Hi -- Have any of you other R.A.T. techs encountered a 500 Hz oscillation in
a McIntosh MC240 amp? The sound comes from both speakers in both stereo and
twin amp configurations.

I hadn't used this amp for a few years, so I hooked it to the test speakers
and put it on the variac to bring it up. All was fine up to about 100
volts -- dead quiet. Above 100 volts there is a distinct oscillation heard
from both speakers that is right about 500 Hz in frequency.

Please e-mail me with any suggestions about where to start looking for the
cause, or post if this is of general interest.

Thanks very much -- Mitch

"The WoodMitch Shop" wrote in
:

Hi -- Have any of you other R.A.T. techs encountered a 500 Hz
oscillation in a McIntosh MC240 amp? The sound comes from both speakers
in both stereo and twin amp configurations.

I hadn't used this amp for a few years, so I hooked it to the test
speakers and put it on the variac to bring it up. All was fine up to
about 100 volts -- dead quiet. Above 100 volts there is a distinct
oscillation heard from both speakers that is right about 500 Hz in
frequency.

Please e-mail me with any suggestions about where to start looking for
the cause, or post if this is of general interest.

Thanks very much -- Mitch






There are a couple of caps that will do so rather nasty things when they
get leaky. Most likly they one that is bad in the driver stage if I
recall correctly. I would guess that caps in the PS have seen better days
as well.

If it were me, I would pack it up and send it to Terry DeWick. By the
time you get done messing with it, you could have had it back and in
perfect shape ready for another 30+ years of use. Plus you would have
guaranteed performance with RTA results printed for you and a warrantee.

Terry hangs out on the audiokarma.com McIntosh forum. He is the only
person I trust with my vintage McIntosh componenets.

Follow his instruction to the letter when packing that gem.


r

Mitch,

I've don't know of specific problems that cause that high a frequency
oscillation, but I would start by looking at all of the power supplies,
ideally with a scope, for excessive ripple. The voltage doubler caps
can be a problem, and IF they need replacement, always replace both of
them at the same time. Problems that are the same in both channels
usually, though not always, mean a common power supply is the issue.


I'd also consider looking for corrosion on the tube pins, especially
the small 9 pin tubes. I do NOT suggest any sprays or treatments on the
pins. I'd use some fine (240-400) grit sand-paper on the tube pins
(Don't be too aggressive, you want to remove corrosion, not metal.) then
brush them off to remove any residual grit. Next, plug them in and out
of the sockets a few times to clean up the sockets.

WARNINGS: 1) be careful not to bend the pins out of alignment - use a
tube pin straightener if you've got one. 2) this method of pin cleaning
may lead to a long, boring thread on this news group.

Regards,
Tim Schwartz
Bristol Electronics

The WoodMitch Shop wrote:

Hi -- Have any of you other R.A.T. techs encountered a 500 Hz oscillation in
a McIntosh MC240 amp? The sound comes from both speakers in both stereo and
twin amp configurations.

I hadn't used this amp for a few years, so I hooked it to the test speakers
and put it on the variac to bring it up. All was fine up to about 100
volts -- dead quiet. Above 100 volts there is a distinct oscillation heard
from both speakers that is right about 500 Hz in frequency.

Please e-mail me with any suggestions about where to start looking for the
cause, or post if this is of general interest.

Thanks very much -- Mitch

I do NOT suggest any sprays or treatments on the
pins. I'd use some fine (240-400) grit sand-paper on the tube pins

Tim - Have you tried one of these fiberglass brushes for removing corrosion?
They're in the form of a retractable mechanical pencil type of a thing, may
be called a "nick-sander" or a "prep-pen". Sold at auto parts stores, less
than about 4.00. Will last for years, removes tarnish like you wouldn't
believe, doesn't damage the metal. Been using them on vcr mode switches ,
relay contacts, and the like for years. Most of the time you won't even need
any sandpaper or emery cloth. I use them with De-Oxit, but that's not a
requirement.

Mark Z.



Tim Schwartz" wrote in message
...
Mitch,

I've don't know of specific problems that cause that high a frequency
oscillation, but I would start by looking at all of the power supplies,
ideally with a scope, for excessive ripple. The voltage doubler caps
can be a problem, and IF they need replacement, always replace both of
them at the same time. Problems that are the same in both channels
usually, though not always, mean a common power supply is the issue.


I'd also consider looking for corrosion on the tube pins, especially
the small 9 pin tubes. I do NOT suggest any sprays or treatments on the
pins. I'd use some fine (240-400) grit sand-paper on the tube pins
(Don't be too aggressive, you want to remove corrosion, not metal.) then
brush them off to remove any residual grit. Next, plug them in and out
of the sockets a few times to clean up the sockets.

WARNINGS: 1) be careful not to bend the pins out of alignment - use a
tube pin straightener if you've got one. 2) this method of pin cleaning
may lead to a long, boring thread on this news group.

Regards,
Tim Schwartz
Bristol Electronics

The WoodMitch Shop wrote:

Hi -- Have any of you other R.A.T. techs encountered a 500 Hz oscillation
in
a McIntosh MC240 amp? The sound comes from both speakers in both stereo
and
twin amp configurations.

I hadn't used this amp for a few years, so I hooked it to the test
speakers
and put it on the variac to bring it up. All was fine up to about 100
volts -- dead quiet. Above 100 volts there is a distinct oscillation
heard
from both speakers that is right about 500 Hz in frequency.

Please e-mail me with any suggestions about where to start looking for
the
cause, or post if this is of general interest.

Thanks very much -- Mitch

It's not about efficiency it's about the learning experience. Paying
someone else to service your tube amps is like paying someone else to
service your wife.

The first thing I would do is a visual inspection for bad electrolytic
caps. Usually when they fail they will expand. Then either the blue
plastic coating that is spiraled around the body of the cap will break
and start to come apart, or the brown bakelite end of the positive side
of the cap will begin to "pooch out". Both of these are quite
noticeable.

wrote in news:1110500104.755367.51350
@z14g2000cwz.googlegroups.com:

It's not about efficiency it's about the learning experience. Paying
someone else to service your tube amps is like paying someone else to
service your wife.



You obviously have not met my brother in law. He is a brilliant guy when
it comes to law, history, and the like, but has no clue about which end of
a screwdriver to grab. He has no business in a tube or ss device of any
sort. He can change flashlight batteries in a pinch but it takes him about
5 minutes to do it, not the seconds it takes most people. He is the reason
AOL as well as many other overly simplistic things exist today.

r


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

R wrote:
wrote in news:1110500104.755367.51350
@z14g2000cwz.googlegroups.com:

It's not about efficiency it's about the learning experience.
Paying
someone else to service your tube amps is like paying someone else
to
service your wife.



You obviously have not met my brother in law. He is a brilliant guy
when
it comes to law, history, and the like, but has no clue about which
end of
a screwdriver to grab. He has no business in a tube or ss device of
any
sort. He can change flashlight batteries in a pinch but it takes him
about
5 minutes to do it, not the seconds it takes most people. He is the
reason
AOL as well as many other overly simplistic things exist today.


Yes, there are such people and while it's not my goal to make their
lives any tougher, such people should buy modern pro amps like Arny
says, or better yet, a Bose package and have it professionally
installed.

I bought my last Corvair from such a person. Bought it on a whim, ran
it out of oil. I put in another engine and swapped the PG out for a
manual, drove it for a year-then he sees it on the street and wants to
buy it back from me. Told him no, sold it to a woman who ran it into a
tree and wound up in the burn ward. Can't win sometimes.