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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 15th Dec 2019, 1:29 pm   #41
orbanp1
Heptode
 
Join Date: Nov 2009
Location: London, Ontario, Canada
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Default Re: Philips AG4126/7 blown thermistor

The idle current is with no audio signal, so turn the volume down while adjusting.
Do set the idle current to the value stated on the schematic.
As you mentioned, do verify the "cold" ( at 25C) resistance of the thermistor.
If you are out of adjustment range on the R22 pot, you could change the value of R18, the resistor in series with that pot.
If the idle current is too high, increase R18 to e.g. 2.2k from the original 1.8k, if it is too small, decrease it to e.g. 1.5k, and see if it works like that.

Regards, Peter
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Old 15th Dec 2019, 6:29 pm   #42
Dodgy Geezer
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Default Re: Philips AG4126/7 blown thermistor

OK - all that sounds quite possible - though I found that the idle current was 0mA with the volume turned right down before...

I have ordered a used AG3310 - a bit expensive, but we shall see if it works.

I suspect that the record player originally failed with a dry joint at R18, which sent too much current through the thermistor and eventually blew it. So the player was put in store, and the cartridge degraded through damp...
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Old 20th Dec 2019, 12:54 pm   #43
Dodgy Geezer
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Default Re: Philips AG4126/7 blown thermistor

Happy to report that the AG3310 has arrived, and works. Connected it up, and there is now a recognisable audio signal through the amp and out of the loudspeaker.

There is a very wonky platter mat which has gone hard though, making records impossible to track. I will cut a bit of neoprene to see if that improves things. Looks like it was a blown thermistor caused by a dry joint in the parallel resistor which stopped the audio getting through. And a dead cartridge due to the length of time in poor storage....

Incidentally, Techman suggested that a 'Cheap Chinese' black and red cartridge might be attached to the headshell. I can't see how - the headshell is a slim piece of plastic bonded to the cartridge, and the connector to the arm is a star-shaped propriety connector also bonded to the cartridge. I wonder if he was expecting me to saw through the dead cartridge and epoxy the back half of the old connector to the front half of a new cartridge - or are there new examples of this connector still available?
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Old 13th Jan 2020, 11:06 pm   #44
Dodgy Geezer
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Default Re: Philips AG4126/7 blown thermistor

New stylus and mat, and the system is now performing perfectly. Another win for the Vintage Radio Forum! Thanks!
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