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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 18th Mar 2018, 11:39 am   #1
richieyork
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Default Akai 4000DS tape stutters..

Hi all

Just wondering if anyone can offer some help here please.
I've picked up a very decent 4000DS and it's giving me a headache - i'm having tape transport issues.

The problem is that whilst it's playing approx every 10 - 12 seconds the tape speed hesitates, which lasts for about a second then continues as normal. It doesn't stop but momentarily falters.

The tape heads/guides/capstan/pinch roller all seem fine and the main drive belt appears to have decent tension.

I'm not a tape deck expert so this is a learning curve. I'd love to get it sorted out since the recording and sound quality is excellent and its a very tidy machine.

Can anyone offer suggestions as to most likely reasons/things to check??

Many thanks in anticipation.

Regards
Richard
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Old 18th Mar 2018, 12:07 pm   #2
Andrew2
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Default Re: Akai 4000DS tape stutters..

If the tape momentarily stops, it's worth checking the reel of tape itself. It could be suffering some kind of 'sticky' problem, causing a short snatch as it comes unstuck. It's quite common on older tapes.
You'll probably find it fouls the heads & guides quite quickly too.
If you do find it is a sticky tape, one cure is to moisten a tissue with isopropyl cleaner and run the tape on FF or REW while holding the tissue against the tape. Clean both surfaces of the tape, and make sure the tape is fully dry before playing.
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Old 18th Mar 2018, 12:29 pm   #3
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Default Re: Akai 4000DS tape stutters..

Hi,

I had one of these back in the late seventies, really good unit for the money.

What I tend to do with this type of a fault is try to look for something that is rotating at the same repetition rate as the wow/flutter. As this only happens once every 10-15 secs then it could be on the supply spool side assuming that it is still 'full'. Does the tape tension appear to alter during the fault? Does the take clutch make the take-up reel snatch or similar. Having a real good look at what's happening with these faults can save a lot of time swopping parts that aren't defective.

Recently I serviced the tape deck on a Panasonic Boom-box. All went well, except for a certain amount of flutter on playback. When I'd re-fitted the belts I'd fitted one belt onto the wrong groove on the flywheel. This meant that the belt barely touched on an adjacent fixing post. Moving the belt to the other groove cleared the fault. In fact you could listen to a Chopin Nocturne without thinking anything was wrong! All I'm saying is it takes very little to introduce a speed fault into a tape deck.

Best regards, SJM.
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Old 18th Mar 2018, 2:00 pm   #4
richieyork
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Default Re: Akai 4000DS tape stutters..

Thanks for the helpful responses so far - i've established the actual recording tape is fine and not sticky so that's ruled out.

When it twitched the tape auto-stop tape tensioner also twitched, if that's any help

I've dropped the speed down to 3 3/4 and the tape now behaves itself, to a point in that it doesn't stall as it was doing thought to my ears there is a lot of wow & flutter - it's very noticeable. It seems to be getting more wowy & fluttery as the take-up reel fills.

So, at lower speed it's not doing the 'twitch' but the speed stability isn't good - this wasn't obvious on 7.5 speed. I'm wondering if the speed twitch was something as simple as a not fully tightened collar on the capstan 'sleeve'? That said, could issues with the main drive belt cause all of this?? I also noticed a large capacitor in the motor circuit - can these create these problems?

Regards
Richard
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Old 18th Mar 2018, 2:20 pm   #5
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Default Re: Akai 4000DS tape stutters..

Almost anything in the transport can cause speed variations, from a stiff supply reel-table to too much take-up tension. Worn pinch roller, sticky tape, badly-adjusted brakes, you name it! It's just a matter of working your way through all the possibilities in a logical manner.
SJM points out that if the 'stopping' is happening every 10 seconds, look for something that rotates at that speed. In this case that might be the tape reels so you would suspect sticking tape or a binding reel brake. This is why I suggested a sticking tape in the first place. However, it seems you have checked for this. Next stop - sticking reel tables?
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Old 18th Mar 2018, 3:16 pm   #6
richieyork
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Default Re: Akai 4000DS tape stutters..

Hi Andy

Thanks for your guidance.

OK, i've refitted the 7.5 speed capstan collar and for the last half hour it's behaved itself in that it's not stalling. I'm wondering if it was simply slipping due to insufficient tightening, if so it's back to school for me

BUT, some flutter is still noticeable on this faster speed (just about liveable with) and very bad at the slower speed.

I've got another 'good' 4000DS in the attic so will drag it down & try a few side by side comparisons as to take-up spool speeds/how much torque is present/supply reel tension. As mentioned i'm new to this tape thing so it's a complete learning curve.

I've made the following deductions:
*The supply reel must have some back tension in order to ensure the tape has good contact with the heads, but im unsure how much tension is correct and how do you adjust it?
*The take-up reel should pull through the tape at sufficient torque to ensure its wound properly & tightly on the reel but again how much torque should there be & how is it adjusted?
I've never dismantled a tape reel mechanism before so its a bit scary but i'm willing to learn!
Cheers for now
Richard
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Old 18th Mar 2018, 3:31 pm   #7
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Default Re: Akai 4000DS tape stutters..

In the old days the normal recommended back tension for standard play 1/4 inch tape was 70 grams. Very few domestic machines are capable of this and in any case are probaly using long play tape.

Take up tension should be the same so that the capstan merely moves the tape, it doesn`t have to haul it across the heads.

Obviously in order to supply constant tension as the reel diameter changes requires some form of servo mechanism which can be electrical control of the reel motors or some form of mechanical braking.

In practice very few non professional tape machines provide constant feed and take up tension over the whole of the reel and in any case all the above goes out the window if the machine uses pressure pads on the heads.

This probably doesn`t really help you much, sorry.
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Old 18th Mar 2018, 6:00 pm   #8
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Default Re: Akai 4000DS tape stutters..

I have had one of these from new. There was always a problem with the tension when the tape was rewound or fast forwarded. It was always too loose which meant that when playing or recording the tape slipped on the supply reel leading to a juddering. I took the original one back to the shop but the replacement and further replacement had the same problem so I concluded that it was a design problem and put up with it.
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Old 18th Mar 2018, 8:00 pm   #9
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Default Re: Akai 4000DS tape stutters..

Richard, go to Vinyl Engine and sign up. It's free and you can download the service manual.

Tape tension is measured using a standard diameter of tape (60mm) with a loop on the end. Using a spring balance (or at a pinch, luggage scales) you activate the required reel whilst holding the scales and take a reading as the clutch slips.

It sounds complicated, but it's not. If you need to make adjustments you take the reel table block apart and add or remove shims.

Sometimes, just taking the block apart and lightly roughing up the felt is all thats required.
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Old 19th Mar 2018, 10:03 am   #10
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Default Re: Akai 4000DS tape stutters..

This machine uses a slipping brake on the supply spool table to apply tape tension and it needs that tension for head-tape contact as it doesn't use pressure pads. Perhaps the brake keeps grabbing?

It uses a slipping clutch on the take-up reel to apply take-upi tension (after the capstan) and it is this which raises the autostop sensor arm. If the supply brake grabs enough to stop the tape and leave the capstan slipping, the inertia of the take up reel will make the sensor arm jump and bounce.

I had the brakes and clutches apart in the mid 70's but can't visuailse details. They might be felt discs and have got grease contamination on them which has hardened.

David
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