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Uniden PC 33 and Mic Squeal

Mikey D.

Active Member
Mar 20, 2018
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I’ve been off of CB for 20 years and I’m new here so please go easy on me.

I have a Uniden PC33 that I got eons ago and I recently took it out to use on a trip to SC.


Initial inspection: the power cord had to be replaced; TR-14 - 2SC1815 and C-103 - 0.001 uF Cap are missing from the board (not by my hands); coil L6 is spread apart slightly; the final is a 2SC2078 (not a 2SC2029 as indicated on the schematic); several components look like they need replacing do to age and I am sure the switches need a good cleaning. Solder joints look OK and no components are cut except those mentioned earlier.


I took some measurements using a CN 801 meter and a 100W 50 ohm dummy load: 4 watts dead key swinging to 5 watts on AVG setting & 4 watts swinging to 12 watts on the PEP setting using the 20 Watt scale on channel 20. Audio testing with a base antenna attached sounded good while monitoring on a different radio – no audio anomaly's heard. There was some splatter on 1 or 2 adjacent channels but I chalk that up to having both radios in close proximity while keying up (I did use 20dB of attenuation). The radio was on frequency – checked that with an MFJ 259B analyzer’s freq. counter.


On the trip: the PC33 was grounded to the vehicle’s interior; had an Astatic RD104E Road Devil power mic with a fresh 9V battery attached and was connected to a Wilson 1000 antenna using a magnetic mount (SWR was 1.5 on channel 20 according to the MFJ 259B analyzer). My first contact copied me, but said that I had a squeal. The mic gain slide switch was at 25%; I lowered the slide switch gradually with no change to the squeal; when I put the switch all the way down I was told my audio was very low. Fortunately I took along a backup radio – a Cobra 25 that I used with no complaints about squeals with the Road Devil attached.


Any ideas on what caused the squeal, the lack of TR14/C103, or a combination of things? There was no squeal when I originally tested the radio in my home so I’m baffled at why the squeal started in the mobile.
 

There are many things that can make that happen. First off, I would go to the battery directly for power/ground. Next, I would double check the plug on the mic cable to make sure all of the solder joints are correct. Having said that, the next issues must be in the radio itself.
 
Pc33's haven't been made in years. More than likely the "squeal" was from the low voltage marginal caps dried out - no shielding or bypass - this affects Mic gain amp TR13 - C96 22uF, C97 47uF and C94 47uF - and there's no guarantees that replacing them will even fix the problem - they are grey in color caps - the 10 volt blues...rated 10WVDC

PC33Micamp.jpg
When you replaced TR14 they ripped out the limiter the squeal comes from RF currents entering in the RF bypass and filter caps located around the board generating circulating currents and squeals.

Expect to replace all of them soon enough.

:+> Andy <+:
 
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There are many things that can make that happen. First off, I would go to the battery directly for power/ground. Next, I would double check the plug on the mic cable to make sure all of the solder joints are correct. Having said that, the next issues must be in the radio itself.

Thanks for the reply Robb. When I tested the radio initially in the mobile a week before the trip I connected it in the same manner as I did the day of the trip - I heard no squeals while monitoring from the mobile (cellphone to home phone). I used that same method to check the audio on my Cobra 25 (connected in the same manner as the PC33) - I found that I had to adjust the Dyna Mic knob on the Cobra to 50% and the mic slide switch to 25% to stop a squeal that I heard when I keyed up. The mic gave me no problems on the trip while connected to the Cobra 25 so the mic shouldn't be an issue.

I concur, it could just be the radio itself.
 
Pc33's haven't been made in years. More than likely the "squeal" was from the low voltage marginal caps dried out - no shielding or bypass - this affects Mic gain amp TR13 - C96 22uF, C97 47uF and C94 47uF - and there's no guarantees that replacing them will even fix the problem - they are grey in color caps - the 10 volt blues...rated 10WVDC

View attachment 23708
When you replaced TR14 they ripped out the limiter the squeal comes from RF currents entering in the RF bypass and filter caps located around the board generating circulating currents and squeals.

Expect to replace all of them soon enough.

:+> Andy <+:

Thanks Andy. I intended to replace TR-14 and C-103 down the road (no pun intended), but if there is more going on inside the radio, I'll just set it aside for now. I like its size though.
 
no mention of one item. do you have talk back installed on the radio? if so disable it and see if the squeal goes away.
 
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The green line that points to both the part on the schematic as well as on the parts layout - this one cap controls the gain in the circuit, you take it out, low mic volume, leave it in there - and it dries out - they look like an electrical short, affecting gain of the mic amp as in overmodulated and squeals.

The line it's connected to is POWERED ON TX but floats at a voltage on RX - cutting off the gain in this circuit so it doesn't add to the problem. And is why the cap is on the emitter side as part of a gain cell for punch to the audio with a recovery lag time set by that value.

So if talkback has been installed - that may lead to this big time...

:+> Andy <+:
 
The green line that points to both the part on the schematic as well as on the parts layout - this one cap controls the gain in the circuit, you take it out, low mic volume, leave it in there - and it dries out - they look like an electrical short, affecting gain of the mic amp as in overmodulated and squeals.

The line it's connected to is POWERED ON TX but floats at a voltage on RX - cutting off the gain in this circuit so it doesn't add to the problem. And is why the cap is on the emitter side as part of a gain cell for punch to the audio with a recovery lag time set by that value.

So if talkback has been installed - that may lead to this big time...

:+> Andy <+:

No talk-back or other toys installed...its just a plain Jane radio with the exception of L6 being spread and TR-14/C-103 removed. I'm gonna reinstall those 2 components as well as replace C-98 and C-94 for kicks and giggles (Mouser). There are 2 other components that I found that I want to replace - R-41 and a disc cap.

Thanks for the info.
 
Here is some more help...

You did not hear this from me...

IF you want to "defeat" the limiter, locate TR15 and VR3 - there is a Diode D13, located by it.

On the Foil side, solder a jumper wire ACROSS it, short it out. This diode is used to locate/isolate/use "negative energy" or energy that is below a threshold set by a resistor divider that TR15 amplifies this effect and uses TR14 to "punch holes" into the audio using the negative bias.

Basically - any modulation that is below 70% or Average Audio Power - meaning the Modulator sends the driver and final a DC straight - bias when you are not talking into that microphone (modulated - work with me here). Tr15 does not conduct - stays off. When you modulate - the audio swings above and below an imaginary threshold line of power (like a sine wave 50% above and 50% below) and affects DC bias - but as a rectified element - meaning a detected, rectified, polarized directional power - the diode starts to empty the steady state bias of a pre-set voltage from TR15's "Happy Place" set by R84 and R85 (3.3K) - this in turn makes TR15 conduct and pull energy from somewhere to do this - as a means to recoup the lost power - by pulling power from TR14 and hence your mic audio arriving in.

Why short it? To keep the negative energy from draining into and staying away from TR15 - the DC Bias and Audio power being sent to the driver and Final it's detecting from. Any negative energy will be washed out by the DC Bias present - so the Limiter recovers quickly - you don't hear the effect.

:+> Andy <+:
 
Andy, when I replace TR-14 and C-103 can I install a variable resistor somewhere in the audio circuitry to control the modulation levels without having to jump out the diode D-13, or can I do both?

Mike
 
no mention of one item. do you have talk back installed on the radio? if so disable it and see if the squeal goes away.

Sorry I didn't answer you directly, but there are no toys in the radio - there is no PA jack on this radio.

Mike
 
Look for R84 and R85 - these two are 3.3K - since you know it looks for a pre-set value - you can raise the bar by lowering the + Feeder to the divider, do the opposite for lowering the bar.

You can change the scale or threshold by adjusting these two values of R84 and R85 - this affects where the limiter engages, and by how much effort it imposes on the audio line.

Try two 10K pots one adjusts one resistor like a variable on each of the two R84 and R85 holes. and use a 1K to buffer the + line so you don't destroy the part TR15 from reverse biasing.

It's quite instructive.

:+> Andy <+:
 
Mikey D, you would be surprised how many guy's will have talk back in a radio and a stock mic and the radio would squeal every time they would key the radio and never say they had talk back and that would be the only reason the radio would squeal.
you can have talk back in that radio even if it does not have a PA plug.

You have Andy helping you and you should get it all set up with his help.
 
Here is some more help...

You did not hear this from me...

IF you want to "defeat" the limiter, locate TR15 and VR3 - there is a Diode D13, located by it.

On the Foil side, solder a jumper wire ACROSS it, short it out. This diode is used to locate/isolate/use "negative energy" or energy that is below a threshold set by a resistor divider that TR15 amplifies this effect and uses TR14 to "punch holes" into the audio using the negative bias.

Basically - any modulation that is below 70% or Average Audio Power - meaning the Modulator sends the driver and final a DC straight - bias when you are not talking into that microphone (modulated - work with me here). Tr15 does not conduct - stays off. When you modulate - the audio swings above and below an imaginary threshold line of power (like a sine wave 50% above and 50% below) and affects DC bias - but as a rectified element - meaning a detected, rectified, polarized directional power - the diode starts to empty the steady state bias of a pre-set voltage from TR15's "Happy Place" set by R84 and R85 (3.3K) - this in turn makes TR15 conduct and pull energy from somewhere to do this - as a means to recoup the lost power - by pulling power from TR14 and hence your mic audio arriving in.

Why short it? To keep the negative energy from draining into and staying away from TR15 - the DC Bias and Audio power being sent to the driver and Final it's detecting from. Any negative energy will be washed out by the DC Bias present - so the Limiter recovers quickly - you don't hear the effect.

:+> Andy <+:

Andy, I decided to go with replacing TR-14/C-103 along with C-98 and C-94 and see what happens with the audio. I'll look into adding that jumper across D-13 if necessary (I forget where I read about doing that - wink, wink).

Mike
 
And...

It may be a good idea to use a spare CB radio as a monitor - this way, you can then verify the squeal or not.

Don't need much for the monitors antenna - a simple wire, or even a test lead with the banana plug will fit in the back of the SO-239 center just fine. 3 feet is a good length.

All you need is to verify the squeal occurs - if you can't re-create the squeal with the engine on or off or when the car is moving then the noise from the other day you had the radio check - may have been from another radio interfering due to a totally separate isolated incident not your fault. Heck, even a local baby monitor or security alarm proximity to the receiving antenna can form an "image" making you sound like you have a squeal. It's more like a tone that rides in on your signal.

It's hard to say, but if you weren't getting any type of squeal before - it may have been from another local that has issues...you were "hetrodyning" on another signal - that was slightly off from your frequency. It happens.

Even worse? They may have told you it was squealing - when you really weren't. People do this just to "***" someone's day...

Nothing serious. The "experiment" I mentioned about the R84 and R85 - are so you experience the reasons why limiters work like they do - they can clean up a dirty - noisy - full of ambient noise level around the microphone - so that you don't sound distorted - which could happen when you overload the audio amp with all the power that microphone has - you could have "so much of you in the audio" the only thing missing is your shoe...

People are not as dumb or ignorant - act more like ingrate - when they know that they are talking to someone that can possibly make their day. Don't do the obvious obnoxious stuff - save that for a battle on another day. I find a lot of those on the channels these days have little patience with someone they don't know - so keep those efforts in mind when it comes to dealing with trying to get the right information from someone.

Even if a friend has a means to use a phone by their radio - call them, then talk while listening and hear from their side - how you sound - that is one of the best methods - is to hear yourself thru someone else's equipment.

:+> Andy <+:
 

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