[TenTec] TenTec Digest, Vol 173, Issue 27
Dan W8KMX
w8kmx at bellsouth.net
Tue May 30 15:08:43 EDT 2017
I would suggest it is an impedance mismatch on the
headphones. I guess you are using low impedance earphones
and maybe you should put in a matching transformer or get a
pair of high impedance earphones to try.
Dan, W8KMX
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Today's Topics:
1. Century 21 audio hum (Josh Gibbs)
2. Century 21 audio hum (Curt Benjamin)
3. 0mniVll (Rodney)
4. Re: Century 21 audio hum (Dukes HiFi)
5. Re: Century 21 audio hum (Josh Gibbs)
6. Re: Century 21 audio hum (Dukes HiFi)
7. Re: Century 21 audio hum (Josh Gibbs)
8. Re: Century 21 audio hum (Dukes HiFi)
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Message: 1
Date: Mon, 29 May 2017 09:34:07 -0700
From: Josh Gibbs <gibbsjj at gmail.com>
To: tentec at contesting.com
Subject: [TenTec] Century 21 audio hum
Message-ID:
<CAGCptuyi2o0JEYTJ1T-WeR+3BsBOuzLv8HhTUbvzcvm=RaLETA at mail.gm
ail.com>
Content-Type: text/plain; charset="UTF-8"
Greetings Ten-Tec List,
I just recently bought my first Ten-Tec rig - a Century 21/
Digital. I really like the radio and I'm having a lot of fun
with it! The one fault I've found with it is a loud hum on
the audio, which is very annoying when using headphones.
I replaced the big 13000uF filter capacitor on the power
board with a new one. No change. I've read the manual cover
to cover a few times, and have been studying the schematics.
When I run the rig from an external power supply, there is
no hum! This isn't the solution however, as the low pass
filter board uses unregulated 23VDC for the T/R switch. I'm
afraid it would damage the radio if I attempted to transmit
while powering it via external 12VDC.
I'd love to hear from other Century 21 owners - do you hear
a loud hum when you plug in headphones? Is this 'normal' for
this rig, or do I have a problem?
Any info or suggestions would be greatly appreciated. I've
posted a lot of pictures of the rig and a high-res scan of
the manual here:
https://drive.google.com/open?id=0B_pjAwEfeYcdYmRaTUhRTnN6SF
U
Thanks!
-Josh WA7FPV
Near Seattle, WA
------------------------------
Message: 2
Date: Mon, 29 May 2017 23:19:42 +0000 (UTC)
From: Curt Benjamin <k8aicurt at yahoo.com>
To: <tentec at contesting.com>
Subject: [TenTec] Century 21 audio hum
Message-ID: <124504664.2715646.1496099982562 at mail.yahoo.com>
Content-Type: text/plain; charset=UTF-8
Josh, direct conversion receivers such as the one in the C21
are inherently susceptible to AC hum from the power supply
since all the system gain comes from the AF stages. The 23
VDC is only used in the T/R switch to bias the PIN diode
open so the voltages coming from the transmit output power
doesn't bias the TR switch closed during transmit, so yes,
it really needs to be there - unless you transmit low power
when powering it off external +12VDC.
Have you checked the health of the bridge rectifier? I
wonder if a partially blown diode assembly could be the
culprit.
Curt, K8AI
------------------------------
Message: 3
Date: Mon, 29 May 2017 21:20:27 -0400
From: "Rodney" <w3krq at dejazzd.com>
To: <TenTec at contesting.com>
Subject: [TenTec] 0mniVll
Message-ID: <9F22922B445D4E098C4F61A6A1485BDC at RodneyPC>
Content-Type: text/plain; charset="iso-8859-1"
for sell a Ten Tec OMNI Vll cw filter ant
tuner,manual,power cord works great looks great a 9 out of
10.sell for 940.00 shipped.
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------------------------------
Message: 4
Date: Mon, 29 May 2017 21:54:20 -0500
From: Dukes HiFi <dukeshifi at comcast.net>
To: Curt Benjamin <k8aicurt at yahoo.com>, Discussion of
Ten-Tec
Equipment <tentec at contesting.com>
Subject: Re: [TenTec] Century 21 audio hum
Message-ID:
<03865765-1808-4226-9D16-9F9C0F846A31 at comcast.net>
Content-Type: text/plain; charset=utf-8
"Have you checked the health of the bridge rectifier? I
wonder if a partially blown diode assembly could be the
culprit.?
Not likely.
Diodes have two failure modes, open and shorted. If they get
leaky momentarily, they then go on to short VERY quickly due
to immense heath brought about by driving the filter
capacitor(s).
The direct conversion design does leave the radio
susceptible to 60 Hz hum. However, the fact that you were
able to get hum-free operation with an external 12 volt
supply suggests that this is not your problem.
I assume the hum is there regardless of volume pot setting.
If so, then this also vindicates the direct conversion
design as the source of the hum.
It would seem more likely that the regulator for the
internal 12 volt line is not working properly. This would
produce 120 Hz hum, not 60 Hz and is easily curable by
fixing the regulator. This would be associated with Q1
(MJE52D) and IC 1 (uA 723). The output there is that which
should concern you as this would present itself as 120 Hz
hum on the audio.
The uA723 regulator is notorious in terms of its relatively
poor reliability, although it HAS managed to survive for 40
years so not too bad.
If you are adventurous, you can replace the uA723 and the
associated transistor with a 7812 regulator. This modernizes
the radio and is a big improvement.
If you are interested, I can tell you how to do it.
Gary
W0DVN
> On May 29, 2017, at 6:19 PM, Curt Benjamin via TenTec
<tentec at contesting.com> wrote:
>
> Josh, direct conversion receivers such as the one in the
C21 are inherently susceptible to AC hum from the power
supply since all the system gain comes from the AF stages.
The 23 VDC is only used in the T/R switch to bias the PIN
diode open so the voltages coming from the transmit output
power doesn't bias the TR switch closed during transmit, so
yes, it really needs to be there - unless you transmit low
power when powering it off external +12VDC.
>
> Have you checked the health of the bridge rectifier? I
wonder if a partially blown diode assembly could be the
culprit.
>
> Curt, K8AI
> _______________________________________________
> TenTec mailing list
> TenTec at contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
------------------------------
Message: 5
Date: Mon, 29 May 2017 20:49:27 -0700
From: Josh Gibbs <gibbsjj at gmail.com>
To: Discussion of Ten-Tec Equipment <tentec at contesting.com>
Subject: Re: [TenTec] Century 21 audio hum
Message-ID:
<CAGCptuzddya9Jy2WCjkWjn93aO6ohtz0erj+9ysUiPLpxT=6uw at mail.gm
ail.com>
Content-Type: text/plain; charset="UTF-8"
Hi Gary,
I attempted to measure the audio output on the scope. I
measured it both on the internal supply and on external.
This is with the volume pot turned all the way down so all
you hear is the hum/buzz:
https://www.youtube.com/watch?v=D7mqyUbNhJw
https://goo.gl/photos/DzcaTGDaGWwTSv4u8
https://goo.gl/photos/mGssTuz695hyw8B97
It looks like 120 Hz to me.
Is there some other test that I can do to tell if the
voltage regulator is the problem? I tried to measure the
ripple of the regulated output and there wasn't much.
I tried grounding out the input audio at the audio power amp
board: it silenced the receiver audio, but the hum was
unaffected. The hum does not get louder when I turn up the
volume - at a high enough volume level (too loud to listen
to!) you no longer hear the hum over the receiver audio. A
couple of the 'tropical fish' caps on the audio power amp
board are very sensitive to my finger - when I touch them
the hum increases. Not sure if this is relevant, just an
observation.
I tried powering just the audio amp board on external 12V.
Hum was as loud as ever.
I'm up for some adventure. This is one of the reasons I
bought this rig - to learn how it works on a relatively deep
level. I've already learned quite a bit and am enjoying the
journey. It's a lot of fun to have QSOs on this old radio
(despite the hum!), and the QSK is great compared to my
Kenwood radios.
Greatly appreciate the help!
73,
-Josh WA7FPV
------------------------------
Message: 6
Date: Mon, 29 May 2017 23:45:42 -0500
From: Dukes HiFi <dukeshifi at comcast.net>
To: Discussion of Ten-Tec Equipment <tentec at contesting.com>
Subject: Re: [TenTec] Century 21 audio hum
Message-ID:
<462B9BB8-FEA0-46D3-8C2F-0873FC107297 at comcast.net>
Content-Type: text/plain; charset=utf-8
That?s not a lot of ripple that?s for sure.
The fact that the volume pot does not reduce the hum clearly
proves that the direct conversion design of this receiver is
not your root cause. of hum.
The fact that you can run the AF stages (including power
amp) on an external supply and still get the hum pretty well
rules out the power supply regulator.
The fact that putting your fingers near some ceramic disc
capacitors (I assume) causes the SAME hum to increase, it
sounds more like 60 Hz pickup than 120 Hz ripple to me.
When you are listening to ?normal? signals, is the volume
pot set quite far CCW to get the volume low enough to be
comfortable? If so, likely your headphones are too sensitive
and you need to put a swamping resistor in between the radio
and the phones to get the hum to acceptable levels, and then
turn the volume op higher to hear the signals.
All circuits pick up some degree of 60 Hz hum. A very
sensitive headphone makes this hum annoyingly strong so the
use of the swamping resistor to require a higher volume
setting for useful listening suppresses the hum and this
method is quite common, especially in the tube days.
The value of the swamping resistor is the highest value you
can use while still delivering sufficient phone volume with
the AF gain set to about 60% of full scale.
Gary
> On May 29, 2017, at 10:49 PM, Josh Gibbs
<gibbsjj at gmail.com> wrote:
>
> Hi Gary,
>
> I attempted to measure the audio output on the scope. I
measured it
> both on the internal supply and on external. This is with
the volume
> pot turned all the way down so all you hear is the
hum/buzz:
>
> https://www.youtube.com/watch?v=D7mqyUbNhJw
> https://goo.gl/photos/DzcaTGDaGWwTSv4u8
> https://goo.gl/photos/mGssTuz695hyw8B97
>
> It looks like 120 Hz to me.
>
> Is there some other test that I can do to tell if the
voltage
> regulator is the problem? I tried to measure the ripple of
the
> regulated output and there wasn't much.
>
> I tried grounding out the input audio at the audio power
amp board: it
> silenced the receiver audio, but the hum was unaffected.
The hum does
> not get louder when I turn up the volume - at a high
enough volume
> level (too loud to listen to!) you no longer hear the hum
over the
> receiver audio. A couple of the 'tropical fish' caps on
the audio
> power amp board are very sensitive to my finger - when I
touch them
> the hum increases. Not sure if this is relevant, just an
observation.
>
> I tried powering just the audio amp board on external 12V.
Hum was as
> loud as ever.
>
> I'm up for some adventure. This is one of the reasons I
bought this
> rig - to learn how it works on a relatively deep level.
I've already
> learned quite a bit and am enjoying the journey. It's a
lot of fun to
> have QSOs on this old radio (despite the hum!), and the
QSK is great
> compared to my Kenwood radios.
>
> Greatly appreciate the help!
>
> 73,
>
> -Josh WA7FPV
> _______________________________________________
> TenTec mailing list
> TenTec at contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
------------------------------
Message: 7
Date: Mon, 29 May 2017 22:12:58 -0700
From: Josh Gibbs <gibbsjj at gmail.com>
To: Discussion of Ten-Tec Equipment <tentec at contesting.com>
Subject: Re: [TenTec] Century 21 audio hum
Message-ID:
<CAGCptuzzGmRW35e0et80NKrBz6Y551f7FGP1q2e_7YP-NGFEGw at mail.gm
ail.com>
Content-Type: text/plain; charset="UTF-8"
Hi Gary,
Yes, I hardly need to turn the volume up at all when
listening to normal signals. There was something in the
Ten-Tec manual about an 'attenuation network' for the phones
to suppress noise. It sounds exactly like the swamping
resistor. I'm going to give that a shot.
Thank you!
-Josh
------------------------------
Message: 8
Date: Tue, 30 May 2017 08:14:43 -0500
From: Dukes HiFi <dukeshifi at comcast.net>
To: Discussion of Ten-Tec Equipment <tentec at contesting.com>
Subject: Re: [TenTec] Century 21 audio hum
Message-ID:
<CB13424E-41BD-40A4-BCF1-61C57746AB47 at comcast.net>
Content-Type: text/plain; charset=us-ascii
I think you will find that result very much to your liking.
73 and enjoy that classic!
Gary
> On May 30, 2017, at 12:12 AM, Josh Gibbs
<gibbsjj at gmail.com> wrote:
>
> Hi Gary,
>
> Yes, I hardly need to turn the volume up at all when
listening to
> normal signals. There was something in the Ten-Tec manual
about an
> 'attenuation network' for the phones to suppress noise. It
sounds
> exactly like the swamping resistor. I'm going to give that
a shot.
>
> Thank you!
>
> -Josh
> _______________________________________________
> TenTec mailing list
> TenTec at contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
------------------------------
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