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Decmate II question
- Thread starter tradde
- Start date
Question: Is it possible the floppies I have that are listed as bootable were created on a different machine where the alignment of the head was slightly off or the head in this machine is slightly off and that's why it won't boot? I am about ready to swap over the floppy drive from the Decmate to this chassis to try that. Sure sounds like the floppy is doing something but I really don't know what. Checked both ends of the floppy cable and no ben pins. Not sure what else to do. Thanks for any suggestions.
That actually worked and my test diskette booted and ran a full system test. I did hit the return key on the keyboard but believe that didn't do anything. I still got the floppy picture but it didn't take long to display a different screen. Waited quite a bit longer with the original floppy and it never did this. I am pretty sure the HD that came with the Decmate has issues. It's an RD53 I think or something of that vintage. It most never does anything. Doesn't even seem to spin up. Will have to try to find some sort of replacement which won't be easy I am sure. Don't want to use the HD from the Rainbow as it likely has stuff on it. Not even sure if that drive would work with this controller. Still progress is progress. Now I'd like to boot CP/M as I've never used it but am fascinated to try it.
bqt
Veteran Member
I don't think you can get out of alignment on the floppies. However, you might consider cleaning the heads. But the drive might just be bad as well.
I was going to say that the RD53 is notorious for being uncooperative when it age. There are some tricks that one can try, but since you later corrected it do an RD32, that's a different story. However, I wonder if you mistyped. I believe there was RD31 and RD32, but they were newer, and I wouldn't expect that the DECmate would know about them. I would have thought it only supported RD50, RD51, RD52 and RD53. Anyway, if you have a rubber mallet, you might consider give the disk a whack or two, to possibly get it unstuck. The spindle can be an issue, but also the heads can get stuck and some mechanical force might solve things.
Oh, and if you think of booting CP/M on the DECmate, I hope you have the coprocessor card needed in that case, as CP/M don't run on a PDP-8.
Or else boot CP/M on the Rainbow.
I was going to say that the RD53 is notorious for being uncooperative when it age. There are some tricks that one can try, but since you later corrected it do an RD32, that's a different story. However, I wonder if you mistyped. I believe there was RD31 and RD32, but they were newer, and I wouldn't expect that the DECmate would know about them. I would have thought it only supported RD50, RD51, RD52 and RD53. Anyway, if you have a rubber mallet, you might consider give the disk a whack or two, to possibly get it unstuck. The spindle can be an issue, but also the heads can get stuck and some mechanical force might solve things.
Oh, and if you think of booting CP/M on the DECmate, I hope you have the coprocessor card needed in that case, as CP/M don't run on a PDP-8.
Or else boot CP/M on the Rainbow.
DougIngraham
Veteran Member
You certainly can have alignment issues with floppies. I experienced situations where drives went out of alignment if you attached them with all four mounting screws. You would look at the aluminum drive castings and swear that shouldn't be a problem. Probably not the situation here. I have not looked into how you align one of these drives. Somewhere I have both an analog and digital alignment diskette. It would take a fair amount of programming effort to setup a DECmate to do the alignment procedure unless there was a field procedure for it and the programs are still available. I imagine that a floppy issue would have been a drive swap as that would take minutes of the field techs time and an alignment would take a lot longer.
Cleaning the heads is a good suggestion and there could be something wrong with the drive, more than just alignment.
DougIngraham
Veteran Member
Where did you get the test diskette. I've not seen one and would like a copy if that is possible. At the moment I am not sure I have a good boot disk. I grabbed images and was able to write floppies that boot. I can do a DIR and see what is on them but the programs I tried to run all give SYS ERR or some message like that. And that doesn't make much sense. When I get back to that project I was going to search for more images to try and look at the programs and see if they differ from the OS/8 versions.
Mine has a Seagate branded drive, pretty sure it is an ST506. Drive spins up but it doesn't appear to do anything after that except try to boot from the floppy drive. The boot code tries the hard disk first if present and then goes to the floppy drive.
CPM is another thing entirely. You need any one of the three APU boards to get the Z80 and you need the special version of CPM for the DECmate. The PDP-8 portion of the machine does all the I/O for the Z80.
I don't think you can get out of alignment on the floppies. However, you might consider cleaning the heads. But the drive might just be bad as well.
I was going to say that the RD53 is notorious for being uncooperative when it age. There are some tricks that one can try, but since you later corrected it do an RD32, that's a different story. However, I wonder if you mistyped. I believe there was RD31 and RD32, but they were newer, and I wouldn't expect that the DECmate would know about them. I would have thought it only supported RD50, RD51, RD52 and RD53. Anyway, if you have a rubber mallet, you might consider give the disk a whack or two, to possibly get it unstuck. The spindle can be an issue, but also the heads can get stuck and some mechanical force might solve things.
Oh, and if you think of booting CP/M on the DECmate, I hope you have the coprocessor card needed in that case, as CP/M don't run on a PDP-8.
Or else boot CP/M on the Rainbow.
Funny thing is that drive worked fine with the Rainbow. So it seems diskettes created with it will work and those not might not. The HD is an ST-251. Don't know if it is original or added later. I read the hand written label on top that said RD32 formatted as an RD-31. The power supply on the original Decmate seems to have issues and that is probably why the drive did not spin up.
Yes, the Decmate has the APU card for CP/M.
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The floppy drive is mounted on a sled so no screws to hold in. So that should not be the issue. I have no idea how to align a floppy and likely would not have the tools to do so anyway. May just have to live with each floppy just dedicated to its machine and not swap between drives.
The test diskette was just the top floppy in the pile I received with the Decmate and Rainbow. It is hand labelled as: BL-HV86A-MV 075591 then DM SYSTEM TESK DISK v4.5. I am not sure I have a way to copy it. It is very limited in what it can do. A full system test, format a hard drive, and a few other basic functions. If I can get MS DOS or something running I might be able to make a copy. But remember this or the other floppy may be out of alignment with spec so you might not be able to read it.
How did you create your diskettes for your work?
Yes, my Decmate has the APU board.
Even Adventure runs. Joy! Now to figure out what's wrong with the other PS. Nothing obvious at a first glance. Nothing appears to have exploded or burned out. I do not have experience with fixing power supplies or even diagnosing. I really don't even like the idea of tinkering inside one unless it's a very obvious problem. Never have learned (and I've tried) how to read an electrical schematic. Any suggestions or well known failures for these?
bqt
Veteran Member
MS-DOS would imply some DDDS floppy drive. Not my first choice, but maybe possible.
If you have any PDP-11 with an RX50 hooked up, and RSX running, then it's trivial to duplicate any RX50.
Probably easy with other systems as well, it's just that I know exactly how to do it with RSX, and have done this many times.
DougIngraham
Veteran Member
I put a 1.44 mb drive and a 1.2 mb drive in an old celeron box and booted MSDOS. Then used the program PUTR to install diskette images on the media. I have a 1.44 mb USB drive on my Win 10 laptop so I could get the images to the MSDOS machine. PUTR can image your diskettes and then it would be easy to email them or download from someplace.
Sounds reasonable. Still trying to locate an older PC for this. Wish I had kept what I had. But recycled it before I got all these DEC machines. I should talk to the guy across the street. He works for Dell support. He might know how to get hold of an older machine. Maybe something being re-cycled. If I can manage to find something I will try to get you a copy of this disk.
What's the best/easiest way to add a small load to a switching power supply to make it run for testing? I've heard of the automotive lamp bulb thing. Is it possible to just use a big/fat resistor? I do not have a load test machine. I tried putting an old disk drive on the 4 pin power line and voltage was there for a split second then dropped off. This seemed odd. Maybe the PS shut down for drawing too much current? I am trying to figure out the best way for me to trouble shoot this.
vrs42
Veteran Member
I think the nice thing about the automotive lamp solution is that they are easy to acquire on short notice, easy to find replacement bulbs for, and they have an adequate power rating. Getting a suitable resistive load might mean ordering parts or prowling scarce pickings at local surplus stores, etc.
I have no expertise when it comes to debugging power supplies.
Vince
I dug out one of my DECmate II's, opened it up, disconnected the two 4-pin Molex connectors and the short, stiff ribbon cable that connects the PSU to the logic boards. No display (VR201) or keyboard connected. Connected to a Variac, fan comes on ~ 45VAC, +5 & +12 'try' to come on ~ 75VAC, but the DC voltage (looking at it with an analog VOM) bounces back and forth between 3-6VDC at about a 2Hz rate. Next step is to dig out the ol' Tek 465 scope and see what it shows.
I have the hand-drawn schematic of the H7842 PSU so that should be somewhat helpful in getting an idea where it's gone wrong internally.
For comparison, I dug out the other DECmate II, but it seems to be in worse shape since it only makes one DC output pulse (attempt) when the Variac AC voltage gets to ~ 75VAC. This may be related to the scorched/burned/overheated indications on two of the pins near the middle of the cable (17-00318-01) that connects the PSU to the logic boards.
Yes, the voltage on mine was bouncing up and down. So when I connected an old disk to it the power stabilized for a second then dropped. I don't have a variac or any such thing. I may try the auto light bulb just to see if it shuts down which I am guessing it is doing due to overcurrent or something. I try not to fiddle with power supplies as they scare me. I know what can happen if I do something wrong. Let me know what you discover. Thanks.
Thought of something just now related to my Decmate II. I know the monitor gets its power from the host. I do know the monitor powered up and had a cursor. Doesn't this seem to imply that the +12v was working? Never could get the floppy to do anything that I remember. I also don't remember seeing the "Decmate II" message ever being displayed on the monitor. So that seems to imply that maybe the +5v was having problems? The faulty power supply is all in parts right now so I can't verify this.
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