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921b sync clone

System 55 • System 35 • Model 15

921b sync clone

Postby Rarecomponentadsr » Tue May 08, 2018 8:15 pm

Not having much luck getting my clone 921b sync section to work.

Does anyone know if pin 10 of the ua796 in the sync section, is actually connected maybe to -6 ? The schematic says blank...(apart from connections to R36 and R57)

Also, is there any other schematic available apart from the Norlins ?
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Re: 921b sync clone

Postby WZOLL » Wed May 09, 2018 10:53 pm

I cant find any other schematics but I believe you are correct with -6v. According to the ua796 datasheet, in the metal can pinout, pin 10 = V- and in the 14 pin dip pinout, pin 14 = V-. I can't quite tell from pictures but it appears that Moog used the metal can 796 in 921b modules. Hope this helps.
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Re: 921b sync clone

Postby Rarecomponentadsr » Thu May 10, 2018 5:59 am

I am using the metal can version ...I will hook up -6v to the pin 10 and see if I can bring the sync to life..
Thanks for your input. I'll report back the results.
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Re: 921b sync clone

Postby dombedos » Thu May 10, 2018 6:09 am

I can definitely confirm that pin 10 is connected to -6; the track from pin 10 of 796 connects R36 and R37, and then continues to the collector of Q5 and to the trimmer R30. It is connected to the -6 line by way of other tracks on the components side.
I have reconstructed the original pcb layout some time ago. Recently, I resumed my long-term project of cloning the 55 after a long stop, and just these days I'm reviewing the 921a and 921b pcb layouts, before etching the plates.

There is another detail in the circuitry around 796 which appears strange to me.
The resistor R102, 1k, should connect pin 8 to ground but, on the actual object there is a track which simply shorts the resistor. The track does not appear to be cut, and the same feature appears on other 921b of different "era's". Does anyone have a possible explanation for that? Should it be an error, is it possible they did not notice it on the long run?

Also, there are other errors on the pinout of IC12, but maybe you already know them as, if I remember well, this has been discussed in the past here.

I'm using the schematics from the service manual; it should be the same of Norlins.

Regards from Italy

Leonardo
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Re: 921b sync clone

Postby Rarecomponentadsr » Thu May 10, 2018 8:12 am

Thanks very much for your very helpful info.

I have connected pin 10 to -6 v but alas the unit is still dead. I suspect, as you say, the fault might lay around
IC12 where the schematics are a little vague. I need to start again I think.

Interesting about R102..it's always good to check and compare the Moog modules if possible.
I will have a genuine Moog 921b soon so I will check what's going on there and report back.
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