Unique problem - one channel silent under different variables

Bought a mint (looking) Cary slp 98 on eBay, connected to two reconditioned Hatachi HMA 7500 mosfet amps, work perfectly for years since reconditioning - biamping NHT 4 towers, bought new. 
When the preamp is turned on, a series of pops through the speakers, then Only the right channel plays. Switched tubes, didn't solve issue. Changed inputs from preamp to left and right amp and immediately both channels worked. Switched off, waited 5 minutes and still both channels working. Came back a few hours later, switched on again (having reversed the inputs to normal) and again same sequence: left channel silent and after switching inputs, both work and remain working until I switch off and come back when the units are cold.
I can understand that that there may be a resistor/capacitor that need to be hot before working, but why would switching input cables from left out to right input of amp and vice versa immediately fix it!
so every time I want to hear music I have to go thro this rigmarole! Why does switching cables fix the problem????
any solution will be greatly appreciated! I'm sure it will end up back at Cary but I wondered  if anyone (Almarg???) has an answer?  Does the popping on switching on, both channels pop, provide a clue?

springbok10 regardless of the problem ALWAYS turn your preamp on and then your amps! Turn on sequence........sources>preamp>amp and turn off sequence is in reverse for the safety of your gear and peace of mind!
Hi Denis,

As you’ve surmised, it sounds like the preamp has a problem that needs to be sorted out via professional troubleshooting. The symptoms you’ve described don’t point to any specific cause, as far as I can tell.

But as Jond indicated it sounds like you are not following the proper turn-on and turn-off sequences (with the power amps being turned on last and off first). Also, it sounds like you are disconnecting and reconnecting the RCA cables that connect the preamp to the amps while everything is powered up, which is definitely bad practice and could conceivably damage your speakers.

That said, I wonder if disconnecting the RCA cables and then re-connecting them WITHOUT switching the channels would have the same effect as what you have observed when you disconnected and re-connected them with the channels reversed. It could be that the channel reversal was not what restored normal operation, but rather a transient of some sort caused by the disconnection and reconnection itself, that might have temporarily zapped the defective preamp circuit back into normal operation.

Or if the problem is temporarily fixed only if the channels are reversed when the cables are disconnected and reconnected, that may just be due to the happenstance of the resulting transient being different when the reconnection for the problematic channel is made to a different amp than previously.

Good luck. Regards,
-- Al
I learned 20 years ago to  turn amp on last, off first. What makes you think I do all these cord exchanges with power on? Did I say that? No, I think you assumed it. With due respect, and I know you are being helpful, if I haven't learned that basic, I shouldn't be  wasting your time posting. Thanks for your input,
Thanks for your thoughts. I have found that if I leave the preamp on constant standby, the issue is resolved. It seems that the unit/tubes/caps/resistors work if kept "warm". Forgive my indignation at the thought of doing anything with power on! To me that's like driving s car with the hand brake on or playing with the innards of a 1500 w amp with the unit on and plugged in :)

It could be that one of the connectors on the cable is a little tighter than the other and one of the RCA jacks on the preamp is a little looser than the other.

When the right combination is used everything is fine, but if the (2) wrong ones are used, you drop a channel.

Maybe try a different set of cables from preamp to amp and see what happens.