Ok, someone has brought me a pair of MC75's to 'wake up' after 10+
years of being stored in the closet. They are unmolested, completly
stock.

I always take a minimalist approach in servicing vintage gear. Bring
them up slowly on a variac, if they work, great, I'll use them as they
are. I never globally replace parts just because they are old and I
certainly don't get caught up in replacing parts (like signal caps)
with the high priced audiophool stuff just because it is 'better'.

I'm sure these amps will come up just fine with a little bit of time
on the variac and I'm very, very reluctant to replace anything.

The owner of these amps is wealthy and I could charge him an amount
that would choke me, but for him, he probably wouldn't think twice
about it. Not that I would do that, but you can imagine the flippant
attitude of some people with too much money.

So what would you do with these amps? I'd vote to bring them up on a
variac and put them back in service if they work good, leave them
unmolested. I think the only way I'd replace any filter caps, is if I
could find the original twist-lock types, I really wouldn't want to
stick in say radial lead types or something that looks totally out of
place.

Any advice? Perhaps somebody has a service 'kit' for updating old Macs
to modern parts?

TIA,
Gilbert

"Gilbert Bates" wrote in message
...
Ok, someone has brought me a pair of MC75's to 'wake up' after 10+
years of being stored in the closet. They are unmolested, completly
stock.

I always take a minimalist approach in servicing vintage gear. Bring
them up slowly on a variac, if they work, great, I'll use them as they
are. I never globally replace parts just because they are old and I
certainly don't get caught up in replacing parts (like signal caps)
with the high priced audiophool stuff just because it is 'better'.

I'm sure these amps will come up just fine with a little bit of time
on the variac and I'm very, very reluctant to replace anything.

The owner of these amps is wealthy and I could charge him an amount
that would choke me, but for him, he probably wouldn't think twice
about it. Not that I would do that, but you can imagine the flippant
attitude of some people with too much money.

So what would you do with these amps? I'd vote to bring them up on a
variac and put them back in service if they work good, leave them
unmolested. I think the only way I'd replace any filter caps, is if I
could find the original twist-lock types, I really wouldn't want to
stick in say radial lead types or something that looks totally out of
place.

Any advice? Perhaps somebody has a service 'kit' for updating old Macs
to modern parts?

TIA,
Gilbert


Get rid of the selenium rectifiers.
It will fail. Spectacularly. Toxically smokey.


--
Alan Gallacher
Born to Tinker!

On Mon, 28 Mar 2005 22:50:34 GMT, Gilbert Bates
wrote:

So what would you do with these amps? I'd vote to bring them up on a
variac and put them back in service if they work good, leave them
unmolested. I think the only way I'd replace any filter caps, is if I
could find the original twist-lock types, I really wouldn't want to
stick in say radial lead types or something that looks totally out of
place.

Replacement can caps are available, if needed. You might consider
just replacing them anyway; they're old.


Any advice?

Check the 12K 2W resistors; they drift a lot. As has already
been posted, replace the selenium rectifier with a silicon.

Good fortune,

Chris Hornbeck
6x9=42

replacing signal caps is one thing, a judgment call

HOWEVER..........

only a fool would not replace the leaking electrolytic caps (power supply,
etc.) and paper caps in this amp, especially since the transformers are
mega-hard to find, and you are just asking to blow one up

and they ARE leaking badly, after 40 years, your MC75 is not special in this
regard, no matter how little it has been used

minimalist is a good idea as far as mods, but not as far as maintenance
issues, such as this critically important repair that needs to be done now.




"Gilbert Bates" wrote in message
...
Ok, someone has brought me a pair of MC75's to 'wake up' after 10+
years of being stored in the closet. They are unmolested, completly
stock.

I always take a minimalist approach in servicing vintage gear. Bring
them up slowly on a variac, if they work, great, I'll use them as they
are. I never globally replace parts just because they are old and I
certainly don't get caught up in replacing parts (like signal caps)
with the high priced audiophool stuff just because it is 'better'.

I'm sure these amps will come up just fine with a little bit of time
on the variac and I'm very, very reluctant to replace anything.

The owner of these amps is wealthy and I could charge him an amount
that would choke me, but for him, he probably wouldn't think twice
about it. Not that I would do that, but you can imagine the flippant
attitude of some people with too much money.

So what would you do with these amps? I'd vote to bring them up on a
variac and put them back in service if they work good, leave them
unmolested. I think the only way I'd replace any filter caps, is if I
could find the original twist-lock types, I really wouldn't want to
stick in say radial lead types or something that looks totally out of
place.

Any advice? Perhaps somebody has a service 'kit' for updating old Macs
to modern parts?

TIA,
Gilbert

Gilbert Bates wrote in
:

Ok, someone has brought me a pair of MC75's to 'wake up' after 10+
years of being stored in the closet. They are unmolested, completly
stock.

I always take a minimalist approach in servicing vintage gear. Bring
them up slowly on a variac, if they work, great, I'll use them as they
are. I never globally replace parts just because they are old and I
certainly don't get caught up in replacing parts (like signal caps)
with the high priced audiophool stuff just because it is 'better'.

I'm sure these amps will come up just fine with a little bit of time
on the variac and I'm very, very reluctant to replace anything.

The owner of these amps is wealthy and I could charge him an amount
that would choke me, but for him, he probably wouldn't think twice
about it. Not that I would do that, but you can imagine the flippant
attitude of some people with too much money.

So what would you do with these amps? I'd vote to bring them up on a
variac and put them back in service if they work good, leave them
unmolested. I think the only way I'd replace any filter caps, is if I
could find the original twist-lock types, I really wouldn't want to
stick in say radial lead types or something that looks totally out of
place.

Any advice? Perhaps somebody has a service 'kit' for updating old Macs
to modern parts?

TIA,
Gilbert

I would talk to Terry DeWick at www.mcintoshaudio.com before doing
anything.

r


--
Nothing beats the bandwidth of a station wagon filled with DLT tapes.

Gilbert Bates wrote:

Ok, someone has brought me a pair of MC75's to 'wake up' after 10+
years of being stored in the closet. They are unmolested, completly
stock.

I always take a minimalist approach in servicing vintage gear. Bring
them up slowly on a variac, if they work, great, I'll use them as they
are. I never globally replace parts just because they are old and I
certainly don't get caught up in replacing parts (like signal caps)
with the high priced audiophool stuff just because it is 'better'.

I'm sure these amps will come up just fine with a little bit of time
on the variac and I'm very, very reluctant to replace anything.

The owner of these amps is wealthy and I could charge him an amount
that would choke me, but for him, he probably wouldn't think twice
about it. Not that I would do that, but you can imagine the flippant
attitude of some people with too much money.

So what would you do with these amps? I'd vote to bring them up on a
variac and put them back in service if they work good, leave them
unmolested. I think the only way I'd replace any filter caps, is if I
could find the original twist-lock types, I really wouldn't want to
stick in say radial lead types or something that looks totally out of
place.

Any advice? Perhaps somebody has a service 'kit' for updating old Macs
to modern parts?

TIA,
Gilbert

There may not be anything wrong with the amp unless it
was parked for 10 years because of a fault, which has been forgotten
about.

A variac alone isn't the right way imho, because its a low impedance
source of mains power.

Then unless you bring up the voltage you won't get the heaters to work, so
no
emission, and if there are tube rectifiers, not much HT

Place a 100 watt light bulb in series with the mains so that if
something is dead shorted, the light brightens, its resistance increasing
ten fold,
and no damaging current flows.
This way you don't need a variac.

I would check the right value of mains fuse,
turn the amp upside down, take the bottom cover off and observe
the voltages quickly, but not recklessly, to determine plate current and
bias voltage, knowing that with a light bulb in series, you will only get
maybe 75% of heater voltages and B+, but enough t reform the electros if
you are worried.
And and you don't waste a fuse.
After the heaters come on, the light should brighten a bit since the B+
current should draw some current.
After 20 minutes, and if there is no further lamp brightening,
place a larger bulb in and repeat.
After that, and beig sure the mains fuse is correct, plug into the mains
and observe
the bias voltages applied to each grid, its stability, voltages across the
ouput tube
bias resistors, ( which shouldn't be more than a volt ), and the bias
currents in the tubes,
perhaps by measuring the DCR of the cathode windings, and working out what

DC V should be there at each cathode for 50 mA of bias current, or
whatever McIntosh recommended.

The amp should be turned off after a few minutes, allowed to cool, then
back on and
kept under close watch for overheating output tubes.

If all is well after 1/2 an hour, probably it is quite OK.

Patrick Turner.

OK, I agree with the "minimal" approach... nevertheless I'd give a look
under the hood before tunìrning the key, maybe some electros have already
given their soul and spread their goo all around. A quick check for shorts,
together with some vacuum cleaning, is not a bad idea. I recently had
serious troubles because of a f****ing ant who suicided between the anode
pin of a 6GV8 and the socket flange bolt. It seems that carbonised pests
are good current conductors.
As per the replacements, I suppose that modern electros and rectifiers can
only improve this great unit. Suitable values (ie, not too big) should be
easily available.

Ciao

Fabio

"Gilbert Bates" ha scritto nel messaggio
...
Ok, someone has brought me a pair of MC75's to 'wake up' after 10+
years of being stored in the closet. They are unmolested, completly
stock.

I always take a minimalist approach in servicing vintage gear. Bring
them up slowly on a variac, if they work, great, I'll use them as they
are. I never globally replace parts just because they are old and I
certainly don't get caught up in replacing parts (like signal caps)
with the high priced audiophool stuff just because it is 'better'.

I'm sure these amps will come up just fine with a little bit of time
on the variac and I'm very, very reluctant to replace anything.

The owner of these amps is wealthy and I could charge him an amount
that would choke me, but for him, he probably wouldn't think twice
about it. Not that I would do that, but you can imagine the flippant
attitude of some people with too much money.

So what would you do with these amps? I'd vote to bring them up on a
variac and put them back in service if they work good, leave them
unmolested. I think the only way I'd replace any filter caps, is if I
could find the original twist-lock types, I really wouldn't want to
stick in say radial lead types or something that looks totally out of
place.

Any advice? Perhaps somebody has a service 'kit' for updating old Macs
to modern parts?

TIA,
Gilbert