Mcintosh 2505 - strange version

[tranguru]

Hi MAC fans,

I have an MC2505 with a circuit setup which quite different form the schematic and the pictures I see on the net.
Serial number seems to be 00355 marked on the side of the chassis - not at the back.
Even some voltages are different.

any idea what kind of version this could be ?

Thanks
Robin

 

[c_dk]

Post a picture of the driver boards. The number you posted is not the serial number.

I think there are 5 schematics for the mc2505.

 

[tranguru]

Hi

Here are some pictures of the driver boards and the pre amp circuit.

I hope I got the picture insertion right.

Thanks
Robin

 

[c_dk]

It is an early one....most likely 68-69 vintage.

Some work has obviously been done before.

Are you have a problem?

 

[jlovda]

I was just helping a friend recap an MC250 with the same three phenolic circuit boards. The boards were bowed. Quote frankly, it looked cheap. What I would call "sub"-Mcintosh. This was the version with the two TO-66 drivers mounted on the chrome chassis, not on the driver boards.

 

[tranguru]

The amp is dead.

I had -40v on the output (before output transformer) on one side and -26v on the other.

One PWR xtor is completely shorted (-26v side - although I would have expected that on the -40v side). the others all look a bit strange.

The amp has a 117V winding only (even thought the diagram on the transformer case shows 2 ==> 220v possible). I suspect that the previous owner plugged it into the 220v supply directly ==> blew the amp.

I also have a voltages on the driver boards and pre amp board which are way off.

Driver board has + 90v supply (on schematic) but I measure +75v (pt 11) - supply for the diff amp.
The real resistor supplying the diff amp has the same value as on the schematic (R34 = 30k)
Pre amp board supply has a similar differences but it works with no saturation so I am going to leave it alone for the moment. this board is supplied by the 2 capacitors you can see on the picture.

So some fixing is still needed ...
any advice?
I intend to use BDT88/87 to replace the drivers and MJ15003 for the Pwr xtors.

TX
Robin

 

[clinic-audio]

Hello Robin

There is many different MC2505 (maybe 5 or 6 versions) and it is really important to have the same service manual to start the job .
Yes , as c_dk said , this is an early version .
1) can you post a picture of the above view without the grill (of course) ?
2) can you send also another sharp picture of the driver board (the one with the both cement resistors) or giving us the pcb number stamped on components side ?
3) are the fuses out ?
4) YES ! MC2505 110v version only does exist !

 

[tranguru]

Hi Clinic-Audio,

here are some details:
PCB numbers:
128-042 for driver board
128-037 for pre amp board

I added some more pictures

I am not sure what you mean with fuses out: there only a fuse in the mains supply (110v) which is OK.

Merci for your help!
Robin

 

[clinic-audio]

Hello Tranguru !

The pcb numbers are usually in 043-xxx
The ref you give are on solder side . I need the numbers "stamped" on components side

I saw that one emiter resistor has been changed and I hope that the value is correct ....

I remember one MC2505 that somebody tried to repair and on one channel there was:
-2 emiter resistor of 0.33 ohm
- 2 emiter resistor of 0,56 ohm on the other one !
- another one with one 0.33 ohm and one 0.56 ohm wich is normal in my point of view with this old MC2505 only !

Waiting for your answer regarding these number

 

[tranguru]

Hi Clinic-Audio,

there is driver board:
- a hand written number: 158 + something I cannot read but looks like a 121
- printed : MEG 0104 probabbly the ref of supplier of the board

pre amp board: only handwritten: 111

Resistor are as per the schematic 0.33+0.56
on channel has been repaired: 0.33 + 0.5 (i will change that )

Thanks
Robin

 

[clinic-audio]

Hello Robin

I have downloaded all your pictures and will send you my answer as soon as possible

:thmbsp: