HT1250 - RXing a PL tone of 110.9Hz causes super loud hum???

[g8tzl2004]

I have a VHF HT1250/GP360 which was manufactured in 2001. Firmware version R03.01.22 and CP 4.3

For receive, I tend to use carrier squelch - with TX encode. Normally on the local repeaters, you can sometimes just hear PL tones in the background, quietly "humming" - hardly noticeable at 67Hz - a bit more noticeable for much higher PL's

However, when I am receiving a PL of 110.9Hz, the "hum" takes off - very very loud against the transmitted audio. I get similar results just above and below 110.9Hz - all very strange!! No problem receiving higher PL's which you might expect to hear? No problem with 82Hz PL's. Its definately not the local repeater as I tested out TXing different PL's on a separate radio.

What's going on? Is it some form of self oscillation?

All my other radios receive the same 110.7Hz PL with no problem at all - its just the HT1250. I'm using an external antenna with the HT1250 so its not RXing any internal processor noise .....or maybe some processor noise is in fact mixing with the 110.9Hz tone to give the strange effect??

Any ideas?

[g8tzl2004]

I think HumHead might have the answer back in a 2002 posting called "MDC Decode HT1250". This involved altering the baseband offset frequencies using the Global Tuner.

Apparantly, using carrier squelch to RX a signal with PL can cause problems!!!

Am I on the right tracks? Was the problem ever fixed with a new firmware update?

[g8tzl2004]

Anybody have any views?

Is it the Baseband Offset frequency causing the super high hum when RXing a 100.9Hz PL using just Carrier Squelch??

Do you select from a fixed range of possible Baseband Offset frequencies using the Tuner software - or can you use any Offset frequency? How do I know what Offset to use? Has there been a firmware update to resolve this problem? Has there been a change in the default Baseband Offset frequency to resolve the problem I have been having?

[wavetar]

The baseband offset thing was only if you were using MDC signalling and PL on the channel, and yes it was fixed in future firmware releases. If you're not using MDC, then the problem likely lies elsewhere, although it couldn't hurt to try a firmware upgrade.

[HumHead]

Actually, the problem was that a channel with MDC and TPL actually used the DPL offset value, frequently resulting in the hum problem.

The same thing applies to CSQ- If your baseband offset frequency is within about 20 Hz of the TPL tone, you will tend to get an RX audio hum.

Since the default CSQ baseband offset is 115Hz it is likely to cause problems between 95Hz and 135Hz, so the 100.9 TPL would fall solidly in that range.

[g8tzl2004]

Thanks HumHead.

I've also just been reading another old post from 2002 called "HT750 Audio Problem" which also gives good info.

If I understand correctly from the "HT750 Audio Problem" comments, I should be able to select BaseBand Offset #7 28Hz which is well away from any PL tones which I use?? ie 28+20Hz = 48Hz

As PL tones start at 67Hz, why is the default not set to, say 28Hz rather than 115Hz which is in the middle of "popular" PL tones??

The PL tones I use range from 77 to 127Hz - any views on what BaseBand Offset I should use? Is 28Hz OK or should I select another value?

I wonder how many HT1250's have been trashed because of the "terrible hum" !!!!

[wavetar]

I wonder how many HT1250's have been trashed because of the "terrible hum" !!!!

Hard to say, but we've sold hundreds, possibly thousands, and have never had a hum issue with any particular PL value, other than early firmware units when using MDC on a PL channel.

[g8tzl2004]

I think its only a problem if you use Carrier Squelch for receive and listen to a signal with a PL tone of around 110.9Hz.

I also checked out a later UHF radio and got the same effect!!!

I'm still not sure why Moto uses a BaseBand Offset of 115Hz rather than, say, a 28Hz offset which is well away from any PL tones.

[g8tzl2004]

There is a very easy fix for the hum when using carrier squelch and receiving a signal with a 110.9Hz PL.

All you need to do is set up the RX Channel with a Decode PL of 110.9Hz RATHER than just setting the field to PL Disable....even though you plan on just using Carrier Squelch rather than PL decode. You then set the Personality screen for RX Squelch Mode to Carrier Squelch (rather than PL). The radio then operates as if NO PL decode on RX was set BUT the hum disappears!!!!

[com501]

There is a provision for base band filter alignment in the software. That's why it's there.

[g8tzl2004]

I was expecting to have to adjust the Baseband Offset frequency but there was no need!!!!!

Just set up the RX side as if you are using PL decode ie. select a 110.9Hz PL decode tone BUT then just specify Carrier Squelch in the "global" Personality RX Squelch Mode screen rather than selecting PL...Super loud hum is now gone when using carrier squelch to receive a signal with a 110.9Hz PL encode tone !!!!

[com501]

That may work, but on my radios I prefer to permanently fix the problem.