My new adventure with my Microbee Premium Series

[CheshireNoir]

I'm going to apply what I learn here to help fix up a dead Dick Smith Wizzard and a couple of IBM PC boards.

OMG! You have a Wizzard as well? There's not many CreatiVision / Wizzard owners out there :-)

(I have done a lot of stuff with the Wizzard over the years. Currently working on a replacement controller for them as the membranes in them are terrible)

When powering from my bench supply set to 12V, I get 11.5V on pins 1 & 2 and at the input to the voltage regulators, and 5V on the regulator output. The computer is consuming around 920mA.

12V will run the regulators pretty hard. I find around 10V is the "sweet spot", at least for me.

Getting that reset working will be the big first step. What happens if you manually reset it? (Try holding down the RESET key on the keyboard for a few seconds.)

Cheers!
John

[The_Huck]

If you are still getting a low on pin 18 of the right hand 25 way pin strip (pin 1 is at the back of the machine, pin 25 towards the front, then have a look at both C43 (0.68uf) and C52 (2.2uF) on the coreboard.
After half a second or so of power on, both of these should have close to 5v on their +ve terminal.  C43 in particular is notorious for failing at this age. If you see either of them showing a voltage of less than 4.0v
then replace them as they have broken down and have a low internal resistance.  The values are not critical, and you can replace them with tantalum or electrolytic parts rated at 10v or greater.
Eg, for the 0.68uF you can use a 1uF - it will just mean that you have to hold the RESET key down longer when you want to reset the machine.

Thanks Ewan.

C43 was at 4.7v and C52 is at 2.2v. 

I swapped out C52 and now I'm getting a high on the reset line.
Should I swap out C43 or is that okay?

Still continuous 1.6v on the composite.

Progress is being made.

---

I'm going to apply what I learn here to help fix up a dead Dick Smith Wizzard and a couple of IBM PC boards.

OMG! You have a Wizzard as well? There's not many CreatiVision / Wizzard owners out there :-)

(I have done a lot of stuff with the Wizzard over the years. Currently working on a replacement controller for them as the membranes in them are terrible)

When powering from my bench supply set to 12V, I get 11.5V on pins 1 & 2 and at the input to the voltage regulators, and 5V on the regulator output. The computer is consuming around 920mA.

12V will run the regulators pretty hard. I find around 10V is the "sweet spot", at least for me.

Getting that reset working will be the big first step. What happens if you manually reset it? (Try holding down the RESET key on the keyboard for a few seconds.)

Cheers!
John

I have a couple of Wizzards. My childhood one is broken and I have another good one. I sold my Spectrum to buy it which some people consider a downgrade but I'm pleased I did it now, considering how rare they are.

I dropped the voltage to 10v for my testing.

Cheers,

[MbeeTech]

I swapped out C52 and now I'm getting a high on the reset line.
Should I swap out C43 or is that okay?

C43 was at 4.7v and C52 is at 2.2v. 

I'd leave C43 - it seems o.k.  C52 was the culprit.

Now you can check the address, data & control lines to see what action you have.  Check the ROM Chip select as well, to see if it cycles up from address 0h to 8000h where the boot rom sits in memory.

The preferred input voltage to the computer is 10V but it will work down to 8v o.k.  12v is acceptable, but it has to disipate more heat out of the 7805 regulators.

[The_Huck]

I swapped out C52 and now I'm getting a high on the reset line.
Should I swap out C43 or is that okay?

C43 was at 4.7v and C52 is at 2.2v. 

I'd leave C43 - it seems o.k.  C52 was the culprit.

Now you can check the address, data & control lines to see what action you have.  Check the ROM Chip select as well, to see if it cycles up from address 0h to 8000h where the boot rom sits in memory.

The preferred input voltage to the computer is 10V but it will work down to 8v o.k.  12v is acceptable, but it has to disipate more heat out of the 7805 regulators.

From what I can see, there are data lines that are continuously up at 5v and some at ground. Also some address lines are behaving the same. I'm not seeing activity as such.
I could note each pin if required. 

I'm just using a scope here but I see a quick burst of data on the ROM chip select, then it sits high.
I guess I'd need a logic analyser to see what it's up to in detail.

Cheers,

[MbeeTech]

If you have a spare 2764 EPROM, go to the repository and download the Universal RAM test ROM ut212

Repository /MbeeTech /Microbee /Software /ROMs /Test /

Replace the boot rom with this and you may get some screen output. It only initializes the bare basics (IE the screen controller).
If you do get a menu up with that, then do a memory test by holding down the G key (for 128k DRAM test) while powering up. You can try any of the options just in case your G key is faulty.
I suspect that you have some faulty DRAM at this point, and running that test will confirm that some of the machine is working at least and you'll be able to see whether the DRAM is working.

[The_Huck]

Thanks again Ewan for the info.
I've just ordered some 2764 EPROMs and I have a TL866 programmer.
I'll let you know how I get on.

Cheers,

[someone]

Someone says beware of using 12V input with a microbee.
Old tantalum caps can give you an unwanted fireworks display with real Roman Candle projectiles.