I have a McIntosh MX113 that has been sitting on my shelf for several years. It was not working when I bought it. The seller indicated that it had a PSU problem. Indeed it did. The main transistor was blown and several resisters were either burned or burned completely through (open). Attached is the schematic. I have replaced all the resistors that were burned or destroyed (R209, burned thru; R208, burned and changed value; R222, burned thru). I replaced the original transistor Q202 (originally 2N3738) with a TIP50. The zener, D207 was broken so I replaced it with a 1N5946B (originally 1N982B).
I have disconnected all loads, including the center channel driver (right side of the schematic). I'm getting the following voltages at Q202:
C - 182 (should be 96)
B - 72 (should be 76)
E - 56 (should be 75)
This uses 2 segmented can capacitors and I have not replaced those (yet).
This is obviously a simple PSU but it has me baffled. I'd appreciate any suggestions about what to look at.
Thanks.
I have disconnected all loads, including the center channel driver (right side of the schematic). I'm getting the following voltages at Q202:
C - 182 (should be 96)
B - 72 (should be 76)
E - 56 (should be 75)
This uses 2 segmented can capacitors and I have not replaced those (yet).
This is obviously a simple PSU but it has me baffled. I'd appreciate any suggestions about what to look at.
Thanks.
Get the whole plan.
McIntosh MX113 Stereo Tuner Pre-amplifier Manual | HiFi Engine
182V being twice 96V, something is wrong on the AC side. Check all five leads.
Q202 is blown (again?).
McIntosh MX113 Stereo Tuner Pre-amplifier Manual | HiFi Engine
182V being twice 96V, something is wrong on the AC side. Check all five leads.
Q202 is blown (again?).
Attachments
Thank you for your assistance.
I do have the complete service manual and schematics. Mine does not show the voltages you have indicated in red. For my learning, how did you determine those voltages?
I have 136VAC at the points you indicate should be 94 volts, so you are correct that there is a problem with the AC. Now to determine what it might be.
I'll check the Q202.
I do have the complete service manual and schematics. Mine does not show the voltages you have indicated in red. For my learning, how did you determine those voltages?
I have 136VAC at the points you indicate should be 94 volts, so you are correct that there is a problem with the AC. Now to determine what it might be.
I'll check the Q202.
I'm getting 35VAC at the 2 points which should be 24VAC. I wonder if there's a problem with the center tap?
With the center tap (red/yellow) unconnected from ground, I get about 42VAC between center tap and ground. I assume this is irrelevant. But with center tap disconnected from ground, I still get 135VAC at the points that should be 94.
The factory plan shows conflicting numbers. I hope somebody who remembers this product can jump in.
Q202 Vbe has to be about a Volt, not 20 Volts.
Q202 Vbe has to be about a Volt, not 20 Volts.
Success! It turns out that the picture of the traces in both versions of the service manual on HiFiengine are the same, but do not match the traces on the board in my MX113. As a result, the Q202 was reversed emitter and base. I had to use a different heatsink and connection to the collector of Q202 to get it correct but it works now. And sounds excellent.
I have not seen this occurrence previously - namely traces that don't match the service manual. I've looked for other versions of the service manual with no luck. And this is a fairly early serial number - covered by the service manual I used.
I have not seen this occurrence previously - namely traces that don't match the service manual. I've looked for other versions of the service manual with no luck. And this is a fairly early serial number - covered by the service manual I used.