Author Topic: How to calibrate an AM radio?  (Read 727 times)
Binarix128
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How to calibrate an AM radio? « on: January 02, 2021, 05:01:15 PM » Author: Binarix128
I need your help guys to calibrate a transistor based AM radio. I got some AM radios in those days of the same model, but one is not working. When I turned on the radio I couldn't barely hear anything, I thought it was the batteries but there were 3V at the input of the circuit, then I realized the problem was in the calibration of three potenciometers, which I'm not sure what each one does exactly.

I could copy the values of a working radio, but my multimeter can not measure capacitance or inductance, so I have no way of doing that. The components are slightly different between the radios but have the same value despite being the same model. I tried copying the positions of the potenciometers but it didn't work. I also tried tweaking the tuner screws but no improvement.

It would help me a lot to have an idea of what those potenciometers do, for a better calibration.

I've tried to tweak the potenciometers, but the audio output is either too low or it's too distorted. If I set it to listen good on a weak station then it oversaturate on a stronger signal, but if I calibrate it on a strong station then I hear almost nothing of weaker signals. That does not happen in the other radios of the same model.

I don't have the enough skills or tools to reverse engineer the circuit either, but I leave you a pictures of the radio circuit.

Edit: I think the problem was the speaker soldering, the wires came off with the touch, so there was a huge resistance forcing the transistor too much to a get an audio output.

Edit 2: I resoldered the speaker but the problem is still there.
« Last Edit: January 02, 2021, 05:28:16 PM by Binarix128 » Logged

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Medved
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Re: How to calibrate an AM radio? « Reply #1 on: January 04, 2021, 01:05:07 AM » Author: Medved
First these are not potentiometers, but adjustable coils (the black thing with the slit is a ferrite cup with a thread, which closes/opens the gap towards the internal rod core), forming a resonant transformers. Two of them are tuned at fixed (most often 455kHz, but sometimes it may vary, for sure within a 450..470kHz range) and form the IF filter, the 3'rh is an oscilltor coil, tuned by the tunning knob to select the station to receive (should oscillate 455kHz above the received frequency).
Second it is highly unlikely the misalignment is the cause of your problem (unless some moron was already "tightening the screws" or "improving the alignment" - ie messing with them without knowing what he is doing), so dont mess with them before you are damn sure EVERYTHING else is working right. You can not align it without dedicated instruments (at least a modulated generator or a counter is required).

If completely dead (even no click on power on), check the audio stage.
If some quiet noise gous out, check whether the oscillator is running: Take a second known working radio, tune it to a station on the upper end of the band.
Then put the tested radio close to the working one, power it on and tune it so the working radio will be squeeling. If it does,the oscillator is running. The tested radio dial should read (ideal case) 455kHz below the station you used.

Being old radios, lying few decades unused, the most suspect are the dried out electrolytic capacitors and often bad contacts (on earphone socket, volume potentiometer and power switch).
So take some known good 33uF capacitor and try to connect it parallel to each electrolyte you find. If the radio comes to life (that includes loud squealing or motorboating), that electrolyte is dried out. If you find more than one defective, replace all of them, chances are all degraded the same way.

Then for the alignment:
The best method is to use an RF generator:

IF alignment:
Set to 455kHz, AM 400Hz 30%. The modulation parameters may differ...
Connect it via about 1nF to the "antenna" terminal of the tuning capacitor.
Turnthe radio on, tune it to the most upper end of the range, you should hear the 400Hz. (You may use a resistor dummy load and an oscilloscope or ac voltmeter capable to measure around 100mV instead)
Lower the generator output voltage so the tone becomes reasonable loudness even when the volume pot is up. The volume should respond to the generator output voltage (so the AGC is out of regultion).
Then locate the last if transformer (uses to be black), take a nonmetalic (or soft nonmagnetic metal; to not break the ferrite or interfere with the field) screwdriver and adjust so you get maximum output.
If it was off, readjust the generator outputvoltage, so you stay out of the agc range.
Then proceed to the first (yellow). Readjust the generator output asneeded during this.
Then recheck the adjustment again from the last stage (the stages interfere to some extend, so if theprevious stage is too far off, it drags the next one with it). Repeat till you are at the peak with all of them.

Oscillator (tuning range/dial) alignment
Then tune the radio to the lowest calibrated point on its scale (somewhere around 520..550kHz).
Set the same frequency on the generator.
Tune the oscillator (red) coil so you get maximum signal.
Then tune the radio to some highest calibrated mark (usually 1600kHz is there), tune the generator the same way.
Locate the oscillator trimmer capacitor on the back of the tunning capacitor: There is a visible metal stripe from it towards the lead marked "O".
Tune it to maximim signal.
Then set the generator back to the low frequency, tune it by the radiotuning knob and check the tuning dial matches the generator. If not, readjust the oscillator coil.
Then in a similar way recheck the top end of the band (setgenerator to 1600kHz,tune it on the radio, check if the dial matches).
Of course, all the time adjust the generator output voltage so you stay out of the AGC range (if the signal is strong enough to make the agc active, it will supress all strength variation and you wont notice where the peak is).

Now when you disconnect the generator, you should be able to tune something.

Antenna tracking alignment:
Disconnect the generator from the radio,
Make a coil from few turns of wire and connect that to the generator (forming a kind of test transmitter; you may need tocrank up the generator output).
Search with the radio tunning for a quiet spot around 700kHz. Set the generator to that frequency (at least 30kHz away from any strong station; so you willhear the generator while out of the agc range).
"Tune" your generator on the radio.
Then adjust the antenna coil for maximum signal by shifting the antenna coil on the rod. If you hear stations around the lower end of the band reasonably and the coilis glued onto the rod, you may skip this step.
Then find similar frequency near 1300..1400kHz, set the generator and tune the radio.
Now adjust the antenna capacitive trimmer (the one leading to the "A" terminal) for maximum signal.
Recheck the tracking again.


If you don't have a generator, but have a frequency counter (or have other means to measure frequency around 1MHz with better than 1kHz accuracy - e.g. another radio "receiving" the oscillator; but it should allow tunning by 1kHz, as the oscillator will be between the normal grid of station frequencies), you may use some existing stations close to the lower band end (the radio should pick them up at least somehow):
Couple the oscillator to the counter, so it displays the frequency. Tune the radio so, the oscillator is 455kHz above the station.
Now the mixer should generate the 455kHz signal from that station, so you may align the IF on it. You may need to start with the first stage, as the is mostly there, in order to hear the station at least somehow. Ocassionally check with somecalibrated radio you are tuning to a correct station (so to really 455kHz) and not somewhere else. And check as well whether the oscillator hadn't drifted away (tuning the 1st IF stage may drag it a bit).
If the station becomes so strong sothe agc kicks in, switch to some weaker one.

Then tune the lowest station you can hear and align the oscillator coil so the scale matches onthe low end.
Similar way tune some station at the top end, align the "O"-scillator trimmer to match the scale on the top end.

Then tune some weaker station around 700kHz to align the antenna coil and some in the 1300..1400kHz range for the antenna trimmer.







« Last Edit: January 04, 2021, 01:09:38 AM by Medved » Logged

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Binarix128
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Re: How to calibrate an AM radio? « Reply #2 on: January 04, 2021, 09:16:11 AM » Author: Binarix128
Thanks for your help! I doubt that the problem comes from the audio part because it works perfectly, it's just oversaturated due to wrong calibration. Well I have no tool that can measure frequency apart from the working radio, I could use my phone as a generator but at low currents not to kill my phone and at maximum 22KHz which is way below the AM range, so it is useless. When I was screwing around the knobs I suddenly started to hear an oscillation in the other radio tuned at the same station, probably the oscillation that should be there that you are taking about.

When I turn the volume know all the way up I can hear some distortion and sometimes a motor sound. The main thing is that the radio picks up the signals perfectly but I can't turn the volume much as it will crap out. So the tuning is perfect, I want to calibrate the things for get the best signal level.

Should I calibrate the radio so the other working one starts to oscillate? Can I copy the values of the coils of the working radio by sticking the meter in resistance mode?
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Re: How to calibrate an AM radio? « Reply #3 on: January 04, 2021, 10:20:38 AM » Author: Medved
So from your description it really seems the radio needs repair (most likely the electrolytics have dried out so need replacement) and it is aligned well.
So DO NOT TOUCH any of the alignment component (the screw like cores in the coils, the trimmers on the back of the tunning capacitor; you keep saying "calibration" but normally the "alignment" is the term used)
...and replace all the electrolytics.
Your symptom description really is pointing to bad decoupling (supply, AGC), which then uses to lead to oscillations, mainly when the volume is cranked up.
Once you replace them, it will most likely start to work normally as it should.
And the problem comes from the audio part too: Bad capacitors does not prevent it from making any noise or playing quietly, it just does not allow full volume without distortion or parasitic oscillations.
Looking at the components, there is one in the AGC (the most top one, leaning against the two IF coil cans; should be around 10..22uF), one supply decoupling (just bottom right beside from the AGC one; same as the AGC one) and and two in the audio amplifier (one coupling from the volume pot to the audio amp input - the dark one between the second stage IF and the headphone connector, about 4.7uF or so; second is the output speaker coupling on the bottom right corner of the PCB, some 100..220uF or so)...
All suffice with 6.3V voltage rating
« Last Edit: January 04, 2021, 10:30:12 AM by Medved » Logged

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Re: How to calibrate an AM radio? « Reply #4 on: January 04, 2021, 10:50:19 AM » Author: Binarix128
I will try to replace the electrolytic capacitors, after all they are just four and it will be easy. I also discovered a broken ceramic capacitor between the speaker and the headphone Jack that seems to have the mark "204", which can be the principal problem, so I'll try to replace it before going into the electrolytic ones.

It's too late, I already screwed up the trimmers and the tuning capacitor looking for the best signal, but I think I can alight it again.

Before the capacitor broke it was damaged so it was working through the parasitic capacitive coupling all that time, hence the low signal.
« Last Edit: January 04, 2021, 11:10:19 AM by Binarix128 » Logged

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Re: How to calibrate an AM radio? « Reply #5 on: January 04, 2021, 11:40:56 AM » Author: Medved
It's too late, I already screwed up the trimmers and the tuning capacitor looking for the best signal, but I think I can alight it again.

That is very bad. Which ones you touched? Is at least the yellow one untouched?
If you messed up just with the red (oscillators) and the trimmers, it could be aligned using existing stations. First align the red oscillator coil for the station on the bottom end to match the scale, the "O" trimmer for some station at the top end of the band to match the scale, then in theory the antenna coil needs to be aligned at a station near the bottom end (but not really at the bottom end, but very likely your selection would be limited there) and then the "A" trimmer for the near top end antenna tunning (and repeat it few times, as everything influences everything, mainly on these very basic radios).
The most problematic is the IF alignment, there you need the 455kHz reference frequency (you need a fixed point). And that is nearly impossible without instruments.
The thing is, in theory the radio will play at whatever frequency the IF string is tuned to (in the past 250kHz was used mainly in car radios). But the thing (assume you wont hit any alignment limit) is the antenna and oscillator tunning has to track each other across the whole band (so the oscilator minus the IF frequency disctating the received station frequency matches the frequency the antenna is tuned on). And for a given input band range to cover, the electrode shapes (so the C vs Angle characteristic) difference between the two sections of the tunning capacitor have to be exact for the desired IF frequency. And because most common is 455kHz, the capacitors are designed for this. If you use different frequency, the antenna tunning wint track the oscillatior frequency along the whole tuning range.

I will try to replace the electrolytic capacitors, after all they are just four and it will be easy. I also discovered a broken ceramic capacitor between the speaker and the headphone Jack that seems to have the mark "204", which can be the principal problem, so I'll try to replace it before going into the electrolytic ones.


Before the capacitor broke it was damaged so it was working through the parasitic capacitive coupling all that time, hence the low signal.

The ceramic capacitor is just a feedback frequency compensation, so the amplifier remains stable. If it breaks in a way just some part chips out, it wont influence anything. But if it will be missing completely, most often nothing really happens (maybe except when you connect headphones on longer cable), worst case the amplifier may start to oscillate at supersonic frequencies and seem to loose power and get extra distortion.
But in any case better to have it there. Are you sure it is "204" and not something like "201" or so? Because with the one just beside the headphone jack I would expect something like the 200pF (so code "201"). To get the value from the code, you write the first two digits and then add as much zeros as the last digit says to get the value in pF, so "204" would be 200nF (too much for a feedback capacitor, if I identified its connection correctly), "201" would be then 200pF (what I would expect at such place; I would expect something between 100..470pF).
In total Ive seen there 4 ceramic:
1) in the base of the frequency converter (oscillator + mixer) transistor coupling the antenna signal to the base, just beside the antenna holder (you may see the wire to the antenna coil from it) Would expect something around 100pF..10nF.
2) is the oscillator feedback to the emitter of the converter transistor (just below the bundle of wires from the antenna coil), expecting about 4.7..22nF there.
3) detector low pass filter (between the tunning capacitor and volume control pot), expecting something along the 4.7..22nF range
4) Audio amplifier feedback stabilization (just beside the headphone jack), expecting something in the 100..330pF ballpark


By the way I see the lead to the speaker broke as well, make sure you connect it to the correct point (it should go where the jumper wire to the jack goes, aka positive battery terminal).
« Last Edit: January 04, 2021, 12:26:32 PM by Medved » Logged

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