I figured out the 4.7 Ohm resistor and 240 uF cap. You must have been
looking at the Century 22 schematic. It uses the same LM380 as the Century
21, BUT it has additional filtering on the 12V feeding the LM380. Century
21 only has a .1 uF cap. Might be a worthwhile mod.
The Century 22 has no internal power supply. In the manual it basically
says to keep the radio away from anything that generates magnetic fields,
as it is very sensitive to them!
I removed the power board and transformer and ran the radio with them on
the bench. The hum/buzz dropped to an almost imperceptible level.
I've concluded that there is nothing wrong with my C21 - the hum/buzz is
inherent in the design when running on the internal PS. The only way to
stop it is to run on external power. Best course of action is to only use
the internal speaker, or else use an attenuation circuit to feed
headphones. Using higher impedance phones might work as well - need to try
that.
I'm also convinced that Ten-Tec did not design the C21 to run on external
12V. The fact that it works was just lucky. When running on the internal
PS, unregulated 23V is used to bias the P-I-N diode, which acts as a T/R
switch. When running on external 12V, there is only 4V present to bias the
P-I-N diode. From my reading on P-I-N diodes, its hard to say if 4V is
really enough... the required bias voltage is dependent on RF power and
frequency. The fact that Ten-Tec used the highest voltage they could get
(unregulated 23V, straight off the filter cap) to bias the P-I-N diode does
not inspire confidence! Seems like they would have arranged to get 12V in
there to bias the P-I-N diode if the thing had really been designed to run
on external 12V. Also the fact that the drive meter and power switch don't
work when running on external power are more indications they did not
design it to run on external power.
73,
-Josh WA7FPV
On Mon, Jun 5, 2017 at 7:00 PM, Byron Cordes <byroncordes@icloud.com> wrote:
> The print I looked at had the 4.7 ohm and a 240 uf cap to filter the 12
> volt supply to the audio amp. Normally the resistor would open up but it's
> a chance. A choke would be a better choice but cost would be much higher
> than a resistor. Try jumping a cap across the 12 volt to ground on the
> audio board. Check grounds on audio board bad solder joint?
>
> Sent from my iPad
>
> > On Jun 5, 2017, at 7:15 PM, Josh Gibbs <gibbsjj@gmail.com> wrote:
> >
> > Thanks Byron. Grounding to PS ground had no effect.
> >
> > I don't see a 4.7 Ohm resistor anywhere... are we talking a resistor on
> the
> > audio board? Here is the schematic for the audio board:
> >
> > https://drive.google.com/file/d/0B_pjAwEfeYcdUk9JWDROazFLVkE/
> > view?usp=sharing
> >
> > All the schematics for the boards are here:
> >
> > https://drive.google.com/open?id=0B_pjAwEfeYcdWVoxTWlMbU9xODg
> >
> >
> > -Josh
> >
> >> On Mon, Jun 5, 2017 at 2:02 PM, Byron Cordes <byroncordes@icloud.com>
> wrote:
> >>
> >> Try grounding ground side of speaker to ground from power supply.
> >> If hum is still there there is a 4.7 ohm resistor that might be shorted
> >> Or a 270 uf cap open both feed the audio amp
> >>
> >> Byron Cordes
> >>
> >>> On Jun 5, 2017, at 3:42 PM, Josh Gibbs <gibbsjj@gmail.com> wrote:
> >>>
> >>> Hi Chris,
> >>>
> >>> The hum is present in the speaker, just not so noticeable because it is
> >>> much less sensitive and much further from my ear. I suspect the speaker
> >> is
> >>> less responsive to frequencies below 120Hz than my headphones, which
> >> helps.
> >>>
> >>> I have tried other headphones. I have also listened to the audio
> directly
> >>> at the output of the audio amp board. Even when I short the input to
> the
> >>> audio board to ground, I can still hear the hum. Even when I powered
> the
> >>> pre-amp and audio amp boards via external 12VDC the hum remained. I'm
> >>> pretty confident the hum is magnetically coupled. The only time I don't
> >>> hear the hum is when I power the radio entirely from external 12 VDC.
> >>>
> >>> Is this normal for this rig, or is there a problem? That is what I'm
> now
> >>> attempting to determine!
> >>>
> >>> The radio works great otherwise, and I likely would never have noticed
> >>> anything if I hadn't plugged in my headphones =)
> >>>
> >>> I've learned a lot while working on this radio, which was one of my
> goals
> >>> in buying it. Big thanks to the the awesome amount of knowledge of the
> >>> Ten-Tec list, and willingness to share it!
> >>>
> >>> 73,
> >>>
> >>> -Josh WA7FPV
> >>>
> >>>> On Mon, Jun 5, 2017 at 12:29 PM, Chris Howard w0ep <w0ep@w0ep.us>
> >> wrote:
> >>>>
> >>>>
> >>>>
> >>>>
> >>>> I'm not an electronics whiz. But to me, if you're getting
> >>>> the noise in headphones but not in the speaker, it sounds
> >>>> like the headphone cord is picking it up.
> >>>>
> >>>> If you have cracked capacitors in the power supply,
> >>>> I wonder if you've got a 120 Hz spark gap going there.
> >>>> The headphone cord is acting as an antenna and picking it up.
> >>>>
> >>>> How to test that theory, turn down the audio on the Century 21.
> >>>> Use the same headphones on some other piece of electronic
> >>>> equipment in close proximity to the C-21 and see if you get the noise.
> >>>>
> >>>>
> >>> _______________________________________________
> >>> 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
> _______________________________________________
> 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
|