Tascam 644 High Speed Recording/Playback Issue

[ley.stn]

Hiya all,

So here's the problem...

Whenever I record in high speed on my 644 I get slow fluctuating speed when I play back the recording. This doesn't occur when I record in low speed.

I have tested this with both type I and type II tapes (90 min). I have also tested playing back the high speed recording on a different tape deck.. the same speed fluctuation occurs, which leads me to believe the problem occurs when the recording is captured, rather than during playback.

Can anyone help me out here? I've searched google a dozen times, but can't seem to find anyone that has this problem. I think the strange thing about this issue is that recording at low speed works perfectly fine.

Any suggestions/ideas would be greatly appreciated. Thanks!

 

[SkywaveTDR]

Well if you think about it from a Technical standpoint, The power supply has the hardest time when at high speed and when all the Bias oscillators are turned on. They do not build this stuff to aerospace grade in soldering nor with parts. This is why on some decks I suggest that the power filter caps be upgraded to 105*C type and 50% higher value for better reserve. I have been known and have in stock 1500uFd caps that would be put where 1000uFd were and then on many units the soldering that is marginal is gone over so the power supply is pretty much worthy of my 6 month warranty. There are many decks I do to solder and see with my magnifiers on joints that are broken completely but they are addressed right away and all the other connector pin also resoldered correctly.
On the other hand these deck do not use the highest grade of motors but also as higher speed results in more bearing heating the lubrication in it might not be sufficient, I pull flywheels out of the bearing and put a good amount of AMSOil into the bearing and on shaft then insert and work it around. Then immediately before and play function is done the capstan shaft is cleaned with denatured alcohol to take the oil off otherwise you will contaminate the Pinch roller and tape with oil. The Pinch Roller is observed and recondition or cleaned with Naphtha which was the 97% solution in Teac RC-1. The motors top bearing- the one you have access to and takes all the load is also oiled and does revolve much better after AMS is in it.
Beyond that, you may have worn brushes in the motor that can take slow speed but at higher speed the current required is not as much as the motor needs. These are voltage controlled motors that do NOT have a feedback FG and so they depend on your setting staying the same but with all the variables such as back tension, take up tension and different tape slip sheets the speed may drift from one part of tape to the other. I see this with non-direct drive motors all the time as they are known not to be that accurate. What you really need is a professional to look into all of the problems and then if nothing works to replace the motor with a new one. Some may look for easy answers and spend pages on forums on here getting info from guys that just do not know and end up wasting a lot of time. I help people fix their own decks all the time and have 8 trainees but there are times the person with the unit doe snot have the equipment needed to diagnose this problem- speed test tapes like MXT111 and wow and flutter meters and frequency counters plus the experience with these.

 

[ley.stn]

Wow, I didn't expect such a technical answer! Thank you. Clearly there is a lot to think about, and ultimately I'm going to have to get a professional look at the unit.

One more question.. do you think using a 60 minute type II cassette will make any difference? In the 644 manual Tascam has said using those rather than 90 minute cassettes will provide a more optimal recording experience, although it doesn't specify that using any other cassette will cause issues.. Probably wouldn't make a difference, but I though I'd ask anyway.

Thanks again for the response!

 

[SkywaveTDR]

As far as cassette deck tape goes, C-60 tape is like 1.5mil tape is in open reel but not 1.5mil tape of course. If your projects fits in a 30 minute tape then that will be fine. Thicker tape always means better longevity tape wise. I use XL II C-90 for my calibration but replace them when I see they are acting as if worn- instruments will show this before ears.
To go further- there was a single tape formula that was better than any Chrome tape made. It was made by TDK. It was a weakened metal tape called HDX. Teac may have called it that in their tape line but be sure it ALL no matter what brand was on the tape was made by TDK. This tape was as tested superior to all tapes I tested in any machine except when it came to a battle with real Metal tape. If you want truly the best tape in your deck get that HDX or maybe they called it HXS.
My answers are rather technical as this is where I am coming from so my use of dude and other such terms may be absent in my answers. If I get too advanced I can explain further but doing tape deck work for this many years has a result that you talk with the terms that are everyday talk with Technicians. I would be just as lost if I would conversed with Aeronautical Engineers as every science has it's own defined set of terms. I say if you do not understand something find out more about it and then go from there as I can promise you there is stuff I don't know even after 47 years doing this. I took some time away from Technical things as a Patrolman and then to work in Radio for 14 years but in the background I was still working on decks all the time.