Meridian Unplugged - Powered by MrTechGuy.co.uk Forums Discussion Meridian - Systems 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.

I have removed the 3 bottom hex screws, the 1 fat screw and the 4 nuts.
Heatsink/PCB assembly will fold over to lie flat on table without doing anything to wiring harness.
I have successfully loosen the 5 NPN hex screws and the 5 PNP hex screws.
Have been unsuccessful in breaking loose transistor from insulator/heatsink.
How much force do I need? Sudden? or Slow?

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

Mr 557
It helps to undo one at a time. I loosen the first one, 3 fastened, and then push a screwdriver just wider than the gap between the devices.at the top corner. Push down sufficiently hard and the end device will just move. Then the next in line. Once you get to the fourth, put a screw back in 3rd device to hold lateral movement, and make fourth one move. Simple. But as one said here, you need to get the hang of it, or whatever you call in in USA English Americanish or broke.
Best of luck! Tell us how it goes.
D.

Success!

Following your recommendations, all transistors were separated from heatsink/insulator without damage.

I took a few electrical measurements before transistor separation. Main supply voltages are +70.1v with 50mv ripple and -69.8v with 10mv ripple. +15v was +14.75v. -15v was -14.71v.

In circuit capacitor measurements were 11.27mF with a Vloss of 1.9% and 0.01ohm ESR for +70.1v supply and 11.16mF with a Vloss of 3.6% and 0.00 ohm ESR for -69.8v supply.

In circuit capacitor measurements of +15v supply output was 649uF with a Vloss of 21% and 0.66 ohm ESR and -15v supply output was 899uF with a Vloss of 6.5% and 0.35 ohm ESR.

I am ordering replacement Nichicon capacitors - 3x 1000uF, 10x 10uF and 2x 1uF. And then I am doubling order in case I decide to replace capacitors on other channel.

Thanks for the help. I am sure 15v supply capacitors should fix oscillations.

Very good!
You should of course double things. The other channel aged just as much. Remember 105^ temp reange, now I am sure you see why I say this. I used Nichicon PS(M) series, and previously also Vishay and Panasonic FM. Perfect fit.
Keep us posted

Hey JamesBrennan4,

I've sent you PM with regards to your 557. Please check your private messages. Thanks!