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Motorola GM300/Maxtrac rear rx pin not passing audio

 

I will start out by saying this is the second gm300 to do this to me. I use it as a repeater receiver. Does very well for what it is. The unfiltered rx audio from the gm300 feeds a masters SC50 micor squelch/PL board then from there it goes to the rlc controller. The actual receive in the radio still works but that pin on the rear will not pass any audio. When the repeater is keyed it is just a dead key. Can’t give the controller any commands either… when this happened the last time I just put in a new GM 300 and problem was solved. They were each in service about a year and a half before this happened. Has anyone experienced the same issue? I have the previous gm300 here with me, but I have not done any digging to see what has failed. The one that’s on the repeater now is still up there till I can find time to go up there and swap it out with something else or get to the bottom of this.. any help is appreciated.
 

Anything else on the Disc line other than the SC50?? Does the audio into the controller come from the SC50 or DISC audio out??? You have to consider both input impedances in parallel if it is ... but if I recall the DISC out is buffered from an OP amp. ? You will need to troubleshoot the maxtrac. ? Check to see if there is audio on the in of the opamp.

Andy

On Sun, Apr 27, 2025 at 8:03?PM Tyler- WZ5TX via <wz5tx=[email protected]> wrote:
I will start out by saying this is the second gm300 to do this to me. I use it as a repeater receiver. Does very well for what it is. The unfiltered rx audio from the gm300 feeds a masters SC50 micor squelch/PL board then from there it goes to the rlc controller. The actual receive in the radio still works but that pin on the rear will not pass any audio. When the repeater is keyed it is just a dead key. Can’t give the controller any commands either… when this happened the last time I just put in a new GM 300 and problem was solved. They were each in service about a year and a half before this happened. Has anyone experienced the same issue? I have the previous gm300 here with me, but I have not done any digging to see what has failed. The one that’s on the repeater now is still up there till I can find time to go up there and swap it out with something else or get to the bottom of this.. any help is appreciated.
 

The rx audio comes from the sc-50 along with cos…I just find it odd that this is the exact same failure and nearly the same time frame. The jumper ju551 in the gm300 is set for discriminator audio out.
i need to get the first one that failed and open it up and trace that pin..I also have a good one I can put side by side and see where it stops letting rx audio pass
 

RX crystal needs to be replaced. Had the same issue with other GM300.



Tony
 

He is saying he gets a valid COS signal, but that IS indeed a common failure.

Andy

On Mon, Apr 28, 2025 at 7:29?AM Tony De via <TonyN2MFT=[email protected]> wrote:
RX crystal needs to be replaced. Had the same issue with other GM300.



Tony
 

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Second Mixer crystal...not the main freq. one.
I had that problem.?? Got the crystals form China.
73 John VE3AMZ
?

From: Tony De via groups.io
Sent: Monday, April 28, 2025 7:28 AM
Subject: Re: [repeater-builder] Motorola GM300/Maxtrac rear rx pin not passing audio
?
RX crystal needs to be replaced. Had the same issue with other GM300.
?

?
Tony
 

Except he said the COS is still active and working fine.?? That would indicate at a distance the radio still works.?? But I agree, if he misstated, that's the most likely suspect.?? It's better to get a junker at a hamfest (and cheaper).?? I found some for $1 each with bad PA's? :)

Andy

On Mon, Apr 28, 2025 at 11:33?AM John J. Riddell via <ve3amz=[email protected]> wrote:
Second Mixer crystal...not the main freq. one.
I had that problem.?? Got the crystals form China.
73 John VE3AMZ
?
From: Tony De via groups.io
Sent: Monday, April 28, 2025 7:28 AM
Subject: Re: [repeater-builder] Motorola GM300/Maxtrac rear rx pin not passing audio
?
RX crystal needs to be replaced. Had the same issue with other GM300.
?

?
Tony
 

I should have been more specific. I don’t get the COS signal from the radio. I get the COS from the masters SC-50 micor squelch pl board… I will get some crystals ordered! Thank you
 

I may need to be a little more clear….:I still get RX audio from the speaker just none from the pin on the rear. All I get from the pin on the rear is dead audio with pl(otherwise the pl encoder would not give a valid cos)
 

So .... Do you get that good audio from the speaker from weak signals??? Can .5 uV signals with audio result in audio from the speaker??? If so, your XTAL is good.

Andy

On Mon, Apr 28, 2025 at 5:01?PM Tyler- WZ5TX via <wz5tx=[email protected]> wrote:
I may need to be a little more clear….:I still get RX audio from the speaker just none from the pin on the rear. All I get from the pin on the rear is dead audio with pl(otherwise the pl encoder would not give a valid cos)
 

Another way you can tell is if you have a communications receiver or a service monitor.?? Tune to 44.645 Mhz and you will hear a carrier emitting from the radios (shields off).?? It may be a little weak so you might have to couple your receiver near that IF Xtal.

Andy

On Mon, Apr 28, 2025 at 5:14?PM wj9jrg via <wj9jrg=[email protected]> wrote:
So .... Do you get that good audio from the speaker from weak signals??? Can .5 uV signals with audio result in audio from the speaker??? If so, your XTAL is good.

Andy

On Mon, Apr 28, 2025 at 5:01?PM Tyler- WZ5TX via <wz5tx=[email protected]> wrote:
I may need to be a little more clear….:I still get RX audio from the speaker just none from the pin on the rear. All I get from the pin on the rear is dead audio with pl(otherwise the pl encoder would not give a valid cos)
 

Yes I am getting the 44.645 signal.
 

You don't have a bad Xtal then.?? You will have to download the schematic and troubleshoot.

Andy

On Mon, Apr 28, 2025 at 5:38?PM Tyler- WZ5TX via <wz5tx=[email protected]> wrote:
Yes I am getting the 44.645 signal.
 

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Yes, I am going to get the scope out trace it from the rear and see where it stops outputting a signal for starters. Since this is the second one to do this, I’m wondering if voltage is either spiking or coming through it from the SC50 or maybe even the controller

WZ5TX
David “Tyler” Bufkin

From: [email protected] <[email protected]> on behalf of wj9jrg via groups.io <wj9jrg@...>
Sent: Monday, April 28, 2025 7:37:37 PM
To: [email protected] <[email protected]>
Subject: Re: [repeater-builder] Motorola GM300/Maxtrac rear rx pin not passing audio
?
You don't have a bad Xtal then.?? You will have to download the schematic and troubleshoot.

Andy

On Mon, Apr 28, 2025 at 5:38?PM Tyler- WZ5TX via <wz5tx=[email protected]> wrote:
Yes I am getting the 44.645 signal.
 

Just glancing at the schematic:?? You can't be missing discriminator audio if you have speaker audio.?? Unless your J551A jumper is bad or something near there (circled).
?
 

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Now I’m even more baffled than before lol. The original receiver is passing audio to the rx pin just fine in both jumper positions.


WZ5TX
David “Tyler” Bufkin

From: [email protected] <[email protected]> on behalf of wj9jrg via groups.io <wj9jrg@...>
Sent: Monday, April 28, 2025 8:27:32 PM
To: [email protected] <[email protected]>
Subject: Re: [repeater-builder] Motorola GM300/Maxtrac rear rx pin not passing audio
?
Just glancing at the schematic:?? You can't be missing discriminator audio if you have speaker audio.?? Unless your J551A jumper is bad or something near there (circled).
?

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