Cb radio fault

[davelectronic]

I have a cb radio with a problem, there is a large 1000uf 16 Volt electrolytic capacitor that keeps venting. Even after being replaced, i since discovered the polarity changes across the capacitor when transmitted. I think this reverse polarity keeps destroying any new fitted capacitors. It is installed in the correct orientation. I was wandering if a diode across this capacitor would stop this reverse voltage issue, there by protecting the capacitor from reverse polarity when transmitting. Any help appreciated thanks.

[Andy Watson]

That's a very unusual design "feature"! Are you sure that there is not some other fault that is causing the reversal? Can you post a diagram of the circuit - at least  the circuit surrounding the capacitor?

[drussell]

What model is the CB and do you have the schematic?

Are you certain that it isn't supposed to be a bipolar capacitor?  If not, there must be some kind of fault in the circuit somewhere.

[james_s]

Where is this capacitor in the circuit? Obviously if the polarity is supposed to be switching like that then it would need to be a bipolar capacitor, but that strikes me as a very large value for that. Is there a schematic available?

[davelectronic]

Thanks for the reply. Yes I was confused why it wasn't a non polarized capacitor. But it's one of the biggest electrolytics on the board. As it blew multiple capacitors I decided to put a meter across it. In RX it's normal, as soon as I key up TX the polarity is reversed, and there is a distinct pop audio noise through the speaker. The radio works ok, then the capacitor vents and have to replace it. I'm almost certain it's a design or layout fault. I will try and find a schematic. The radio is a Midland 78 plus. Its the first model with the orange display. I've had two of these from new, both had the same problem.

[james_s]

I suspect there's a failed component, if it were entirely a design fault I don't see how it would have lasted as long as it did. It could be a fault that makes whatever other part fail.

[davelectronic]

Yes James I think you might be right, although it's done it on two radios. I managed to get the schematic. And a couple of pictures of the pcb.
It's the biggest capacitor on the board. I don't have the radio in front of me, but I believe it's C206.
But the value on the silk screen doesn't make sense.

[davelectronic]

And the pcb image.

[drussell]

It's the biggest capacitor on the board. I don't have the radio in front of me, but I believe it's C206.

I think you mean C212.  It is across the power supply pins on the audio amplifier.  If that was actually going negative, I don't see how it isn't blowing the amplifier IC.  The negative end should be going straight to ground so I would start by looking for broken tracks, bad grounds, etc. through the whole circuit.  Follow the thick black line on the schematic and check how well everything is connected to there to begin with...  everywhere should all be very low resistance to each other spot.

Something must be quite wrong, there should be no way for that to go negative.

P.S. http://pdf1.alldatasheet.com/datasheet-pdf/view/69410/KEC/KIA7217.html

[Shock]

Double check if you got the correct cap on the audio amp as shown in the schematic. Was it replaced prior to you working on it? Is it using the original mic?

If you look at that schematic it's connected to the KIA7217AP which derives power and ground similar to the sample in the datasheet. http://pdf1.alldatasheet.com/datasheet-pdf/view/69409/KEC/KIA7217AP.html

I think you should check the pins and components surrounding that IC and double check the microphone circuit isn't doing anything weird then replace the cap and IC if everything looks good.

But if it is that other cap C212 on the VCC then it could be the microphone or speaker circuit. How are they connecting the speaker? Is it connected to chassis ground?

I mean aside from obvious component failures it seems like something weird is going on. Is it on a clean DC supply? Is anything connected to ground that shouldn't be?

[davelectronic]

Yes it's very puzzling, I'm only a novice and my knowledge only extends so far at the moment. All I know is I had two Midland 78 plus radios from new, good seller, Maplins.
My first note was the loud pop on keying up both radios from new. That lad me to take a look. I knew I'd voided the warranty. On lifting the lid I saw the vented capacitor. I know it's 16 Volts 1000uf. Replaced it with a known good brand capacitor, it continued to pop audioably, until that vented.

The radio received fine and transmitted ok as well. It was just this pop noise on keying up, I decided to put a meter across the capacitor after replacing it, that's when I see the meter go reverse polarity as soon as I keyed the radio up TX.
Midland do a new 78 plus B model now with blue display, I was thinking of getting one. But don't want to go through that fiasco again. I went with TTI radios after that, they never played up once. But I do like Midland radios. I was just at a loss as to what was going on in the offending Midland radio circuits.

[floobydust]

Is this the modulator output coupling cap? If it's a transformer, the (-) goes to the audio power amp output IC and (+) to the transformer.
16V caps on B+ are too low voltage , 25V is what I see on +13.8V

[TheDefpom]

I suppose I should chime in here...

C212 is across the DC supply to the audio amp, so my suspicions start with an excessive voltage feeding the radio, possibly even a power supply fault (if mobile it could be a bad alternator causing it).

Aside from excessive voltage being applied to the radio then I do not see an obvious cause, do you have an oscilloscope to hook up to the DC rail to check for excessive noise/spikes in the supply?

As touched on my floobydust, the cap voltage is too close for comfort even on a normal supply, I would definitely upgrade to 25V as a minimum, make sure you are getting good quality caps, not some budget low quality ones.

If this doesn't help then let us know and I will look into it some more, but at this point I do not see how it could be going to reverse polarity between RX/TX, the ONLY thing that comes to mind is the audio transformer dumping current rapidly.

[DimitriP]

This looks like an FM unit (since I spied an FM deviation adjustment on the schematic).
Tehre is a good change when transmitting and attempting to measure voltages 5 or so Watts @27 MHz might be causing the meter to go a little nuts.

IC203 is receiving power from the the regulator build around DZ202,Q212,Q213,Q213
It shoudl be something "less" than 13.2V.
What is the voltage in receive mode on Pin 1 of IC203?


Straight from the wild guess department:
I 'd put a 0.1uF  in parallel with C212. Maybe the same RF that causes the meter to "measure negative" is also frying the cap.

[floobydust]

Yes James I think you might be right, although it's done it on two radios. I managed to get the schematic. And a couple of pictures of the pcb.
It's the biggest capacitor on the board. I don't have the radio in front of me, but I believe it's C206.
But the value on the silk screen doesn't make sense.

I think C216 is drawn backwards on the schematic, (+) should connect to the modulator autotransformer- like C213 and C218, C212.

[CJay]

As touched on my floobydust, the cap voltage is too close for comfort even on a normal supply, I would definitely upgrade to 25V as a minimum, make sure you are getting good quality caps, not some budget low quality ones.

I reckon the problem could be that there's a significant AC component on the supply rail from the PSU or the PSU regulation is breaking down under load, maybe as a result of being affected by RF  or going into oscillation perhaps.

16V on a 13.8V supply is adequate unless the PSU is woefully poor or faulty, in which case the OP has bigger problems that a lack of headroom in a capacitor rating.

If this doesn't help then let us know and I will look into it some more, but at this point I do not see how it could be going to reverse polarity between RX/TX, the ONLY thing that comes to mind is the audio transformer dumping current rapidly.

RF affecting a digital meter can and will cause all sorts of odd problems on a digital meter, I've seen 40-50V indicated on a digital meter and local FM radio reception wiped out by an oscillating truck 'dropper' when an old school AVO measured the expected voltage, couple of caps on the 7812 regulator fixed that...

Might be worth checking the 8V regulator as well,  Q211

[Milmat1]

Is there is a resistor just to the left of the cap in that pic that looks roasted, Or is it just my poor eyes ?

[DimitriP]

Is there is a resistor just to the left of the cap in that pic that looks roasted, Or is it just my poor eyes ?

It does look a little crispy. Too fuzzy to read the silkscreen.

[JoeO]

Yes James I think you might be right, although it's done it on two radios. I managed to get the schematic. And a couple of pictures of the pcb.
It's the biggest capacitor on the board. I don't have the radio in front of me, but I believe it's C206.
But the value on the silk screen doesn't make sense.

I think C216 is drawn backwards on the schematic, (+) should connect to the modulator autotransformer- like C213 and C218, C212.

This looks like the problem.  Here is a schematic of what is basically the same circuit with the capacitor the other way.  Go to the sixth post to see the schematic.

https://www.elektroda.pl/rtvforum/topic1764965.html

[DimitriP]

C216  !!!!