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Tal 2 motor drive power supply. Long shot.


Si Ricketts
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Hi, I recently bought a pretty rough Tal 2 which included a non working motorised  mount. At the moment I’m trying to feel my way around a list of problems which includes buying a new psu for the drive. The psu supplied with scope is UK made and is rated 12v AC 0.7A. This unit does not have any output and is obviously faulty. Unless I’m patient and very lucky I’m not expecting to be able to buy an original Tal psu, so my question is does anyone have an original unit which may have the spec written on it. I’d like to ensure that any new non Tal unit I buy has the same or better current rating to ensure it’s not overloaded.  I do have the original specification for the scope but of course those kinds of specs  aren’t included.  Thanks Si

35E13741-A94E-4F40-BBDC-6117A379F49C.jpeg

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Writing as a former electronics engineer: it definitely says "12v AC", but why?  One could easily get 12v AC from a transformer, which is not a flat object, wheras a 240v to 12v DC power supply would look more like the object in the photo...

Do the insides of the PSU, or the motor, offer any further clues?

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16 minutes ago, Cosmic Geoff said:

Writing as a former electronics engineer: it definitely says "12v AC", but why?  One could easily get 12v AC from a transformer, which is not a flat object, wheras a 240v to 12v DC power supply would look more like the object in the photo...

Do the insides of the PSU, or the motor, offer any further clues?

The Tal2 uses an AC synchronous drive rather than dc stepper motors.

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6 minutes ago, Cornelius Varley said:

The Tal2 uses an AC synchronous drive rather than dc stepper motors.

If that's the case, the supply presumably consists of a step-down transformer, and the problem can be solved by finding where there is a bad connection and fixing it, or replacing the faulty transformer with a new one that gives 12v output with a minimum of 8,4 watts.

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Thanks for the replies,

Thanks Geoff, the pictured transformer is actually a block about the size of a block of butter, sorry about the photo.  It’s also a sealed unit and very solid so maybe filled with resin. Theres no way I can get inside without destroying it. You have made me think though as the only thing I haven't done is check the mains cable continuity between the plug and trans but if a conductor has broken it'll probably be right where the mains cable enters the trans.  I could cut the cable close to the trans and check. My concern is I've seen a picture online of the Tal psu and it’s about 3 times bigger than the one I've got, hence I was concerned about the current rating. I did read somewhere that the original Tal psu was problematic and my broken one may be a replacement. Hence the question.
 

Thanks Peter, I had a poke around the Douglas site and will probably contact them once I've confirmed the spec. 

This will teach me to take on a old basket case. Hopefully someone may be able to confirm what she needs..
 


 

 

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18 hours ago, Cornelius Varley said:

The Tal2 uses an AC synchronous drive rather than dc stepper motors.

That 'synchronous' makes me wonder if the synchronous motor is self-starting. If it isn't, maybe the power pack is OK and the motor has a kick-starter you have overlooked.  (I have a synchronous mains clock that needs a smart tug on the setting knob to start it running.)

Your PSU has a British label on it which suggests that, as you suggest, it is a replacement.  You need a multi-test meter to check this gear out.  

For a competent person, making up a new PSU should not be hard. It just has to supply the right voltage and an adequate amount of current.

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Many thanks  Geoff for taking the time to reply, yes I've done all the normal checks with a meter and had a look around for off the shelf transformers, both the packaged and bare units providing a Primary AC mains tap and secondary tap of 12V AC.

There are a number of 12V AC options here. 

https://pulsardevelopments.com/catalogsearch/result/?q=+12v+ac+adaptor

I've also found this chassis mounted trans which I could easily make up. Its 50VA which should give just over 2A output for the 12V AC side.
 

https://uk.rs-online.com/web/p/chassis-mounting-transformers/0504672

This or something similar would be my ideal choice and I could easily make this up. Any thoughts on the 2A output being good enough

 

 


 

 

Edited by Si Ricketts
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I have had a look at the PSUs.  The panel mounted one is only 3VA so clearly unsuitable.

You should be able to select a Pulsar unit that is:  UK plug, 12v AC output, and delivers at least 8,5VA e.g. 8.5, 12, 15 or 20VA (and is in stock🙂).  The more powerful ones seem to cost more. You also are expected  to select the style of output plug (2.1 or 2.5 inner)  I realise that neither plug may fit your motor, but...

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Thanks again Geoff, after some more browsing last night I  found another more recent TAL manual which stated the power rating of the psu at 10W which works out at 0.8A.

I think this chassis trans should do the job?

It’s a dual output 12VAC with a total 20VA/0.8A rating.
 

https://uk.rs-online.com/web/p/chassis-mounting-transformers/0504571

 

 

 

 

Edited by Si Ricketts
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The chassis transformer you cite will do. But wouldn't it be less work to use one of the ready-made units? You do NOT have to match the wattage of the original TAL unit. Any 12v transformer capable of outputting 0.8A OR MORE will do just as well.  It will output 0.8 A when connected to your motor, regardless of its capacity.

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I like a bit of project Geoff, that’s why I bought the Tal…..😉 Making up the psu will be fine as I've got all the other bits required to make a nice job of it. Thanks again for your help and advice.

 

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I re-built my TAL-2M Mount (used the members help here). The electrical part was easy. My "Power Box" was working, but I opened  it up and just found a standard 12 volt transformer, fuse & switch.  Well constructed, like the scope.  i wonder why it came with a replacement UK Power Brick?  Could you also have a damaged gearbox or worn front Nylon bearing?  My gearbox was fine (I re-lubed it), but that Nylon bearing was gone and I machined a Bronze replacement bearing.  The motor runs on little current.  

 

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Thanks star man, that’s good to hear. Whereabouts is  the nylon bearing located? Any pics? The new chassis transformer arrived and I jury rigged it to test the motor and clutches etc. a couple of bits to complete the enclosure are on their way. I’m currently working through a few things. Motor works fine, I had to adjust the spur gear to eliminate back lash. Next step is to mount up the scope and check and adjust the main clutch. It hasn’t been stored very well over the previous years so lots of fettling to do.  After this it’s a proper optics check and collimation. (First time for me) Will no doubt be posting again soon. 

Edited by Si Ricketts
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Good, these scopes are worth it ! I posted an image (took a while to find it) on 01-13-2021 under my Advert of TAL of Two Cities-2M Mount."  I'm afraid my 73 year old brain did not recall how to attach it directly. I used my mini-lathe to modify the part slightly.  I am in the States, but may have another part. I used a small fine toothed  blade to cut the nylon bearing in two and pass fitted the bronze or maybe Brass one in place and used epoxy to hold it. You would need to carefully unscrew the motor from the gearbox.

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Great thanks, I’ll take a look at the pic. The only wear I can find is on the small clutch shaft where it enters the motor.  Will take a proper look when I strip it completely for a repaint. At the moment I just want to get it pointing skywards.  Although the weather over here is pretty appalling at the mo. 

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