Here is a link to the service manual for the receiver.
It could be a dry joint solder connection on the LF connector
If the connector is OK then the amp section block diagram on p.38 shows that the outputs to all the speakers go through protection relay contacts.
For the Left Front speaker it is relay 1350. It may be that the relay contacts are dirty or are not making proper contact, i.e. no "follow through" when operated, (the operating lever contacts 'push' the make contacts to ensure a good connection). If they are OK you have to follow the audio path back to the power amp etc to see if you can locate the problem.
Hopefully this is of some help.
First disconnect the power from the receiver. Then hold the power on button operated for about 10-15 seconds to dissipate any residual power in the receiver.
Once this is done you can open the receiver to gain access to the boards.
Be careful when you are 'working' on the boards that you don't damage any components.
Here is an image of the relevant section of the main board that you have to look at.
(click on image to enlarge for better viewing)
Find relay RY 1350 on the board, then find contact #4 of the relay.
Place the Ohmmeter test leads between it and the centre conductor of the FL connector (remove the speaker cable plug) and test for low resistance continuity.
If that is OK test for the same between contacts 4 & 5 of the RY1350 when the relay is operated by hand (if you have access to the relay armature push it to operate the contacts. If it is a "sealed" relay it is more difficult to test.
The circuit is on p.68 of the manual