Upgrading a 2MHz 32K Bee

[IanMcLean]

Hi forum and Ewan,

I have an original 32K 2Mhz Bee here, still running Basic 5.10. I also have the EDASM ROMs installed.
I have been contemplating upgrading this to the "IC" model spec. This would entail the following:

   
   

1) Changing the crystal from 2MHz to 3.375MHz
2) Upgrading the Basic ROMS from 5.10 to 5.22e
3) Installing the upgraded char ROM (to allow 64x16 and 80x25)
4) Installing the Telcom ROM.

The main advantages of this upgrade for me would using the Telcom ROM for running a terminal at standard baud rates, and having the 80x25 screen mode. This would be a very handy addition to this old Bee to make it more useful.

However, I realise there is a non-technical issue I seriously need to consider.
This Bee is in excellent condition. Like all these old machines there are a few things, like some possibly dry joints on the right hand side of the core board connector which occasionally require me to reseat the core board to get the machine to boot cleanly. Also possibly a cap or two that need replacing (the main filter cap is still original I think - the big orange metal can one), and a few semi-dodgy keys on the keyboard, all of which I can and should get around to addressing. I know these ROM based models, especially the earlier ones, are becoming as rare as rocking horse poop in working order now, and command quite a price in the retro market/eBay.

So I will ask a rather contentious question, that is likely to get answers on both sides of the fence, but I want to get a feel for peoples thoughts on this.
Should I keep this Microbee in its current state, or should I upgrade? Would upgrading it reduce its value? I am never likely to sell this machine, so I'm not concerned about that, but I do want to respect history.

If I do upgrade, then there are still some issues to consider. The instructions for performing the upgrade in the Microbee Technical Manual are a fairly poor scan quality. I am not sure I want to do this based on these scans. Is there a better more hi-res scan of this anywhere? Would someone in the know like to perform this upgrade for me? Ewan? I must admit I am a little nervous about modifying this old Bee myself for fear of f'ing it up.

[Ernest]

Hi Ian,
Personally I would go for it, however there are a few more factors you need to consider.
1) You will need to cut tracks and alter the output pick off from the Oscillator divider, otherwise you will end up with the processor running too slow. Currently its using a divide by six to go from 12Mhz to 2Mhz when you fit the 13.5Mhz crystal you need to only divide by four to get your 3.375Mhz.
2) Your original Character PROM will likely be a 2516 and if you are producing a new character EPROM yourself it will likely be a 2732, only the later boards had the jumpers available to change between the two so once again some track cutting and changing of signals to drive the new character EPROM.
3) The originals had Z80's and PIO's that were capable of 2Mhz and may not run at 3.375Mhz reliably, if they have an "A" suffix then they will be rated at 4Mhz so will be good.
Your contact problem on the RH side of the Coreboard may be the 25 pin X4 pins or socket these were notorious on the older boards.
If you are going to do any work on the Microbee I would look at changing both the plug and socket of both the X4 and X1 connectors.

Ernest

[someone]

Hi Ian,

If you don't care about your bee's resale value then make the mods.

We're here to help you enjoy your microbee experience.

As for breaking something, it can be fixed an you'll learn something along the way.

I highly recommend that the tag tantalum capacitors be replaced with low inductance electrolytics because the tag tantalum capacitors can spontaneously ignite with an accompanying flame and projectiles.

The replacement of the large orange filter capacitor would have likely lost much of its capacity. The replacement you use will depend upon the type of power supply you're going to use.


Cheers

Someone

[ChickenMan]

..... The instructions for performing the upgrade in the Microbee Technical Manual are a fairly poor scan quality. I am not sure I want to do this based on these scans. Is there a better more hi-res scan of this anywhere?

What is the actual document name and can you give me a page number ?  I assume you got it from the Repository here ?  I can always scan my original copy.

Also, from The Catcher v2_03

MOD TO CHANGE CLOCK TO 3.375 MHz.
(Required for 80 x24 char format. Factory fitted to IC Bees).
1. Remove the 12MHz crystal and replace it with a 13.5 MHz crystal.
2. Remove IC32 or cut pin 8 of+ and remove. (0/P of divide by 6 chip clock signal to CPU, PIO etc).
3. Connect track where IC32 pin 8 was to IC30 pin 10 (Divide by 4 0/P) of IC26 74LS161.
4. This mod gives 13.5 / 4 = 3.375 MHz main clock instead of the old 12 / 6 = 2MHz signal. It also gives 13.5 MHz instead of 12 MHz to IC12 and a
1.4 MHz char clock to the 6545 instead of the previous 1.25 MHz. This clocks out the characters faster, allowing 80 to fit across the screen.
5. Install new basic 5.22e.

[IanMcLean]

Thanks everyone for the replies. Sorry for the long delay in replying back. I had some trouble for a while logging onto the forum but I've obviously got that sorted out now.

After thinking about it, I probably would have elected to keep this Bee in original condition if not for one problem. I may have to go with the new adapter that Ewan has made to use an external keyboard, as there are a few keys on this Bee that are rather dodgy. They have already been stripped and serviced once only a couple of years ago, but as many of us here know, sometimes we are just fighting a losing battle with some of these old key-switches.

If I do that then the Bee is no longer in original condition. The core board connectors could be changed for new ones at the same time. If I do that then I may as well do the update in crystal speed and char rom, update to Basic 5.22e, and add the telcom rom as well.

I am just thinking of asking Ewan nicely if he would mind doing these things if I package up the Bee and send it down to him. I don't have a vacuum de-soldering gun here, and there is no way I would trust myself not to lift pads trying to change the connectors using flux and wick on these nearly 40 year old boards. I really want a Hakko vacuum de-soldering station, but I don't have the money to invest in that right now.

[ChickenMan]

Sounds like the best option is to send to Ewan, he is more than capable to do the conversion and check it out fully for you. A Hakko de-soldering station can cost you more than the Microbee is worth also.

[IanMcLean]

Ewan has kindly agreed to do this service and upgrades for me.

Ewan also  told me this morning that he is putting out the keyboard upgrade kit again soon, the same as what came with the Premium+. So, newer key-switches that can be used with the original key caps. I'll do this update when available as well to get the inbuilt keyboard working smoothly. Even doing this makes the Bee non-standard, giving me the excuse to do the other upgrades as well anyway ;-)

[someone]

Hi Ian,

Someone think's you've made the right decision for yourself.
Even if you've got a industrial quality vacuum desoldering station it takes a fair bit of practise to become proficient with it.

Someone doesn't have any sentimental attachments to standard production microbees and has pimped up microbees with fantastic features that were never released into the wild!

To someone it's the adventure that counts and not the beholden treasure (or trash).

Cheers!

[g67wjjk]

I just realised that this is basically the same question I asked ... oops, I should've checked around a bit more.

[g67wjjk]

2) Your original Character PROM will likely be a 2516 and if you are producing a new character EPROM yourself it will likely be a 2732, only the later boards had the jumpers available to change between the two so once again some track cutting and changing of signals to drive the new character EPROM.

This has been confusing me a little bit when reading the HW NB details.

I've got a 12MHz crystal in my Microbee 32 and the Z80 chips are suffixed A, but the character EPROM is a 2716 and not a 2516.

When I was comparing the 2716 and 2732 pinouts with the HW NB details it didn't make any sense.