Problem w/ Tascam 48 speeding up

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lo.fi.love

lo.fi.love

Functionally obsessed.
Hey folks, long time no post....

I stored my Tascam 48 in my closet for about a year and a half + just brought it out again. It's developed this strange problem where the tape will speed up during playback + recording, sometimes a little bit but sometimes more than 50%. When it *really* speeds up, adjusting the pitch dial does practically nothing to control it.

I've noticed that this usually happens when the machine has been running for more than a half hour. Not when it's "cold". So I would almost suspect a cracked solder connection - ?? I have no idea where to begin troubleshooting this. Does anyone know what this may be a symptom of?

I'll really appreciate the help! This was really heartbreaking to discover and I hope I can get it right again!
 
Does the problem go away if you push the pinch roller harder against the capstan?
 
lo.fi.love said:
How do you suggest I try this? Just with my finger on the underside of the roller?
Click to expand...

I'll have to check the 48, but usually pinch rollers have a cap of some kind that you can press against. Pressing against the rotating surface of the roller itself isn't going to help.

What can happen sometimes is if the pinch roller isn't engaging properly, either because the rubber is failing or because it's not being given sufficient pressure, the tape tension can overcome the capstan and cause interesting problems with the speed. It's something that might be worth checking if you can, before assuming the electronics are faulty.

EDIT: And eww, it looks like the pinch roller is hidden under a plastic cover. You may need to remove that (and there must be a fairly easy way to do that because the pinch roller is a replaceable part)
 
+1 to that. Just pull the head cover and then push on the pinch roller mounting screw toward the capstan. IIRC that is an original pinch roller, and as the rubber hardens it doesn't conform to the radius of the capstan thereby reducing the contact patch of the tape to the capstan.

Can you tell if the capstan itself is speeding up? If think if it was the be able to hear a speed difference in the motor.
 
it could also be the take up tension is changing when the 48 is hot.Look and see if the right tension arm is moving higher when the tape speeds up.
 
Good point, but unless extreme that still shouldn't cause the tape to speed up. The tape speed should be effectively maintained by the capstan and pinch roller.

I still think the first things to check are the condition of the pinch roller (and if you can, Jeff, the pinch roller pressure), and to listen to see if the capstan motor is changing speed in conjunction with the change in tape speed.

If it is the former either get a new pinch roller or have Terry Witt resurface the one you have and check/adjust the pinch roller pressure. If it is the latter then it is a more complicated issue with the capstan servo board and or control PCB and the first step in troubleshooting will be to check for proper voltages to those boards both when the machine is cold and when it is warmed up (and presenting the undesirable symptoms).
 
I have personally seen tension being the issue at least twice.Once when a solder joint got flaky on the final drive transistor for the take up motor,and once when the same transistor failed when hot. Both would run in play at a 'spooling' speed.I would suspect the capstan motor/pinch roller first-but posted the tension issue comment for clarity. Observing the tension arms during failure would show you that.Actually, the capstan motor cannot regulate the tape speed if the take up,(or back tension) is too high.Either one out of spec. will cause W/F tape speed fluctuations.The capstan motor is a fairly low torque motor.
 
I haven't had time yet to look at it, but I'm going to report back once I dig deeper into this. It usually happens after a couple/few hours of constant use so it's the sort of thing where I'd need to be working on music all day to catch it happening...
 
Hey everyone, thanks for the input. It just happened, I've been recording for the past 6-7 hours, and I definitely believe it's the pinch roller. It's old, hard, and smooth, and it's just not gripping the tape well in the more heavily-played, smoother passages of the reel. I've used CaiKleen RBR on it since I got the machine from Cory a few years ago. I think it's finally just bit the dust.

It's a relief, considering I won't have to open the machine to remedy this. :)
 
It's important to understand the pinch roller does not grab the tape. It is a passive "idler". What it DOES do is pinch the tape against the capstan shaft which IS the driving surface.

When a pinch roller is new and properly soft and it presses against the capstan shaft, the pinch roller conforms to the radius of the capstan increasing the degree of wrap...the contact patch of the tape against the capstan. As the rubber pinch roller ages and hardens that degree of wrap decreases because hardened rubber no longer properly conforms to the capstan shaft and there is a commensurate decrease in the contact area between the capstan and the tape.

Send your pinch roller to Terry Witt, Jeff...you'll spend less than you would on a new one or NOS on eBay or whatever and end up with a better part. I'm not surprised its time to replace it. My 58 was in need if a new pinch roller and it was about 4 years older than the 48 I sold you...and I sold that to you about 4 years ago was it?

So when the tape is speeding up can you press the pinch roller harder against the capstan and bring the speed back down? Have you checked the pinch roller pressure?
 
sweetbeats said:
When a pinch roller is new and properly soft and it presses against the capstan shaft, the pinch roller conforms to the radius of the capstan increasing the degree of wrap...the contact patch of the tape against the capstan. As the rubber pinch roller ages and hardens that degree of wrap decreases because hardened rubber no longer properly conforms to the capstan shaft and there is a commensurate decrease in the contact area between the capstan and the tape.
Click to expand...

This is what I meant... the rubber is old and it's lost its ability to press against & conform to the capstan. I said 'grab' sort of colloquially maybe because I pictured it grabbing the runaway tape... heh. The cause of the problem became clear after manipulating the transport with my hands + inspecting the pinch roller.

I bought a new roller from eBay before I got your message bout Terry Witt. I'll keep that in mind for next time. And yeah, I think you sold this to me almost five years ago? I can't remember exactly. But I love this machine, you did a terrific job of setting it up + educating me on using it, and I've gotten hundreds of hours of pleasure from this thing. Thanks :)
 
In fact, I recently redid my studio, and my 48 and my Otari 2T are the only items of gear I continue to use from my old setup (except mics). I capital L-U-V my 48 :)
 
Follow up...

The pinch roller arrived today, and the problem persisted even with the new roller.

I played a tape and when I moved the tension arms with my hands it felt like neither side was tight enough. I looked in the manual (note: hard copy not PDF, I can't give copies out) and discovered the tension adjustment trim pots underneath the removable splicing block. One quick adjustment solved everything. I like how Tascam hid the controls in plain sight, I thought that was really clever.

Anyway I'm still glad I got the new roller, the old one needed to go.
 
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if you can grab a tentelometer off ebay. Too high of a tape tension will wear out your heads.
 
Yeah, I was thinking that I should do that. It's working but it doesn't seem 'correct' now either.
 
Cool.
Glad you figured it out.
So. Imguessing. You sold the. Ampex set up?
I recently sold off al my decks except for a 48.
It needs a new roller also.

Since you've had both decks and the. Other stuff being the same, how would you compare?
 
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