I've posted this on another forum but I hope someone here has seen this before as it has been a long 4 weeks of trying to troubleshoot this.
I got a Color Classic and when it arrived I was able to plug it in, hit power, and it fired right up. Only issue I saw with it was that it was an RB monitor instead of RGB but since I planned to go through a complete recap I wasn't all that worried about troubleshooting the monitor. I shut it down, pick up my daughter from school, and when I get home to fire it up again it acts like someone shorted the power supply. Hit power and it blinks the led, chirps, and shuts off. Alright, time to replace the caps.
I replace all of the capacitors in the analog board and the motherboard and it does the same thing. Hit power and it blinks. Maybe a bad cap job on the mainboard? Pulled it and it sits there constantly chirping and the HDD does not spin up. It acts exactly like a switcher supply that has been shorted on the output, however, it have that problem as troubleshooting has determined .
It appears that the power supply thinks it is unloaded, even with an LC 550 motherboard and stock HDD installed. If I place another diode and 62 ohm resistor in series with DP15 and RP34 between the feedback winding of the supply transformer and pin 6 of the TDA4605 the computer operates, without this in place pin 6 falls below 7.25V and the supply shuts down because of the discontinuous output. Boots, runs, doesn't seem to have a problem, but the supply is loud as it is running at about 3KHz vs the almost 100KHz of the HV supply and the low frequency causes distortion on the CRT. The TDA4605 is running in discontinuous mode as though it is unloaded. Link to pin 8 scope trace shows how the HV supply runs continuously like it should but the LV supply does one giant pulse. When loaded it adds pulses like it should but it is still discontinuous. This is a pic of the supply being heavily loaded. It continues to regulate and seems to be getting necessary feedback.
I've compared the startup of both supplies and I can't find anything blatantly obvious as wrong. I've resoldered every component related to the bad supply, checked capacitors and resistors and I've checked the transformer for issues and come up with nothing. I've swapped the TDA4605 and optical isolator from both supplies and nothing changed. For about 7ms on startup the supply oscillates normally like it is supposed to and once the rails get up to 12v and 5v it switches to discontinuous mode for however long it runs thereafter.
Has anyone seen this before? Any ideas of where to look?
I got a Color Classic and when it arrived I was able to plug it in, hit power, and it fired right up. Only issue I saw with it was that it was an RB monitor instead of RGB but since I planned to go through a complete recap I wasn't all that worried about troubleshooting the monitor. I shut it down, pick up my daughter from school, and when I get home to fire it up again it acts like someone shorted the power supply. Hit power and it blinks the led, chirps, and shuts off. Alright, time to replace the caps.
I replace all of the capacitors in the analog board and the motherboard and it does the same thing. Hit power and it blinks. Maybe a bad cap job on the mainboard? Pulled it and it sits there constantly chirping and the HDD does not spin up. It acts exactly like a switcher supply that has been shorted on the output, however, it have that problem as troubleshooting has determined .
It appears that the power supply thinks it is unloaded, even with an LC 550 motherboard and stock HDD installed. If I place another diode and 62 ohm resistor in series with DP15 and RP34 between the feedback winding of the supply transformer and pin 6 of the TDA4605 the computer operates, without this in place pin 6 falls below 7.25V and the supply shuts down because of the discontinuous output. Boots, runs, doesn't seem to have a problem, but the supply is loud as it is running at about 3KHz vs the almost 100KHz of the HV supply and the low frequency causes distortion on the CRT. The TDA4605 is running in discontinuous mode as though it is unloaded. Link to pin 8 scope trace shows how the HV supply runs continuously like it should but the LV supply does one giant pulse. When loaded it adds pulses like it should but it is still discontinuous. This is a pic of the supply being heavily loaded. It continues to regulate and seems to be getting necessary feedback.
I've compared the startup of both supplies and I can't find anything blatantly obvious as wrong. I've resoldered every component related to the bad supply, checked capacitors and resistors and I've checked the transformer for issues and come up with nothing. I've swapped the TDA4605 and optical isolator from both supplies and nothing changed. For about 7ms on startup the supply oscillates normally like it is supposed to and once the rails get up to 12v and 5v it switches to discontinuous mode for however long it runs thereafter.
Has anyone seen this before? Any ideas of where to look?
