[Vintage-Audio] preamp troubleshooting

[JM/CO]

You have, most likely, a bad transistor somewhere in the signal path. The
C-26 has no integrated circuits, making it an easily repairable unit. All of
the transistors that were used to construct this thing are readily
available. Test procedure would be roughly as follows:
1) Obtain schematic / service manual. Available from McIntosh.
2) run a 1KHZ sine wave into both channels of the aux. or tuner inputs.
3) Set your generator level around 0.5 volts.
4) With  an oscilloscope, probe the signal path from input toward output. If
in doubt, reference the good channel.
5) Loss of test signal at some point will indicate the defective stage.
6) Examine the individual parts associated with this stage. Start with the
transistor, itself.

The above assumes that you have cleaned all switches and controls, and done
the "tap and probe test" to search out mechanically intermittent
connections.

Good luck.
Chuck N1LNH
-----Original Message-----
From: Rick Larson <rick@mnbrewers.com>
To: vintage-audio@mailman.qth.net <vintage-audio@mailman.qth.net>
Date: Wednesday, August 21, 2002 11:12 AM
Subject: [Vintage-Audio] preamp troubleshooting


>My McIntosh C26 preamp started cracking, poping, hissing, ...
>in the left channel.  Praying, turning it off and on again, and
>crying didn't seem to help.  Finally, no sound in the one channel.
>
>Earlier in the year, I replaced two power supply caps to remove
>a hum from both channels.
>
>I want to get this working again.  Tools I have include: DVM,
>soldering iron, c26 service and owners manuals.  I can read a
>schematic.
>
>I can get access to a scope and function generator if needed.
>
>The service manual has voltage test points that I can test.
>
>Should I short the inputs first?
>Would simple voltage checks be enough?
>Other ideas?
>
>I could scan in the schematic if someone would like to help
>"if the voltage is not V at this point, part Y might be bad..."
>
>If anyone hasn't seen the inside of a Mac, it is beautiful.
>After working on old tube gear and other 'kits', I'm in awe in the
>detail and neatness of the workmanship (craftmanship?).
>
>Anyone in the Twin Cities want to stop by? :-)
>
>BTW, I swapped in my trusty Hafler 101 to the joy of my 8 year old
>"I love that one Dad" he said.
>
>rick
>--
>mac, marantz, dynaco, bozak, magnapan, fisher, hafler, leach, swtp,
>teac, jbl, plus some new stuff
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