Hi,
I'd better close this one down before it gets too much further. So after
thinking about this and coming to much the same sort of conclusion as
you suggested it was time to pop the covers and make a measure on the
inside. The culprit revealed itself almost immediately, in the shape of
a mod. Reference the wiki at
http://tentecwiki.eqth.net/doku.php?id=562pop A mod to prevent speaker pop.
Apparently a factory mod puts a 470k from pin 5 of the audio IC to
ground to reduce speaker pop on first cw dot or audio. Mine had a 330k
leaving all of about 5 volts on pin 5. Removing this makes for far far
far nicer audio and no negative peak clipping which now occurs pretty
much symetrically at around the TDA1015 limits, actually quite a nice
sound indeed. The problem is of course the thump is back. It's quite a
loud crump on the first audio or seemingly even between longish pauses
in speech. I think this calls for a little more investigation. to try
and fix the thump without killing the audio. It's possible a higher
value R will work. A job for tomorrow.
Thanks again
Martin, HS0ZED
On 09/07/2018 22:12, MadScientist wrote:
I would suggest that the TDA1015 has failed. This would be the same as if one
of the transistors in ay complimentary symmetry audio stage went open.
Youcan test this by checking voltage on pin 2 of U10. It should be roughly half
the voltage at pins 3 and 5. Pin 7 should also be about half that of 3 and 5.
Also, look at the audio waveform at pin 7 and 6. They should be about the same
in appearance . The signal path between pin 7 and 6 is only the tone control so
amplitude and shape of the waveform should be about the same at both pins.
You can also eliminate a lot of things by looking at the waveform of the
sidetone (carrier level set to zero) when keying in CW. This signal should pass
through U10 unchanged in appearance but altered in amplitude, with symmetrical
clipping.
TDA1015’s are commonly available on eBay right now.
Try to purchase from a USA or UK seller to reduce chances of getting an empty.
Gary
On Jul 9, 2018, at 8:44 AM, Martin Sole <hs0zed@gmail.com> wrote:
Hi,
I'm sure I read something similar to this recently but I can't locate it now. I
think the discussion centered on failing capacitors on the audio board. Anyway
my problem here is that I have started to hear quite high levels of distortion
on the received audio of my Omni V. Looking with a scope and using a nice big
wire wound 10 resistor as a load on the external speaker output with scope
across it I see clipping of the negative peaks once they reach about 2.5v peak.
Positive peaks can go much much higher before clipping, around 7v. I would
expect to see a bit more symmetry.
The fixed level of audio out is clean so I think everything prior to C37 should
be okay. From there the audio circuit is simple, if not crude, a TDA1015 with a
simple tone control between sections.
So, a leaky capacitor?
Cheers
Martin, HS0ZED
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|