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mnemo

1050 Unknown Thing at J6

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Hi, just a quick question. I recently acquired an Atari 1050. It has a 3-position switch with red and green LEDs, probably for Write Disable/Enable and a "1050 Turbo" board. While removing the 1050 Turbo and some cables I noticed something under the front right of the main PCB. It is connecting two pins of J6, which seems to be the connector for the read/write head. Does anyone know what it is and what purpose it has? As you can see, one of the legs came off, so it can't be active now. It's not connected to anything else.

 

Could it be a reed switch? What for?

 

Sorry for the blurry pictures, I wasn't able to make the phone cam focus correctly.

 

komp1.jpg

komp2.jpg

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It might be C3, a 47pf capacitor. But it should be missing from the top side mounting spot for it. They might have run short on a suitable sized one and had to move to the bottom side to place this one that they could get that day? I'd be using a multimeter on it to test for reed switch function, diode function, etc. And I don't think I have one that measures capacitance just yet either, I should actually find out and then get one as it's about time.

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I finally managed to make better photos and identified the component, it's a Telefunken germanium detector diode, type TFK OA172.

 

Why it was soldered to the J6 connector of the 1050, I still have no idea.

 

 

tfk1.jpg

tfk2.jpg

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CR10, pins 2 (cathode) and 4 of J6 instead of pins 1 and 3 for the capacitor. It's oversized to say the least for a fit on the top side where there should be a labeled spot for it and all that assumes it's a factory mod in the first place. Just noticed on the sams schematic, pin 0 is ground for the shield, a zero based pin count is first for me, leave it to Atari. No wonder people get slid into the ditch working on these. That guy backengineering that board for the Sams schematic had a handful to figure that one out and keep it all usable. Nothing wrong with your pictures at all, I wish I could do as good myself.

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As a replacement for CR10 that would make sense, but as you can (or cannot really) see in the second picture of my original post, the wires are connected to pins 3 and 4, not 2 and 4. But connecting pins 3 and 4 makes no sense, right? Since the diode was not actually connected to pin 2, but hanging loose in there... maybe someone tried to make a fix and someone else then removed one end of the connection? But if that was the case, it would make more sense to remove the diode altogether. I just don't get it.

 

I still have to try to write with the drive - maybe that might reveal a problem. After I tried that I'll post here.

 

image.png.c43b6ea6e98f8dd839c2d238f3f2754b.png

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It wasn't too clear which was the actual connection here and I wasn't sure which pin to start counting from either. Getting a board in hand might have fixed that, but finding out it's a zero based count wasn't known here until my second post. It won't write well without the diode installed on pins 2 (cathode) and pin 4. If it was wrong to start with then that might explain why it was cut due to inadequate performance. But pin2 and pin4 are the erase head which takes a good deal of RF power to null the data that is being overwritten, the center tap on the write head (not clearly shown as such on your sobola schematic) is half wave DC biased by that diode to make the new data field desired to magnetize the material beyond it's hysteresis curve. The DC bias is added to the read/write head at that center tap and causes the strong magnetizing signal to be at the extreme in both directions alternately with the RF mostly used by the erase head. So if the diode isn't there it could work anyway, but the performance is bound to suffer and possibly catastrophically. I would restore it as it was supposed to be if it were mine and yes only YOU can count which pin it was actually soldered to. Your sobola schematic is correct as to CR10. R2 should have a note of a 2 Watt rating on that resistor as it's a quite large resistor and a significant change from the norm. RF power is required during both erase and writing the new data, further the RF power is needed by both the erase head and the read/write head at the same time. Atari engineers have done a brilliant job of keeping the confusing circuit simple and quite robust at the same time.

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