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DMcMahon · 27th Jul 2020, 7:02 pm

The main function of the Hum Flap is to shield the Playback head from electrical interference such as hum during Playback.

I have never seen a pad in any Hum Flap across many machines, this maybe is because the metal flap is not supposed to contact the tape itself.

I have found a third contact point on the back of the tape. There is a black plastic block that the little copper sprung arm (that pushes against the Hum Flap) is mounted to.

There are 2 raised vertical curved ridges on the block roughly inline with the Record head, when Play is selected, the ridges push the tape forward, so these ridges will be making contact with the back of the tape when Playing. As the ridges are plastic and curved would think that normally they should not damage the back coating.

By manually pushing forward all the way, the metal Hum Flap I can see that both sides of its vertical edges make contact with the tape and maybe push the tape forward a little bit. These edges are angled out at 45 degrees so the tape contact is maybe across a sharp corner edge at that time. However when Play is selected the Pinch Wheel pushes the tape forward onto the capstan shaft and possibly moves the tape enough to move it away forward of the Hum Flap right edge. Similarly the plastic ridges may also move the tape forward enough to clear the left hand Hum Flap edge, it is nearly impossible to actually see, so it is possible that during Playback that the tape does not make contact with the Hum Flap.

A test I am going to try is to find a short section of the tape that has no line on the back then run that section through in Play one or more times to see if a line starts to be made on the back of the tape. If line can be created then repeat the test with the Hum Flap removed. If no line with Hum Flap removed then that would suggest the the Hum Flap is contacting the tape and rubbing off some of the back coating. If line still being created with no Hum Flap then that could suggest that the plastic ridges are making the line.

We will see later.

David

27th Jul 2020, 7:02 pm	#7
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,603	Re: Strange Line on Back of Tape The main function of the Hum Flap is to shield the Playback head from electrical interference such as hum during Playback. I have never seen a pad in any Hum Flap across many machines, this maybe is because the metal flap is not supposed to contact the tape itself. I have found a third contact point on the back of the tape. There is a black plastic block that the little copper sprung arm (that pushes against the Hum Flap) is mounted to. There are 2 raised vertical curved ridges on the block roughly inline with the Record head, when Play is selected, the ridges push the tape forward, so these ridges will be making contact with the back of the tape when Playing. As the ridges are plastic and curved would think that normally they should not damage the back coating. By manually pushing forward all the way, the metal Hum Flap I can see that both sides of its vertical edges make contact with the tape and maybe push the tape forward a little bit. These edges are angled out at 45 degrees so the tape contact is maybe across a sharp corner edge at that time. However when Play is selected the Pinch Wheel pushes the tape forward onto the capstan shaft and possibly moves the tape enough to move it away forward of the Hum Flap right edge. Similarly the plastic ridges may also move the tape forward enough to clear the left hand Hum Flap edge, it is nearly impossible to actually see, so it is possible that during Playback that the tape does not make contact with the Hum Flap. A test I am going to try is to find a short section of the tape that has no line on the back then run that section through in Play one or more times to see if a line starts to be made on the back of the tape. If line can be created then repeat the test with the Hum Flap removed. If no line with Hum Flap removed then that would suggest the the Hum Flap is contacting the tape and rubbing off some of the back coating. If line still being created with no Hum Flap then that could suggest that the plastic ridges are making the line. We will see later. David