KrisBlueNZ
Sadly passed away in 2015
Hmm. OK, well you've confirmed that C1~6 and the bridge rectifier diodes are OK.
Have you screwed the board fully to the chassis? There's a connection on the common of C1 and C2 that looks like it's supposed to be earthed to chassis. If C1 or C2 are faulty, the fuse will blow if that connection is earthed, but won't blow if it isn't.
And you've checked the six diodes in the oscillator circuit.
And you replaced the transistors, but it's possible that the replacement transistors failed and have now failed short. Can you measure collector-emitter resistance without power on the circuit. They should read open circuit or high resistance from collector to emitter.
What type of multimeter do you have? Is it digital or analogue?
You've checked the 15k resistors. The 0.2 ohm emitter resistors are very unlikely to be the problem. The two 3 uF capacitors are also not likely to have failed. Inspect them carefully though, for any signs of problems.
I doubt that the feedback transformer (T2) would have a problem.
That leaves the big inductor (external to the board) and the transducer, and the transistors.
I think it would be worth desoldering the transistors - all four of them - and carefully inspecting the insulating washers and the insulating collars for the screws. Replace the insulating washers if there is any doubt about their condition. You can use mica washers with white heatsink goop, or get grey-coloured flexible silicone-impregnated washers that have good heat transfer.
You said you replaced the insulating collars because they were cracked. Were they fitted in between the heatsink and the PCB, poking upwards into the screw holes in the heatsink? That's how they probably should have been fitted. What did you replace them with?
While the transistors are out, you can check for any collector-emitter leakage (use resistance range). When they're back in circuit, and screwed in, check resistance from the transistor case to the heatsink metal, for both heatsinks, using a high resistance range. Should read open circuit, obviously.
Have you been screwing both heatsinks to the chassis each time you've tested the board?
It's hard to know what to suggest next. The oscillator depends on the characteristics of the big inductor and the transducer in order to oscillate at all, so it's not possible to diagnose individual parts of the oscillator circuit.
Does anyone else have any suggestions here?
Have you screwed the board fully to the chassis? There's a connection on the common of C1 and C2 that looks like it's supposed to be earthed to chassis. If C1 or C2 are faulty, the fuse will blow if that connection is earthed, but won't blow if it isn't.
And you've checked the six diodes in the oscillator circuit.
And you replaced the transistors, but it's possible that the replacement transistors failed and have now failed short. Can you measure collector-emitter resistance without power on the circuit. They should read open circuit or high resistance from collector to emitter.
What type of multimeter do you have? Is it digital or analogue?
You've checked the 15k resistors. The 0.2 ohm emitter resistors are very unlikely to be the problem. The two 3 uF capacitors are also not likely to have failed. Inspect them carefully though, for any signs of problems.
I doubt that the feedback transformer (T2) would have a problem.
That leaves the big inductor (external to the board) and the transducer, and the transistors.
I think it would be worth desoldering the transistors - all four of them - and carefully inspecting the insulating washers and the insulating collars for the screws. Replace the insulating washers if there is any doubt about their condition. You can use mica washers with white heatsink goop, or get grey-coloured flexible silicone-impregnated washers that have good heat transfer.
You said you replaced the insulating collars because they were cracked. Were they fitted in between the heatsink and the PCB, poking upwards into the screw holes in the heatsink? That's how they probably should have been fitted. What did you replace them with?
While the transistors are out, you can check for any collector-emitter leakage (use resistance range). When they're back in circuit, and screwed in, check resistance from the transistor case to the heatsink metal, for both heatsinks, using a high resistance range. Should read open circuit, obviously.
Have you been screwing both heatsinks to the chassis each time you've tested the board?
It's hard to know what to suggest next. The oscillator depends on the characteristics of the big inductor and the transducer in order to oscillate at all, so it's not possible to diagnose individual parts of the oscillator circuit.
Does anyone else have any suggestions here?
