[arm-allstar] Echo heard on Rx Audio

[Danny K5CG]

Dusty,

I added a relay hat on top of hamvoip Pi3B+ node and a 60mm fan on the back of the CDM I use for a transmitter. When PTT is asserted, one of the relays powers up the fan to get a head start on the heat dissipation. There is also a temperature sensor (a Maxim DS18B20+) stuck to the heatsink which feeds a GPIO pin input. A script that runs every minute checks the temperature. When it gets hot the script will turn on a second relay to power the fan in parallel. This will keep the fan going between transmissions when needed. When PTT is off and the temperature drops, then both relays are off. It has been trouble free for a couple of years now.

Relay hat like this https://www.ebay.com/itm/175241257133
CDM fan bracket https://www.ebay.com/itm/173895487669

73 
Danny, K5CG 
HH 550-000-0609 
SKCC 14257

----- Original Message -----
From: "ARM Allstar" <arm-allstar at hamvoip.org>
To: "Kevin Custer" <kevin at kc-wireless.com>, "ARM Allstar" <arm-allstar at hamvoip.org>
Cc: "Dusty Smith" <w4msi at outlook.com>
Sent: Sunday, July 2, 2023 3:20:34 PM
Subject: Re: [arm-allstar] Echo heard on Rx Audio

Excellent advice. I changed that out this afternoon. Removed the high value inline resistors and added the 10k / 1k L network and the audio is still a little hot but perfectly manageable...more so than before and no more echo. Thanks a million for that advise. 

New question, more out of curiosity than anything. How hot would be too hot for the heatsinks on a transmitter...specially these CDM1250's. I've noticed during a long qso, the transmitter's heatsink temp is up around 115-120 degrees. After qso ends, it'll fall 10degrees pretty quickly, within a minute or two and continues to cool down from there. Just wondering about possible overheating problems and curious what the max safe temp range might be. Haven't been able to find any specifics on that in my initial searching. 


Dusty Smith
—W4MSI—

-----Original Message-----
From: Kevin Custer <kevin at kc-wireless.com> 
Sent: Sunday, July 2, 2023 10:08 AM
To: ARM Allstar <arm-allstar at hamvoip.org>
Cc: Dusty Smith <w4msi at outlook.com>
Subject: Re: [arm-allstar] Echo heard on Rx Audio

Since you reference the discriminator audio is really hot - I'd conclude that it's leaking into the transmit audio directly because of cable cross-talk or another pseudo path.

Instead of using a large series resistance to reduce the audio level, consider a L network where you have something like a 10k through resistor and a 1k to ground.  This will keep the audio path low impedance but accomplish necessary reduction without using high value resistors.  Put this resistance at the connector on the radio so it reduces cable cross-talk as well.

Kevin


On 7/1/2023 10:50 PM, Dusty Smith via ARM-allstar wrote:
> Hello all, hoping to gather some info/pointers on where to look to try to solve this issue.
>
> I've had a repeater running with hamvoip on a pi4 for a good long while now...no major issues that I haven't been able to overcome at least thus far.
> This evening I swapped out my radios on the repeater for some different/newer rigs.
>
> The old radios that have been working fine were Motorola Maxtracs and I've replaced them with some Motorola CDM1250 units.
>
> All seems to be pretty much equal on the surface, however, I've noticed that there is a reverb/echo on my Rx Audio. Any audio coming through links seems to be normal, but if I'm keying into the receiver of the repeater...then the output of the repeater has this echo. It's not a real loud echo, kinda faint, but it's definitely there and can be noticed especially if you are speaking louder. Others on the same frequency/repeater can hear it and if I do the parrot mode I can hear it.
>
> This wasn't here on the old radios. Same controller, same interface, etc......there is a new interface cable but it's made up the same way as the old one just different connections for the new radios. I've tried adjusting audio levels and such and if I turn it way down (simpleusb settings) then you can't hear the echo anymore, but the audio is just too low in general. So really the echo is probably still there you just can't hear it at that point. If I go into "alsamixer" and turn off automatic gain control, it seems to get better but it doesn't take it away entirely.
>
> One other note, the "flat/filtered rx audio" on pin 11 (where I'm getting my rx audio from on the back panel pinout of the CDM1250) seemed to be extremely hot audio. With it piped directly into my soundcard interface (modified usb fob) I could turn the rx audio level in simpleusb all the way down to 0 and it was still overmodulated by a lot. So I added in 2Mohms worth of resistance to the rxAudio line going from radio to interface (2 x 1Mohm resistors inline) and that got it down to a reasonable level.
>
> Ok I think that's all I can think to include in this question for now. If you need more info please ask. And thank very much in advance for any assistance.
>
>
> Dusty Smith
> -W4MSI-
>
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