Not really a vintage product, the Mackie M1400i dates from the 90’s; but an interesting repair.
The M1400i is a Pro Audio amplifier, 700 watts per channel into 2 ohms, or 1400 watts into 4 ohms when run in bridge mode. It was sent in with the instruction that “someone else” had tried to repair it but failed. No problem, I have spent 35 years cleaning up other peoples failed repairs on Pro Audio equipment.
The service manual is readily available from the web, such as my favourite
https://elektrotanya.com
I stripped the amplifier down, and a quick investigation revealed R7, R9, R10, R11, R13, R14, R15, R16 and R18 were open circuit, whilst Q28 was short circuit.
After replacing these, the amplifier quiescent current set up perfectly, but the fan was starting instantly at full speed, it should be a 4 second delay unless the unit is hot, and the front panel LED said the unit was overheating.
After a long search, I found that Q89 was switched hard on constantly no a.c reaching D57, so no reverse bias. Unfortunately, the service manual does not provide the print layouts, and I was getting nowhere trying to follow the path of D57 back to the a.c. inputs.
To follow the print, I had to depopulate parts of the board. Doing so, I followed the a.c. to the fuse area, and so I lifted the two fuses to follow the a.c.; see the attached photograph.
Someone had previously removed the fuses with a crowbar, and ripped numerous pieces of fine print.
20 minutes with Kynar wire, superglue and polyurethane lacquer, and I had a nice neat print repair.
Reassembly gave a blip from the fan at switch on, and no overheat LED. The amplifier checked out fine for power, noise and distortion. Another happy customer
Kevin