I'd like to share the information I've found out about the C32 Auto-On circuit 045-121 with external relay used on the early serial number C32's. When I was working on one I found the information scarce and even McIntosh had no helpful information. The one I worked on had no relay and the power was run directly through the front panel switch, so I wanted to fix that.
First, here is information about the original relay (pictures attached).
- Brand is Potter & Brumfield
- PN is KR-3862
- Coil Resistance is 10k ohms
- Coil Voltage rating is 110V DC
- Under test with an original 045-121, I measured 118V coil voltage and 11mA current through the relay coil.
Both McIntosh and Audio Classics say that this relay can no longer be sourced. I also could not find a similar relay anywhere.
Here is an alternative I came up with and tested on the C32 before installing an OEM relay I was lucky enough to find on eBay. The alternative setup worked with no issues in my short term testing.
- Use Omron relay G2R-1A-T DC48.
- This is a 48V DC relay with 4.17k coil resistance.
- This isn't widely available either, but there might be similar chassis mount 48V DC relays if you can't find this one.
- You will need to replace resistor R4003 (680 ohm 1W) on the Auto-On pcb with a 2.7K 2W resistor so that the voltage on the relay is 48V.
- There is room to mount the relay near where the original was, but you will need to find a method to mount it since the holes don't line up with the original holes.
I just wanted to put all this out there for the archives, but welcome any comments or questions.
First, here is information about the original relay (pictures attached).
- Brand is Potter & Brumfield
- PN is KR-3862
- Coil Resistance is 10k ohms
- Coil Voltage rating is 110V DC
- Under test with an original 045-121, I measured 118V coil voltage and 11mA current through the relay coil.
Both McIntosh and Audio Classics say that this relay can no longer be sourced. I also could not find a similar relay anywhere.
Here is an alternative I came up with and tested on the C32 before installing an OEM relay I was lucky enough to find on eBay. The alternative setup worked with no issues in my short term testing.
- Use Omron relay G2R-1A-T DC48.
- This is a 48V DC relay with 4.17k coil resistance.
- This isn't widely available either, but there might be similar chassis mount 48V DC relays if you can't find this one.
- You will need to replace resistor R4003 (680 ohm 1W) on the Auto-On pcb with a 2.7K 2W resistor so that the voltage on the relay is 48V.
- There is room to mount the relay near where the original was, but you will need to find a method to mount it since the holes don't line up with the original holes.
I just wanted to put all this out there for the archives, but welcome any comments or questions.
