Do I own the 40th MC275 ever made ?

old mc275

New Member
Dear Mcintosh friends,

This is my first post and I need some advice on an original MC275 I recently bought please.
If Clint Eastwood saw me there in that second hand store and would have asked me “Ask yourself, punk, is this your lucky day” my answer would have been “Yes, Yes, Yes, Mr. Harry !!! “ :rockon:
The chance of finding this amp here in Holland is kind of slim.
The chance to buy it for the price of a dinner for two even slimmer.
The amp was in a very average cosmetic state, lots of dust, dead insects etc. , but after some brushing ,vacuuming and mild cleaning turned out to be quite acceptable.
While carefully cleaning up the chrome I stumbled upon the top serial number which turned out to be 27510041 !!!!!! (see pic 3 and 5)
After that I stopped, according to some of the articles on this forum the first MC275 ever built had sn 27510001.
Did that mean I had the 40th MC275 ever made in front of me ? ( oops 41th :stupid:thank you Rodeodave )
This was the time I decided to join this forum and ask for some advice because it is very hard to get some reliable information here in the Netherlands.

The amp has been to the clinic for the last time in 1973 according to the stickers, the insides look fine and original but has not seen some voltage for a long time, so until now I only did visual inspections read a lot of articles on this forum and used a multimeter on a powerless amp.
These are my first findings :

As far as I can tell all the transformers are ok.

3 out of the 4 220 ohm 5W resistors are blown.

The thermistor is gone.

The rectifier diodes seem ok.

L2 has been very hot but still conducts.

A KT88 socket has once been replaced

No cracked or damaged capacitors except maybe 2 out of the 4 big power supply caps look suspect, paper parts and brownish stuff visible through the holes.

One of the two 330pf caps in the feedback circuit, C2 C13, the ones that look like resistors, has a very matte surface, it looks like it has been hot.

R65 according to the schematics should be 500 ohm 5W but is a 1/2W 1K.

C31 on V1-A and C32 on V1-B both 1.8pf are missing, looks they were never there.

So far no severe problems as far as I can tell.
I have got a step down transformer 220-110V 9A and a variac at home.

My questions are these :

I know that these amps are collectable, does this low serial number add value ?
Should I leave it alone and sell it to a rich asian guy and retire? :banana:
If this is not the case how should I proceed, even if I replace all of the dead parts, all the kt88’s are suspect and I am not even thinking of firing the amp up with the variac with these tubes in place so I can do voltage readings.

I hope some members of this forum can help me out on this.
Thanks in advance.

Robert.

P.S. English is not my native language so please forgive any wrongdoings.
 

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I think that could be the 40th MC275. This is what my serial number list shows:

10,001 May 1961-10,147 June 1961
100E1 June 1961-572E9 July 1973

That's a very cool piece of McIntosh history you have there. I'm not sure what an early serial number like this does to the value but if it was me I would fix it up and enjoy it. It would be a great story to tell your friends while listening to beautiful music being played on a 40 year old piece of history.
 
Very Cool!

You've got yourself a nice piece of gear for what could turn out to be a fantastic restoration project.

Congrats!
 
Hi Robert, congrats, a really nice find! If you have the skills to restore the amp I would definitely recommend to keep it!

That one tube with the white getter material has definitely got a bad vacuum. Looks like a Genalex though, or GEC. Check ebay, you'll be surprised what people pay, even for broken ones, they seem to be collectibles...
The other tubes might be fine, shiny getter and all...who knows.

I'd like to know what type of 12AX7 is under the metal shield, might be a nice Telefunken ECC83 with smooth plates...?

About the serial number, so if 1 is the first, 2 the second, 10 the tenth, shouldn't 41 be the 41st then?

Be sure to check out my thread on restoring to MC75 monoblocks here: http://www.audiokarma.org/forums/showthread.php?t=289952, I have collected lots of infos and links, also for what parts to choose and where to get them ect. The MC275 and MC75 almost the same inside. For schematics http://www.berners.ch/McIntosh/de/Frame_Matrix.htm and also AK are a great source.

As for how to proceed, do a lot of reading here in the McIntosh Forum, many things have been discussed before and many people have done outstanding projects, like AK member sloober rebuilding an MC30 here: http://www.audiokarma.org/forums/showthread.php?t=255523 for example.

Have fun!

Oh, and post more pics! Bedankt!
 
I meant to say earlier, that I think it's cool that the amp still has the McIntosh Clinic stickers on it. That alone is a piece of McIntosh audio history.
 
Wow.. what a find!!

I'd restore it and run it. Those babies are worth 1000s! If you bought it for peanuts, then you'll have a healthy restore budget. Terry has done similar amps that have gone through fires or floods, and from what I can tell in the photos, they look almost new.


-Gregory
 
You will probably get a quick response if you e-mail Mcintosh customer service and ask them to verify serial number production orders, or if we wait just a bit there's a chance that an employee or two will happen along here... :)

Hopefully that amp will be brought back to its original glory shortly! Congrats!
 
Thank for all your kind responses so far,

Of course Rodeodave you are correct in the fact that I may own the 41st MC275 ever made, math and English is multitasking and women are better at that.:D

The bad KT88 is a Genalex, but is has been broken straight of the base and was badly arced.
I wonder if it still is a collectable.

The reason I believe the other output tubes are also bad is the fact that 3 out of the 4 220 ohm 5W resistors are dead.

There is no Telefunken ECC83 smooth plate in V1 but a Chinese one.
I searched my old tube stock and only found a used non compatible ECC82 smooth plate.:tears:

Thank you for pointing out the thread on the restoring of your MC75’s.

I have been using “berners” schematics before I posted my first post.

I agree with Chuckworb that it is cool that the clinic stickers are still on it, I can post some close-ups, if there is interest, and I will never ever remove them .

If you guys want more pictures please specify what parts of the amp you are interested in, because the size of the pictures on the forum is limited.

I am still puzzled by some tech things I mentioned in my first post :

R65 according to the berners schematics should be 500 ohm 5W but in this amp it is a 1/2W 1K.

And the fact that the C31 on V1-A and C32 on V1-B both 1.8pf are missing, looks they were never there.

Are these some later modifications or “pre berner”.

Thanks,

Robert.
 
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Hi Robert,

Attached you'll find a service manual for serial numbers 275-10001 to 275-10147. This version doesn't have the small 1.8pF capacitors yet. R65 is listed as 500R 5W. Does the 1k resistor in you unit look stock or was it replaced?
What I found to be very useful are the resistance charts for the unpowered unit, just make sure you follow the advice to short out the main capacitors.

Hm, what pictures would I like to see...well, a shot where one can see the whole internals, a closeup of the coupling-stage capacitors, and of course pics of the fully restored unit! :D
 

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Common failure points besides the 220 ohm 5W resistors that will open if a tube arcs.
R16, 17, 18, 24, 27, check these for correct value, can be done in circuit.
C6 and C7 should be replaced if they are bumblebees (.047 striped).
 
Thank you Terry and David,

I took your advice and did some more measurements.

R16-18K = 17,6K R41 = 17,5K

R17-27K = 25,5K R42 = 25,4K

R18-30K = 29,3K R43 = 29,7K
Seems OK

R24-12K = 10,88K
R27-12K = 11,34K
They should be matched 1%
Not OK

R49-12K = 10,8K
R52-12K = 10,83K
Seems OK

R65 looks like stock, please have a look at the pictures.
 

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Lucky again,

I’ve found 10 NOS twistlock caps with the wrong values but almost the exact size of the originals, only 8mm shorter.

Ideal for restuffing.

Managed to open one without cutting it open, removed the innards and closed it again:D.

Two axial JJ caps 1x 47uF 500v and 1x 100uF 500v will fit in nicely.

Guess I will order 4 of each, or is there a strong reason to use the original 30uF value ?
 

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Just ordered the caps to start working on the restuffing of the NOS twistlocks I bought.

4 x JJ 30µF, 500V Axial, found the original value at last.

4x JJ 100µF, 500V Axial

5 x F&T 15 µF 450 V Axial to replace the 12µ 250v electrolytics C10, C11, C20, C21 and C28

2 x F&T 100 µF 100 V Axial to replace the 100 µF 3v electrolytics C3 and C14.

I will try to take some pictures of the restuffing process and attach them later
 
This is a "better late than never" update.

Thanks to all the good advice and information on this forum I finished the restoration some 6 years ago.
I have been enjoying my MC275 ever since and never had any problems with it !
Thanks again.

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