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Clarifier problem

D

dxing.bruce

Active Member
Apr 15, 2012
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Hey: Guys.

I have a TRC-465 which is the same chassis as the Uniden PC122XL.

The issue I'm having is when I try to clarify someone on USB or LSB it sounds like
their under water. Not the normal Chipmunk or lower tones, this actually sound
very warbled as I'm turning the knob. If I go super slow I can fine tune them in but it's
not easy as when the clarifier is working normally.

I'm thinking poor voltage regulation in the circuit.

This is a stock radio, no mods or tinkering around had been done to it.

Any thoughts ?

Thanks in advance guys.


Bruce.

Toronto. Canada.
 

could be lack of power supply filtering due to a cap going south due to age.
could also be a crystal getting flaky due to a cold solder joint.
could also be temperature related.

there are a few possibilities, and test equipment will lead you toward the culprit.

do you have a freq. counter with a BNC connector on the front?

LC
 
Thanks for the input guys.

I sprayed the pot with good contact cleaner, no help.
The radio is indoors so temperature is not the culprit.

I do have a Freq counter with a BNC connector as well a digital multi meter,
dummy load, good power supply, Good vswr / power meter.

I will keep trying to see if there are defective components.

Thanks.

Bruce.
 
First thing I would do is to "tune in" the radio's 16 MHz PLL frequency and have a listen. This is the one that feeds into the two mixer stages, one for transmit and one for receive.

Setting a radio (or service monitor) to 16.27 MHz and the radio to channel 1 will let you hear if the warble is coming from the radio's PLL synthesizer. Set the receiver for a SSB mode and tune in the carrier. If the PLL is okay, you'll hear the steady carrier tone. And if you hear the same kind of warble that was distorting the receive audio, this is the cause.

That's where I think it's coming from. Since the PLL's frequency is controlled by a tuning voltage, this suggests that the PLL is not feeding a steady DC voltage into the radio's VCO (voltage-controlled oscillator), but feeding a DC voltage with some audio-frequency "ripple" riding piggyback on the DC. Looking at the VCO tuning voltage with a 'scope might reveal this. Or not. Only takes a tiny AC voltage to modulate the PLL this way. Could make it hard to see on a 'scope. More effective to "listen" for this fault instead.

Two best guesses for the culprit, if this turns out to be the problem.

First would be the PLL chip. Make sure the power at pin 11 is 5 Volts. Can't blame the PLL chip if the power supply feeding it is bad.

Second would be the capacitors that serve to filter the VCO tuning voltage where it comes out of the PLL chip. C63, C64 and C65 are usually tiny tantalum-electrolytic capacitors. My copy of the schemo is so smeared I can't read the capacitance values. We gotta have a better copy of that diagram somewhere, but for now that's what I have to work with.

(edit) My Cobra 146GTL schemo says these three capacitors are C68,69 and 71. Hmmm.

Good luck. Troubleshooting a PLL problem is tricky without a 'scope. Always had one around to use. Never bothered to learn how to do this without one.

73
 
Last edited:
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Thanks again guys for the input, I will doing tests on my next off days & see what the cause is. I really appreciate all the info.

Bruce.

Toronto. Canada
 
I have 4 trc-453, 2 pc122 and 2 trc-465 radios like this and they all have the same board. I recap all of mine then make sure all solder joints are good and the radios work fine. if yours has the original caps could be causing you the problem you have. also I have found that if these radios set for a while such as months the solder joints get bad and have to go over them when they have the original parts in them just to get them working. at least that was what I found with mine. after a recap and making sure all the solder joints are good at the pll and crystal areas they will start to work as they should. for some reason the board has such small traces they will go bad after a long idle period. this is just what I have found. I bought a cheap magnifying set up that hooks to my computer and look at all the joints with it and you can see all the bad solder spots very well. you can hold it in your hand and move it across all the joints and you will find a lot of them that the trace is so small in areas and then you can clean them up and resolder them with out eye strain.
 
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I want to thank all you guys for your help.

I've been super busy at work & just had time to work on the TRC-465 last night.
I replaced all the caps in the " IF " & " RF " sections & all is good now.

Thanks again.

Bruce.

Toronto. Canada.
 
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