When tape won't erase....and why?

[hammerofhathor]

So I've read that alot of people erase tape for future use by using the machine that tape was recorded on. I've got a bunch of one pass tape that I've used, good RMGI 911 that I'd like to use again. Until I get the bulk eraser I thought I'd try to erase some with the deck I made the recordings with.

So, I recorded over the tape, and upon listening back, I can still faintly hear what was on there before.

Now, I made these tapes before I knew anything about storing the reels tails out, and the recordings are made pretty hot, I like the needle in the red quite a bit. But the only instruments used are drums and sax, only two mics, so I was surprised to find the tape wouldn't completely erase.

Is this common, or is my machine suspect?

Also, can one erase tape with the Han D Mag? These are 7" reels of 1/4" tape.

Thanks for any info!

mark

 

[jviss]

Take a look.

(I'm new here, but I'd like to build up some rep. points, so, being an EE, I'll take a whack).

First, did this deck ever properly erase?

Does the deck have a separate erase head (probably does)? Take a look at it. Is it clean?

I doubt you can effectively bulk-erase a tape with a head demagnetizer.

Here's a piece of the internet repairfaq I found:
http://www.repairfaq.org/REPAIR/F_audiofaq2.html#AUDIOFAQ_014

4.13) Previous recording not erased


In this case both the original and new audio appear on the tape. The most
likely cause (assuming your deck doesn't have some fancy sound-with-sound
or sound-on-sound modes that may be engaged) is a faulty erase head or its
driving signal.

The erase head precedes the record head and probably uses the same high
frequency signal as that for record bias to totally wipe the previous
recording. (However, on really really cheap tape recorders, erase may just be
performed by a permanent magnet.) If the new recordings are really distorted,
the bias oscillator itself may not be working. The erase head is either part
of the REC/PLAY head assembly or a totally separate head. Check for broken
wires to this head as well. If you have an oscilloscope, monitor the signal
during record. The erase head could also be defective or really dirty.

 

[jpmorris]

What deck is it? If it was designed for the old Ampex standard, SM911 might take more power to erase than the erase head is putting out.

I did have this problem with my Studer, actually - what I did was record a test tone, and then play it back while adjusting the erase drive circuit until it was working fully. Mine has a single erase head for both channels - if yours allows L & R to be erased independently, you would have to tweak both circuits. (I had a job figuring out which circuit was actually driving the erase head as the Studer can have either full-track or stereo erase heads).

 

[ofajen]

So, I recorded over the tape, and upon listening back, I can still faintly hear what was on there before.

How faint is "faint?" My 3M M-23 1/4" machine lists a S/N ratio of 62 dB (using Scotch 203) comparing a 3% distorted tone at 400 Hz to noise. The erase spec says a 3% distortion 1K tone will be reduced by 68 dB. If the S/N and erase specs don't change much between 400 Hz and 1K, all that means is that a really loud tone will be erased to 6 dB below the noise. However, a coherent signal is not that hard to pick out at only 6 dB below the incoherent noise. Again, how faint is "faint?"

Also, have you done a full record-side electronic alignment to make sure the bias and erase circuits are set up to meet spec? That might improve things.

Cheers,

Otto