My new adventure with my Microbee Premium Series

[The_Huck]

Thanks Ewan. Looking forward to it.

I'm watching your Centre for Computing History interview which is very interesting.

Cheers,
Huck

[The_Huck]

The power/video/cassette cable arrived today along with a boot disk. Thanks again Ewan.
I can't wait to try it out.

Cheers,
The Huck

---

After plugging in the power and video, along with the drive cable I fired up the Bee.
No signs of life unfortunately.

I'll start by measuring voltages and going from there. I can't see a power LED on the computer unit so I'm not sure if power is actually getting to it.

I'm assuming the video output is standard composite?

When I power up the drive unit by itself, I can hear a drive spin up.
If I power the drive unit with everything connected, I see the access LED light on the left drive.
I put in the boot disk and power cycled but I did not hear any boot type action from the drive. It sounds like the drive did seek though.
The drives are in unknown condition so I will also confirm they work on a PC. I assume they are PC style drives?

[MbeeTech]

After plugging in the power and video, along with the drive cable I fired up the Bee.
No signs of life unfortunately.

I'll start by measuring voltages and going from there. I can't see a power LED on the computer unit so I'm not sure if power is actually getting to it.

I'm assuming the video output is standard composite?

When I power up the drive unit by itself, I can hear a drive spin up.
If I power the drive unit with everything connected, I see the access LED light on the left drive.
I put in the boot disk and power cycled but I did not hear any boot type action from the drive. It sounds like the drive did seek though.
The drives are in unknown condition so I will also confirm they work on a PC. I assume they are PC style drives?

There is no power LED on the computer.
Seeing that you have a light on 'A' drive when it is all plugged in means that you have power o.k. to the drives and the computer.
If there was some problem with power in the keyboard, neither of the drive LEDs would show (just like if you disconnect the 34 way drive cable).

The video output is composite - correct. The easiest way to get a video output is to use the AV input of a TV, and although this might have sync issues with a not so steady display, at least you will know if 
you have something on the screen.  With the computer not booting from disk, if the display is working correctly, you should see a block cursor at the top left of the screen, and nothing else.
If you get nothing on the screen at all, then there is something wrong with the computer and the first thing to do there is open the case and remove the top board from the 2 x  25 way pin strip sockets
that connect the top board to the main board - unplug & plug back in multiple times.  If the unit has been sitting for some time, it may have poor oxidized connections on these pins. Unplugging and plugging
the board in again a few times can get over this problem.
I wouldn't worry about testing the drives until you get something on the screen.  Most likely the drives are o.k. and when the computer is working properly, it will boot from the drives o.k. as well.

[The_Huck]

Thanks for the reply Ewan.

I've re-seated the top board several times to no avail.
I checked the voltage regulators on the right side of the top board and both are outputting 5V.
I removed the top board and checked the CPU had 5V and the clock. Both good.
The reset pin doesn't go high so something is holding the system in reset. Bear in mind I did these tests without the top board installed. I assume it should come out of reset even so?
I guess a faulty CPU could hold down the reset line? I may just try another CPU.

[MbeeTech]

Thanks for the reply Ewan.

I've re-seated the top board several times to no avail.
I checked the voltage regulators on the right side of the top board and both are outputting 5V.
I removed the top board and checked the CPU had 5V and the clock. Both good.
The reset pin doesn't go high so something is holding the system in reset. Bear in mind I did these tests without the top board installed. I assume it should come out of reset even so?
I guess a faulty CPU could hold down the reset line? I may just try another CPU.

The CPU's RESETn signal is generated on the coreboard, so you will have to have the coreboard in place to check that the reset pin on the CPU is o.k.
Note that the RESET key on the keyboard does not actually drive the CPU's reset signal on a 128K (DRAM) machine.  Instead, there is a Power On RESET generated on the coreboard that drives the RESET line, and the Keyboard
RESET key (with a 1 second debounce circuit) drives the NMI signal which causes the CPU to jump to location 0x66 to do a soft reset so that the contents of the DRAM are not lost due to refresh cycles stopping if a proper RESET is done.

[CheshireNoir]

There are special "Coreboard Riser" PCBs you can use to lift the top "Core" Board out of the way so you can access all the signals. I have heard of people using a bunch of dupont male to female cables as a substitute, but I can't imagine they would be great for signal integrity.
Someone here might have a spare set of the PCBs for sale. I'm not sure if the gerbers are out there. If they're not, I can probably remake them pretty easily. I have some ideas on how to make a "better" version with tap points for key signals.

I know I found my set invaluable for tracking down an issue with the reset.

The reset signal comes out on the edge connectors that go up to the Core Board, so you can check it there. Pin 18 of the RHS X5 connector. I think Pin 12

Also, if you haven't yet grabbed them, there are schematics and board layouts for all the various models of main board and core board which I found invaluable when tracking through the issues I had seen. They're in the repository.

Depending on what tools you have access to, there's other things we can do. I found an oscilloscope and an EPROM burner to be genuinely useful tools. If you don't have them, see if there's a "Maker" space nearby that you can join and borrow them from. I've got a RAM Test ROM burned that helped me pin down some bad 4164 chips on my DRAM Core board. (Also in the repository)

If you do have a scope, check to see if there's a video signal coming out of the composite plug on your cable. It could just be your output device is playing up. (Ask me how I know about that particular issue :-D )

Hopefully we can help you get it working sooner rather than later.

Cheers!

John

[The_Huck]

Thank you for the helpful comments.

I have a scope and an EPROM burner. I'll grab the schematics and do some more digging.

I know what you mean re output device John - that is certainly a trap. I am outputting to a USB composite capture device connected to my PC. It seems to be pretty good and I've used it for a lot of different machines but I did think there could be an issue with compatibility there so I tried it on my Sony Bravia TV and it showed a blank screen as well. Maybe I need to crack out the Commodore 1084

I can see a flicker of white when I power cycle the machine but I guess that doesn't mean much. The screen is all black.
I'll look at the reset and the composite output with the scope and let you know what I find.

Cheers,

[MbeeTech]

There is also the option of sending your machine to us for repair, but I understand that fixing retro stuff yourself is part of the hobby.

[The_Huck]

I noted only 9V on pins 1 & 2 on connector X5 and the schematic indicates it should be 12V.
When I measure the output of the power supply unloaded, I get 11.5V. When I plug the computer in, it drops to 9V again.

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.

With the core board installed, I checked the composite with the scope and I'm getting a constant 1.66V at the output.

The reset is still showing as low.

Note: I'm using schematic 8501-3-01 whereas my board is 8501-03-04CR so connector X4 on the schematic translates to connector X5 on my board. It appears there may not be a schematic for my revision of board.

Now I need to figure out what's holding the reset low.

---

After plugging in the power and video, along with the drive cable I fired up the Bee.
No signs of life unfortunately.

I'll start by measuring voltages and going from there. I can't see a power LED on the computer unit so I'm not sure if power is actually getting to it.

I'm assuming the video output is standard composite?

When I power up the drive unit by itself, I can hear a drive spin up.
If I power the drive unit with everything connected, I see the access LED light on the left drive.
I put in the boot disk and power cycled but I did not hear any boot type action from the drive. It sounds like the drive did seek though.
The drives are in unknown condition so I will also confirm they work on a PC. I assume they are PC style drives?

There is no power LED on the computer.
Seeing that you have a light on 'A' drive when it is all plugged in means that you have power o.k. to the drives and the computer.
If there was some problem with power in the keyboard, neither of the drive LEDs would show (just like if you disconnect the 34 way drive cable).

The video output is composite - correct. The easiest way to get a video output is to use the AV input of a TV, and although this might have sync issues with a not so steady display, at least you will know if 
you have something on the screen.  With the computer not booting from disk, if the display is working correctly, you should see a block cursor at the top left of the screen, and nothing else.
If you get nothing on the screen at all, then there is something wrong with the computer and the first thing to do there is open the case and remove the top board from the 2 x  25 way pin strip sockets
that connect the top board to the main board - unplug & plug back in multiple times.  If the unit has been sitting for some time, it may have poor oxidized connections on these pins. Unplugging and plugging
the board in again a few times can get over this problem.
I wouldn't worry about testing the drives until you get something on the screen.  Most likely the drives are o.k. and when the computer is working properly, it will boot from the drives o.k. as well.

There is also the option of sending your machine to us for repair, but I understand that fixing retro stuff yourself is part of the hobby.

That could be an option.
I have limited knowledge in this area but I'm keen to learn some of these troubleshooting techniques.
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.

[MbeeTech]

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.