Amp issue, terminal or simple?

Hello everybody.

I have a quick question regarding the health of my amplifier. Firstly its a Denon PMA-350/ii from around 1993, it has outputs for two pairs of speakers 'A and B'. The problem is that the left channel on 'Speakers B' produces little or no sound when playing music. This can be correct, however, by switching 'Speakers B' off and on again several times in a row, or by increasing the volume slightly, then the speaker will suddenly boost in at normal level. I've tried different speakers on this channel and different music sources too, but the problem still persists in the 'B' left channel. There's no issues with any of the other channels or any other area of the amp (that I'm aware of).

I was wondering if anyone had any knowledge about this problem and whether its terminal, or something quite simple and common. I've had the amp for about 4 months now (Bought used of course but in overall great condition for its age), and I've only just noticed the issue in the last few weeks or so. It may have been there prior to purchasing it but I wasn't aware of it. (I only payed £60 and got a 12 month guarantee with it anyway if something can't be fixed).

Thanks to any replies.