Introduction
12/02/2024
Back in 2022 I built a replica of Project Expansions sound cartridge for the Acorn Electron and it crossed my mind that there is plenty space on the board to add a small Bluetooth transmitter. While I was drawing the circuit I decided to reduce the amount of 74 series logic and add 32K of battery backed sideways RAM. The 74 series logic would be replaced with a couple of small 16V8 GAL IC's.
By late 2022 I had the board designed and built and the GAL chips programmed but then came my hobby hiatus and it was put away in a box.
The build so far..
and this is when it was packed away - fully built and ready for testing.
Mid January 2024, I finally unboxed it to begin testing and it didn't go well. There are three basic areas to test, the ROM/SRAM, the sound generation and the bluetooth transmitter. On powering up there was a constant hiss/noise from the speaker so that wasn't right, the ROM/SRAM worked ok - ROM 13, the Sound OS, could be seen and worked OK and the SRAM could be loaded etc. The Bluetooth I would give a miss until I resolved the sound issue.
The Expansion ROM (13/D) and the two Sideways RAM banks (6 & 7) showing up with the *ROMS command.
My initial thoughts on the sound issue was that I'd made a mess of the 74 series logic conversion to GAL equations so I went through each equation to double check (and to remind myself) what I had programmed. I convinced myself that there were no errors but it still didn't work.
At this point I remembered I had built a GAL Checker, this would let me cross check the program statements to the actual input/outputs of the programmed chip.
To my surprise I found two outputs on the GAL not agreeing with the statements in program. The two OR statements were being interpreted as NOR - some head scratching later and a bit of Googling and getting nowhere I decided there must be something in the compiler that was doing this.
The last two OR statements were being compiled as NOR functions.
Ploughing through each menu option in WINCUPL I found this:
an option to 'Deactivate Unused OR Terms'. This was ticked - could this also deactivate Used OR terms? I removed the tick, re-compiled and re-tested on my GAL checker and it worked! ORs were ORs and NORs were NORs. That took a bit of finding...
Now with the compiler doing what it was supposed to I popped the reprogrammed GALs in the board and on powering up I was met with silence..a good sign.
After a *SDON command and loading up the SNOWMEN it played loud and clear.
Testing the Bluetooth proved very simple. I turned on the bluetooth speaker and it automatically paired. There is a pairing button but it won't get used much as the card is searching for a speaker on power up.
Below is a sound sample through the on board speaker and then the Bluetooth speaker.
With all the testing of the sound card I am noticing that sometimes there is a power on delay.. possibly a bad decoupling capacitor dragging down the 5V supply..I'll need to look into this:
The first thing I did was to try the new cartridge in a basic Electron with a Plus 1 - that worked fine.
I tried the original cartridge in the clone - that worked fine
I then measured the current consumption of both the original and my cartridge - 125mA / 180mA
and the 5V on the cartridge in my clone Electron (Where I was carrying out the tests)- 4.88V
and the 5V on the original cartridge in my clone Electron - 4.92V and the 5V on the cartridge slot without the sound cartridge - 4.93V
Conclusion - I am reaching the current limit of the Electron PSU in the Clone Electron and it can't handle the in-rush current. When this PSU reaches its maximum current capacity the voltage seems to take a few seconds to reach its 5V. I think it wouldn't take much more for the over current to trip.
To prove this I looked out a Mean Well PT-45 power supply, it is rated a 5V 3A & -5V 0.3mA and has a mains input of 230VAC so it's not suitable as a direct replacement to the Acorn power supply although it is very similar in size.
I temporarily wired the PT-45 into the clone and it consistantly powered on with no delay. I'll need to look for a 3A DC-DC converter so I can fit it inside the clone or come up with some way to double insulate this supply.
