Meridian Unplugged - Powered by MrTechGuy.co.uk Forums Thread Like Summary - 557 disassembly

Has anyone taken apart a 557 to the point of being able to replace components on one of the amplifier PCB?

I have removed the top, leaving the bottom, front and back panels and the two headsinked sides.

I can almost envision removal of a heatsink side panel and attached amplifier PCB. How do single wire connectors detach? Do you unattached supply and speaker wires before or after heatsink side is loose? Separation of heatsink from PCB looks straight forward. Reassembly of power transistors to heatsink looks like a nightmare.

Amplifier problem - works correctly for first 10 minutes, then I can hear distortion and a 4KHz squeal from speaker. After another few minutes, total silence from that amplifier.

Troubleshooting - when I hear squeal, I can also see 500 KHz, 1v RMS oscillation on output (yes, I see this oscillation on oscilloscope - of course I am not hearing it). Reducing AC Power from normal 115vac to 105vac will revive amplifier. All internal supply voltages are normal. No signal output on speaker terminals is 27mV.

Hi,

Well if you can measure signals it looks like you are not a novice. The heatsink detaches by removing 3 screws on the bottom sides and another fat screw to the front panel. Do not attempt to remove the wire harnesses unless you are particularly bored and then some. to detach the board from PCB you may have some problem as they are normally quite stuck. Don't desolder. Release ONE screw from a C3821 on the edge and pry sideways by ever so carefully inserting a smooth object like a flat screwdriver in the gap between a fastened one and the unbolted. It will snap off the isolator, then do for the next. The last one in each row you can do be putting the one-before-last screw back in finger tight, and then pry the last tranny the same way, they are very close. Them release the next row, the PNP. The rest just lift off. You will have a few small nuts to release and then it pulls right off.

The oscillations are caused by instability around the regulators, remove the 1000u/16V caps, the green ones, and you will find they have all but gone. Trash them anyway and put proper 105^ parts. I use Nichicon or Panasonic FM. Low footprint and will last much longer than the originals. To me the Muse in that location was just audiophile fluff. Replace also the smaller caps stabilizing the output of the 317 regulators, 10u if I am not wrong. Again put 105^ because they are so close that they overheat.

If all you hear is oscillations, just put it all back together and it should be fine. If you are a nutcase like me then replace the reservoir cans, but the footprint is tricky to find. Kemet make something quite similar, do check first that they are 20% under printed value or just don't bother. They are good quality.

You used to be able to download a tech doc from the meridian support site, they did detail replacement of which capacitors. But do take my word for it, the Muse parts are not up to the job and you will benefit by putting a more industrial type as mentioned.
That's about it.. I do have photos, I may post them in the gallery if my wives don't threaten blue murder if I don't come to bed.

All the best,
David

Wives (pleural) and a penchant for high maintenance hifi. That’s dedication.....

Thank you.

Will begin the disassembly tonight.

You need quite a bit of force but DON'T damage the insulating pads between the heatsink and the transistors!

There is a certain nack, once you've done one, the rest follow fairly easily...

Good luck,

Steve