80-8 alignment

[hatbeardglasses]

So I just finished aligning my 80-8, my first time attempting this operation. I've recorded a few projects on it before doing an alignment, and they all came out sounding fine, but it was time to find out what was really going on.

It seemed to go relatively well, some significant adjustments needed to be made (one track that had been unusable came back into spec).

There were a few issues that confused me a bit, particularly setting the bias level and the record eq.

I had a hard time figuring out how to center the meters for bias adjustment. Some of the tracks, the reading would go off the scale and I'd have to recenter the meter. I wound up turning the trim pot all the way down, and basically starting from 0 (which I take to be 0 bias current), and nudging the input control so the needle would stay on the scale. I guess the hard part was figuring out what to do from where the pots already were, as if I turned clockwise, the needle would drop all the way to zero. Starting from fully counterclockwise and turning up until peak, then down 3 db would put the trims about where the other ones were starting from, so I satisfied myself with that.

The one thing that I don't think was working properly was adjusting the record eq. Using 15 khz, when I switched to the monitor head, the first few tracks read a couple db higher than the input, and when I tried adjusting the trim, the needle didn't move. This was true for the record eq, and the peak adjust. After three tracks of the same behavior, I just abandoned this part.

I recorded some Rhodes piano on track 1, which had been giving me trouble (meters read sine tones with a little wobble), and it sounded pretty good, so I stopped there.

Anyone have any insight for adjusting bias or record eq?



thanks in advance.

-David

 

[Beck]

David,

Did you do a full calibration starting with mechanical head alignment? Differences in high frequency levels between tracks can indicate that the head has uneven wear so that one half of the head (for example tracks 1-4) is leaning away from the tape, so it doesn’t have as good a contact with the tape as the other half containing tracks 5-8. (But in reality it’s not usually divided up exactly in half… you might have just the first one or two tracks noticeably different than the rest).

On the other hand, 1 or 2 dB is within spec for the 80-8 and most other decks. I think the frequency response for that machine is 40Hz – 18 kHz +/- 3dB, reference to 1kHz (maybe 400Hz)@ 0VU. So if 1kHz (or 400 Hz… whatever the manual says) reads 0VU on your meters, it’s normal for higher and lower frequencies to read as much as 3dB up or down from that. And the elements for each track in the head won’t necessarily have the exact same response, especially on an older head with some wear. A new head or relap of the old heads can make things a bit tighter, however the individual elements in each track may still be slightly different.

If the EQ trimmers are not doing anything they may be nonfunctional due to oxidation. This happens on some older machines with the cheap open trimmers. Cleaning them with Caig DeoxIT can bring them back to life, but you might need to replace them if the carbon is worn off. The service manual should list their values.

As for the wobble, it’s normal for tones on a calibration tape to show less than perfectly steady on your VU meters. You’ll see it better on mechanical VUs. People with LED bar graphs may get freaked out if the setting is right on the edge, which causes an LED to go in and out… even though the change isn’t actually a whole dB. Bar graphs aren’t as precise with tones.

With bias adjustment the VU meter is not being used to indicate an absolute value, but a relative one. That is, so many dB down from peak for a given type of tape. So, you don’t need to start the process with the tone reading 0VU. Set the VU meter lower so that the peak doesn’t go off scale. Otherwise you won’t be able to see how many dB down you are. I know some manuals say to, but I don’t like to use the input level knobs to adjust the meter. They should stay at nominal input level, which I believe is 7 on that deck… the manual will tell you. Adjust the VU meters with the internal VU adjustment trimmer (try -10VU). Later you realign them for 0VU using your reproduce calibration tape after you’re done setting bias. This is how you would have to do it if your machine didn’t have input level controls, which many don’t.

(While we’re on the subject level pots… It’s a good idea to spray some fader or tuner cleaner inside those front panel input level controls and work them back and forth a few times. If they are dusty they will jump around as you adjust them, which can throw off any procedure you’re attempting. If they don’t pop and crackle when you turn them they’re probably ok.)

Just remember you’re using the deck's meters for a completely different purpose when setting bias, like you would an external meter.

 

[hatbeardglasses]

Good to hear that a couple db off 0 is within spec, I didn't even try tracks 1-5 after the first few eq adjustments did nothing. From what I can tell, the wear on the heads doesn't look asymmetrical, but the machine has been moved a few times since I've had it. I don't have an oscilloscope, and hadn't really looked into the finer points of head alignment.

as far as adjusting the VUs by the internal trimmer, that pot is on the back of the card, and you need the extender card to get to it, which I haven't been able to find anywhere.

by the way, I think the MRL tape I'm using I bought from you, thanks for that, and for your insight.

-D