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Post by sfi355 aka Billy Bob on Jan 26, 2017 3:55:42 GMT -5
I lost receive audio on AM and on SSB, both internal and extension speaker no sound. Yes I tryed other speaker's, The radio show a signal on the meter when receiving both am/ssb. The Radio will transmit on AM 4 watt carrier plus modulation, as well of power out on LSB/USB. I hooked a Speaker to The PA Jack and switch the radio to PA Mode, and yes I can Hear myself just fine. I replaced the 10 volt cap (c186), no luck. Any Help would be great.
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Sandbagger
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Post by Sandbagger on Jan 26, 2017 7:44:00 GMT -5
I lost receive audio on AM and on SSB, both internal and extension speaker no sound. Yes I tryed other speaker's, The radio show a signal on the meter when receiving both am/ssb. The Radio will transmit on AM 4 watt carrier plus modulation, as well of power out on LSB/USB. I hooked a Speaker to The PA Jack and switch the radio to PA Mode, and yes I can Hear myself just fine. I replaced the 10 volt cap (c186), no luck. Any Help would be great. Try the simple things first, have you tried a different mic? |
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Post by sfi355 aka Billy Bob on Jan 26, 2017 10:06:26 GMT -5
Yes,
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Sandbagger
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Post by Sandbagger on Jan 26, 2017 11:43:40 GMT -5
I was hoping for the simple receive wire in the mic broken.... Oh well. It looks like the tests you're tried give some clues as to what is NOT wrong. Usually when you lose receive audio, the first thing that comes to mind is the audio chip. But you have both AM transmit audio and PA, both of which use the audio chip, so that's not the problem. But now the fun starts. Signal on AM and SSB is there so the receiver is working. But no audio on either mode. If you have a scope, this will be the best way to trace the signal from the detector through the volume control and on to the audio chip. I would suspect that the problem may lie in the audio pre-driver transistor or the squelch circuit. But you need a scope to see it for real. I'm not a big fan of blind shotgunning parts. but there are a couple of suspicious 10V caps that could short and cause havoc. I couldn't find a schematic of the Grant on CB tricks, but I am looking at the Cobra 148, and there is a cap marked C72. If that shorts, it will cut off supply voltage for the audio predriver transistor, so that's worth checking out. |
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Post by 2600 on Jan 27, 2017 1:27:18 GMT -5
Since it's not C186, maybe the squelch circuit has a fault. I would try unsoldering and isolating the center (collector) lead of Q53, 2SC945. It's the squelch switch transistor. If this gets your receiver audio back, resolder that center pin. Now power the receiver, and see if shorting across the two outside pins of Q53 restores receiver audio.
If it does, Q53 is okay, and the squelch circuit is staying muted. Trouble is upstream from Q53.
If shorting the outer two pads on Q53 WON'T bring back receiver audio, Q53 is probably bad.
Just a guess.
73
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Post by sfi355 aka Billy Bob on Jan 27, 2017 23:56:51 GMT -5
No luck on shorting out the 2 outer pin on TR53 Transistor, I pulled TR53 and it tested Good on the Transistor tester.
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Sandbagger
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Post by Sandbagger on Jan 28, 2017 11:26:57 GMT -5
No luck on shorting out the 2 outer pin on TR53 Transistor, I pulled TR53 and it tested Good on the Transistor tester. Have you checked C72 for a short? |
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Post by sfi355 aka Billy Bob on Jan 28, 2017 20:55:51 GMT -5
Thanks for all the Help. It ended up being a open (BURNT) White Wire off the LOW/HIGH Switch to the VR3 POT area. 73 To all that Helped.
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Sandbagger
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Post by Sandbagger on Jan 28, 2017 22:07:22 GMT -5
Thanks for all the Help. It ended up being a open (BURNT) White Wire off the LOW/HIGH Switch to the VR3 POT area. 73 To all that Helped. Wow, how does a wire that connects low level audio actually burn? I'm glad you got it working again, but I'd be scratching my head figuring out how if failed..... |
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Deleted
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Post by Deleted on Jan 28, 2017 23:07:23 GMT -5
Thanks for all the Help. It ended up being a open (BURNT) White Wire off the LOW/HIGH Switch to the VR3 POT area. 73 To all that Helped. Wow, how does a wire that connects low level audio actually burn? I'm glad you got it working again, but I'd be scratching my head figuring out how if failed..... Dammit Dave! That was my next question. I was given a spare TRC-421A which was hooked up backwards. I replaced the reverse diode, but noticed burnt I mean incinerated. I replaced it with hard wire and the radio came back to life. But by next question is, how the hell does that happen? |
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Post by 2600 on Jan 28, 2017 23:31:16 GMT -5
Way cool! Glad you found it.
Sometimes the most baffling faults have the simplest cause.
Just goes to show the "calibrated eyeball" can be the most valuable tool on the bench.
Congratulations!
73
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Sandbagger
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Post by Sandbagger on Jan 29, 2017 15:23:24 GMT -5
Wow, how does a wire that connects low level audio actually burn? I'm glad you got it working again, but I'd be scratching my head figuring out how if failed..... Dammit Dave! That was my next question. I was given a spare TRC-421A which was hooked up backwards. I replaced the reverse diode, but noticed burnt I mean incinerated. I replaced it with hard wire and the radio came back to life. But by next question is, how the hell does that happen? Well, your question is much easier to answer. I assume you meant that a PC trace on the board incinerated and you replaced it with a hard wire. In that case, after a polarity reversal, and someone jumped out the fuse (which would have instantly blown), the PC trace was subject to a DC short (through the now shorted protection diode), dropping many amps of current and the PC trace became the fuse. Remove the shorted diode and repair the trace and no problem. That's a far cry from a circuit that should never see more than a few milliamps on it, and which somehow burned a wire. |
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Post by sfi355 aka Billy Bob on Jan 31, 2017 13:38:27 GMT -5
I cut the coating off to check out the copper, What I found was the wire was missing strains of wire at one point of the run, So I will take it as a wire when made was subpar to begin with.
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Sandbagger
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Post by Sandbagger on Jan 31, 2017 19:30:08 GMT -5
I cut the coating off to check out the copper, What I found was the wire was missing strains of wire at one point of the run, So I will take it as a wire when made was subpar to begin with. I've seen that happen before. So maybe it was a broken connection just waiting to happen...... |
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