[ICOM] 756PRO audio distortion

[Eric Dobrowansky K2WD]

Hi Roger, 



Two things to check: 



1.  Check the bias voltage and current of the 2SC1972 driver pair on the PA board.  I had a stubborn PRO2 here not too long ago where the tx audio would start to distort at times, especially the harder it was driven (ie: compressor on). The bias voltage on the base of the transistors was dropping from the normal 0.8v to down around 0.4v, thus acting like the transistors were beginning to turn off, thus distorting the audio. Yet power output was still fine.  Come to find out, one of the 2SC1972 drivers was intermittently shorting due to thermal breakdown. 



2. I also have seen on another PRO where the resistor in series between the +8v rail, and the junction of the latter two resistors which feed the driver base pins, had developed a cold solder joint. If I recall, there is a feedthru point in series with that resistor which goes from the bottom to the top of the PA board. I resoldered the resistor and the feedthru junction, and all was well once again.  




You will have a hard time finding replacement 2SC1972s nowadays. Most likely you will need to buy them thru a parts broker.  I made the mistake of buying a set from an eBay vendor in Hong Kong.  They turned out to be counterfeits!   Not only were the rebranded parts (which would not have been all that bad in itself, such as when someone rebrands an NTE as an original number), but they actually used a transistor which had the wrong pinout (CEB instead of  BEC) ! 





Simplest thing to do is connect a milliamp meter at the bias adjustment point as per the service manual, and another voltmeter to simultaneously monitor the base voltage at the transistors.  Then, record a 20 second voice message in one of the tx audio memories, and constantly play it until the problem arises. 



The 2SC19 72s are known to fail and produce the type of audio issue you are experiencing. 





Good Luck, 


Eric 
K2CB 





You will have a hard time finding replacement 2SC1972s nowadays. Most likely you will need to buy them thru a parts broker.  I made the mistake of buying a set from an eBay vendor in Hong Kong.  They turned out to be counterfeits!   Not only were the rebranded parts (which would not have been all that bad in itself, such as when someone rebrands an NTE as an original number), but they actually used a transistor which had the wrong pinout (CEB instead of  BEC) ! 





Simplest thing to do is connect a milliamp meter at the bias adjustment point as per the service manual, and another voltmeter to simultaneously monitor the base voltage at the transistors.  Then, record a 20 second voice message in one of the tx audio memories, and constantly play it until the problem arises. 



The 2SC19 72s are known to fail and produce the type of audio issue you are experiencing. 





Good Luck, 


Eric 
K2CB