[sdiy] Freebass 383 ROM anyone?
gordonjcp at gjcp.net
Wed Apr 13 09:59:38 CEST 2016
On Wed, Apr 13, 2016 at 05:23:23AM +0000, Ullrich Peter wrote:
> Ok, this sounds like bigger problems in the box...
> I would first analyse all the necessary levels and signals around the microcontroller:
> Power supply and ground, crystal oscillator, reset
The PSU is healthy, the clock is clocking and the reset is resetting. That's one of the first things I checked.
One interesting thing is that when it does its filtersweep noise you can't get anything out of the squarewave setting. Turns out, the squarewave is generated by a comparator with one side fed from the DAC multiplexer. So what's the DAC putting out? Hee-haw. Okay, why's that then? Well it's not getting /WR pulses from the CPU. Let's look back around the CPU.
There's a thing - unless it's using some funny quad-state TTL then something is amiss in the CPU section. Some of the data lines are stuck high, some are sitting around 2.5V, and some switch between 0V, 1.25v, 2.5V and 5V. Okay, well two of the affected ones go to the CPU, the D3 and D4 pins of the EPROM and the A3 and A4 pins of the 74LS373 address latch with a bunch of those 0-ohm resistors linking to the rest of the circuit. Let's pull a couple of links and wave the sillyscope at it again. Oh dear, it's still doing it. Pull the EPROM and see what happens. No, but the 74LS373 is warm - not hot, but just slightly warm. Well, that's okay I've got loads of those. Ah, no I don't, I've got loads of 74LS573s and they're different with the inputs and outputs all down the same sides. What I have got is a bunch of old Storno radio repeaters with 12MHz 80C31s I can cannibalise - balls, 74LS573.
So until I've got enough to stick a Farnell order together worth sending off, I'm stuck for 74LS373s. I did briefly entertain the notion of sticking the ROM into one of the Storno repeater processor boards to see what would happen - at least see if it started pulsing /WR low which would suggest it's trying to clock data into the DAC.
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