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Midwest Scientific 6800, SS-50, PSU help!

fdiskitup

fdiskitup

Experienced Member
Joined
Mar 2, 2022
Messages
247
Location
Chicagoland, USA
I was extremely lucky on craigs list and found this wonderful hombrew SS-50 system from the original maker, a very nice guy who spent an hour explaining the parts to me.
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I began the cleaning and checkout process, stripping it down to remove the spiders etc. Before I try and power it on.

The only thing that looks bad is a big capacitor on the power supply side. 19,000 uF 20V DC.
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First question, can I replace this with a 19,000 uF cap with higher voltage rating , say 75V ?

Second question, Should I buy a new cap from mouser, newark etc , or the exact same vintage part from ebay new old stock (40year old) ? Will electrolytics have survived?

And should I replace them both or all? They seem really expensive.
 
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Also with this machine came the Hazeltine 1420 and a LOBO dual 8” drive.
On the topic of the PSU capacitors, do these need reforming ?
 
Good score!

Yes, you can replace a capacitor with a higher-voltage part than the original. Just check the physical dimensions and mounting arrangements.

New vs New Old Stock... This is really a matter of personal choice I am afraid. Up to you.

If you purchase new, you just "plug and play"... If NOS, I would reform. Electrolytics have a shelf life...

If a capacitor looks physically damaged, is leaking or bulging, it will need to be replaced. Otherwise, try and reform. Any that fail the reform will require replacement. Yes, these are expensive, and this suggestion keeps the costs down...

If it is a power supply capacitor - and is a large electrolytic - I would (personally) reform it first. That is my recommendation. Take it or leave it...

If the PSU is linear, and you have a Variac, you could bring the Voltage up very slowly, whilst monitoring the capacitors. That would avoid removing them. However, I would check the capacitors for short circuits and high leakage with a multimeter first.

Dave
 
Thanks for your reply, that has clarified things significantly.
my day job is working with batteries so I have access to some very neat programmable potentiostats - I’m going to try reforming, stepping up the voltage based on the current drawn. There may be graphs.
 
I bought 2 NOS capacitors, date code indicates 1977!
I gave them a very gentle reforming.
I put it all back together and put it outside on the drive. I powered on the psu with a bare backboard no cards, nothing.
8V line reads 9.2V (a bit high)
12V line reads 16.25V and
-12V reads -16.25V
All seems a little bit high to me, is that just because there is no load ?
Any suggestions how to load a S-50 bus ? Junk hard disk or cdrom …ECD901B4-B2CB-4EC0-84F9-A1E6BF39FE7F.jpegB911AA70-1F29-484D-B7C0-EB9BE26A0942.jpeg
 
These aren't regulated voltages, so they will be a bit high...

The Voltage regulators should be on the cards themselves (if memory serves me correctly).

Use car lamps (in series if necessary to match the supply voltage) and of the appropriate wattage to apply a suitable load to the supply.

Dave
 
I got some good reassurance from Adrians Digital Basement ! 4 mins in
On the SWTPc 6800 he got 9.42V On the 8V line, and +15.2V and -15.1V. With just the CPU card and the backplane logic.
Now I’m more confident to plug my cards into the bus…
 
On the subject of electrolytics, (which I realise is moot now) I would think that keeping the voltage rating to a bit above the actual voltage would be best. Those two 19,000 μF capacitors are (I'm guessing) for the +12 Volts and -12 Volts, which you measured at 16.25 Volts. So 25 Volt capacitors would be appropriate. If you can't get 19,000 μF, then a pair of 10,000 μF in parallel will be fine, and maybe even more economical. There's some opinion (I can't find a proper reference so YMMV) that capacitors shouldn't be 'undervolted' by a long way as this leads to lower capacitance. https://eevblog.com/forum/beginners...o-using-capacitors-with-higher-rated-voltage/
Since this is transformer + linear regulators ESR won't be an issue IMHO, nor will temperature. Also, because you have a transformer, you are saved from RIFA problems, which is nice.
 
Progress on this computer: I checked out the Backplane voltages and plugged in the SS-50 to the SS-30 board. The 7805 voltage regulator is giving about 5.10 V with not much ripple.
I tried to backup the INTEL D2716 EPROMS on the CPU card, but failed to read them - probably a problem with my ROM reader(?).
I installed the CPU card back on the backplane and powered the system up.
there is some activity on the data and address lines, but I did not have a serial card and terminal hooked up.
 
I found the power supply manual, expected is approximately 16V on busses labeled 12V !!! and 10V on the 8V buss.
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Attached here is a scan of the Midwest Scientific Instruments Power supply PS-1 and SI-1 serial card documentation.
I couldn't find it anywhere else. I'll put it up on archive.org.
 

Attachments

  • Midwest Scientific Instruments Serial Interface Board Model SI-1.pdf
    5.3 MB · Views: 3
  • Midwest Scientific Instruments Power Supply Model PS-1.pdf
    3.6 MB · Views: 3
  • Midwest Scientific Instruments Mother Board Model MB-1.pdf
    2.5 MB · Views: 2
  • Midwest Scientific Instruments Interface Adapter Board Model IA-1.pdf
    5.2 MB · Views: 2
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ITS ALIVE ! and its only taken me 3 months…
I literally cant make it do anything else.

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Minimum diagnostic configuration - Gimix “ghost” cpu board , msi6800 backplane, msi ss50 to ss30 interface card and ss-30 msi serial card, plus there is a homebrew baud rate generator on the ss-30 bus. No memory except the 1kb of scratch ram on the cpu board.

What took me so long to figure out was why there was no baud rate signal between mainboard and the interface board: the baud lines were physically cut on the interface board so they could be used as address lines for the 6809.

Is flexbug just a monitor for loading flex os from a disk ?
 
Only 3 months! Some of us have had projects on the go for 10 years!

Well done on getting this far...

Difficult to say what flexbug actually is. It clearly has a 'Dump' command.

Some machines had a basic monitor to enter values into memory, Dump memory and execute software at a particular start address. They may also contain a disk bootstrap.

What type of EPROM is fitted? That would give us a clue about the size of the monitor.

You should be able to specify a start address with the D command.

I would try all of the letter commands and see what happens!

The line that starts with FFF0 is the 16 bytes at the top of memory. This will be EPROM fir a 6809 CPU. Some of the bytes indicate the reset, non maskable interrupt and maskable interrupt vectors. I will have a look tomorrow when I have a bit more time.

Dave
 
It stopped working !

it was working in a limited way, I turned it off, I went away for a few hours and when I came back and rebooted it wont give me anything from the terminal.

possibly it cooled down and there are some poor connections, or possible borderline IC's, caps ?

it might be my EPROM Chips are borderline - Intel D2716 there was another thread about these -https://forum.vcfed.org/index.php?threads/read-2716-uv-eprom.1241971/heres the link
it might also be speed related 6809 working with 6800 era serial card ?

I think I will try booting from the old roms on the cpu board.
 
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I got it back, nobody move…
(Deoxit + wiggling, moved the cpu card to another slot, rebooted the teraterm pc).
The ss50 and ss30 pins are all gold so connections should be good.

Typing
D hhhh gives a dump of the memory at the hhhh address specified !
R gives the register values
B gives breakpoints
S scrolls out the entire memory 0000 to FFFF in about 8 minutes , interesting things dont show up until E800 and F000
It seems similar to the PSYMON, part of the Percom disk low level system monitor shown in the second picture.

How do I Load, typing L just returns to the prompt, do i need to specify where to load?


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.
 
I am not too convinced myself. The fact that the 'L' command displays a '?' implies to me that this command is illegal. Unfortunately, most monitors generally do similar things - and the command letters are pretty close as a result.

Yes, I had some fun and games with my Cromemco backplane and cards when I started to resurrect that! Yes it works, no it doesn't, yes it it works again, oh no it doesn't... Eventually, I beat it into submission!

Dave
 
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