Cassette torque meter - UK Vintage Radio Repair and Restoration Discussion Forum

ronbryan · 25th Feb 2018, 6:50 pm

Does anyone have an audio cassette torque meter that I could borrow for a week or two, please?

I have an Akai GXC-75D (auto-reverse single motor mechanical deck) that stalls on rewind (fast forward is ok) and I would like to see if the cause is excess stiffness in the 'paying-out reel table block' or if a clutch in the take-up reel drive is slipping and is unable to deliver the required torque.

For rewind, the reel that is not driven has to drive a belt to the mechanical tape counter/rotation sensor (magnet and reed switch) and does feel a lot stiffer when twiddled with rewind selected with the power off. I have cleaned all of the tyres and fitted three new belts. It is just possible that the new tape counter belt is presenting a much greater side-load to the bearings and causing the stiffness.

I was once told by a Swedish engineer that 'one measurement is worth a thousand opinions', hence my request for the torque meter, but any suggestions would be welcome.

Ron

TIMTAPE · 26th Feb 2018, 12:57 pm

Quote:

Originally Posted by ronbryan

For rewind, the reel that is not driven has to drive a belt to the mechanical tape counter/rotation sensor (magnet and reed switch) and does feel a lot stiffer when twiddled with rewind selected with the power off. I have cleaned all of the tyres and fitted three new belts. It is just possible that the new tape counter belt is presenting a much greater side-load to the bearings and causing the stiffness.

I was once told by a Swedish engineer that 'one measurement is worth a thousand opinions', hence my request for the torque meter, but any suggestions would be welcome.

It could be a combination of factors.

The reel drive rubber idler tyre may have gone somewhat hard and so slipping, especially in rewind. If there is a fast wind clutch, it should slip in stalled mode rather than the idler slipping. If the machine poorly meets that stalled spec, that is probably your main problem.

Yes the side load. For machines running a belt to the counter from the take up table it's common for the rewind to go weak (slip) before the fast forward does due to the extra drag.

ronbryan · 26th Feb 2018, 6:21 pm

Thanks for your reply, Tim.

The fast wind clutch slips in 'rewind' mode, but the f.w. clutch tyre (driven by one of the capstan flywheels) does not slip before the f.w. clutch itself slips, so the tyre would seem to have enough grip.

I tried an experiment of increasing the fast wind clutch spring pressure, by adding a small shim under the compression spring. The 'rewind' nearly made it to the end of the reel before stalling, but the f.w. clutch slip torque was too much for the speedy 'fast forward' and at the end of tape the clutch tyre started to squeal as it stalled instead of the clutch lining slipping. So, not a viable work-around.

I will try refitting the old slack counter belt, putting less side load on the 'paying-out reel' bearings and see what that does. Of the three belts that I fitted, this one was the most awkward, requiring the greatest amount of dismantling to get the belt around the solenoid and various brackets.

Ron

emeritus · 27th Feb 2018, 12:09 am

Household Glycerine can renovate some types of synthetic rubber that has gone hard. I have used it successfully on the friction drive of an 8mm cine projector where the variable speed drive was a metal wheel pressed against a rubber-coated metal disc, and it completely eliminated slipping. It only needs to be applied for a few minutes and then cleaned off.

ronbryan · 27th Feb 2018, 12:42 am

Thanks for the tip about using glycerine on synthetic rubber to revitalise it.

In this case I have resolved the problem by making the effort to refit the original tape counter belt, which is quite slack compared to the new one, which with hindsight is much tighter than I would have expected. The reduction in side-load on the counter and reel table bearings has allowed rewind to complete without stalling.

I will measure the new belt and see if the supplier (in Germany) has made an incorrect selection, although without the dimensions of an original unstretched belt I can't see it going very far.

Ron

kevinaston1 · 28th Feb 2018, 8:40 pm

Torque meters, along with test tapes, are impossible to find now, apart from silly prices on a certain market website.

To measure torque, I dismantled a cassette hub to get a 1cm diameter pulley, and left a short length of cassette tape on it. Make a loop on the free end, and pop in a suitably rated spring gauge; 0-100g or 0-250g - instant measurement of force in in g/cm.

I got my set from Rapid Electronics (just a satisfied customer). Look up order code 06-2177 for a 0-100g Spring Scale.

Regards.

Kevin

25th Feb 2018, 6:50 pm	#1
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,960	Cassette torque meter Does anyone have an audio cassette torque meter that I could borrow for a week or two, please? I have an Akai GXC-75D (auto-reverse single motor mechanical deck) that stalls on rewind (fast forward is ok) and I would like to see if the cause is excess stiffness in the 'paying-out reel table block' or if a clutch in the take-up reel drive is slipping and is unable to deliver the required torque. For rewind, the reel that is not driven has to drive a belt to the mechanical tape counter/rotation sensor (magnet and reed switch) and does feel a lot stiffer when twiddled with rewind selected with the power off. I have cleaned all of the tyres and fitted three new belts. It is just possible that the new tape counter belt is presenting a much greater side-load to the bearings and causing the stiffness. I was once told by a Swedish engineer that 'one measurement is worth a thousand opinions', hence my request for the torque meter, but any suggestions would be welcome. Ron

26th Feb 2018, 6:21 pm	#3
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,960	Re: Cassette torque meter Thanks for your reply, Tim. The fast wind clutch slips in 'rewind' mode, but the f.w. clutch tyre (driven by one of the capstan flywheels) does not slip before the f.w. clutch itself slips, so the tyre would seem to have enough grip. I tried an experiment of increasing the fast wind clutch spring pressure, by adding a small shim under the compression spring. The 'rewind' nearly made it to the end of the reel before stalling, but the f.w. clutch slip torque was too much for the speedy 'fast forward' and at the end of tape the clutch tyre started to squeal as it stalled instead of the clutch lining slipping. So, not a viable work-around. I will try refitting the old slack counter belt, putting less side load on the 'paying-out reel' bearings and see what that does. Of the three belts that I fitted, this one was the most awkward, requiring the greatest amount of dismantling to get the belt around the solenoid and various brackets. Ron Attached Thumbnails Last edited by ronbryan; 26th Feb 2018 at 6:40 pm. Reason: Added fast wind clutch photos.

27th Feb 2018, 12:09 am	#4
emeritus Dekatron Join Date: Jan 2012 Location: Brentwood, Essex, UK. Posts: 5,345	Re: Cassette torque meter Household Glycerine can renovate some types of synthetic rubber that has gone hard. I have used it successfully on the friction drive of an 8mm cine projector where the variable speed drive was a metal wheel pressed against a rubber-coated metal disc, and it completely eliminated slipping. It only needs to be applied for a few minutes and then cleaned off.

27th Feb 2018, 12:42 am	#5
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,960	Re: Cassette torque meter Thanks for the tip about using glycerine on synthetic rubber to revitalise it. In this case I have resolved the problem by making the effort to refit the original tape counter belt, which is quite slack compared to the new one, which with hindsight is much tighter than I would have expected. The reduction in side-load on the counter and reel table bearings has allowed rewind to complete without stalling. I will measure the new belt and see if the supplier (in Germany) has made an incorrect selection, although without the dimensions of an original unstretched belt I can't see it going very far. Ron

28th Feb 2018, 8:40 pm	#6
kevinaston1 Hexode Join Date: Aug 2014 Location: Featherstone, West Yorkshire, UK. Posts: 386	Re: Cassette torque meter Torque meters, along with test tapes, are impossible to find now, apart from silly prices on a certain market website. To measure torque, I dismantled a cassette hub to get a 1cm diameter pulley, and left a short length of cassette tape on it. Make a loop on the free end, and pop in a suitably rated spring gauge; 0-100g or 0-250g - instant measurement of force in in g/cm. I got my set from Rapid Electronics (just a satisfied customer). Look up order code 06-2177 for a 0-100g Spring Scale. Regards. Kevin