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Old rig in need of some TLC perhaps?

Posted: Saturday 23rd Jan 2010, 6:40
by Tipsy
Hello, I'm a bit of an odd-ball ecentric type and have a habit of collecting things other people throw away - usually consumer electronics or mechanical devices but could be anything that I think might serve me one way or another.

Many moons ago I got into CB radio use which were illegal at the time but after having my collar felt, the rig confiscated and getting a stern interrogation/talking to by HM Customs and Exise I moved onto other hobby's.

So without boring you too much, I'll get down to the subject in hand - an old President Jhon Q AM/FM CB that's seen better days and has obviously been tampered with inside before getting consigned to a loft for who-knows how long. It came to me without the mic, aerial, power lead, brackets, clips etc but that didn't stop me seeing if it would light up. :P

In short, it kept blowing fuses but after reading lots of posts on here I ended up replacing a diode and also realised the power connector has previously been fitted upside down (therefore invalidating the +ve and -ve legends stamped into the chassis :banghead: ) and I'm happy to report I can now turn it on. 8)

With power connected and it switched on, the PWR/SWR meter lights up, the channel display works, the TX led is constantly on and there's a faint white noise to be heard through the internal speaker (dependant on control settings).

So, that's the history. Do you think, given the above, the probability of repairing the rig is good? Inside, it's obviously been previously worked on and there's a free-floating board that I presume has been added to increase functionality. The switches on the front have too much free-play and hinder the extreme switching positions as well as messy solder joints here and there.

Re: Old rig in need of some TLC perhaps?

Posted: Saturday 23rd Jan 2010, 7:35
by The Defpom
Most radios can be repaired, given the right amount of time investment, and parts availability, the tricky thing is having the test gear available to be able to find the faults, at the minimum you will need a power meter with a dummy load (or tuned antenna), a multimeter, a mic which is known to be good, a 50MHz frequency counter, and ideally you would have an 100MHz oscilloscope and RF signal generator as well.

A circuit diagram for the radio is always a big help too, I just checked my collection, and I do not seem to have one for the John Q.

Re: Old rig in need of some TLC perhaps?

Posted: Sunday 24th Jan 2010, 6:09
by Tipsy
Hi Scott,

Yes, I'm aware troubleshooting electronic faults may require the use of the expensive test equipment you mention (but I think I've done alright so far with my cheap multimeter :lol:). Perhaps some more experienced members might be able to help me out - give me a few pointers/alternatives - if it does come to that? Maybe even pay someone to do the more tricky repairs, I don't know. At the end of the day, I got nothing to lose trying. Time is not limiting factor but money is.

I'm hoping, with an aerial (or dummy load) and mic connected, I might find the rig is working as it is, I'm just not sure if it's normal for the TX led to be on without them connected? As I said, there's white noise from the speaker whereas I've come to expect white noise in RX mode. Additionally, the internal speaker is a lot quieter than an external speaker which could be signs of a faulty circuit somewhere.

I found the diagrams on another site (cbtricks I think) and I'm happy to send you a copy if you'd like. NOTE, if searching for info: President have misspelt "JHON".

Re: Old rig in need of some TLC perhaps?

Posted: Sunday 24th Jan 2010, 14:15
by zodiac
Hi Mike,
If you unsolder the collector pin of TR6 that should turn the TX LED out till you find the fault.
If the radio has gone into TX on its own it'll burn the final transistor out without an aerial connected.
The diode you replaced, had it just gone short circuit, if so there won't be that much damage if any.

Re: Old rig in need of some TLC perhaps?

Posted: Sunday 24th Jan 2010, 14:54
by The Defpom
Going by the diagram if the TX LED is on, then the radio is transmitting, as it is powered from the RF output, either that or TR11 has failed.

Check the voltages on TR21 (D14), this is the TX/RX switch and should provide 9V or so in RX, and 0V in TX, D14 is the RX voltage regulator, as this switch is electrically close to the main DC input, it is possible that this part has been damaged if it was reverse wired, it may not have failed completely, it may just be leaky.

D15 is a 9V regulator, it will pay to check that also, as well as IC4, which is a 5V regulator.

I would also replace C111, as if that part fails it could drop the TX input line low, forcing the radio into TX.

Re: Old rig in need of some TLC perhaps?

Posted: Tuesday 26th Jan 2010, 8:49
by Tipsy
Is it possible there are two different models of the same name, a US and European model? Perhaps this is why it's misspelt "Jhon" on mine? Have I got a fake?? :doh:
I just found a US ebay ad for a President John Q and it's not the same radio I have. ... 518fc80ecb

The circuit diagram shows a DIMMER yet it's absent on mine as you can see... ... TA#gallery

One slight difference on the rear, I have a two pin power socket.

Re: Old rig in need of some TLC perhaps?

Posted: Tuesday 26th Jan 2010, 8:56
by The Defpom
Well that is interesting !

OK, can you find the main PCB's number and tell us what that is, it is likely that it is the same as some other radio inside, if we know the PCB number we will be able know what to base repair advice on.

Re: Old rig in need of some TLC perhaps?

Posted: Tuesday 26th Jan 2010, 9:38
by Tipsy
I do apologise Dave, I didn't answer your post! :oops:

Yes, the diode (I'll try and identify it on the diagram for you ... it's not my strong point but I'll willing to learn, if I may beg your patience) had provided a short by the power socket, hence the blown fuses with the rig turned off.

Scott, I see each board has a clearly identified part number, I hope this is what you're asking for. As it happens, they're all marked UNIDEN and a previous person has written/scratched "Catch 22" on the solder side.

PC 242 AC is the main board
PC 870 AA is off the Channel Selector
PC 743 AA is the free floating board which appears to be part of...
PC 686 AA (carries a 15.810 crystal I think) was mummified in insulating tape and tucked away.

EDIT: Dave, I believe it was D16 that shorted.
EDIT II: No, scratch that. On the actual board, it's printed D23 (and was fitted on the solder side with card for insulation).

Re: Old rig in need of some TLC perhaps?

Posted: Thursday 28th Jan 2010, 0:27
by Tipsy
I've searched long and hard yet cannot find any info on my board. :(

Plenty of UNIDEN PCnnn schematics except PC242

Meanwhile I've got hold of a Fidelity 4-pin mic that I can use for this President rig but again, without diagrams, I don't know if I need to re-wire it?

Re: Old rig in need of some TLC perhaps?

Posted: Friday 29th Jan 2010, 3:47
by zodiac
D16 is the protection diode and when it goes short circuit it will just keep on blowing the fuse every time you replace it with a new fuse that is until you put a fuse in that's too strong, then you will do some damage.
The mike is Pin 1 screen, Pin 2 audio, Pin 3 TX, and Pin 4 RX.
The audio wire is the wire with the screen wrapped round it.
PC 743AA is the FM board added to make the radio switch between AM and FM, the board is used in other Uniden AM radios as well to give FM.


Re: Old rig in need of some TLC perhaps?

Posted: Friday 29th Jan 2010, 4:54
by Tipsy
Hi Dave, I'm certain those schematics are not for my board - the diode is D23 on my board.

Either way, I've made a little progress on the diagnostic side with a mic connected - it's definitely fubar'd with constant TX illumination (which changes from red unkeyed, to green keyed), PA not working (presumably rewired for upgrades) and producing other abnormalities - flickering lights, static over the speaker when switches are wiggled and a transistor overheating very quickly. It's all beyond my capability so I think I'll give up at this point. I may try remaking all those bad solder joints and tinkering here and there but I don't hold much hope now as I've got other projects I can turn to.

Thanks and best wishes to you and Scott,