Help, wierd remote control behaviour on McIntosh MX121 receiver.

[CHexclaim]

Hello and as usual thank you for helping other people.

I have on the bench a McIntosh MX121 that came in saying "the remote control is not working". Indeed, testing it aparently showed that except that it works well only when the unit is in standby and power on is pressed in the remote. The unit powers on ok but then, it responds to no other command from the remote.

So, issues with the electronics on the IR receiver is unlikely. Maybe somthing regarding a difference between the power supply to the IR receiver between standby and operation.

Anyhow, the unit inside is very complex, it is a A/V control center. So, having the service manual or schematic would make the job much straight forward and professional. I haven't been able to find this information yet. Can anyone help with the service manual, shcematic or tips on what can be wrong.

Much appreciated, Carlos.

Indeed, testing it

[David_AVD]

It appears to have a SMPS for the standby (backup) power supply and a linear main power supply.

It could be that the capacitors in the SMPS are on the way out and the increased current draw once it's powered up is causing more noise on the rail that supplies the micro / IR section.

[CHexclaim]

Thank you for your reply

So the SMPS would power all the uP for display, IR, switching, etc, and for the signal electronics there would be a linear PS?

There is a main big transformer and caps what look like linear regulators and there is an SMPS. I'm attaching pictures.

There is also, at the back of the unit, a board with i/Os for control. And, the connector has a label IR in.

CH

[David_AVD]

Normally the standby power supply runs at least one of the micro-controllers and the IR sensor so it can switch the main transformer on from standby.

The display is often partly powered from the standby and partly from the main power supply.

The large transformer usually powers all of the analog circuitry, power amplifiers and other stuff like the DSP.

[wasedadoc]

There is also, at the back of the unit, a board with i/Os for control. And, the connector has a label IR in.

Some equipment has provision for plugging in an external IR receiver £dongle" so that the main unit can be controlled in a cabinet with closed opaque doors or similar situations where only the dongle can see the remote control. See page 6 of the owner's manual available at https://www.audioservicemanuals.com/m/mcintosh/mcintosh-mx/51107-mcintosh-mx-121-owners-manual