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PT-25 audio distortion - Printable Version

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RE: PT-25 audio distortion - Arran - 12-21-2017

If the voice coil in the speaker isn't rubbing in any way then a gassy output tube can also cause this problem, it isn't a common problem but it does happen.
Regards
Arran

RE: PT-25 audio distortion - morzh - 12-21-2017

Bob has a point asking about alignment: specifically a very bad IF misalignment can cause distortion, although to an experienced person discernable from the one caused by clipping/waveform distortion.

RE: PT-25 audio distortion - WyTex - 12-21-2017

Good morning fellas -

I performed an alignment by ear several times which did increase the volume and clarity but did not resolve the underlying distortion. Each component replaced has made an improvement but there's still an issue that I haven't isolated. A couple of those resistors I replaced were past 20% out of tolerance and I was sure was the cause.

I worked from home yesterday but spent the day in the radio shop working on this and listening to AFRS Christmas broadcasts on my 66s. A great time but didn't get to the last two resistors. Kind of got side tracked a few times. One is a 4.7meg but my tester does not measure above 2M. Been thinking of getting myself a new Fluke.

I don't have a permanent magnet speaker to use as a test.
Could the volume pot be the cause, and if so, how to test?

I'm also not entirely sure the speaker isn't the cause.

Regards,
Paul

RE: PT-25 audio distortion - Phlogiston - 12-21-2017

(12-21-2017, 10:58 AM)Paul Wrote:  Good morning fellas -


Could the volume pot be the cause, and if so, how to test?

Yes

VOM

You will probably have to remove the connections. Turn the control end to end, Test resistance from both ends to center. The change in resistance should be smooth and continuous. The rate of change depends on thr taper of the pot - Audio (log) or linear taper.

RE: PT-25 audio distortion - Radioroslyn - 12-21-2017

+1
The volume control is listed @ 500,000 ohms alias 500K or .5Meg. More than likely it has an audio taper. This means the w/ the control turned up half way from the low side to the wiper should read about 50K, and from the wiper to the high side 450K. See pic below.
Linear taper w/the control 1/2 up would be 250K. What that does it put most of the audio gain at the first 1/4" of rotation. Audio taper makes the audio level spread out over most of the controls rotation.
Bad or wrong value pot can mess with the avc circuit causing distortion.
GL

RE: PT-25 audio distortion - morzh - 12-21-2017

If you suspect the pot, then find a station that is received well enough to be heard well but not too loud at say half pot scale, disconect the pot, and connect two resistors, one between the low point and wiper being, say 50K and the othe between the wiper and the top point being 450K (values are approximate, don't have to be exact) and see if the distortion persists.
I doubt it, but possible.

RE: PT-25 audio distortion - WyTex - 01-05-2018

Just a quick update to finish this off. Vol pot tested good so I jumped in a permanent magnet speaker and the sound was much better so I'm concluding that the old, original speaker is bit worn out. Polished the cabinet, replaced the plastic dial cover and called this project done.

Thanks all for your help with this. I learned some new testing skills, got rid of "black market" contact cleaner and have another Philco on the shelf.

On to the next one!
Regards,
Paul

RE: PT-25 audio distortion - morzh - 01-05-2018

Testing the speaker is the furst thing you do if you cannot explain distortion. It is the cheapest and fastest way, not requiring much doing.
Then if it doesn't help you start thinking deeper.