Pioneer RT701 noise in rt channel on low speed only!

So this one has me stumped.

My Pioneer RT701 just this afternoon started making very odd noises through the rt channel, sort of a sound VERY similar to when you clean your stylus with a brush and your input is on for the phono. It is very irregular, it can go a couple minutes quiet, then a few seconds in varying intensity before quiet again.

Now as I found by accident this is ONLY on 3.74ips. When I push the button to engage high speed 7.5ips it goes deathly silent as it always has been.

Even stranger it makes the noise on 3.75 even with no tape playing, just sitting powered on.
You can see the vu meter move when the noise occurs.

How in the world can this NOT happen when I switch to 7.5ips?

Surely that should ONLY change the speed of the Capstan?

I took the cover off, checked all connections, blasted any pots I could see with deoxit but no change.

Anybody have any ideas?

6db47fb2 f8db 415d a4f7 49a7b6ed12b0Ag insider logo xs@2xuberwaltz
Could be noisy motor controller. Been a very long time but isn't the capstan spinning selected speed whether the pinch roller is engaged or not? So if the clock or whatever circuit controls speed has a fault causing it to generate this noise it would do it whenever its running, whether tape is moving or not. Just my guess. Open it up, hope its something easy like cleaning dirty connections. It is a vintage deck after all. Nice one though. Wish I'd never sold my Pioneer RTR.

With case off I can see Capstan change speed when I push the 7.5ips button and it goes deathly silent.

Thinking of what you said it does sound like the speed circuit may have developed a fault, bad resistor or similar. 

Still most odd.

Unfortunately I have already checked all connections I can see or reach and sprayed what pots I could see with deoxit.

I fear it is something a little more involved.

And there is NOBODY in this area who works on tape decks of any description!

Good job got a lot of 7.5ips tapes.....
You can try using component testing freezer spray to help locate a faulty component.
Joined as that is where the tape deck experts hopefully reside and will see what words of wisdom arise there.