Deaf President George

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47bw022
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Deaf President George

Post by 47bw022 » Tuesday 15th Nov 2011, 9:08

Radio Make and Model: President George
PCB Number: PB-334AA
PLL Number:
Test gear available: Signal Generator, Analouge Voltmeter, Digital Voltmeter, Frequency counter, Osciloscope, access to test station with Sinad meter.
Symptoms: Radio is deaf, can just hear audio from signal generator way down in the noise at 10uV

What is working: Anything else

What has been tried: 2SC1674L has been replaced by new part. Looked for cold soldering on pcb. Tried to realign it with no better result. Checked voltage for carrier oscillator, checked for frequency on AM.

Did it suddenly happen ?: Reported that it happend suddenly.
Anything else that may be of help?: Got the radio from another operator who owned it when the failure happend.

The radio do receive, as i do get very loud signal in. But nothing compared to other radioes that i got. Which also can be seen as the largest signal audioble is above atleast 10uV.

I hope you all can be able to help me on this issue.

Best regards

47BW022
OZ2DA

Dennis Andersen

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Re: Deaf President George

Post by 47bw022 » Wednesday 16th Nov 2011, 1:18

Mesured voltage at some of the transistors on the upperside of the pcb.
Used chassis as minus/ground.
All mesurements are done with squelsh open.

Q8 - 2SC1674 - Base = 3,3V - Collector = 6,3V - Emitter = 4,7V

Q213 - 2SC1675 - Base = 0V - Collector = 7,2V - Emitter = 4,0V

Q412 - 2SA950 - Base = 7,8V - Collector = 0V - Emitter = 7,2V

Q413 - 2SC3242A - Base = 7V - Collector =7,8V - Emitter = 7,8V

Hope it helps.

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Re: Deaf President George

Post by 47bw022 » Thursday 17th Nov 2011, 3:30

Today I've been doing the following.

Changed C486 and C487 both 330uF 10V to 330uF 35V.

A bit overkill at the voltage ratings but that's what my parts pusher had.

Changes : None.

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Re: Deaf President George

Post by KC7NOA » Thursday 17th Nov 2011, 5:59

Doesn't seem like your getting much help so I'll throw my thoughts in....

I would start by bypassing the lowpass filter and inject signal to the first stage ... try to devide and conquer ...

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Re: Deaf President George

Post by The Defpom » Thursday 17th Nov 2011, 9:40

Unfortunately the circuit diagram I have for that radio is rubbish, I cannot make out most of the part numbers etc.

But the first couple of things I would do is try removing the AGC function in case that is staying turned on, this can be done by lifting R37, Q18 is the AGC control transistor.

The RF gain control goes to Q8, does the RX sensitivity change with changes to the RF gain ? if not I would be looking at that control and Q8 for correct function.

D3 and D4 are the AGC shunts to protect the front end, sometimes they get leaky and need to be replaced, so I would be looking to do that.

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Re: Deaf President George

Post by 47bw022 » Thursday 17th Nov 2011, 10:11

Thanks for the replies KC7NOA and Defpom.

I'll try to bypass the lowpass filter.

I've uploaded a schematic diagram that I hope is better than the one you got Defpom.

RF control button on the front does change the sensitivity at the signal. From absolutely nothing at ccw to just barely audioble at full cw.

Strong signals get through, mean very strong signals, low signals i dont get. On other radios where i have s-6 noise i don't have any s-meter reading, but at the large signals i get s-meter.

I'll try to lift R57 to see if it does anything and report back here. From there i'll look at the diodes D3 and D4.
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Re: Deaf President George

Post by The Defpom » Thursday 17th Nov 2011, 10:38

Thanks for the diagram, that was slightly better ! the resistor to lift is R37, not 57.

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Re: Deaf President George

Post by 47bw022 » Thursday 17th Nov 2011, 10:45

Hi Defpom

Yes after looking at the diagram i had an idea that it might not be R57 as it goes to ground ;)

Will lift R37 tomorrow and see what happens. I do hope that it'll do something.
The D3 and D4 I have found on the schematic as well, I just have to find them on the pcb tomorrow and have to find some replacements as i don't have those diodes at my qth at the moment. I don't even know if i can find them locally yet, but i'll look for them.

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Re: Deaf President George

Post by 47bw022 » Thursday 17th Nov 2011, 23:01

D3 and D4 seems to hold.

There are through the right way and blocked the other way. Both soldered loose and tilted from solderjoint so that i only messured the diode.


Still trying to locate R37 on the pcb though, thats a little nasty resistor that keeps hiding from me :)

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Re: Deaf President George

Post by The Defpom » Friday 18th Nov 2011, 7:01

I found a mistake on the PCB layout, there are two r27 on it, one below q8 and one just to the right of it, one of those will probably be r37, see which one connects to d22/r569 and then q18

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Re: Deaf President George

Post by 47bw022 » Friday 18th Nov 2011, 9:07

Thanks Scott

Will look for that tomorrow after work :)
I noticed that fault on the layout too, and it pussled me a bit. I've done some tracing on the pcb aswell and think that it might be a smaller project now finding that resistor.

But I'll let you know when it's done.

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Re: Deaf President George

Post by 47bw022 » Saturday 19th Nov 2011, 23:44

Hi Scott

I've lifted (removed) resistor R27 (R37) the one near Q8.
I seems to have to do with the rf control button on the front as this stops working which also fits with the agc.

The result was :
No change when using the rf control knob on the front of the radio.
The radio became even more deaf.

Conclusion. The agc do work, but does et work as it should.
The rf control knob only starts amplifying on knob position from 3 o'clock to turned all up.


best regards

Dennis
47BW022
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Re: Deaf President George

Post by 47bw022 » Sunday 20th Nov 2011, 8:48

new info

I've messured Q8 again.
Fun part as i see Q8 as a transistor for amplification, correct me if im wrong.
But when i messure Q8 with the rf control knob turned fully counter clok wise it gives me higher voltage as if i messure it with rf knob fully clock wise.
Will that say that the amplification actually lowers when the rf knob is fully clock wise ? and shouldn't it be the other way around ??

best regards

Dennis

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Re: Deaf President George

Post by 47bw022 » Sunday 20th Nov 2011, 8:56

And another strange thing, but that might just be how it's supposed to be, but the squelsh seems a bit odd aswell.
There is just a tiny opening where it lets noise through.
When it fully open (counter clock wise position) it closes as if i move it way up clock wise.
But when i move it clock wise it comes to a point were it opens up, for then a short bit later at moving clock wise it blockes the noice again.

Is it supposed to be like that with the asc that's implementet in those President radios or is it here the fault perhaps are ??


best regards
Dennis

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Re: Deaf President George

Post by lordkevin8 » Wednesday 19th Dec 2012, 21:36

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