DD M3b speaker terminals relays not engaging

I’ve been working on this amp for some time now and it has fought me every step of the way. It’s been a great learning experience for me and I thank the folks on here for their help and advice.

The amp came in with a blown output section. It has the 3S tech driver board that proved to be a challenge. The amp kept blowing outputs and ICs while testing and the problem turned out to be a shorted output inductor. I rewound the inductor and finally got the output section to oscillate but now the relays won’t engage to send audio out to the speaker terminals.

Before when I was testing the relays would happily engage and soon after one or two outputs would fail. Now that the problem with output section is resolved the relays want some attention.

I checked Q5 on the power supply PWM board and it test good in diode check with the DMM. On the main board in front of the PWM board in the power supply section, I did find that Q275 was shorted. It’s a K30A transistor. My search on google doesn’t return any reliable sources for this transistor. It is either too New or Obsolete. I was wondering if there is a suitable alternative for this transistor that I may get quickly and reliably.

From what I find in my searches it is a SGD Jfet. I’m not sure what this transistors function is in the circuit. It appears to connect to pin 5 of the PWM board through a 1uf 50v cap.

How critical is this transistor and could this be the reason my relays are now, not engaging? Advice would be welcome.



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Turns out after checking and replacing the aforementioned components, I also had blown trace between the quenching diode for the relay and pin 1 of the power supply PWM card. It was hidden under some glue amongst the rail caps.

The amp is working now. This one made me work for it.

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