Making some progress with the 16K Microbee

CheshireNoir · 16-11-2021, 12:50 PM

Splitting off from my introduction post, (And somewhat mirrored from MSPP) I've started working on my third 'bee. This was the one missing the "A" ROM.
I finally got a replacement 2532, which came on a very slow boat from China. It promptly didn't work. (I'd ordered two and both appear to be fakes)
Thankfully the person burning the ROM for me (2532s won't burn in a TL866II+) found a spare and was able to successfully burn me the ROM.
I got it home, dragged out the 16k 'bee and carefully dropped in the ROM and fired her up...
Nothing. No synch. Nada.
This was not actually a surprise. I figured this system would at least fight back a little :-)

I noticed the voltage seems a bit low near the SRAM (4.6v as opposed to 5v at the regulator) but that may be normal. The resistance between GND and +5v is a low 100ohm. (Closer to 2.7kohm on my PC85 across the same regulator)
I did do some more investigation, and discovered the C ROM was actually missing two legs! Just gone. No legs in the socket so I can only assume someone plugged it in incorrectly and severed the legs, then didn't notice when they reseated it. I soldered the legs back on and... BEEP! (This is why I always save the legs from resistors and capacitors)
It beeps! Huzzah!
Still no video synch. Before I go mad swapping compatible components around, I am assuming a "Beep" means the ROM is initialising and the Z80 is executing enough code to generate the tone? (If so, time to break out my scope and start probing around the Video encoder).

Tonight, time willing, I'll get the logic probe on the board and see what, if anything, is showing activity around the Z80 and the 6545. If I see life, I can also swap some key components from the PC85, as I know that one is working fine.

Cheers!

Chesh

***ChickenMan*** · 16-11-2021, 03:46 PM

Well done, if you were after a 2532 you just had to ask, I have a stack of them here and can burn them also no probs Smile

as just burnt out a set for my Super-80.

The beep comes from Basic loading and being initialized so thats good news as a lot of the computer is actually working. First thing I would do is swap the 6545 from your PC85 to see if that gives any video as they are known to fail.

CheshireNoir · 16-11-2021, 04:34 PM

Thanks ChickenMan, for the offer. I didn't expect the 2532s, I ordered to take as long and to promptly be fakes :-)
Good to have confirmation about the beep. I'll swap across the 6545 as a first step. Crossing fingers that resolves the issue. If it does, I'll source a 6545. :-)

Here's hoping it'll be that easy :-)

Chesh

CheshireNoir · 16-11-2021, 10:32 PM

OK, so in the process of working on it, it's got "better".
I swapped over the 6545 with the PC85 'bee and I got no picture still. I put the original 6545 in the other 'bee and still no picture. Swapped them back and I got a picture in the 16k 'bee all of a sudden. Not a great picture, but a picture nonetheless.
It was garbage on the screen.

I tried swapping the 6545 from the 32k 'bee and got an even better picture, but putting back the original kept the same picture. There's a blinking cursor in there that moves around.

After a few seconds it changed to stripes and no cursor.
On the weekend I'll try swapping over the connectors as they don't look the greatest currently, and I may swap out the socket for the 6545.
I feel I'm getting close...

Chesh

***ChickenMan*** · 16-11-2021, 10:49 PM

Arr your getting closer. Looking at the pic of the C rom above, the legs look very corroded so suspect the sockets are corroded also and as you suggest, maybe time to change the sockets and clean up the IC legs with a Brass wire brush also Smile

CheshireNoir · 18-11-2021, 10:10 PM

Anyone know why there would have been a wire bodge across D14 on the underside? It didn't look "factory".

someone · 19-11-2021, 07:19 AM

From your text and photos, it appears that your 6545 CRTC is at least partially working.
When MWBASIC boots, it firstly initialises the Z80 PIO, then the CRTC registers, fills the screen RAM with spaces and loads the PCG with INVERSE characters.
So, if you're getting a stable picture of any kind then the 6545 CRTC is at least partially working. Seeing a flashing cursor is also a good sign; even better if it moves right and left with keystrokes..

If there's junk characters on your screen then there's and issue with char RAM access.

The boot beep come later immediately prior to the opening cold boot MWBASIC caption displayed.

Having an EPROM Emulator, a stack of EPROMs (or NVRAMS) with programmer is most practical way to fix bees as the enables one to write and use small diagnostic programs the simplest containing just one byte 0x76 - The HALT instruction.

CheshireNoir · 19-11-2021, 12:28 PM

(19-11-2021, 07:19 AM)someone Wrote: From your text and photos, it appears that your 6545 CRTC is at least partially working.
When MWBASIC boots, it firstly initialises the Z80 PIO, then the CRTC registers, fills the screen RAM with spaces and loads the PCG with INVERSE characters.
So, if you're getting a stable picture of any kind then the 6545 CRTC is at least partially working. Seeing a flashing cursor is also a good sign; even better if it moves right and left with keystrokes..

If there's junk characters on your screen then there's and issue with char RAM access.

The boot beep come later immediately prior to the opening cold boot MWBASIC caption displayed.

Having an EPROM Emulator, a stack of EPROMs (or NVRAMS) with programmer is most practical way to fix bees as the enables one to write and use small diagnostic programs the simplest containing just one byte 0x76 - The HALT instruction.

Thank you someone,
That description of the bootup steps helps a lot.
I've replaced all the sockets for ROMs and for the 6545, as well as the connectors between the base and the core board.
I'll try this 6545 in my other Microbee tonight, which should either confirm the 6545 is OK or give me something to work on. (I swapped the other way to no change but realised I didn't try this 6545 in a working 'bee. D'oh!)
Assuming the 6545 is good, I'll concentrate, initially, on seeing if I'm seeing "sane" logic out of the 74 series between the video out and the 6545. There's only two chips there and I should be able to probe them with the logic probe without having to bust out the oscilloscope.
This has been a fun little project so far. So glad I have a spare Microbee to help me fix this one though.

Chesh

CheshireNoir · (This post was last modified: 20-11-2021, 03:24 PM by CheshireNoir.)

Ahahahaha!
I am an idiot! Helps when you plug the composite out from the 5 pin connector actually into the composite in on your TV.
I have a lovely rock solid picture now. Looks like replacing sockets and connectors was the right call.

Next step: Keyboard. Are there any modern switches with the same footprint (Not worried about the keystem) out there?
(XPosted to MSPP)

EDIT ...and the Retro gods taketh away. Stuck it back in the case and I'm back to no video. WTF?

***ChickenMan*** · 20-11-2021, 04:40 PM

Good hear your got the Bee finally working as it should, but then when you re-cased, it stopped. Obviously something has moved while casing, reseat all your IC's and clean the legs of all IC.

As for Keyboard, best solution is one of these - https://microbeetechnology.com.au/keyboa...ntkits.htm I have installed both Std and Prem versions and if I can do it, anyone can. Pretty simple really.