[aprssig] Does anyone have a fix for the intermittant tone encoder on the TH-d7ag 70cm side?

Keith VE7GDH ve7gdh at rac.ca
Sun Mar 18 11:09:48 EDT 2007

Tom N2YTF wrote...

> The problem is that this dropout also happens fairly randomly while
> talking into the ht. I first noticed there was a problem while talking on
> the ht and walking along within half a mile of and with line of sight to
> my clubs repeater.  The motion of me walking plus the volume of my
> voice would combine to make me drop out somewhat randomly. If I
> whispered and did not move I could minimize, but not eliminate the
> problem, but then I was inaudible and still would drop out. Some hams
> I have spoken to could not figure out why they were in and out from
> their mobiles while running 5w to a good sized roof mounted antenna
> well within range of their repeaters.....then when they tested their
> d7ag rigs they also found that the tone encoder on 70cm was their problem.
> Again, the problem only occurs on 70cm, and I do not know if the tone
> drops out or is just somehow improperly modulated for a moment.  Either
> way, the d7ag screen does not reflect any problem, you have to test the
> rig against a second receiver. The problem happens much less frequently
> if the d7ag is connected to a dummy load.  To observe the problem, use
> a rubber duck I have not been able to develop a fix.

OK, I've got it now. It went through my mind that it could be a problem with
high SWR and RF getting back into the rig and messing up the CTCSS encoder,
you but said "it happens less frequently if connected to a dummy load".
Unless you have a bad match on your dummy load, it would tend to rule it out
as being an RF related problem. I'm leaning towards it being an audio level
/ mechanical problem, but if that was 100% true, going to the dummy load
shouldn't have made any difference.

Here are some things you could try. First talking into it and tapping on
both mechanical energy, but with a bit of troubleshooting, you might be able
to determine if the problem occurs because of something mechanical (talking
loud or tapping it) or if it's something else. If you could obtain a
connector the same as used by the mic connector and plug it in. Hit the PTT
on UHF with it set to transmit a sub-audible tone to your local repeater and
monitor the repeater with another radio. Try tapping the radio and see if
you can determine by monitoring the repeater on another radio if the CTCSS
tone cuts out while transmitting. This might help determine if it's a
mechanical thing or related to the actual level of audio being picked up by
the mic.

Do you have a deviation meter? You could measure the transmitted deviation
of the D7. I don't have any figures in front of me, but the deviation of the
transmitted CTCSS tones should be very low. Somewhere around .75 KHz
seems to ring a bell. Then talk into the mic and see what the deviation
looks like. It shouldn't be more than 5 KHz, but talking quietly or loud 
should produce quite a range. By whistling into the mic going from a low 
frequency whistle to a high frequency whistle (or sweep the range with a 
tone generator hooked up to a speaker), you could determine if it's related 
to the frequency of the audio or the level of the audio.

If you could hook up a scope to the audio output of a receiver, you could
also watch the audio and see if you can see the CTCSS actually cutting out
when you talk into the mic or tap on the radio. Perhaps there is some
interaction and the deviation of the CTCSS tones goes down when you talk
into the mic. If you could go this route, it would be a good comparison to
try and determine why it happens on UHF only by doing the same test on VHF.
You would think the audio circuitry would be shared on VHF and UHF. I don't
know how much or any circuitry would be shared on VHF ad UHF beyond the
audio stages. Another thing to keep in mind is that it would only take a
shift of a few Hertz of the CTCSS tone to mess things up. However, pinning
that down would be a lot harder unless you could observe it while tapping
the radio with a dummy mic plug connected so there would be no audio from
the mic while you tap the radio.

I see the D7E has a setting for switching the VHF deviation to a narrow
setting. Does your radio have that in "Access Menu 1–5–7"? If so, you could
toggle the setting and see if it had any affect on the transmitted CTCSS
tones on VHF. I presume because isn't mentioned in the manual that there
isn't a deviation setting for UHF on any model.

One more thing... does it make any difference if you set it to "tone" or to
"CTCSS" With tone, it will only transmit the tone. With CTCSS, the tone
decoder will be active on receive. You wouldn't think that it would make any
difference, but you could just give it a try to see if it makes any
difference or not.

I'm sure that more could be done, but the above could be a start to try and
see what's going on.

73 es cul - Keith VE7GDH
"I may be lost, but I know exactly where I am!"

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