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Tandon 848 Thinline strangeness

T

Tibs

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Dec 10, 2016
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I'm going to start working through the circuit, but if anyone has an idea, I've got a really weird one (well, to me).

One drive works fine, and speed is correct. Works, boots, formats, etc.

The other drive:
When a double sided disk is in, which was what I had when I did drive 0, the speed test just hangs, but when I adjust the RPM it starts to report at 380. The lights on the drive board (Honestly I don't know what one means, unless its double sided detect, stop blinking as well once it falls below this speed. The other looks like an index indicator. When using a single sided disk, it works fine at 360 RPM.

I was thinking it was an index sensor, but since on double sided it works at 380 (reports), and on single it reports 360 (different index sensors I know).

I tested voltage out of the regulator and its giving 12 volt like it should. I'm going to study the schematics and the manual and see if I can determine if there is a logic chip that is bad.

Just curious if someone else might have seen this before. Trashing the logic board just seems wrong. Its also throwing drive not ready errors, but not always.
 
Not on the 848, but I've seen drives that will drop READY if the speed drops below a certain point. That's probably what's happening here--so there's likely an RC time constant that's drifted (look for a one-shot).

Which version of the 848 do you have?
 
Chuck(G) said:
Not on the 848, but I've seen drives that will drop READY if the speed drops below a certain point. That's probably what's happening here--so there's likely an RC time constant that's drifted (look for a one-shot).

Which version of the 848 do you have?
Click to expand...

Its not the E version, I know that much.

I found a tech bulletin that talks about a resistor being in the wrong spot due to artwork errors that caused the index to be missed. I'll check all the tech bulletins because there are at least two that reference this. I think the other was around speed reliability.

I do know the two drives are not identical, because the working one doesnt have the same jumpers as the not working on around the drive speed R133. I'll report back on models and what I find with the bulletins.
 
If your drive matches the schematic over at Bitsavers, have a look at U17, pin 12 and see if its behavior matches the <> 380 RPM situation. Could be that either C36 or R36 has drifted in value.
 
Chuck(G) said:
If your drive matches the schematic over at Bitsavers, have a look at U17, pin 12 and see if its behavior matches the <> 380 RPM situation. Could be that either C36 or R36 has drifted in value.
Click to expand...

Will do. Studying the service guide, learning how these work. They are 848-2, rev M. Odd that one drive does not have jumper pins for many of the motor settings but the other one does. I'm going to first verify all the programming is right, then I'll test the index lines of both double and single sided drives. The speed test in software may not be accurate, but the scope will be. Seems ready logic and motor control are in two separate sections, so I need to determine if its one or the other before going further.

I do have an alignment disk for double sided drives, so will be able to do the other tests. Goin to go read the bit savers stuff too. I actually enjoy working on floppy drives so I'm demented in a way.

Edit: Also going to check the load arm, since I am using two different brands/type of disks and getting two different speeds. I cant imagine the drive would run at two different speeds based in which index sensor is being used.
 
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Chuck(G) said:
One of the benefits of a big old line-operated spindle motor! You never worry about the rotation speed.
Click to expand...

Yeah, its funny you say that because I remember you saying that and it just kept going through my head.

I went and put the critters up, and walked by the drive, took a look and C36 looks like it may have failed. Drive is programmed right as well.

One of the two legs has some green crud around it. So, if I don't get solid speeds on the scope I'll look at that area first. I just don't trust software for speed testing.

Ok, I see now, DS1 and DS2 are the two status LEDs. Once is ready and one is the index pulse. Makes sense what I'm seeing with them.
 
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pski said:
What would we do with our TM-848s without Chuck(G) around?

http://www.vcfed.org/forum/showthread.php?53777-TM-848-8-quot-Floppy-Drive-Error-Codes

Thanks, Chuck!
Click to expand...

No kidding. I read that thread and I also have found that sensor works on the TM-50s. And there are other drives as well that use it. I have a pile of them I ordered when I ran across the first bad Track 0 sensor.

But hey, thanks to everyone here, I got to fire up Xenix 3.2 today after making some disks. Kind of cool. Now to get the elusive hard drive controller.
 
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