Jupiter ACE Kit /sorry half off-topic! :)/

[Prime]

Right I now have a CPU clock having changed R7 for a 4K7, now to make it work

Looks like the machine is running but I'm having video sync problems, I've tried running through a USB video capture device and through an LCD TV via a SCART adapter.
The USB capture device has a screen full of pixel lines each offset relative to the previous one, as if the hsync where not being recognized and the pixel stream is just wrapping, at least I was able to verify the machine was actually running by typing VLIST and seeing a screen full of pixels.

The LCD TV just won't sync, if I pull the phono plug out so only the centre connector is still connected it almost syncs and I get valid characters though the image dances all over the place.

I tried scoping the output, and with nothing connected I seem to get valid video with +ve going video and -ve going sync cetred around +2.5v (I guess set by the 2x1K potential divider), with the video lead the signal is much lower and the sync seems to pretty much disappear. I guess this might be an impedance issue ?

Ok this circuit :

http://jupiter-ace.co.uk/hardware_ebvdoboard.html

Fixes the video, something to bear in mind if a second run of boards is done.

Cheers.

Phill.

[1024MAK]

When I built the replica kit that I bought from eBay, I had to experiment with the base resistor (R7) for the transistor (Q2) that supplies the clock signal to the CPU. Q2 also ended up being a BC547C.

The values in use are:
R7 = 68K ohms
R8 = 330 ohms
C9 = 33pF

Rather than fitting a modulator, I used a transistor, some resistors and capacitors to provide an emitter follower amplifier. The composite output then goes to a phono socket.

Although I have a heatsink, I opted instead to use a Recom DC-DC converter. This works well, but it does not like the spark you get when plugging in an already powered up ZX81 power supply into the 3.5mm mono jack socket

Here are some pictures of mine.

Mark

[1024MAK]

Close up photos...

First, the composite video circuitry with the emitter follower amplifier.

[1024MAK]

CPU clock transistor and it’s resistors and the Recom DC-DC converter:

[1024MAK]

Jupiter ACE - optional modifications

Add a /ROMCS signal to the edge connector
Needs two 1N4148 diodes and two 1K ohm resistors. Needs two track cuts. One cut through the track coming from pin 15 of the 74LS138 (U29) and the second cut is through the track coming from pin 14 of the same chip. Sorry no photo of these track cuts, as they are under the socket for this chip on the top side.

Connect the anodes of the diodes together and to the edge-connector pad as shown in the photo. Then the cathode of one diode connects to the track that goes to pin 20 on one ROM chip (U1). The cathode of the second diode connects to the track that goes to pin 20 on the second ROM chip (U2).

Next, connect a 1K ohm resistor between pin 15 of the 74LS138 (U29) and the track that goes to pin 20 on the first ROM chip (U1). Then connect a 1K ohm resistor between pin 14 of the 74LS138 (U29) and the track that goes to pin 20 on the second ROM chip (U2). Now by applying +5V to the new /ROMCS signal on the edge connector, it is possible to map another ROM or RAM in place of the standard ROMs.

/INT line modification
The /INT line connection on the edge-connector cannot be used by external peripherals to interrupt the CPU due to the /INT line being directly connected to pin 11 of U19 (74LS00) which is an output pin.
A 470 ohm resistor can be inserted to allow external peripherals to interrupt the CPU.

Cut the track for CPU pin 16 on the bottom side under the Z80 CPU (see photo) and add a 470 ohm resistor on the top side under the socket for the CPU to replace it (see photos). Be careful that the resistor lead does not short out on any tracks or component legs.

/NMI and /BUSRQ modifications
Despite having connections on the edge-connector, both /NMI and /BUSRQ are connected directly to the Vcc +5V rail. Hence cannot be used by external peripherals. So the next modification involves cutting the PCB tracks between the Vcc +5V rail and the relevant CPU pins (pins 17 and 25). Then adding two pull-up resistors (both 10K ohms).

The track for pin 17 (/NMI) is cut on the top side close to the edge of the socket for the Z80 CPU (see photo), then a 10K ohm resistor is soldered on the bottom of the PCB between pin 17 and a nearby Vcc +5V track (see photo).

Find the plated through hole just below capacitor C7 that connects the Vcc +5V rail on the top side of the PCB to the /BUSRQ track (to CPU pin 25) on the bottom side. Cut the track on the top side by drilling out the top of the plated through-hole so that the bottom side no longer connects to the Vcc +5V rail on the top side of the PCB. But don't drill all the way through. Just drill enough to break the connection. Sorry, no photo, as it is under the stick-on keyboard. Now add a 10K ohm resistor. Connect it between the through-hole you just worked on on the bottom side (the track for /BUSRQ) and a nearby Vcc +5V track that is also on the bottom of the PCB (see photo).

Now your Jupiter ACE can use far more complex expansions. Assuming that someone is interested in designing and building them...

[1024MAK]

If you have read my posts above earlier today, please note that I have extensively edited them to correct some errors, remove some mistakes, and to expand the content so that the posts make more sense.

Most benefit from being carried out during construction, as they are more difficult to do after the board is built.

Note also that some of the component numbering varies between different schematics and between the board in the kit that I bought and maybe a real original Jupiter ACE.

I forget what the intended values for these capacitors were. But here are the values that I used:
CP1 = 220uF 16V electrolytic (smooths the +V input)
C1 = 100nF ceramic
C10 = 1uF ceramic
C20 = 100uF 16V electrolytic (filters the +5V for the composite video)
Plus I added a 100nF ceramic in parallel with C20.

Mark

[makeit2day]

What is the value of L in the circuit diagram.
Thanks

[1024MAK]

What is the value of L in the circuit diagram.
Thanks

Ahh, alas, I don’t know!

It was an inductor that was supplied with the kit. I did work out the value, but have forgotten

However, the value is not critical. Use a type described as a RF choke in an axial “resistor like” form. The DC resistance should however be less than 10 ohms. I would suggest a value between 4.7uH and 100uH.

Mark

[Danjovic]

You can add backporch capacity with some spare gates and have a video signal with correct output impedance and amblitude with mere 3 resistors (link1) (link2)