Reparing a Dec RE81/RA82 Hard disk unit. Help required :) - Page 1
Forum section: electronics / general

author: mancausoft
author_rank: Contributor
timestamp: March 09, 2019, 05:29:09 pm
content: Hello,We start to try to run our VAX 11/780, and we start trying the Hard drive unit.This vax have one RA81 and one RA82. Plus we have another RA81 from another Vax.All three disks are faulty, we are use the diagnostic serial (yes they have a diagnostic serial menu running in a pdp11 inside the unit) and we discover the problem the speed sensor.The speed sensor it's just a led with a photo transistor that read the gap in a metal ring (you can see the photo on the attachment)the problem is that i can't find the datasheet for the sensor a LG07-CD but I have the schematics of this disks I found the part but i don't understand the symbol of the transistor:I speculated it was a darlington, but i'm not understand the part where the collector and the emitter of the second transistor are connected together, it's make no sense to me. It's have some particular meaning?We fix one sensor removing the no more transparent window on the sensor... and we was able to backup all data of the disks.I will try to made a new sensor using a 3d printed enclosure with a led and a photo-transistor SMD, but i don't know white type of transistor i can use.I have one of this sensor and i will measure the bandwidth (i think with an homemade encoder wheel and a motor to see when it stop to see the gap)But how i can measure the max out current of the sensor without risking to break the sensor?EDIT: other photos of the all restoration process here:https://www.facebook.com/pg/MuseoInformaticaFunzionante/photos/?tab=album&album_id=1657047637665489

author: edavid
author_rank: Super Contributor
timestamp: March 09, 2019, 05:36:17 pm
content: Quote from: mancausoft on March 09, 2019, 05:29:09 pmIt is just drawn in a sloppy way. Ignore that line.QuoteJust measure the pullup resistor, then you will know how much current you need to pull down the signal.

author: mancausoft
author_rank: Contributor
timestamp: March 09, 2019, 07:10:06 pm
content: Quote from: edavid on March 09, 2019, 05:36:17 pmas I had thought wellQuotesimpler than I thought, I should have even the schematics with the value of the resistor but at the moment i can't found where is it.Thanks a lot.

author: floobydust
author_rank: Super Contributor
timestamp: March 09, 2019, 08:13:26 pm
content: check the tiny holesslotsare not plugged with dust, they look dirty and would block light.edit:If the optical switch can be taken apart, you can replace the part. There could be a cover on the back of it. I find the LED usually goes dim. Inside the housing are two flat-packaged LED and photo-transistor.The photo-transistor is not critical.I did see parts similar likeSG23FH,orOmron EE-SV3but they do not have flying leads.edit 2: similar thread for RA82 but the guy hacked it apart to fix ithttp://www.vcfed.org/forum/archive/index.php/t-23852.htmlThe phototransistor is also wrong on that schematic. I think it has an extra transistor stuffed in there to make it a Darlington (see thin blue wire). 470R pullup to 'LS14 with the LED driven hard at 82 ohms to 5V. They were trying to get fast rise/fall times. You can pop off the cover and take it apart if need be.https://photobucket.com/gallery/user/ballsandy/media/bWVkaWFJZDoxMDQxMTA4NDU=/?ref=https://photobucket.com/gallery/user/ballsandy/media/bWVkaWFJZDo3MDgxNjkzMQ==/?ref=

author: mancausoft
author_rank: Contributor
timestamp: March 09, 2019, 09:59:19 pm
content: theQuote from: floobydust on March 09, 2019, 08:13:26 pmThe holes are filled of some transparent plastic, that now is no more transparent, this is the problem, in one we remove this plastic with a needle and it's start to work, but is not easy to remove, and was a bit easier in that sensor because the plastic was more damaged.EDIT:here the detail, is not dirty, i think is degraded plastic:QuoteYes, I want use a flat-packaged, but i was thinking on a 3d printed.I almost read all the catalog of the Omron sensor and all was all different size or orientation, not easy to adapt.I'm checking the asiantech site, and the SG267V2 can be adapt, but i'm aware about the distance of the two sensors that should be 5 mm but in the datasheet of the SG267V2 is only 4.2 (the omron 3.4), i don't know it there is enough clearance for the metal disc with all the vibration and so on...But there are other on asiantech with 5 mm of gaps, i will try to see if is possible to create an piece to mount the sensor to the screw hole on the HD. (I think it's not wise to make new holes or changes to the disk shell)