I always wanted one of these back in the 1970's. Found one as is on Ebay so I thought I would give it a try at repairing it. I tried to keep it as original as possible. I did however, design a couple of PCB's to fix a couple issues.
Craig, I remember repairing Marantz receivers where the speaker protection relay would arch when the and burn up. It was caused by a diode in the bias circuit that was intermittent and caused the dc to slam to the minus or plus rail. When the relay opened, it provided a short to the speaker and a good path to ground for the arch.
Fantastic! I admire your approach of attempting to maintain the original design with tasteful enhancements. When and where can we buy the pcb’s you built and incorporated!?!?!? Thanks again for sharing 👍
Dear Sir, I would like to ask a question about your protection circuit. What is the electrical current specification for the 1.75uH inductor? Or if you made it yourself, what is the AWG, number of turns, and diameter? I would be very grateful if you allow me to use your design to add to my amplifier. Thank you very much.
I bought a used one sometime ago that was dead, I changed out the fuses because they were blown but I have not even powered it up lol.... I guess haven't really been curious about whether it works or not until now.
If the speaker fuses were blown.. don't bother testing it. You've got something shorted somewhere. I find most often it's a pre-driver failure. It won't always cook the output devices, BUT I recommend you check every transistor on the main board, and all of the outputs. The first TO-3 output transistors (RCA 410's) are drivers for the main outputs. Those usually don't go bad. If it's just the main fuse that went bad, it may have been inrush current trying to reform the old filter caps. Bring it up slowly on a variac or use a dim bulb to limit current. You'll likely need new filter caps. Easy to do and easily available.
Great video. I'd be interested if you would be selling the unpopulated protection board. I'm restoring 2 PL400's, and really like your design. Thanks Richard
Yes they did, and they are a bit more popular and command more money . I have a 700b myself that I am doing all the white oak and a few other upgrades .
