Pioneer SX-1280 question

[LocknLoad]

I have an opportunity to buy a Pioneer SX1280, real nice condition cosmetically, sounds great as well - with one exception.

When cold, the unit is turned on, there is a scratchy sound and loss of volume in the left channel only. I don’t think this is as simple as dirt/dust in the control. Volume control seems to have no impact on the sound, nor does the balance knob. Switching the stereo/mono switch to mono corrects the condition. Additionally, it consistently goes away (in stereo) and plays normally when it warms up - say 5-10 minutes or so. No distortion, no scratchy noise, no volume difference between channels. I’ve played it 5 hours straight and it was fine. Turn it off, let it cool, and it’s back until it warms up again.

Before I decide if I’m going to buy this beast I thought I'd ask if anyone thinks this is a small problem, or potentially a larger one. I’ve used contact cleaner as best I can through the front panel, but it seems to make no difference. Generally speaking this was well cared for and not abused.

I've got some experience with vintage receivers but I'm not an audio tech (I've got a Marantz 2285B, 2252B, Pioneer SA-9500 II, and a Luxman R-1050, but this one has me stumped.

If you have any ideas based on my sketchy description, can you let me know what you think?
Thanks!

 

[leemelone]

If the stereo mono switch corrects the problem, I would try working it back and forth a bunch of times. Switches that are rarely used are typically the ones that tend to need cleaning the most. Spraying contact cleaner thru the front does no good because the switches are sealed.

 

[rcs16]

As said above, work all the switches in the signal path, source selectors, tone, tape monitors, adapter, muting, even the balance control.
You can even check bias, by feeling how warm the heatsinks get after 20 mins, should feel the same warmth on both heatsinks. If they are cool they are out of adjustment.