Akai GX4000-D fluctuating speed

[Nickersby]

Hi,

Looking for advice on fixing this issue. Playback speed is fine until the tape has played about 3/4, then the speed starts fluctuating, as if it can't cope with the weight of tape on the take-up spool.

I've cleaned and lubricated the idler wheels, and also disconnected and tested the motor run capacitor, which read 2.6uf. Could the capacitor be faulty even though it measures the correct capacitance? What should I try next?

Thanks for any help.

 

[vitorzoom]

Some old tapes can have this kind of problem.
Try other tapes from different brands and observe the result.
The end of the tape is where you notice the most fluctuation in sound due to problems with the tape or transport.

 

[sweatybetty]

How is the pinch roller? Are the capstan and pinch roller clean? Is the rest of the tape path clean? It doesnt take much buildup to cause problems like you describe.
The takeup reel is only going to go as fast as the pinch roller allows it to. If the tape between the pinch roller and takeup reel is not drooping then I would look for a dirty tape path.

 

[Nickersby]

Thanks for the suggestions. I have been using some old tapes, so maybe that is the problem. I've ordered a brand new RTM tape so we'll see how it works with that.

 

[vitorzoom]

If you are using Ampex 456 tape for example, this is most likely the cause as this tape is sticky. Or Sony ULH among others

 

[vitorzoom]

If you are using Ampex 456 tape for example, this is most likely the cause as this tape is sticky. Or Sony ULH tapes among others

 

[titanstats]

Hi,

Looking for advice on fixing this issue. Playback speed is fine until the tape has played about 3/4, then the speed starts fluctuating, as if it can't cope with the weight of tape on the take-up spool.

I've cleaned and lubricated the idler wheels, and also disconnected and tested the motor run capacitor, which read 2.6uf. Could the capacitor be faulty even though it measures the correct capacitance? What should I try next?

Thanks for any help.

Have you taken a really close the idlers to make sure the rubber hasn’t hardened? There’s a lot going on back there, worth a look. I had one with a similar problem, couple of the idlers looked fine but were too hard, as torque increased they slipped. Couldn’t hurt to change the cap, it’s ancient at this point and only costs a few bucks; doubt it’s the root of your problem, but it should be swapped anyway.
As others have said, if you’re using old backcoated tape, that’s your problem right there. It usually doesn’t take 3/4 of a reel for that problem to make itself known though.

 

[Nickersby]

I've tried a brand new tape and wound it 3/4 of the way on before recording anything (having thoroughly cleaned everything again first) and the problem is still there, so I can rule out sticky tapes. The pinch roller and idler wheels look to be in good condition, the machine is like new so I don't think it's been used much. However, I've no experience with these things so I'm not sure how hard or soft an idler or pinch roller should be. Is there a simple test (such as digging a thumbnail in to see if it leaves an impression)? Alternatively, can anyone recommend a UK supplier of replacement wheels? Thanks again for all the suggestions so far.

 

[audiojones]

The Akai GX-4000D is a belt driven deck. Have you checked / replaced the capstan drive belt? It’s around back, maybe it’s just stretched or dry rotted from age.

 

[sweatybetty]

You should be able to push your fingernail into the rubber but it shouldnt leave a mark.
Maybe here? https://www.donberg.co.uk/catalogue/video_spares/video_recorder_pinch_rollers/

 

[vissal]

Hi,

Looking for advice on fixing this issue. Playback speed is fine until the tape has played about 3/4, then the speed starts fluctuating, as if it can't cope with the weight of tape on the take-up spool.

I've cleaned and lubricated the idler wheels, and also disconnected and tested the motor run capacitor, which read 2.6uf. Could the capacitor be faulty even though it measures the correct capacitance? What should I try next?

Thanks for any help.

I have the exact same problem. Have changed the belt and pinch roller which does not have any effect. However if I gently increase the pressure on the pulley. The speed is rectified. I wonder if there is an option in increase the pressure of the pulley. There is a large spring which pulls the pinch roller towards the capstan. I wonder if that is the problem.