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Replacement tachometer sensor for DEC RA80/1/2 drives ...

SpareTimeGizmos

New Member
Joined
May 24, 2020
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Location
Silicon Valley
I have three DEC RA8x drives that have failed (all of them fault with "spin error") because of bad photo-interrupter tachometer sensors. After talking to a few friends, it sounds like this is a pretty common fault. Photo sensors like this are fairly common, even today, but the specific parts DEC used are weird and unobtainable. So I designed a little PC board that uses an ITR9606 photo interrupter, a 2N3904 and a resistor as a replacement. Works great - gives a beautiful 5V P-P clean waveform and with the PCB it's a mechanical drop in replacement. Just screws right onto the original mounting holes and plugs into the original connector.

I've put the PCB design up on OSH Park

https://oshpark.com/shared_projects/z8DSkQsP

If anybody needs one you're welcome to order some PCBs and build your own. The ITR9606 is as common as dirt, and you can buy bags of ten on Amazon for a few bucks.

RA8xTach (small).jpgRA8xTach Waveform.jpg
 
Welcome Bob, and bravo! Now, one for the RK05, please. And the rest of the electronics to allow the RK drives to spoof the transducer count to allow interchange of 12 and 16 sector disks.
Does the RK05 have a similar problem? I didn't know that, but I do have a couple of RK05 drives that I want to get working sometime. Unfortunately I have to fix both my RK8E and my RK11D first, though.
 
I'm not aware of frequent transducer failure on the RK series drives, but there are several versions of the 14" platters (DEC + others) that use different sector numbering. Being able to read an OS/8 platter (16 sectors) on a PDP-11 machine that expects 12-sector platters would be very helpful. HP uses a similar platter set up for 24 sectors, so helpful there too.
 
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