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A few years ago, a family member asked me to look at a set of bookshelf speakers that stopped working. Inside I noticed obvious signs of a cooked component on the internal power supply, which was a separate board from the amplifier. The power supply outputted 9V AC to the amp circuitry.

At the time, I had no substitute 9V AC supply to swap in, so I used a 10V DC supply and soldered it up in place, figuring the extra volt wouldn't hurt and would simply compensate for voltage drop across the bridge rectifier which was no longer needed due to the DC supply. It worked fine for some time.

Recently I inquired about the speakers, and learned that they stopped working again. Inside, I see the 10V DC supply is working fine but 2 diodes of the (discrete) bridge rectifier are burned up on the amp module.

Assuming I want to stick with a DC-rectified supply, and not revert to AC with replacement bridge diodes on the module, what is the best course for the repair?

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  • \$\begingroup\$ Should be fine to put the 10 V DC supply in and bypass the rectifier; rectified 9 V AC produces around 11 V peak on the output (formula: 9 V·√2 - 2·Vf), so the circuitry after the rectifier can definitely handle the 10 V input. \$\endgroup\$
    – Hearth
    Commented Dec 16, 2023 at 16:46
  • \$\begingroup\$ @Hearth That sounds easy enough. Can you elaborate on the method of the "bypass" in this case? Is it as simple as swapping the 2 burned diodes with wire? Or do I replace all 4 diodes with jumpers? \$\endgroup\$
    – Jamesfo
    Commented Dec 16, 2023 at 16:49
  • \$\begingroup\$ If the device wants to rectify AC into positive and negative supplies, you can't simply power it with DC. Reverse engineer the circuitry to know if it recticies the AC into single DC or dual DC supply. Also, 9VAC has a peak voltage of 12.7, so 10V DC is less than that anyway. \$\endgroup\$
    – Justme
    Commented Dec 16, 2023 at 16:49
  • \$\begingroup\$ If the diodes burned, there could be fault in the amplifier which is overloading the power supply. What was the current rating of the diodes which burned, and what was the rating of the original supply? \$\endgroup\$
    – Chester Gillon
    Commented Dec 16, 2023 at 16:51
  • \$\begingroup\$ @Justme If the device "worked fine for some time" after replacing the AC supply with a DC one, it either doesn't need a negative supply or it generates one by some other means. \$\endgroup\$
    – Hearth
    Commented Dec 16, 2023 at 16:51

1 Answer 1

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Should be fine to put the 10 V DC supply in and bypass the rectifier; rectified 9 V AC produces around 11 V peak on the output (formula: 9 V·√2 - 2·Vf), so the circuitry after the rectifier can definitely handle the 10 V input.

My guess is that the diodes burned out because they were being used at a higher average power than normal--since each diode in a single-phase bridge rectifier is only used half of the time, they only need to handle half of the device's average current consumption. By swapping to a DC supply, two of the diodes now need to handle the full current used by the device, double what they did before, and the other two don't see any current.

Remove the burned out diodes (you can remove the good diodes too, they won't be doing anything) and just connect your DC supply to the rectifier's + and - output terminals.

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  • \$\begingroup\$ Thanks. I did this, and after first power-up I could see the 10V supply was under heavy load. It turns out that the main IC on the amp circuit was damaged and drawing excess current, but the heat sink on it prevented signs of damage from appearing on the PCB. So either this component failed first and then caused the diodes to blow as fuses, or my initial hacky repair of feeding DC caused problems for the chip, cooked it, and then the diodes in turn. \$\endgroup\$
    – Jamesfo
    Commented Dec 17, 2023 at 17:04
  • \$\begingroup\$ Unlikely your initial repair caused the problem--it's more likely that it was there to begin with. \$\endgroup\$
    – Hearth
    Commented Dec 17, 2023 at 17:05

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