President Madison CB radio repair

[steviewonder]

Hi there folks!
Looking for a bit of help here,I have the above radio,with a odd issue.
Firstly,there's no TX output,although the red tx light illuminates.The output stages seem fine,as when a damp finger is placed on l46 in tx mode,10 watts appears on a wattmeter,varying from 29-50mhz.Connected directly with a patch lead to another radio,a signal 9S is achieved,although just spurious noise.The PA doesn't work,either.when the PA is on,the speaker turns on,but still with the cb audio....There's very little mic audio,quiet and very distorted if you shout down it!, if you switch to ssb (still in PA mode) there's no audio at all,apart from hiss.I have had a good poke around with a multimeter,but getting no-where fast,and there's little info available in the service manual department.Anyone any ideas,or had this problem? It's a odd one,for sure.
Oh,nearly forgot,It's the PC-411AD board,with mechanical clock. Any assistance greatly appreciated !

[petehall347]

Re: President Madison CB radio repair

maybe its the mic or its connections

[steviewonder]

Re: President Madison CB radio repair

Thanks for the reply,
It's not the mic,the solonoid clicks as it should,it's certainly a board issue.

[stj]

Re: President Madison CB radio repair

does it have a relay? if yes, dont trust a click to mean the contacts are good.

[stj]

Re: President Madison CB radio repair

[redwire]

Re: President Madison CB radio repair

It's a fairly hairy layout and design. A lot of the time a gorilla will have been in these "fixing" or doing mods and making a complete mess.

You have to be systematic and start at one stage and work your way though it.

I'd make sure the PSU is making 13.8VDC and would first get the audio section working. I inject music or a sine-wave at the volume control wiper (=middle of the audio stages) and see if TR43+IC6 TA7222P is working OK. I have replaced a lot of these IC's. The voltages on IC6, input pin 4 should be 0V, pin 9 output is around 1/2VCC through caps to spkr. It also branches off to TR41, TR42 as a Class A amp that drive the TX modulation.

I couldn't quite figure out the mic preamp. It's common to have a mic wired wrong and the radio actually works.
With a scope or freq. counter you can check the IC1 MB8719 PLL output at TP10.
The radio is kinda old so expect failed (low/open value) electrolytic capacitors.

Service manual here

[CapLeaker]

Re: President Madison CB radio repair

My first thing to check in your case would be the PLL. If you have a signal generator, you can inject a signal into each IF stage and see if these are working o.k.

[steviewonder]

Re: President Madison CB radio repair

Many thanks for the replies,
Last night I managed to repair the PA section,For some reason,someone had added a diode to the relay,which went to the middle pin of the PA switch.Redwire,yes it is a mess inside that will get tidied up,will add pics.I have replaced many caps already,particularly the 10v ones.The 8v switches as it should.Basically now working fine,Apart from the no TX.The voltages rise on the base pins of the driver/finals on TX,but seems there's no RF there,according to my rather naff counter!

[CapLeaker]

Re: President Madison CB radio repair

Many thanks for the replies,
Last night I managed to repair the PA section,For some reason,someone had added a diode to the relay,which went to the middle pin of the PA switch.Redwire,yes it is a mess inside that will get tidied up,will add pics.I have replaced many caps already,particularly the 10v ones.The 8v switches as it should.Basically now working fine,Apart from the no TX.The voltages rise on the base pins of the driver/finals on TX,but seems there's no RF there,according to my rather naff counter!

there should be a protection diode across the coil of the relay.

[steviewonder]

Re: President Madison CB radio repair

The blocking diode is present,yes,The other diode was added by persons unknown,which stopped the PA working.Will do some checks on TP10,and TP1,and see what we have there...

[redwire]

Re: President Madison CB radio repair

I don't see a relay anywhere on the schematic? Second generation went to a LED digital clock.

Sometimes you find a mod to make the radio generate a distinctive chirp or squawk (ping) upon key-on or key-off.
The TX switch (off) causes the audio section to oscillate briefly as the TX starts or stops. It was a common mod on i.e. Browning base stations by people who wanted to stand out, a positive feedback capacitor added.

For troubleshooting the TX, I usually test the PA/driver/pre-driver transistors (with one leg lifted) as there are inductors that can test like a false shorted junction. I go here first because the finals are high failure rate parts.
But you can troubleshoot the other direction, from the PLL TX OSC at TP1 through to IC5 Siemens SO42P mixer. I'm not sure why it uses a mixer there.

These old radios are a lot of fun to fix actually because they are a challenge and so much analog lol.

[steviewonder]

Re: President Madison CB radio repair

It appears the relay is there to disable the speaker on tx,and to disable/enable the PA speaker socket.It's quite a bizzarre add-on !..Now,with the radio set to UK FM channel 19, (the breaking channel),we have 35.580Mhz at TP1,TP10 has has 2.23-2.30 mhz,that varies,however i'm sure i was seeing in the region of 6 Mhz there last night?..It receives stations fine.I did replace the SO42P mixer first,as seemed a likely suspect,as all its pins had turned black.Every last bit of conductive glue has been removed.Here's something curious-connected directly to another radio via patch lead,it will Transmit around 5 watts...I thought no harm in seeing what we get,as wasn't expecting any RF out at all.There it transmits in the 30Mhz region,but the receiving radio shows 9S of signal and noise-on the same channel.Go one channel away on the other radio,then nothing...Back on a normal antenna,No TX at all.It's not the easiest of radios to understand,And seems un-neccesarily complex in most areas!.I am suspicious of the PLL,however as it receives stations,and the red TX light appears,seems to suggest it's fine,Maybe?

[redwire]

Re: President Madison CB radio repair

Uh, what frequency is Ch. 19 in the UK? The 35.580MHz seems a bit high.

In the radio a 7.8MHz IF frequency is used, the S042P TX mixer takes 7.8MHz and 35MHz to make 27MHz for TX.

USA Ch. 19 is 27.185MHz plus 7.8MHz = 34.985MHz expected at TP1. Divider IC1 pin 11,12,13,14,15,16 = 1,0,0,1,0,1 for Ch. 19 it should be 34.985MHz at TP1 (AM) and stable 1.650MHz at TP10.
You measure 35.580MHz which would be 27.780MHz or "27.780Mhz is used by Italian CB Radio Group PG (Packet Group)" not likely? and the frequency moving around at TP10 tells me the synthesizer is out of lock.

The DCV on TP9 (3.4V?) the VCO Control Voltage I would measure, it might say whether the PLL is trying to pull the VCO on frequency up or down or just lost and wandering. 5.00V is highest freq. Ch. 40 and 3.4V typical.

If TP10 is moving around in frequency I would think the VCO loop with IC2 is unstable and out of lock. So I would look around the synthesizer for a problem, what confuses me is the extra crystal oscillator. Yes another one.
Check the 11.1125MHz xtal oscillator after (on little board PC-645 I think) it feeds the frequency-tripler (TR29, TR30)- at TR20 emitter. This oscillator shifts according to AM/USB/LSB and controls TX frequency. The Clarifier and TX Freq. pots also skew this stage. I guess the radio switches to this for the PLL during transmit to jump up in frequency?

I would be looking at aligning the synthesizer section, assuming there is no total failure of one of the oscillators.

[steviewonder]

Re: President Madison CB radio repair

Hi Redwire,
Ch 19 in the UK is 27.78125.Here's some results: DCv at TP9 at it's lowest,3.38v on ch1,moving up approx .3v steps up to ch.40.Frequency over at TR20 emitter was in the order of 35 MHZ,varying.From there,moved over to the 7.800 xtal,but in that area a good few components had been bodged back on the wrong side of the board.These were removed,checked,and put where they should have been to start with.Once done,it seemed there was no action around the 7.800 xtal,However ,a wet finger test (on TX,always worth a try?) on TR32 produced perfect tx and audio,to the other radio,at the far end of the room !,Bit of poking here revealed R151 (22k) to be completely open circuit.!!!.....Not what i was really expecting at all.It's connected between the Collector and base of TR32.so A RESULT!!!!! Many thanks for your help here,I would have been at it forever! (feels like I already have already) !

[redwire]

Re: President Madison CB radio repair

An open resistor at that xtal oscillator is really tough to track down, that's a great find
I'd get lost with the three xtal oscillators in this radio. Vaguely recall that President was a higher end radio, some people thought it was hype. That was a very long time ago lol.

[steviewonder]

Re: President Madison CB radio repair

Apparently,back in it's day,it was regarded the best of the best,along with the Cobra 2000gtl, (same internals,nearly),It probably wouldn't compete well with today's offerings from Icom,Etc.It does seem to be incredibly stable,with a distinct lack of drift.
Many thanks once more,for the assistance!

[CapLeaker]

Re: President Madison CB radio repair

tough find! congrats!