Philips N4450 open reel machine flywheel ringing

[chartz2]

Hi,

I’m working on this very odd machine. Pure Philips logic and manufacturing, so different from my Revoxes!

First, the mechanics.
All three motors work (I had a dead capstan motor which I had to replace), and in spite of so-so belts, especially take-up belts which are slightly loose, I am able to play a tape, in both directions.

My issue is the following: I can hear the flywheels ring slightly at high speed.
I am wondering what causes this. The capstan belt appears to be fine with good grip. I can hear no wow. I’m certain nothing rubs on them, and they were cleaned and oiled with Mobil DTE (sintered brass bearing oil).

Would a slightly off motor pulley alignment (height) do that?

Any ideas?

Cheers,

Jacques

[chartz2]

Anyone please? ?

[Sideband]

Only thing I can think of that might affect it....there are nylon brushes resting gently against the flywheel belt. These are to stop belt vibration. They pick up a lot of dirt and debris over time and can become sticky. Back in the day, we just replaced them. Now that is not an option so cleaning them in spirit will probably have to do.

[chartz2]

Thanks. Disengaging both brushes doesn’t change the ringing sound. I too had thought of that!
Perhaps a bad belt then, slightly slippery. I’ll try another one from MFB Freaks.

[Analogue man]

Although I cannot be sure I think that I used to have a N4450 which had a quietly ringing flywheel (that is to say I definitely had an N4450 can't be sure about the ringing). As the machine spent most of it's time in the garage/workshop I never worried about it if it did.

[ricard]

I won't pretend I really know, but I have noted that on some machines the flywheel belt seems to jump and twist, which might explain the ringing. If the motor pulley and flywheel are not properly aligned I could imagine this happening. And Philips tended to use square section belts (but don't know about the N4450) so they shouldn't twist like round section belts might.

[DMcMahon]

I personally find that it is somewhat difficult to fit square section belts (particularly long thin ones) without them ending up looking twisted along their length.

David

[chartz2]

Not a problem here it seems. I’ll check its position for good measure.
Thanks.

[Reelman]

Originally Posted by ricard

……And Philips tended to use square section belts (but don't know about the N4450) so they shouldn't twist like round section belts might.

The original belt driving the flywheels was definitely square section. I seem to remember a very metallic ringing from the flywheel when I accidentally allowed a wire to touch it.

Peter

[woodchips]

What a problem!

If you lightly tap the flywheel with a small screwdriver does it ring? If not then possibly something else is making the sound.

Whilst running with the ringing audible what happens if you touch your finger on the flywheel? The finger should damp the ringing, does it? If press slightly more heavier can you hear a change in the note of the ringing as the flywheel slows down? Easier to hear if touch and remove the finger so modulating the speed and hence ringing sound.

You replaced the capstan motor, is the replacement the source of the ringing being that it will be running faster than the flywheel. What happens if you remove the capstan belt?

If you have an audio card in the PC and a microphone capture some sound, can then calculate frequency and then does it relate to any speeds of the transport?

[chartz2]

Thanks.
Yes the flywheels ring like bells with a gentle fingernail tap. They always do, even when rotated manually. They both ring slightly. I’ll wait till I get the new belt, although the present one is in good condition.
The motor can’t run faster than the flywheels, it runs them!
The ringing disappears at 9.5 and 4.75 ips.

[Michael Maurice]

Originally Posted by chartz2

The motor can’t run faster than the flywheels, it runs them!
The ringing disappears at 9.5 and 4.75 ips.

Yes it does, the motor runs very fast compared to the flywheel,because the flywheel has a bigger diameter than the motor pulley.

[chartz2]

Are we speaking angular speed or actual speed?
Some machines have sensors at the flywheel, but this it regulated on the motor’s pulley.
So any feedback/servo regulation comes from what?

[Chris55000]

. . .Speed Control is by means of circuitry on plugin unit P207 "SPEED CONTROL UNIT", which gets the tacho pulses from the pickup head K7 mounted at the right–hand side of the capstan motor mounting bracket – problems often occur here because the grub–screws securing the capstan motor pulley work loose, causing the pulley to shift out of position!

. . .Head K7 should not be subject to any wear as there's a fractional distance between the head K7 and the motor pulley – I haven't got a '4450 so I can't say for certain what the clearance should be!

Chris Williams

[chartz2]

Hi,
That was a rhetorical question Chris.
I know how the motor works. The head is very close to the magnetic wheel and regulation works fine.
I got the new belt, but it’s too tight. I’ll leave the old one for now.
However, fast forward and rewind work impeccably. The small DC motors actually have quite enough torque!