Joining the even cooler kids: Premium 128K

[CheshireNoir]

Hey Folks, I just scored a Microbee Premium 128k to go along with all my other systems. 

Before I power it up, is there any maintenance I should be looking at?

I've already cleaned off a lot of the grime that was on it. Looks nice on the inside. Someone has already worked on it as the big cap has been replaced, and there's an additional earth tap (Obviously someone has used a 'scope on this before me.)

Cheers!

John

[CheshireNoir]

Well, I figured "What the hey", and plugged it all in. I'm getting the square cursor, but nothing more. It's not booting off the gotek and holding down M doesn't seem to get me into Monitor.
Oh well, another project for me!

John

[ChickenMan]

Well done on acquiring a Prem 128k

The big earth link between the coreboard and mainboard is standard, fitted to all Premiums. Also, they were normally powered from the floppy disk case which have a Switched Power supply and the original large 4,700uF cap was seen as a short circuit and would turn off. So a much smaller (1000uf ) cap was used. Again standard issue.

[Ernest]

Well, I figured "What the hey", and plugged it all in. I'm getting the square cursor, but nothing more. It's not booting off the gotek and holding down M doesn't seem to get me into Monitor.
Oh well, another project for me!

John

Hi John,
What does the Coreboard look like?
Your other standard system with the DRAM Coreboard should be fully compatible with the Premium.
You should be able to quickly isolate if your lack of booting is Coareboard or not, by swapping over the Coreboards.

Not being able to get into the monitor could be as simple as the "M" key not working (not uncommon with the white shaft keys used with the premiums).

Ernest

[MbeeTech]

Your other standard system with the DRAM Coreboard should be fully compatible with the Premium.
You should be able to quickly isolate if your lack of booting is Coareboard or not, by swapping over the Coreboards.

While this is mostly correct, early (first & second revisions) of the premium baseboard did not have jumpers W2 & W4
for enabling / disabling  F000h active (VIDEOACTn) and NMI reset and had 2 pins cut off from the coreboard right hand 25 way
pin strip.   If the baseboard is a later revision with the 2 jumpers mentioned above on the baseboard (they are cut-able links actually)
then you should be able to change the coreboard over for a known working one with no problems.
If it is an early version, you can still do it, but the reset button will not work.

[CheshireNoir]

Thanks Ewan and Ernest.
I've barely done more than open the beast, wipe off the grime and quickly plug it in.
Thankfully another collector nearby has also just got a Premium so between us we hope to get a working system that we can use to get the rest working from.

Maybe with some luck and a tailwind I'll get to have a closer look at this system on the weekend. As I have the 128k standard, at least I have a working system I can check signals on against things like the FDC controller chip. I also have that RAM diagnostics ROM still.

I haven't even checked the keys for continuity from below, so that'd probably be a good first step.

I'll get there, probably sooner rather than later. I'm just excited to have one! :-D
(Colour! At last!)

John

[CheshireNoir]

So I got carried away last night.
The "M" key is definitely dead, (Millions of ohms when depressed) which could explain why I'm not getting into the monitor. I'll probably wire in a temporary momentary switch there as part of testing.
The Core board is a 8342-4-04CR (So late model)
The Main board is a 8501-3-04CR (So mid model)
ROM version is BN56.
If I swap the core board from my 128k standard (8342-3), will I need to add the earth strap? Doesn't look too hard but it might get a "temporary" one until I can find the right connector.

Anyway I'll potter on over the next few evenings. I have the RAM tester ROM, and a proper Oscilloscope as well as the board lifters so I'm much more prepared this time round.

John

[Ernest]

Hi John,
As per Ewan's post above, if the Premium's 128K Coreboard has all 25 pins present on both the X1 & X5 pin strips, you should be able to swap between.
I know your other 128K Coreboard is adapted from a Starnet Coreboard but you have populate it to work as a normal 128k Coreboard with FDC,
so as such they should be sufficiently interchangeable for testing.
The earth strap will help a lot with stability & reliable operation however once again not normally critical from a testing / troubleshooting point.
You can just use a decent alligator clip / jumper lead just make sure it is well insulated (you don't want to short anything out).

Using the board lifters can be a bit hit and miss with the Premium and DRAM, (the ground lead definitely helps with that)so only use it when you really need to scope out the hard to get to signals.

PS. Ensure the battery has been removed from the Premium Main board if it has the Real Time Clock present, Very left adjacent the X1 25 pin connector.
If it is there look for damage to this area from leaking battery.

Ernest

[CheshireNoir]

Thanks Ernest.
I've swapped across the boards and voilà!
   
Obviously that'll help me isolate this a lot. I'll check the RAM next.

(Now where did I put that CGA to VGA adapter?)

John

[Ernest]

That's great John,
Now you have proven half the system, you could try the ROM from the new Coreboard in your known working Coreboard to prove that is OK.
Then I'd move to the FDC WD2793 and confirm that. Finally the White PAL, all easy as they are plugins.
Once those are all confirmed I'd then put the new Coreboard in with the TEST ROM and see how that goes.

Ernest