Spectrum +2 - no video or audio

[mthomason]

Hi everyone!

So I just managed to get a Spectrum +2 (grey, Issue 3 board) listed "for parts" on ebay as the power supply was dead and it couldn't be tested.

I've found a replacement power supply. First problem was the power LED would flash on then go dead every time I plugged it in. I've solved that (actually not quite sure *how* I solved it, I just took it apart and gave it a good clean, reassembled it, and now that works!). The tape deck now works where it didn't before I did that.

However, i'm not getting any video output from the TV socket. I've got an RGB lead on order to see if there's any output there instead, but I'm doubting that because I'm not getting any keyboard clicks over headphones plugged into the audio socket either.

I've got most of the cheaper parts that might be needed on order, including most of the transistors and caps needed to replace, and a spare TEA2000. However, I'm wondering if there's a nice flowchart-style "test this, then this" diagnostic guide for the +2 (the +2 service manual seems quite sparse compared to the original 60-page 48K one, alas), or if anyone has any advice or other pointers. Otherwise I'm just going to make a spreadsheet of all the part numbers and check each off as I test it, I guess!

My electronics skills are at the "can wire an arduino up to things" stage (I've just finished a circuit to make one turn a Raspberry Pi on and off, for example), and can solder through-hole pins (as long as I don't have to get into surface-mount stuff I'm fine!). I know how to use a multimeter to test for voltage, resistance, etc but for most components I'd have to look up what the relevant test is to see if it's working properly.

If it comes down to it I'll just plod on through each component in turn, as I was expecting to have to do that from the start (I decided from the start ths would be a learning project!) but am mostly just looking for any pointers in helpful directions.

Thanks in advance!

[mthomason]

Update:

After replacing a couple of capacitors and transistors, there's now both sound and video!

Except the sound is a constant trilling in the background

And the video is this:



Using the cursor keys (in the hope of moving up and down the menu even if I can't see it) make no noticeable difference, neither does hitting reset.

[1024MAK]

Hi

So with ZX Spectrums there are some basic checks and tests. How to proceed very much depends on the symptoms.

The following assumes you have a +2 grey that has a +9V (nominal) DC 2.1mm/5.5mm ‘barrel’ input socket. As opposed to the later +2A or +2B machines that have a DIN power input socket.

Can you please confirm the following voltages are in spec:

• +9V nominal input, between +7.5V to +12V. Note if using a regulated PSU as low as 7.5V is fine, if using an unregulated PSU the minimum input is recommended to be +9V or greater.
• +5V +/-5% (so 4.75V to 5.25V)
• +12V +/-10%

If these are all okay then a bit of explanation on how a ZX Spectrum works...

Computers need a master clock source to allow them to get all the timings correct. In a ZX Spectrum, the ULA (or gate array / custom chip) does this.

The ULA in your machine must be generating a clock, as it’s producing a video picture. But we don’t yet know if the Z80 is receiving a clock.

The ULA does not need or care if there is a working Z80 or even one on the board. It has internal timing and logic systems that will generate a video picture regardless. To get the data for the screen display, it reads the content of some (eight in a +2 grey) of the RAM chips. If the Z80 is not running the machine code program in the ROM, then the contents of the RAM will just be random data (at power up the content of RAM is random).

So we know that the ULA is doing something. The next step very much depends on what test gear you have (or have access to).

Mark

[mthomason]

Thanks for that

Yup, it's a grey +2, barrel input socket.

I've got a multimeter, I've tested the 9V input and 5V from the regulator. I haven't checked the 12V yet so will get that done today!

Thanks again!

- Matt

[mthomason]

Oh, I forgot to mention:

The reset button does absolutely nothing, visibly. Which also makes me lean towards the Z80 (either being faulty or not receiving everything it needs), that being the only thing I believe it's connected to?

[mthomason]

12V appears to be present - I traced it to being available on R9, and one of the pads there is happy enough....

Except R9 itself is missing altogether. Before I get excited and shove in a 1K resistor here, is there a good reason why this would be absent?

Onto the Z80 - some rudimentary tests show the right voltages in the right places for 5V, HALT, WAIT, RESET. The address lines and clock line also seem busy, although I can only test that atm by checking they're somewhere between 0 and 5V and assuming that's down to pulses. CLK is reading ~3.2V

Thanks!

- Matt

[Ast A. Moore]

Except R9 itself is missing altogether. Before I get excited and shove in a 1K resistor here, is there a good reason why this would be absent?

The video output of the +2 is configurable. It can either mimic that of the original 128K Spectrum or the +2A/+3 machines. R9 is only required for the latter configuration.

[mthomason]

The video output of the +2 is configurable. It can either mimic that of the original 128K Spectrum or the +2A/+3 machines. R9 is only required for the latter configuration.

Thank you! I can happily leave that alone then

- Matt

[mthomason]

Update - turned out to be faulty RAM! I got enough low memory working for it to function, but don't have enough good chips to fix it 100%.

However, I've gotten hold of a 100% (at the time) working board (literally cheaper than buying enough new RAM ICs to be certain of a fix) which I've now switched in instead, with the intention of using the original to cannibalise for spares if I need them.

... and it turned out I needed them a few days later, when the video on that board died. Replacing TR8 and TR9 with the ones off the old board fixed this up in a jiffy

Thanks for all the help!

[Ast A. Moore]

Hey, glad you were able to sort it out!

[mthomason]

Hey, glad you were able to sort it out!

Thanks

And now I have, it's time for me to think forwards to what I can *add* to it I'm already contemplating how to get it onto wifi via the serial port, either via an ESP8266 or a Pi Zero W. Will write that up if/when I get it working (I know others have, but half the fun is figuring out my own way )

[1024MAK]

Keep in mind the serial port has to use the Z80 to do the parallel to serial / serial to parallel conversion and bit bash the port, as there is no UART chip. Just a simple input/output port and voltage level converter chips.

This is fine for attaching relatively slow devices like printers, or uploading/downloading files to/from a PC where the data flow is mostly all in one direction and not much else needs the processing power of the Z80 at the same time. But less than ideal for more demanding applications.

In any event, have fun and good luck

Mark

[mthomason]

This is fine for attaching relatively slow devices like printers, or uploading/downloading files to/from a PC where the data flow is mostly all in one direction and not much else needs the processing power of the Z80 at the same time. But less than ideal for more demanding applications.

Eep, thanks! For now I'm thinking just a simple downloader that connects to an ESP8266 over serial, tells it to connect to a wifi network, Then does an HTTP download of a specified URL. So hopefully that'll not tax it too much!

I really want to start tinkering a little with the Z80 bus at some point, so maybe putting a real UART on there would be a good project for me next

- Matt