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Trying to use a FD501 floppy drive

Sealtest

New Member
Joined
Oct 8, 2017
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9
Hey all, this is my first "working" vintage machine. Long story short, but I'm the local junk guy. I was driving a back road and found a hoarder-style junk shop run by a little old man since the 70s. Over a few weeks I kept finding bits and pieces of old machines like a 128k macintosh (no keyboard or mouse, and needs the PS rebuilt), 2 Vic-20's + 1541 floppy drive, and other bits. I kept finding Tandy bits until I found a hidden room that was hidden due to junk. Inside I found this Tandy trs-80 color computer 2 all set up with a jcp tv, two fd-501 drives (one with card one without) all under a mountain of junk and animal feces.

So I have cleaned up the Machine itself and it has its quirks with the keyboard, but has gotten better. I have been working through the booklet online that teaches basic programming and introducing you to how to use the machine.

So here is the problem. The machine works and can do basic stuff, but it freaks out when I connect the FD501. The screen goes into a bunch of colors and patterns that don't make sense, like how a nintendo game would mess up. I have cleaned all contacts and have checked for cold joints, with no luck. My drive was running every time I turned on the machine but then stopped, but did run again when the machine was on and I pushed the card in to ensure contact (it did not like this, and probably isn't good for the machine).

Are there any troubleshooting things I should be doing or common issues like bad components I should look out for? I used to work on vintage stereo gear and older CRT TV's so it shouldn't be too bad. It seems something is missing.
 
That’s a neat find, @Sealtest ! It sounds like it was a ton of work to get it, so kudos to you for your efforts.

My guess is that there is a chip on the board that has a shorted pin. I’d start with taking some high quality pictures of the PCB from the disk interface. If you want to post them here, folks who are CoCo experts can probably chime in with their thoughts.

Good to have you here!
- Alex
 
So here is the problem. The machine works and can do basic stuff, but it freaks out when I connect the FD501. The screen goes into a bunch of colors and patterns that don't make sense, like how a nintendo game would mess up. I have cleaned all contacts and have checked for cold joints, with no luck. My drive was running every time I turned on the machine but then stopped, but did run again when the machine was on and I pushed the card in to ensure contact (it did not like this, and probably isn't good for the machine).

You definitely don't want to be plugging in cartridges when the machine is on. That's a really good way to melt either the CPU or the SAM chip, and the latter in particular is getting hard to find these days.

So at the most basic level the first thing I'd suggest trying is starting from the very basics: disconnect the controller box from the drive cable and see if the machine will power on to the BASIC prompt without a drive attached. The CoCo doesn't attempt to automatically boot from a disk drive when powered up so it won't care if there's no drive. Probably a long shot, but it's at least possible if there's something wrong with the floppy drive like an internal short it could be disrupting the operation of the computer.

(Re: the drive running, I believe the particular model of drive that was packaged with the FD-501 controllers automatically spins briefly when powered on or when a disk is changed, so I don't think the activity you're seeing is being triggered by the computer. Spinning when you jam the controller in hot is probably just the result of a transient being passed through the control line.)

Assuming the drive controller box still gives you garbage when plugged in by itself, well, there's not a lot going on inside it. The ROM chip would probably be my first guess for a single component failure. I don't think the ROM even really relies on anything else in the cartridge working, it's pretty much hanging directly on the address and data lines from the computer. So unless something has gone badly wrong enough somewhere else on the board so it's dumping trash constantly onto the data bus I'd think the thing would be able to get to the BASIC prompt.
 
You definitely don't want to be plugging in cartridges when the machine is on. That's a really good way to melt either the CPU or the SAM chip, and the latter in particular is getting hard to find these days.

So at the most basic level the first thing I'd suggest trying is starting from the very basics: disconnect the controller box from the drive cable and see if the machine will power on to the BASIC prompt without a drive attached. The CoCo doesn't attempt to automatically boot from a disk drive when powered up so it won't care if there's no drive. Probably a long shot, but it's at least possible if there's something wrong with the floppy drive like an internal short it could be disrupting the operation of the computer.

(Re: the drive running, I believe the particular model of drive that was packaged with the FD-501 controllers automatically spins briefly when powered on or when a disk is changed, so I don't think the activity you're seeing is being triggered by the computer. Spinning when you jam the controller in hot is probably just the result of a transient being passed through the control line.)

Assuming the drive controller box still gives you garbage when plugged in by itself, well, there's not a lot going on inside it. The ROM chip would probably be my first guess for a single component failure. I don't think the ROM even really relies on anything else in the cartridge working, it's pretty much hanging directly on the address and data lines from the computer. So unless something has gone badly wrong enough somewhere else on the board so it's dumping trash constantly onto the data bus I'd think the thing would be able to get to the BASIC prompt.

I have an interesting update on this. I did order a replacment controller chip off the bay for it, but have not received it.

What's interesting is I went back to where I got this system and I found two more fd-501 drives WITH the cards. One card does boot and one does the same as the other, which is interesting.

Regardless I now have a stack of 4 drives, and no floppies for them (as far as I know). The programs I have are homemade and probably to an IBM machine.
 
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