721 platters and motor

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joea
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721 platters and motor

Post by joea » 19 Nov 2019 16:52

I just picked up a 721 that does not spin. Acts as if no power.

Noticed when I got it home that the platter does not match the other 721 I have (also not working at the moment), which has a much earlier serial number. The differences at a glance appear to be the rubber mat embossing and the strobe marks, the later on having several more rows of marks. The motor also has a black center section, the earlier one a "natural" aluminum finish.

While I dimly recall some threads on the strobe mark variations, nothing is popping out at me now. I've attached pictures for clarity. Hopefully.

Comments?

Tinkaroo
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Re: 721 platters and motor

Post by Tinkaroo » 19 Nov 2019 18:16

You forgot the pictures.

There were two platter versions. The second had 4 tiers 33 and 45 at 50 hz and a second set for 60 hz. The first had one tier only for 33 RPM.

Have you checked that none of the safety caps or fuses are blown?

Will it start if you move the arm manually?

joea
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Re: 721 platters and motor

Post by joea » 19 Nov 2019 18:31

Tinkaroo wrote:
19 Nov 2019 18:16
You forgot the pictures.

There were two platter versions. The second had 4 tiers 33 and 45 at 50 hz and a second set for 60 hz. The first had one tier only for 33 RPM.

Have you checked that none of the safety caps or fuses are blown?

Will it start if you move the arm manually?
Oops. I spaced and did a "save draft" and after messing with how to post it, must have dropped the pics. Attached.

It does not start if I move the tonearm manually. I've not checked anything else yet. I did print out the schematic and will start on it soon, but have a 1219 still in the "fix-it" jig, so need to clear that out first.
Attachments
older-platter.jpg
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older-motor.jpg
(78.84999999999999 KiB) Downloaded 71 times
newer-platter.jpg
(54.53 KiB) Downloaded 67 times
newer-motor.jpg
(76.66 KiB) Downloaded 72 times

Tinkaroo
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Re: 721 platters and motor

Post by Tinkaroo » 19 Nov 2019 19:06

I think the motor is the same but perhaps a different paint scheme on the top part.

There is a little switch box which may be KS4 that needs to make a circuit to start when contacted by the linkages beneath. You may want to check of that is happening or if the contacts need cleaning perhaps. You can see a photo of it here:

https://old-fidelity.de/thread-10338.html

joea
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Re: 721 platters and motor

Post by joea » 19 Nov 2019 21:00

Tinkaroo wrote:
19 Nov 2019 19:06
I think the motor is the same but perhaps a different paint scheme on the top part.

There is a little switch box which may be KS4 that needs to make a circuit to start when contacted by the linkages beneath. You may want to check of that is happening or if the contacts need cleaning perhaps. You can see a photo of it here:

https://old-fidelity.de/thread-10338.html
I have to brush up on my German. Or hunt up that "translate page" thinglet that must still exist.

GuidoK
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Re: 721 platters and motor

Post by GuidoK » 19 Nov 2019 23:03

There are 2 versions of the 721 which have different platters. The 2nd gen has the platter with the 4 rows of strobe points and the 1st one has the single, intergrated one (which I think looks much better but maybe not so convenient in use)

joea
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Re: 721 platters and motor

Post by joea » 19 Nov 2019 23:34

GuidoK wrote:
19 Nov 2019 23:03
There are 2 versions of the 721 which have different platters. The 2nd gen has the platter with the 4 rows of strobe points and the 1st one has the single, intergrated one (which I think looks much better but maybe not so convenient in use)
I agree the single row looks nicer.

Are there any significant differences that would affect problem resolution? Such as circuit board, wiring, part numbers? I have not scoured the manual on my own as yet.

GuidoK
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Re: 721 platters and motor

Post by GuidoK » 19 Nov 2019 23:54

joea wrote:
19 Nov 2019 23:34

Are there any significant differences that would affect problem resolution? Such as circuit board, wiring, part numbers? I have not scoured the manual on my own as yet.
I have no idea, I only have the 1st gen.
I imagine that if there are any technical differences, they are very small.
When I did some research on which gen to get, I didn't find any major differences except the platter/mat (but this is a long time ago).

But I expect your problem won't be particular difficult to faulttrace. I suspect it's the internal switch or wirebreak or such.
I don't expect it's a PCB problem as your strobe light also doesnt light up.

sotpal
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Re: 721 platters and motor

Post by sotpal » 22 Nov 2019 01:37

joea wrote:
19 Nov 2019 16:52
I just picked up a 721 that does not spin. Acts as if no power.
In addition to what has been said for the switch,
there is also a fuse inside the transformer box of the 721.

mrow2
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Re: 721 platters and motor

Post by mrow2 » 24 Nov 2019 08:23

I’ve read in the past that the power supplies go bad. Someone I met a few years back who was good with electronics had adapted an older laptop power supply for his. It should be checked in any case, especially If the trouble is not a simple matter of switch cleaning or repair. If you replace it you would want to be sure it met or exceeded the specs of the original. This gent said the one he used was much better than what was needed in his 701.

The platter with 4 rows of strobe marks is the easiest to read, my opinion. Two pairs of markings, one set for 50hz, the other for 60hz systems. The other one is a little more difficult to view at 33, and to my eyes it doesn’t seem very satisfactory at 45rpm.

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