Electronics > RF, Microwave, Ham Radio

Fixing 1962 Philips L4X25T SW receiver

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Cliff Matthews:
I picked-up a Philips L4X25T last week, cleaned it up, loaded it with 6 new C cells and crossed my fingers.. (didn't work this time). What I get is a tremendously loud tone around 500hz and no switch, control or any thing I wiggle inside affects it. Quite likely this thing is going to need some recapping, but it looks worth the effort. Since the tone seems somewhere around the speakers resonance, I thought I'd ask what are the chances that one of the TO-1 output transistors is bad? The schematic shows an output stage that looks very odd..

Radio Tech:
A 1962? That is over 50 years old. First thing I would do is start replacing those old electrolytic capacitors.
I am not saying this as just being a "parts changer" but from experience.

Cliff Matthews:

--- Quote from: Radio Tech on February 03, 2016, 01:10:08 pm ---A 1962? That is over 50 years old. First thing I would do is start replacing those old electrolytic capacitors.
I am not saying this as just being a "parts changer" but from experience.

--- End quote ---
Agreed (and subscribed, thanks!), but since the audio stage is totally DC isolated and the tone is so mind-blowing, wouldn't checking those transistors be a deal-breaker? (It'd be a shame to re-cap if those transistors were pooched and not reasonably obtainable).

Cliff Matthews:
The prospect of getting OC-74's on ebay looks good.. http://www.ebay.com/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR0.TRC0.H0.Xtransistor+oc74.TRS0&_nkw=transistor+oc74&_sacat=0

Radio Tech:

--- Quote from: Cliff Matthews on February 03, 2016, 02:18:48 pm ---
--- Quote from: Radio Tech on February 03, 2016, 01:10:08 pm ---A 1962? That is over 50 years old. First thing I would do is start replacing those old electrolytic capacitors.
I am not saying this as just being a "parts changer" but from experience.

--- End quote ---
Agreed (and subscribed, thanks!), but since the audio stage is totally DC isolated and the tone is so mind-blowing, wouldn't checking those transistors be a deal-breaker? (It'd be a shame to re-cap if those transistors were pooched and not reasonably obtainable).

--- End quote ---

Yes I agree, checking the transistors can not hurt at all.  Also any high value resistors  should be checked, just to be sure they are in tolerance.  Also I would check that transformer to make sure there is no shorts, looks like a pc mount? May be able to lift component leads to get a good check.

And thanks :)

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