vinylisfinyl
Member
Greetings, fellow stereo curators!
I need some assistance from you today regarding high DC voltage on the left channel in our beloved STA-78 receiver. Dad bought it new in the late 70s and passed it on to me, but when he tried using it this summer in our garage it let out the magic smoke after a loud bang. Since I studied EE in college and love working on old electronics, I investigated and found TR613 (power amp) completely shorted and R623 roasted alive.
I ordered a NOS 2SB509 off eBay for TR613 and installed it, also replacing R623 and D61 which were toasted pretty good. After powering the unit up I found there were no shorts or smoke (yay!) but also found 5V across the left channel speaker outputs (whaaaa?!!!).
Further probing has found absurdly high voltages at all of the left channel transistors all the way back to TR605 but not before, and 5V on the main output line. I attached an image with red numbers to show what I am measuring. The right channel voltages are all very close the factory specs (in the second attached image), and no high voltages are coming into the amp board on either channel.
I saw that TR605 had correct emitter and base voltages but high collector voltage so I figured it was leaking and replaced it with a KSA1013. But when I powered the unit back up with the new TR605 in place I found the exact same high voltages on the left channel!
I am admittedly stumped where to look from here. I thought about another transistor leaking, but since high voltages are even seen at TR605 (which is a “pre amp drive” according to the service manual) I am questioning that. C605 and C607 are not shorting according to my DMM. The 2SB509 I put in is a rev 6J not a D7G as original unit, but I can't image how that would cause an issue like this since I'd think a 509 should be a 509 regardless of revision...right?.
I will continue searching for bad caps or resistors, but am interested in getting your feedback on what should be my next area of investigation. If this unit was on your bench, what part of the circuit would you try looking at next?
Thanks in advance!
I need some assistance from you today regarding high DC voltage on the left channel in our beloved STA-78 receiver. Dad bought it new in the late 70s and passed it on to me, but when he tried using it this summer in our garage it let out the magic smoke after a loud bang. Since I studied EE in college and love working on old electronics, I investigated and found TR613 (power amp) completely shorted and R623 roasted alive.
I ordered a NOS 2SB509 off eBay for TR613 and installed it, also replacing R623 and D61 which were toasted pretty good. After powering the unit up I found there were no shorts or smoke (yay!) but also found 5V across the left channel speaker outputs (whaaaa?!!!).
Further probing has found absurdly high voltages at all of the left channel transistors all the way back to TR605 but not before, and 5V on the main output line. I attached an image with red numbers to show what I am measuring. The right channel voltages are all very close the factory specs (in the second attached image), and no high voltages are coming into the amp board on either channel.
I saw that TR605 had correct emitter and base voltages but high collector voltage so I figured it was leaking and replaced it with a KSA1013. But when I powered the unit back up with the new TR605 in place I found the exact same high voltages on the left channel!
I am admittedly stumped where to look from here. I thought about another transistor leaking, but since high voltages are even seen at TR605 (which is a “pre amp drive” according to the service manual) I am questioning that. C605 and C607 are not shorting according to my DMM. The 2SB509 I put in is a rev 6J not a D7G as original unit, but I can't image how that would cause an issue like this since I'd think a 509 should be a 509 regardless of revision...right?.
I will continue searching for bad caps or resistors, but am interested in getting your feedback on what should be my next area of investigation. If this unit was on your bench, what part of the circuit would you try looking at next?
Thanks in advance!