MC502 and C504 Clean Up

c-atle-79bay

Super Member
Well I'm not sure of good title so I'll go with the above. I picked these up yesterday, got home, hopped on CL saw these for $500 and emailed then. I kinda figured anything MC for that price was a score. The look great, only a little bit of maring on the MC502 top edge. But very dirty inside.

Known Issues:

-bad power switch on MC502
-bit of transformer hum(will inspect physical chassis connection first)
-no feet
-lots of static in knobs
-one channel is less dynamic than the other in very limited testing (think it' related to dirty pots)

So here are the before photos.

Any advice on deoxit of the knobs? Should I go from the top/bottom and front? How do you remove sticky knobs without damaging the glass?

Here are some photos of the amp
 

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Dusty MC502

Hello c-atle-79bay !

See my pictures and you will see what result you can simply get only with air under pressure !

Take off all the dust in the unit and on the pot before to clean the gain potentiometers !

Than all electrolytic capacitors has been changed and everything go back to normal function !:banana:

Regarding the power transformer there nothing to do except to replace it ....

We have replaced 53 power transformers on MC502 who was sold in France under warranty !
Today warranty is over ...:sigh:

To remove knob is easy , just pull on them , that's all
 

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Yep pull harder. Got it nice and dusted out, removed 1 sponge bob square pants bandaid, and looks good. Want to tighten down the pots a bit, it seems really obvious how to get the glass out, but have others had to pry the pieces holding the glass of of the chassis side rails? I didn't have the nerve for it yet.

Otherwise, runs about silent, tested with head phones tonight and the only thing I heard at maximum volume was that sense of too quiet, something there just not audible, weird.

Will get the glass figured out and get to tightening down the pots then I should have it going back together and off the bench.
 
Yep pull harder. Got it nice and dusted out, and looks good. Want to tighten down the pots a bit, it seems really obvious how to get the glass out, but have others had to pry the pieces holding the glass of of the chassis side rails? I didn't have the nerve for it yet. Will get the glass figured out and get to tightening down the pots then I should have it going back together and off the bench.

That's what you call "in need of decontaminating."

The front panel should come off with two screws on each end, and probably two low profile screws on both the top and bottom of chassis. First pull the knobs. Gripping them with something rubber (like gloves) seems to work well. Beware of removing the glass. You might have an additional project on your hands.. i.e. new foam tape, etc. The pots can be tightened with a deep-well nut driver.

MC2205_Love_nutdriver.jpg

Deep-well driver tightening pots.



-Gregory
 
So this one has brackets that hold the glass to the chassis. I removed all the 4 screws but it did not budge. Perhaps a bit of prying is in order?
 

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Well I'm not sure of good title so I'll go with the above. I picked these up yesterday, got home, hopped on CL saw these for $500 and emailed then. I kinda figured anything MC for that price was a score. The look great, only a little bit of maring on the MC502 top edge. But very dirty inside.

Known Issues:

-bad power switch on MC502
-bit of transformer hum(will inspect physical chassis connection first)
-no feet
-lots of static in knobs
-one channel is less dynamic than the other in very limited testing (think it' related to dirty pots)

So here are the before photos.

Any advice on deoxit of the knobs? Should I go from the top/bottom and front? How do you remove sticky knobs without damaging the glass?

Here are some photos of the amp

Damnit $500 for both? I hate you.

To be honest most of those issues are prolly just corrosion in the pots as mentioned. My sister's BF has an MX113/MC2100 combo with similar issues and a simple back and forth working of each pot for a few minutes solved 80% of the issues. I didn't have any deoxit handy. You are a lucky bastard I'll tell you that.
 
Deoxit has cured most everything. Amp is basically silent, and ready to go. Do have to tighten the pots but that can wait a bit.

The premap however has 1 remaining issue, the right side of the phono stage (1,2) is noisy when heard through headphones with no inputs, and is volume dependent, meaning when I turn up the volume the noise increases. Rushy water/wind sound. I've read in other posts that this means I should replace the capacitors on it.
 
Anyone with insight on the glass removal and how to separate the bracket from the chassis rails? Oh and do the bulbs ever stick?
 
Swap the 5534an phono op amps side to side, they are in sockets. See if noise swaps sides.
 
Would do that but so far have been skunked on the schematic or service manual. Berners has it but it's a favoured one, and I'm not favored.

Anyone have this? Thanks.
 
Thanks Larry, but its the C504 (preamp) that I need. Want to check c_DK's suggestion of swapping the op amps (however it is that I do that).
 
I am sure youcan buy a schematic from the factory. Mine cost $10.00 back in the day. Came in a notebook.

If you take off the top cover, with back towards you, phono stage is under silver cover/shield on rh rear of chassis. Two 5534s are in center of board in sockets.
 
Did the op-amp swap, noise changed sides. So faulty op-amp? Can modern op-amps be swapped in? IDK about these things.
 
I swap in new TI or Onsemi 5534an. I am always concerned about oscillation rolling in non stock ICs.
 
c_dk, Is either one better than the other?

I also buttoned everything up last night and gave a listen, still bench. One channel is much less dynamic than the other, when moving the balance/loudness control to the right or left, the bass is very weak on the left hand side, and very strong on the other side via my AR 4ax speakers.
 
Can not say if one is really better than other.....all kinds of opinions out there, not much empirical testing.

Did use a client's c29 to observe lower noise floor between old signetics and new onsemis.

Without a scope and distortion analyzer be tough to trouble shoot ......amp issue or preamp?
 
Was thinking more and I don't know if the issue, weak channel, is a pre or amp issue. I will have to run straight through the amp to be certain tonight.

Another observation. The right channel is more dynamic, but also has more noise as you turn up the right channel gain. The left channel is less dynamic, but has less noise as you turn up the gain.

Purely for the fun of testing I'll try the same test with the amplifier opamps on the right/left channel.
 
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