guys i need help. I've checked everything out in this circuit and cannot find the problem. all voltages are correct if i isolate pin 4 on ic4 from ground radio comes out of transmit and recieves normal. any ideas at all will be greatly appreciated. also checked all switching transistors too. I'm totally stumped
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galaxy pluto stuck in transmit with out mic plugged in
- Thread starter brent stroud
- Start date
Unhooking pin 4 of IC4 will cause the voltage feeding the transmit-switch transistor TR38 to rise, shutting it off. This is not a fix, but shows that TR38 is not totally shorted, only that the base terminal of TR38 is being pulled down to a voltage low enough to switch it on when it should be off. Most-common cause of this symptom is D77, a diode in line with the base terminal of TR38.
Odds are that if TR38 is good, unsoldering the base lead will return the radio to receive mode. With IC4 pin 4 reconnected, of course.
D77 needs to have a forward-voltage drop of over 1 Volt. Later radios use a diode that has two diodes in series inside it, for a forward-drop voltage of about 1.2 Volts.
The schematic for older radios like this one show a type 1N4148 for D77. If D77 fails as a short, we use three 1N4148 in a series string to replace it.
Found that using only two of them didn't work all the time. Three of them does.
73
Odds are that if TR38 is good, unsoldering the base lead will return the radio to receive mode. With IC4 pin 4 reconnected, of course.
D77 needs to have a forward-voltage drop of over 1 Volt. Later radios use a diode that has two diodes in series inside it, for a forward-drop voltage of about 1.2 Volts.
The schematic for older radios like this one show a type 1N4148 for D77. If D77 fails as a short, we use three 1N4148 in a series string to replace it.
Found that using only two of them didn't work all the time. Three of them does.
73
IC4 could have shorted internally.
you might try removing it and checking for continuity with other pins.
LC
you might try removing it and checking for continuity with other pins.
LC
IC4 almost never goes bad.
Almost.
But we have seen D77 cause trouble many times.
Just playing the odds.
73
Almost.
But we have seen D77 cause trouble many times.
Just playing the odds.
73
replaced d77 still same issue. i isolated the base of tr38 tx dropped but no rx then. I'm totally clueless voltages on ic 4 are crazy. i replaced ic4 as well also replaced both 2sa1282 transistors as well as the 5.1 v zenor diode
thought this as well. caps in the affected circuit are all good . thats whats so frustrating. I've exhausted everything possible that's why i have asked for any suggestions you all may have believe me i appreciate any thoughts anyone might have
I had a similar issue, checking TR38 and TR36, they showed good. I could remove TR37 and it would come out of transmit, I think I’m telling this correctly... I think.
Issue turned out to be TR37, the 945 piece. I also replaced it with a “new” one, from Mouser. As a matter of fact, I replaced it 3 times and still had the same issue. I contemplated taking my aluminum baseball bat to it right there on my bench.
In a Chevy Chase moment like Christmas Vacation, one last feeble attempt to find the problem, I pulled a 945 from a parts chassis. And can you guess what happened? Problem solved.
So instead of taking the ball bat to the radio, I took the entire lot of 945’s from Mouser and beat them to dust and pulp. Ordered a new batch from RFP and haven’t ever looked back.
My point - make sure your new parts are real and not fakes.
This also explained why so many TR32 replacements didn’t restore limiting or would result in absolutely no modulation. Hmm, damn Darkside let me down on that one...
Issue turned out to be TR37, the 945 piece. I also replaced it with a “new” one, from Mouser. As a matter of fact, I replaced it 3 times and still had the same issue. I contemplated taking my aluminum baseball bat to it right there on my bench.
In a Chevy Chase moment like Christmas Vacation, one last feeble attempt to find the problem, I pulled a 945 from a parts chassis. And can you guess what happened? Problem solved.
So instead of taking the ball bat to the radio, I took the entire lot of 945’s from Mouser and beat them to dust and pulp. Ordered a new batch from RFP and haven’t ever looked back.
My point - make sure your new parts are real and not fakes.
This also explained why so many TR32 replacements didn’t restore limiting or would result in absolutely no modulation. Hmm, damn Darkside let me down on that one...
SOLVED THE PROBLEM. d81 was the problem. thanks so much guys for all your suggestions. if i can be of any help let me know I'm Brent from Stroud's Electronics.