M-2 resurrected!

I bought an M-2 off the bay a while back. Cosmetically it was very sharp, not a mark anywhere, but I could not get an idle reading on the L/H channel. DC offset and 80+/- were right on. R/H channel adjusted fine. The heat sinks on the L/H would not get warm, certainly not as warm as the R/H side.

First thing I noticed was the 1.5 amp 10V fuses were both blown. I replaced them, crossed my fingers and plugged it in. Switch on, poof...

I looked for the obvious, chased a few circuits using the schematic, but I knew I was in way over my head.

In comes Avionic to the rescue! With Dave's technical help I first removed all the sony bond from the power supply and replaced the caps (Sir byrd helped me order the caps).

I never knew the M-2 had the dreaded goo. This one does but only on the power supply caps.


All cleaned up, not hard but time consuming.


New caps (not bad for a rookie?)


Poof, fuses still blow and still no L/H idle signal. Goo needed gone and caps should be replaced anyway so certainly value added work.

Dave and I decided to switch around the 3 bridge diodes as two of them were not original and we thought this may be causing trouble. Negative, but I replaced all three anyway as the schematic showed them to be the same.

Dave and I started thinking about this 10V circuit. It seems to supplement the 83V rail voltage to get it up to 93V. Kinda of strange, but we were thinking maybe the 83V was trying to back track into the 10V circuit. Bingo, replaced two diodes and fuses stopped blowing, but still no idle signal on the L/H.

We decided to replace the trim pots with some nice Bourns units. Very nice modification but still no idle.

I had checked every thing on the board but Dave recommened I remove some of the resistors to check them out of ciruit. This I did and they all measured per speck so I reinstalled them. Bingo! I got an idle signal and it adjusted just fine. There must have been some bad solder joints although they sure looked fine.

Listening to the amp now it is mighty fine! Heat sinks are nice and warm on both sides.

I want to personally thank fellow AK'er Avionic as I could never have done this without his excellent guidance. May much Karma come your way Dave!



Life is good!

We are getting there.Slow but sure.:thmbsp:

avionic said:

We are getting there.Slow but sure.:thmbsp:

Click to expand...

We are there, all measures perfect and this is one sweet sounding amp. It drives the Soliloquy 6.5 speakers with authority. I still prefer my Primaluna Prologue 7 mono blocks in the mid range and high frequencies. What can I say, tubes are magic! However, this M-2 really drives the 6 woofers to perfection so now I have them bi-amped. The M-2 has adjustable gain so getting a nice even balance is possible.

Resulting sound quality is exceptional! Tube magic with SS bass authority. I really like the results....

stoutblock said:

We are there, all measures perfect and this is one sweet sounding amp. It drives the Soliloquy 6.5 speakers with authority. I still prefer my Primaluna Prologue 7 mono blocks in the mid range and high frequencies. What can I say, tubes are magic! However, this M-2 really drives the 6 woofers to perfection so now I have them bi-amped. The M-2 has adjustable gain so getting a nice even balance is possible.

Resulting sound quality is exceptional! Tube magic with SS bass authority. I really like the results....

Click to expand...

All that phone time paid off! Frick'n Awsome.You did a great job !!!

PS Nice setup!!!

Thanks Dave but you are my hero!

Listening to Yes - Fragile, with Roundabout cranked. Man this amp really drives the woofers on these speakers! It balances surprisingly well with the tube monos driving the mids and tweeters. Solid wall of sound!

:yes:

Sweet deal! how did those filter caps work out?

stoutblock said:

Thanks Dave but you are my hero!

Listening to Yes - Fragile, with Roundabout cranked. Man this amp really drives the woofers on these speakers! It balances surprisingly well with the tube monos driving the mids and tweeters. Solid wall of sound!

:yes:

Click to expand...

That settles it, I'm going to Pull out Fragile and crank up the M-2 in my office.

Sir.Byrd said:

Sweet deal! how did those filter caps work out?

Click to expand...

The big cans adapted just fine. Looks factory. Last mod is to replace the speaker terminals with some 5 ways. Might add a IEC power plug so I can use a better power cable.

It seems to supplement the 83V rail voltage to get it up to 93V.

Click to expand...

They bump up the driver rail voltage to 93vdc.The outputs rail voltage remains @ 83vdc..

have we come to the conclusion that that earlier glue is also bad? i thought it was just the stuff they used on the M-80 and such.

If the glue gets dried up and crusty it attracts dust and moisture.The three together accelerate corrosion on metallic surfaces.

cdfac said:

have we come to the conclusion that that earlier glue is also bad? i thought it was just the stuff they used on the M-80 and such.

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It's more pervasive than just the M-80, and certainly not limited to Yamaha. I recapped an old Sansui for a friend and found it there as well.

The glue I saw in the MX-2000 is snow white and seems a different formulation.

as it is here, though, it doesn't appear to pose any immediate problem, right? i seem to remember seeing it darker red and crustier in the units where people have had problems and posted pictures. i'm asking for future reference, whenever i pick up my next vintage Yamaha and inspect it.

Nice M2 you have! I have a M4 and loved mine, at least for the 6 hours I heard it and then it quit working. Tuned out it was the left channel and the dc-offset and bias trim pots some I'm replacing all of the trim-pots on both sides with bourns.

cdfac said:

as it is here, though, it doesn't appear to pose any immediate problem, right? i seem to remember seeing it darker red and crustier in the units where people have had problems and posted pictures. i'm asking for future reference, whenever i pick up my next vintage Yamaha and inspect it.

Click to expand...

There was some area that were a darker brown and crusty. Had to replace the caps in any case.

lha1992 said:

Nice M2 you have! I have a M4 and loved mine, at least for the 6 hours I heard it and then it quit working. Tuned out it was the left channel and the dc-offset and bias trim pots some I'm replacing all of the trim-pots on both sides with bourns.

Click to expand...

Yes, I also replaced trim pots with bourns.

Old:


New:

Last night I replaced the speaker terminals with some new units. Had to remove the rear plate and drill out the holes just a little.

Old:


New:

They look purdy good..:yes:

lha1992 said:

Nice M2 you have! I have a M4 and loved mine, at least for the 6 hours I heard it and then it quit working. Tuned out it was the left channel and the dc-offset and bias trim pots some I'm replacing all of the trim-pots on both sides with bourns.

Click to expand...

Ahem!

avionic said:

They look purdy good..:yes:

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Thanks, certainly gives me new options on speaker cables...