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McIntosh - Hints and Kinks

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I've already contacted Mc. They haven't any more of the 19kc transformers. And, just to show that Murphy still gets to be in charge, one of the 6 leads broke off *at* the coil, up inside the ferrite, and is 100% inaccessible.
 
MQ101 Repaired

dewickt said:
No real history on the EQ101, a transistor or two and a power supply filter, only had 2 on the bench in 3 years.
Click to expand...

The second transistor, a BC137 had blown, shorting the power supply. I was amazed to see the row of vintage Siemens transistors inside. The replacement for the BC137 is a BC237 which comes in a standard packaging. Some bending of the leads are needed to get it to seat nicely.

Thanks Terry and MRZ80!
 
dewickt said:
Either figure out the wiring, ohm the leads, and wire them back, or order a new one from the factory. You would be surprised what they still have in stock, I just ordered a 162-031 19K coil for a MR73, retail at $6, better than telling the customer his tuner is scrap.
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Ok, I should really be in bed, but your comment about the MR73 got me digging out the schematics. Aside from the collector and base voltages on the amp transistor ahead of the 19kc transformer, the circuit topology looks identical on the MR73, the MX112 and 113, and the MR77 and 78... so I'm wondering if maybe a 162-031 would be a workable substitution for the 162-055. Heck, at $6 I might as well give it a shot, huh?
 
Ahha yes, the sign of a learning technician, make do with what is available, check the pin out of the new can, the numbering and layout is a bit confusing.
 
dewickt said:
Ahha yes, the sign of a learning technician, make do with what is available, check the pin out of the new can, the numbering and layout is a bit confusing.
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Hot soldering iron and a can do make do attitude. That's me! :)

And doggone it, Mom Fantastick'ed ALL the lettering off the 275 :( . At least I don't have to worry about destroying it cleaning it up.
 
Terry
Have you ever repaired a MC7205? I just picked one up cheap that has the entire left side out. I'm assuming that one channel has shorted and the power guard protects the entire heatsink (2 channels). This was purchased from an ex-McIntosh rep. He indicated that heard a pop, then smoke, which sounds like a resistor/transistor shorted. This happened with less than one watt output while listening to a TV source. My question is, does McIntosh stamp their part numbers on the transistors or will these be easy to source elsewhere? I would tend to believe McIntosh sources these transistors from motorola. Or do you suggest using McIntosh replacement parts? I do have the equipment to match parts but may not be worth the effort if its just a couple transistors. Your thoughts?
 
My first suggestion is to get the service manual, the one for the 7205 gives a fairly complete parts list with the generic part numbers included. I buy most of my parts from the McIntosh parts department, saves time on finding a source then coming up with a minimum order, one stop shopping.
 
dshoaf said:
There was a discussion some time back regarding MX-110s and I/F cans with where they were rebuilt to eliminate a small silver-coated cap in the can's bottom. Might search with those elements.....I've got an MX-110 with the symptoms that will need attention at some point.

Cheers,

David
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David,

Here's one http://www.audiokarma.org/forums/showthread.php?t=67711&highlight=windings

And another http://www.audiokarma.org/forums/showthread.php?t=153556&highlight=windings

Murray
 
Smoked MC7205

This is my first newer McIntosh amp I've repaired and I couldn't believe the number of parts damaged by a shorted output. The circuit board was also burnt to a crisp underneath R8 resistor (flameproof, yeah, right). I have probably repaired a half a dozen McIntosh amps over the years but have never seen such a catastrophic failure as this in an MC series amp. I have a poor quality schematic found on the berners site so troubleshooting is difficult except for the obvious. I requested a schematic from McIntosh but not sure if these are free or not.
These are the known parts shorted or out of tolerence due to the short.
I am replacing all the outputs on the one channel as preventive maintenance even though one side tests OK. What concerns me is the board is double sided and some of the output transistor connections are made by the solder only since the pins were not turned thru the circuit board. This is probably why the output shorted as some of the solder did not fully penetrate the holes and appear to be a "cold" joint. Will probably order all of these from McIntosh unless they charge me $10 a piece for outputs.

1 6644600 47MF 35V C1
2 7004700 DIODE D4,D5
2 13222300 MPS4124 Q1
1 13222400 MPS4126 Q2
1 13226100 A1145 Q7
1 13226200 C2705 Q6
1 13704200 47 OHM 5% 1/4W R12
1 13706500 430 OHM 5% 1/4W R7
1 13707000 680 OHM 5% 1/4W R2
2 13708000 1.8K OHM 5% 1/4W R6,11
1 14103300 220 OHM 5% 1/4W R8
3 NOT LISTED 2SA1302 Q8,Q26,Q10
3 NOT LISTED 2SC3201 Q9,Q11,19
2 13222300 MPS4124 Q6,Q44
2 13222400 MPS4126 Q1,Q43
1 13225000 3068 Q7
1 13225100 1238 Q3
1 13225400 C2302 Q5
1 13225500 A1016 Q2
1 13226100 A1145 Q18
1 13226200 C2705 Q17

Edit:

MCM Electronics is a very good source and at 30% of the cost.
 
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Hello Terry and everyone,

About the MA 230:

If I were to tube roll some EH7591s in a matched quad, instead of the currrent Westinghouse/RCAs, I understand that the MA 230 would have to be rebiased to reflect the new EHs.

Is this something I can do myself, and if so, how and with what? Thanks in advance.

By the way this thread is remarkable. You guys are engaged in something really valuable here. Thanks!

:thmbsp:
 
Owners manual gives procedure, each tube has a test point and associated pot for adjustment. The manual calls for each tube to be set for .7 Volt at the test point, it will take several passes to get them close, I usually set them for .6 a bit on the light side. A small screwdriver from the top will access the adjustment, start with the pots CCW for minimum voltage for safety, a 10 minute warmup volume set to 0.
 
Shipping Carton Cleanup and re-use

Many of us are reusing our shipping cartons many times, some even modified to hold different models, I have been known to get out my 1" wood drill (blade type) and drill new holes for the shipping feet.
Another thought is what about cleaning up your box with 10 different shipping labels and assorted layers of tape - you can remove old labels and tape from a box with a heat gun or hair dryer, heat makes the adhesive soft and release it’s grip, the box may be well used, but now it is clean and ready to go another trip.
 
Terry thank you for your directions and advice. imho this thread makes many valuable contributions to member not just now but into the future as an archive of knowledge and experience not easily passed down or accessible otherwise.
 
New McIntosh glass vs the old glass

Hello Terry:

I installed new glass on my 2255 amp and mr 77 tuner several years ago. The new glass from McIntosh doesn't diffuse the light evenly like the old glass did. To see if I didn't do it right I put the old glass back on and in spite of the all bubbles in the old glass the light was perfectly even. Is this an art or a science? I have noticed new Mac gear in the showrooms that don't have even lighting across the board. The back of the new glass doesn't appear to be the same as the old glass.

Gene
 
Might try some games with the light diffuser, look at how the pattern of white paint is on the backside to make it glow behind the lettering areas, possibly more white. A material you can experiment with that is easily removable is White Out.
 
MC7205 Followup

Terry,

I replaced all the smoked parts. But, after firing back up with the variac, the outputs on both sides of the left channel go into a thermal runaway. Its the input board but I dont understand why it happens on both channels. McIntosh forgot to send the service manual so its frustrating trying to read a blurred copy I found on Berners.ca The bias is not high because the variac reads less than 1/2 amp. I'm afraid to leave this on too long for fear of smoking the outputs again.
Do you have any experience with this amp?
 
Minor experience, turn bias completely off, the last part of the turn should do nothing, this will let you know if the bias circuits are working correctly. Last one I worked on had DC offset, was one of the 6 lead pack transistors, check them all with a Simpson.
 
I have suspected some of those but don't have a data sheet, do you know the part number, 1237 does not show anything.
 
Terry

You were right, it was the 6 lead pack transistor. I found out the real part number from McIntosh and was able to test the device. This 6 pack lead device is a 2SC3064 differential amp and is no longer available from McIntosh. I'm about to match it up with some standard transistor devices but am concerned since these matched pairs. Don't know why McIntosh cant come up with a solution.

Edit:

DO NOT ASSUME the ribbon cables can be swapped from Front to Surround on the preamp board of a MC7205. The pos 59v pin is swapped with the ground and could smoke a channel, had I not used a variac, this channel would of been smoked. THE SCHEMATIC IS IMPORTANT on this amp, follow the cable connections carefully!!!
 
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