TR4 overheating 6A board
Posted: Sun Jul 24, 2022 4:52 pm
Hi all, I'm trying to repair a 6A board.
I first checked the resistances to 0v at the lower RAM, +5, -5, 12v and there was no low resistance;
so I plugged it in and resister R79 got very hot and smoked.
I replaced TR4 with a C2236 which I've found on some 4A boards; I did swap the base and collector legs around making sure that they didn't short.
I also replaced TR5 with a ZTX751.
I then tested the machine with my bench PSU at 11v with a 0.75A overload protection, and the voltage drops from 11v to just over 5v.
As TR4 burnouts are often the 4116 lower RAM problems, I removed them all (I expect some are damaged anyway), but it's still the same.
After watching JoulesperCoulomb brilliant YouTube video, I learnt that if for any reason the 12v is missing, that TR5 switches on and then TR4 can burn out.
I believe I've tested every resister, diode and capacitor in the power circuit and can't find anything wrong. I removed one end of R61 that goes to the coil and then TR4's base, just so I could test the 7805 was producing 5v, and it was about 4.8v.
I have now replaced TR4 again, with a used (but tests ok) ZTX651 but it's the same.
I have ordered some ZTX653 transistors, just in case it's still TR4 that is faulty.
Any suggestions please?
I first checked the resistances to 0v at the lower RAM, +5, -5, 12v and there was no low resistance;
so I plugged it in and resister R79 got very hot and smoked.
I replaced TR4 with a C2236 which I've found on some 4A boards; I did swap the base and collector legs around making sure that they didn't short.
I also replaced TR5 with a ZTX751.
I then tested the machine with my bench PSU at 11v with a 0.75A overload protection, and the voltage drops from 11v to just over 5v.
As TR4 burnouts are often the 4116 lower RAM problems, I removed them all (I expect some are damaged anyway), but it's still the same.
After watching JoulesperCoulomb brilliant YouTube video, I learnt that if for any reason the 12v is missing, that TR5 switches on and then TR4 can burn out.
I believe I've tested every resister, diode and capacitor in the power circuit and can't find anything wrong. I removed one end of R61 that goes to the coil and then TR4's base, just so I could test the 7805 was producing 5v, and it was about 4.8v.
I have now replaced TR4 again, with a used (but tests ok) ZTX651 but it's the same.
I have ordered some ZTX653 transistors, just in case it's still TR4 that is faulty.
Any suggestions please?