1st IF a little better but not by much. This is effective when you get a lot of strong signals even with the RF gain cut all the way back down.
:+> Andy <+:
IF you need to do anything -
Don't worry - no one using a typical CB can really tell the difference - only radios with NARROW filters like the HR2510 - may take issue with your off-frequency carrier...
You do have a means - install a Varicap - else you can sub in different NPO's until you get it right.
I myself have only changed this part 2 or 3 times - that was due to age.
IF you have a receive issue, take a look at C161 - that one is for the 1st IF - that one is a tricky one to get right.
They install it on the bottom of the board, not a big deal - but I prefer a clean install - they use about 33pF to pare down the output of the IF from L16.
The output of L16 can peak out pretty high, and if you're having issues with IF noise, scope TP3 - and you'll see why they used a padding cap like C161 to smooth off, or trim down - the output level into the C10/R5 (TP3 header wire on R5 1K) so you're not getting more noise into the signal than you should. Can push the FET pretty hard SK192 (Q24).
So if you feel the "RX" isn't quite right - it may not be from the Xtal base frequency and it being off. It may be how the Image is getting sent thru IF. Instead of trying to parse to the "center" frequency by playing with C111 or using a Varicap - check L16 and C161 - you can have some leeway to make the skirt of the RX on the 1st IF a little better but not by much. This is effective when you get a lot of strong signals even with the RF gain cut all the way back down.
:+> Andy <+:
Uh, okay.
The difference between 28.1850 and 27.1847 is 3/10 of a kHz, or 300 Hz.
300 Hz divided by 27 million Hertz (or so) would be the percent error of your reading.
That would be 0.000011 percent.
If the counter is reading low by 1/10 of a kHz and the radio is also 2/10 of a kHz low, this would get you this reading.
I'll predict that the counter has not been calibrated recently. No telling how accurate it is.
But this reading is perfectly acceptable for an AM-only CB, even if the counter is reading exactly right. This much error in frequency won't be enough to hear any difference at the other end.
73
The difference between 28.1850 and 27.1847 is 3/10 of a kHz, or 300 Hz.
300 Hz divided by 27 million Hertz (or so) would be the percent error of your reading.
That would be 0.000011 percent.
If the counter is reading low by 1/10 of a kHz and the radio is also 2/10 of a kHz low, this would get you this reading.
I'll predict that the counter has not been calibrated recently. No telling how accurate it is.
But this reading is perfectly acceptable for an AM-only CB, even if the counter is reading exactly right. This much error in frequency won't be enough to hear any difference at the other end.
73
That's an error of 3/10 of a kHz. Not enough for anyone to hear a difference on AM.
The schematic does not show an adjustment for this.
73
The schematic does not show an adjustment for this.
73
I modded the board with these parts added and I got it to adjust freq.
The holes on the board are already there for the parts - IIRC.
Wasn't a biggie; pretty easy - really.
Recommend using NPO caps for the fixed ceramic caps - tho.
BTW - I was under the impression that when the center of the carrier freq was off by more than 150hz that it started to be noticed on RX/TX?
The holes on the board are already there for the parts - IIRC.
Wasn't a biggie; pretty easy - really.
Recommend using NPO caps for the fixed ceramic caps - tho.
BTW - I was under the impression that when the center of the carrier freq was off by more than 150hz that it started to be noticed on RX/TX?
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Yes, the multiplier of that got worse - so 10MHz not a big deal, 17MHz yes it's greater - at 27, with 10.7 mixing in...you got it!
Can make a carrier signal sound noisy.
Can throw you off the frequency a narrow band receiver is listening in for, (HR2510 is the example)
:+> Andy <+:
Can make a carrier signal sound noisy.
Can throw you off the frequency a narrow band receiver is listening in for, (HR2510 is the example)
:+> Andy <+:
Before the Pll days, CB radios were typically off frequency by a much larger amount. The addition of a “delta tune” switch was applied on many 23ch radios. The point is, that switch was rarely used because it made little or no difference. That larger frequency variance was sometimes used as an advantage back then if you had a SSB radio. In the case of “who is dumping the dead carrier”, it was difficult to determine that question on AM with any reliability. Signal strength is not enough plus that's a variable. For those that had a SSB radio the offending carrier became obvious. An AM carrier is like a signature, but the variations were much greater in those days. Making a note on everybody’s carrier on SSB eliminated most of “dead key bandits” in our area.
Log: John Doe...down 900Hz, drifted up 200Hz…. Couldn’t tell any difference on AM
Log: John Doe...down 900Hz, drifted up 200Hz…. Couldn’t tell any difference on AM
Ahh, but we ALL know..... because he mentioned in post #105 that it was showing 27.1847. 
~Cheers~
~Cheers~
Did you ever find out the board model number yet?
Better do that first.
Did you every check your freq counter to make sure it is accurate?
Otherwise, don't even bother to mod it yet - it might already be on freq.