Akai 747 w/DBX question

@fretfile  

Thanks 

I had the Akai 747 back when it first came out and I realized then that parts would be hard to find. To the best of my knowledge,  the recorder is equipped with dbx type 1 which is a different beast from dbx type 2. I never tried an outboard 224 type 2 reduction, but it may not be an ideal match. 

I would demag the entire tape path. The key in demag'ing is to move the unit sloooowwwwly. If you move it fast across the heads very little demag will happen and you can actually increase the mag level on the heads.

it will vibrate a little since the magnetic flux field is interacting with the metal parts on the head or rollers.

and as one other poster noted, do not use a bulk tape eraser.

Ok guys. I’m about to de-mag my tape transport and heads. Hope I don’t blow anything up. 

Hey thanks for the info. I really love my reel to reel and I have had one since the 70s and I want to keep it in top working condition. I have it serviced once every two years so I Know the heads have not been de-maged at least since then, if even. 

 @cleeds 

Well well well what do ya know. That’s good! Ok I feel better now. I will perform the operation and see if it makes a difference. I am not really having s problem per se but if you guys recommend it needs to be done, and I have the right equipment to do it why not. 

barnettk

... this device is called "Han-D-Mag"

That is the gold standard in head de-mags and is still being manufactured by R. B. Hannis.

Ok so this device is called "Han-D-Mag" and the instructions are on the actual device.. go figure. This thing looks like it was made in the 1970s at least.  The instruction say " plug in, Bring energized probe close to work, then withdraw slowly to a distance of 12’ then unplug form power source" so your instructions are more right then the way I was doing it. 

@cleeds 

you are prob correct. I was following instructions from a YouTube video if I remember right. Let me dig this thing up and see if it even has an on/off switch. It was given to me so it does not have any instructions. 

barnettk

I have a demagnetizer, and the way I did it was as follows: Start about 6 feet out and slowly work my way to the head (without touching it) then slowly work my way back away from the head.

That is not the correct way to use a head de-mag, according to the instructions I’ve seen that accompany them. The de-mag should be placed close to the head - not touching, as you state - and then energized. After rotating it closely around the head (or tape guide), the de-mag should then be gradually drawn away from the head, and then switched off.

also I was careful to remove the reels when I did it. Now this was about a year or two ago.. maybe longer. I put that thing in a drawer and have not looked at it since. Scared the crap out of me haha. 

sleepwalker65355 posts01-09-2019 10:26pmI hope you weren’t using a bulk tape eraser. Those are way too powerful to demagnetize a tape deck, and have pretty short duty cycles  

No its a head demagnetizer with the rubber coated tip. 

I hope you weren’t using a bulk tape eraser. Those are way too powerful to demagnetize a tape deck, and have pretty short duty cycles  

@johnss 
"Incidently the Vishays do wonders for MM and MC cart loading positions, but not trying to hi jack the thread. "

I am perfectly fine with talking about whatever so it all fits IMO. 

I think these are all good ideas, and the more I think about it I would rather make the deck sound better nativily without adding additional components. Sounds like a better option to me. I was considering re-capping it anyway. So I will see how much my tech will charge me to get that done (adding the Vishays, and replacing the caps. 

Question 1: In regards to replacing  the wire from the head to the PCB with Mogami Console wire. What will this do for me? allow me to add the Dolby A since the ones I have seen have what appears to be a console connector instead of line level?? I have never heard of doing  this so excuse my ignorance on that. 

Question 2: In regards to head demagnetization. Do I need to damag the erase head or only the playback and record heads? I have a demagnetizer, and the way I did it was as follows: Start about 6 feet out and slowly work my way to the head (without touching it) then slowly work my way back away from the head. The first time I attempted this I just got nervous because the closer I got to the head the device seemed to vibrate in a way that, well frankly scared me. Plus its a time consuming process and it was getting pretty warm.. I guess thats normal but I was just afraid I could possibly do some damage to the deck, by  possibly causing some harm to the machines other parts, boards, etc. 

I would suggest demag'ing the heads every so often. Otherwise you will lose high freq response. Just make sure you use a demager with a protective tip and move the demager SLOWLY, you want remove magnetic fields from the heads not add them.

I demag the heads on my decks before doing any serious recording.

The stock load resistors on the 747 are carbon film. you may think the deck is quiet now at 7.5 ips, but wait til you listen to it with Vishays, you'll think you got 30% more musical detail coming through.

The other big benefit the 747 deserves is bypassing the audio stage coupling caps with film and foil caps.

Incidently the Vishays do wonders for MM and MC cart loading positions, but not trying to hi jack the thread.

@cleeds  

there are are some that think demagnetizing tape heads is not s good practice. Their words. Not mine. I was just curious to know how he felt about it. So you make a good point. I will never try it “myself” if it gets done it will be part of the bi annual service my deck gets. Good point tho. 

barnettk

I was interested in the DBX mainly to get the bump on dynamic range which also by proxy reduces the noise however like I said the faster recording speed definitely improves things for sure. The Dolby A I think would improve the highs.

Dolby ’A’ will improve the highs and DR.

Not really a fan of demagnetization. There are two camps on that topic and it seems to be a little dangerous for me to try it myself even though I do have a demagnetislzer. Your thoughts on that.

I was not aware that there are two camps of thought about demagnetization. There’s no question that tape deck parts can become magnetized with use over time, which is obviously not good for the deck or any tape played on it. Demagnetization is actually part of routine tape deck maintenance. That it can be improperly done doesn’t mean that it should be avoided, imo.

@johnss

i have ave heard of the Doing the Vishay. Have not pushed it however. I’m not having a problem with much noise since I have been recording at 7.5. At the faster speed my recordings sound much much better. I was interested in the DBX mainly to get the bump on dynamic range which also by proxy reduces the noise however like I said the faster recording speed definitely improves things for sure. The Dolby A I think would improve the highs. The lower octaves are fine. I am also considering having re-capping the deck. It’s probably time since they have never been replaced or st least not since I have had the unit. I just feel that the highs have dropped off a bit. Not really a fan of demagnetization. There are two camps on that topic and it seems to be a little dangerous for me to try it myself even though I do have a demagnetislzer. Your thoughts on that.

I would take a pass on the dbx unit. If you really feel you need noise reduction would look at one of the pro units, dolby A 363, or a Dolby SR unit.

Other three options would be
1. get a set of half track heads for your 747
2. install a Vishay foil resistor at the loading position on the PCB where the signal from the head first goes to the board.
3. replace the wire from the head to the PCB with Mogami Console wire.

items 1-3 above will each reduce (increase S/N ratio) by a few db at 7.5 ips tape speed. 

The Dolby A seems to be more of a console device. 

Yes the 224 will but not sure about the Dolby A

I own a 224, but haven’t used it in a while. It’s compatible with any line level deck. 

Sleepwalker.
Have you used the dbx224?
Looks like it should work with any r2r correct?
Certainly cheap enough to try one out.

Im also seeing a couple of the Type A systems but nothing that looks like it will work with my consumer electronics. The Type A encoder/decoder looks promising, but Im not sure I can find one. Both good looking options. Thanks again. I will let you know what I come up with. 

I found it. I was not searching for the type II. Nice 

Are you sure its the 224 and not the 234?

@sleepwalker65 

Right. I actually remember that Dolby made a stand alone component to provide the DNR. I had forgotten that you can buy both devices. I think I will look into the 224 because I do like the Dynamic range extension that is gives as a benefit. This way if I decide to go to another deck or the one I have breaks to where It can't be repaired due to lack of parts etc I can still have the feature. The last 747 I had stopped recording. The part needed according to the repair shop I took it to (still have my doubts that was the problem) was not able to find the part and neither was I with the part number he referenced.. anyway my point is that when they break beyond repair not much you can do but buy another machine. So I will see if I can find the 224. Hopefully there is one floating around out there. 

As Cleeds alluded to, when dbx type II is incorrectly set up (it is dependent on proper level settings for optimal results), you may experience pumping or breathing artifacts. The Dolby “A” noise reduction that Cleeds also referred to is a commercial product that also uses compression, but is usually not found on hifi tape recording gear. 

Thanks for the info guys. 

I've never been a fan of DBX NR, which is not completely transparent even when it's working, and which can have "pumping" effects even when it is. Sure, if you're using cassette or 8-track it may have some value, but I don't think it has any place on an open reel deck. If you need NR with a reel machine, there's Dolby A for that.

dbx type II noise reduction is just that, besides preserving more of the source dynamic range than Dolby NR, it also lowers the noise floor substantially (10db iirc). You don’t need to have in-built dbx circuitry on any tape deck to benefit from it though - an external dbx model 224 or similar will give you all the benefits without replacing the tape deck. A bonus is being able to use the dbx NR unit with any of your R2R or cassette, or even 8-track tape decks for that matter.