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Old 2nd Jul 2022, 9:13 pm   #1
toxtethogrady
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Default Hitachi SDT-7785 Music centre - tape belt issue

Good evening. My usual repair items are 50s Bush radios and 8 bit computers but I've come across one of these and its a bit beyond me at the moment ! All is fine except the tape deck - if I should have posted this in the Tape section I apologise.

The eject button does not open the cassette door (which is empty) , the other buttons except record go down but the mechanism does not rotate. Not surprising as on disassembling, the cassette mechanism has some hanging shreds of belts and lots of horrid black slime. Is the lack of belts responsible for the door not opening - I know its a "power assisted" deck but I don't really know what that means.....

Can anyone help with where to start please? I have found a service manual online but it is plagued with many pop up ads and I can't find anything about belt replacement.

Thanks in advance for any help, cheers, Steve
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Old 2nd Jul 2022, 10:14 pm   #2
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Default Re: Hitachi SDT-7785 Music centre - tape belt issue

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Originally Posted by toxtethogrady View Post
Is the lack of belts responsible for the door not opening - I know its a "power assisted" deck but I don't really know what that means.....
Hi Steve and welcome!

"Power assisted" means that the buttons only need a light push. Pushing one causes cogs etc. to engage with a pinion on the capstan flywheel, which then provides the mechanical power to move the heads into position, etc. This became common on better machines in the late 1970s.

The older or more basic machines had buttons which did the work directly, often requiring a very firm push (and a steadying hand to stop the whole machine sliding around on the shelf!).

There will be an interlock whereby the door can't be opened unless the mech is in stop mode, i.e. heads and pinch wheel retracted and brakes applied to the reel tables. This is to prevent damage to the cassette and the heads etc.

If the belts have gone, the mech probably isn't in the aforementioned state, so the door will be locked. Manually rotating the flywheel in the right direction while pressing stop will probably release the lock.

The goo is a pain to shift, and gets everywhere. Cif cream cleaner is suprisingly adept at getting it off, but obviously it can only really be used on a dismantled mechanism, because it has to be washed away with plenty of water and the components thoroughly dried. It's abrasive, so if it gets into bearings etc. and is not rinsed out, it will cause no end of trouble.

Good luck! It will take ages but it's all good fun.
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Old 2nd Jul 2022, 10:24 pm   #3
toxtethogrady
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Default Re: Hitachi SDT-7785 Music centre - tape belt issue

Thanks Nick, that makes sense. Was thinking of using isopropyl alcohol on the goo, would that be very wrong?

Service manual doesn't seem to say what belt(s) I need to get or how they run - or can I get a more complete manual somewhere ?
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Old 6th Jul 2022, 11:52 am   #4
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Default Re: Hitachi SDT-7785 Music centre - tape belt issue

If ipa doesn't shift it, try white spirit tho' it is a bit smelly tho' it does dissipate in time.
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Old 6th Jul 2022, 12:44 pm   #5
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Default Re: Hitachi SDT-7785 Music centre - tape belt issue

De-odourised (?) white spirit is an excellent product, so much nicer to use than the ordinary stuff. Toolstation sell it, amongst others.
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Old 6th Jul 2022, 1:09 pm   #6
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Default Re: Hitachi SDT-7785 Music centre - tape belt issue

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Originally Posted by toxtethogrady View Post
Service manual doesn't seem to say what belt(s) I need to get or how they run - or can I get a more complete manual somewhere ?
No, you need to be clever and work it out for yourself

Ask us for help after the big clean up.
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Old 6th Jul 2022, 2:30 pm   #7
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Default Re: Hitachi SDT-7785 Music centre - tape belt issue

I have a count of 70 pages for the SDT-7785 service manual. Don't know how that compares with your downloaded version. I could scan and email missing pages to you if it helps. Send me a PM if you think it might be use.

Dave
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Old 6th Jul 2022, 6:07 pm   #8
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Default Re: Hitachi SDT-7785 Music centre - tape belt issue

I've downloaded all the pages of the SDT–7785 S.M. and from what I can tell it seems to be all there!

To find the belt sizes, cut a length of thin string just long enough to stretch between the pulleys, etc., where it threads, measure it's length and divide by π (3.142) to give the calculated diameter, reduce the figure just calculated by 5% to get the belt diameter needed!

For example, if you measured 200 mm between the capstan and flywheel pulleys with a piece of string that just fits between them, the diameter needed is 0.95 × (200/π) = 60 mm.

If the belt for the capstan/flywheel is of the flat type, then a width of 5 mm will be adequate in most cases!

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Old 23rd Jul 2022, 12:55 pm   #9
toxtethogrady
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Default Re: Hitachi SDT-7785 Music centre - tape belt issue

So....I've taken deck apart, cleaned the black goo off, at least as much as I can, it was EVERYWHERE. Found a diagram, obtained a replacement set of belts.....

Having big trouble getting the main belt in place (and to stay in place) on the flywheel - the edge of which is completely flat, there's no bevel or lip at all to keep the belt in place. Any tips please? Is it best to put the belt round the motor end first and stretch it over the flywheel, or drape round the flywheel then stretch over the motor - neither seems to work for me. The belt kit was a direct replacement so I'm fairly sure I've got the right belt, you just seem to need the dexterity of a safe cracker to put it on .....
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Old 23rd Jul 2022, 6:12 pm   #10
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Default Re: Hitachi SDT-7785 Music centre - tape belt issue

Often the motor pulley is wider at the middle (barrel shaped) to help keep the belt running true. If the belt keeps dropping off when properly in place, it suggests that the replacement belt is too large a diameter. That or a belt tensioner is missing, although I don't think that tape deck has one. The belt should be a slight stretch to fit and then retract to stay in position. The only other thing that comes to mind is if the motor pulley is not vertical along with the flywheel from the chassis due to the motor mounting being at an angle for some reason.

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Old 23rd Jul 2022, 8:00 pm   #11
toxtethogrady
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Default Re: Hitachi SDT-7785 Music centre - tape belt issue

It's not so much that it drops off, but that I can't get it on in the first place. The flywheel is only about 5mm wide and is completely flat, I can't seem to stretch the belt round it in such a way that I can get it over the motor pulley without it slipping off the flywheel, I seem to need an extra couple of hands. I'll persevere though, it's just so fiddly. Thanks for the reply, if I can get it on in the first place I'll let you know if it stays on when it's running
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Old 24th Jul 2022, 6:36 pm   #12
toxtethogrady
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Default Re: Hitachi SDT-7785 Music centre - tape belt issue

Right, progress made !
Replaced all the belts and cleaned the heads , everything appears to work mechanically ok, rewind , FF, play. Initially the output from the tape section was REALLY quiet - stumped me for a bit, then I spotted on the block diagram the "playback mute" switch - checked and the record button was indeed stuck, so some switch cleaner on the switch on the board and worked it a few times and now sound is loud so I guess that was the problem.

Minor issue still - when you close the door with tape in the metal "prong" that goes across the left side of the tape (I think its the thing that detects whether the tape has the erase protection tab in place) doesn't latch over the tape properly so the left side of the tape won't sit right till you lift the prong over manually - I presume this is something I created when disassembling/assembling the deck , I can't quite see what though.

There's a fair bit of hiss through one channel even when not playing a tape - you can hear the same when using the tuner it just isn't so obvious with music playing. Presume this must be in the output stage somewhere.

Not quite there yet but definitely progress - thanks chaps !
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