Hi,
A few weeks ago, my TA-2A started to have no sound at all. I removed pre-out & main-in jumper and directly connect another preamp to the TA-2A to check if the main amplifier works. Sounds came out with that setup, that means the main amplifier section is OK.
After tracking down, I found that the voltage inputs to the digital switch IC U204 (LC7818) have issues. The voltages to the pins 11-15 are all high (18V) and do not drop down to low (0.6V) to activate switches. So, I checked the logic board and check the voltage outs from U503 MPU. It looks like working fine because output voltages in pin 55-51 change from 0V to 5V when I pushed the front panel source switches. Now, I got stuck at the resistors and TR parts between U503 and U204. The voltage at the base of Q521 reads 5V, not 1.25V, when pin 54 shoots out 5V! No voltage drop at R595. Even when I changed the source switch, there are no voltage drops at the bases of all TR's. I mean 5V at the base of Q524 when I selected "tape source", 5V at Q523 for "AUX source", etc.
Does that mean that all the five resistors R594-598 are shorted? Or there's problem in other places? If so, which parts should I look at?
Thank you.
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A few weeks ago, my TA-2A started to have no sound at all. I removed pre-out & main-in jumper and directly connect another preamp to the TA-2A to check if the main amplifier works. Sounds came out with that setup, that means the main amplifier section is OK.
After tracking down, I found that the voltage inputs to the digital switch IC U204 (LC7818) have issues. The voltages to the pins 11-15 are all high (18V) and do not drop down to low (0.6V) to activate switches. So, I checked the logic board and check the voltage outs from U503 MPU. It looks like working fine because output voltages in pin 55-51 change from 0V to 5V when I pushed the front panel source switches. Now, I got stuck at the resistors and TR parts between U503 and U204. The voltage at the base of Q521 reads 5V, not 1.25V, when pin 54 shoots out 5V! No voltage drop at R595. Even when I changed the source switch, there are no voltage drops at the bases of all TR's. I mean 5V at the base of Q524 when I selected "tape source", 5V at Q523 for "AUX source", etc.
Does that mean that all the five resistors R594-598 are shorted? Or there's problem in other places? If so, which parts should I look at?
Thank you.
An externally hosted image should be here but it was not working when we last tested it.
If the picture above is not shown, please use this link:
https://onedrive.live.com/redir?resid=691775B642BC559F!1639&authkey=!AJMcwCDYg4RakTE&v=3&ithint=photo%2cJPG
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