Question About Cause of 500 Hz Oscillation in McIntosh MC2..

Archived from groups: rec.audio.tech (More info?)

"The WoodMitch Shop" <mitchell@cwo.com> wrote in
news:c5tXd.14$804.261@news-west.eli.net:

> 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
>
> mitchell@cwo.com
>
>
>

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

Archived from groups: rec.audio.tech (More info?)

>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" <toschwartz@worldnet.att.net> wrote in message
news:422EF4DD.219D3137@worldnet.att.net...
> 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
>>
>> mitchell@cwo.com

Archived from groups: rec.audio.tech (More info?)

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.

Archived from groups: rec.audio.tech (More info?)

R wrote:
> calcerise@hotmail.com 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.