McIntosk MC 275 SS

[virginio]

can the measures be made without load? that is, without diffuser necks ... thank you

 

[virginio]

If you have a short interruption in mains power, long enough that the reservoir caps can deplete but the thermistor is still hot, so it cannot limit inrush current when power comes back on. Such a scenario often takes out the rectifier bridge or one of the diodes if it is built from 4 diodes. Disconnect from mains, wait until caps are depleted ( measure voltage across caps to make shure they are at zero ) then test each of the 4 rectifier diodes for shorts with ohm meter. If a diode reads near zero ohm in both directions, it is gone ... in which case I would replace all 4. Type like 1N5408 should be good enough.

the 220vac mains failure was very rapid. The amp currently turns on (all filaments are on) but it feels bad and a loud background noise ...

 

[ahankinson]

You are probably better off taking this to an experienced tech. I know this goes against "DIY" but you seem to be without a schematic, and not to be able to perform some basic tests to troubleshoot.

That, along with the very high expense of replacing McIntosh transformers if they happen to burn out while you are troubleshooting, means that you will probably save money in the long run by taking it to a tech.

 

[rayma]

Definitely agree. A decent tech could fix this very easily and safely. This is not a good unit for learning on.

 

[virginio]

Hi I have identified the fault and solved it. I also thank everyone who advised me to take the amp to service which is obvious. Since the spirit of these forums is to help each other as far as possible (limit in which my request fell given the simplicity of the circuit and the type of fault), not having found the diagram, I proceeded to draw the part of the circuit affected by the fault ( power supply) and I am attaching it with the components affected by the fault highlighted. I hope it can be a car in the future .... Thanks.

 

[rayma]

Then two resistors were also bad?

 

[EM EM EL]

I have a service manual for the Gordon J. Gow edition from the mid-Naughties that is probably like your Stainless Steel chassis version. Miniature valves are 12AX7s and 12AZ7s?


For safety's sake, first test for the -150 VDC supplying the 12AZ7 cathode followers (4th and 7th from the left facing as in your middle picture. Each is fed from two 18K Ohm resistors in series; measure at the far end from the valves' cathodes (pins 3 and 8).


If that's OK, you can start looking for some new B+ capacitors. Or, for rigour, put a scope to the output of the rectifier bridge, then order the capacitors. I can highly recommend Hayseed Hamfest, but Mc might be willing to sell you some.


All good fortune,
Chris

Hi Chris, can i ask for a copy of the schematic diagram for McIntosh275 Gordon Gow Edition of you have pls? Thanks in advance. My email is marlon.luna@ymail.com