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Old 20th Oct 2017, 6:00 pm   #1
Pete_kaye
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Default Philips 50 IC 323

Just got this from a shop . Very little response. A quiet hum when plugged in . A few clicks when volume control rotated . A hint of a station but otherwise zilch .Also when switched off after 5 mins , a prolonged squeal . Any advice before I proceed would be welcome .Any one else repaired one? I noticed how easy it is to flip the mains selector to 110 volts .
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Old 23rd Oct 2017, 2:21 pm   #2
Pete_kaye
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Default Re: Philips 50 IC 323

After a long chat with Trevor(G3VL ) , I decided to proceed and quickly found a cracked track on the board using my sawn off toothbrush and tapping technique . It works well now . All the foam packing inside had turned to dust and was removed but not replaced. The piano key switches were damaged and loose but refitted with hot glue ( not very well) and function . The sound is good and the only oddity is that it will pick up a distorted version of radio 3( 92 FM) in the 16 M band. Alignment? Doer s anyone know if this s et has panel light .I couldn't see one and it needs it IMO.

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Old 4th Dec 2017, 3:05 pm   #3
Pete_kaye
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Default Re: Philips 50 IC 323

Could someone explain why the meter on this shows 'high' {9/8]on VHF stations but volume is less than LW where the meter shows 'low'{2/8}?
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Old 4th Dec 2017, 8:04 pm   #4
orbanp1
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Default Re: Philips 50 IC 323

Hi Pete,

I take you have the schematics of the radio!
Also have the TBA570 datasheet at hand!

That indicator is a very curious way of signal strength reporting.
Certainly the TBA570 application does not suggest it.

Pin 5 is the demodulated AM audio output, it is an emitter follower output after a peak-detector in the IC. It has the signal average as well as the full audio signal on it. The indicator shows the signal average, the meter itself does the "audio filtering".
Pin 14 is the same emitter follower output (through a different resistor), now filtering is applied to it, and that signal is used internally as the AGC-signal in AM mode. In FM-mode the AGC is shorted to the ground, there is no gain regulation in the IF-amp, in FM-mode the IF-amp works in a "limiting" mode.
In the FM-IF there is another amp stage in addition to the first differential amp. The AM-IF uses just one diff-amp stage. Pin 5 still gives the rectified signal strength (now FM-IF) from the output of the first IF diff-amp stage.

There is no provision to set individual indicator sensitivities for each of the bands.
R638 sets the zero of the indicator, R639 sets the sensitivity.
It sure looks like a compromise solution to signal strength reporting.

Regards, Peter
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Old 5th Dec 2017, 1:51 am   #5
Synchrodyne
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Default Re: Philips 50 IC 323

Even without translating the text, this graph suggests that with the TBA570, one might expect higher meter readings on FM than on AM. There is quite a gap at the 1 mV input signal level.

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I imagine that in the FM case, the measurements were done with the two-transistor front end shown in the application note.

Here are the respective FM and AM quieting curves.

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Cheers,
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Old 5th Dec 2017, 3:45 pm   #6
Pete_kaye
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Default Re: Philips 50 IC 323

Thanks for the very helpful replies from far and away. I am happy to know that the effects I observe are normal for this set.
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