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Post by tubefan on Jun 11, 2023 13:01:59 GMT -5
Have a mark 4a and when you key it has its ping. Receiver cuts off and meter goes to right. But I still hear myself coming through the speaker. Almost like talk back on these newer radios. Any idea what to check?
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Post by 2600 on Jun 12, 2023 0:21:13 GMT -5
That's not a common fault. First thing I would do is change the relay in the transmitter. This is what shuts off the receiver while transmitting. If the receiver won't cut off, this is the first place to look for an explanation.
And if swapping the relay won't help, have a VERY close look at the relay socket. BRIGHT light and magnifier. Can only remember one time that a relay socket had deteriorated and caused leakage current between separate circuits.
But that's the only other thing besides the relay itself that comes to mind.
73
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Post by tubefan on Jun 12, 2023 15:38:58 GMT -5
That's not a common fault. First thing I would do is change the relay in the transmitter. This is what shuts off the receiver while transmitting. If the receiver won't cut off, this is the first place to look for an explanation. And if swapping the relay won't help, have a VERY close look at the relay socket. BRIGHT light and magnifier. Can only remember one time that a relay socket had deteriorated and caused leakage current between separate circuits. But that's the only other thing besides the relay itself that comes to mind. 73 Will check that out. Thanks |
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Post by tubefan on Jun 23, 2023 22:32:21 GMT -5
Update. Tried the transmitter with another receiver. Works fine. So the talk back effect is coming from the original receiver not the transmitter or relay. Wonder if the ping cap is causing a problem. Will have to remove it and see what happens. The receiver is cutting off and meter goes to the right but I can hear myself coming from the speaker.It actually sounds like someone put talk back in somehow. It's not full loud and volume has no effect while I'm talking. Just loud enough that I can hear it.
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Post by tubefan on Jun 23, 2023 22:54:17 GMT -5
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Post by tubefan on Jun 23, 2023 23:04:18 GMT -5
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Post by 2600 on Jun 24, 2023 0:24:06 GMT -5
The ping cap is the fat blue electrolytic with the positive end facing the front of the radio. The positive lead is soldered to a foil trace just under or alongside the 6AQ5 speaker-amp tube. Have a close look to see if that lead has bridged to an adjacent foil trace. This could do it.
73
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Post by tubefan on Jun 24, 2023 9:37:45 GMT -5
I removed that ping cap that is in the pic. With my goggles I searched the entire circuit board and everything looks good. Could not find any solder bridges. Everything looks factory except that ping cap that I removed. Here is something I tried and I find it strange. I removed the 6aq5 and I still hear myself from the speaker. Then I removed the 12ax7. Again I still hear myself from the speaker. Of course there is no volume with either removed. But when I transmit I hear myself clear through the speaker like talk back. Strange one. Is there a cap in the audio circuit that if went bad might cause this?
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Post by tubefan on Jun 24, 2023 9:44:31 GMT -5
Found my problem. The big can cap. I could hear a slight hum coming from speaker. So for kicks I jumped in a 40x450 and the first point no change. Second no change. Third was the charm . Hit it with the clip lead and hum went away. Keyed up and talked and NO TALK BACK!! All new caps on order. Thank you for the help and suggestions. Hopefully if someone else encounters this problem they find this post.
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Post by 2600 on Jun 25, 2023 0:47:59 GMT -5
Our usual remedy is to change all the electrolytic caps before attempting any troubleshooting. Amazing the problems that magically go away when all the failed electrolytic caps get replaced.
Can't remember seeing this symptom with that cause. Maybe because we change those caps first, and then look for performance faults.
Had a customer with a Mark 4A, said he only wanted the 'hum' in the receiver fixed. Didn't want any of the routine updates done. Took it home, then brought it back because the transmitter was acting psycho. Channel display would advance in the wrong order, channel frequencies were wrong. The receiver didn't "hum" any more, but he wanted the transmitter to work, as well.
Thought he needed the CPU that runs the selector and channel display. Changed all the small electrolytics in the transmitter and all the "cpu" problems magically went away.
I had already quoted him a repair price that included the expensive replacement for the transmitter's CPU. Final repair cost turned out a lot lower, once we saw that chip was okay.
I'd say that's the last time we offer a customer a "partial" update on that model. Just don't need the grief.
73
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