exxos's Atari, Amiga & retro forum

ah, the joys of learning to fault find a new system.

You'll get it done in no time :)

PhilC wrote: 18 Jan 2024 17:01 ah, the joys of learning to fault find a new system.

You'll get it done in no time :)

Fingers crossed :)

I think it's the clock, so got to do some tracking down on the schematics, trouble is I have Karate tonight so will have to wait until later on.

TBH it's no more complex than a TRS-80 so should be relatively easy to work out.

It's always the clock circuit. Hehe. Is it a crystal and capacitors or a crystal oscillator?

stephen_usher wrote: 18 Jan 2024 19:28 It's always the clock circuit. Hehe. Is it a crystal and capacitors or a crystal oscillator?

Crystal and caps and a couple of resistors feeding logic gates. Got to figure out the schematic, it isn't the best...

At leats there is one though :)

The crystal is oscillating BTW.

Yay! We have an oscillating crystal! :-) That's definitely a good start.

rubber_jonnie wrote: 18 Jan 2024 16:52 Only trouble is, it doesn't power up, so I'm looking at the service manual, as it has some oddities I'm told that you need to do before troubleshooting. I can say that 5v does appear to be everywhere it should be, and I don't see a clock at the CPU.

The thing you don't want to happen once your build is completely finished...

:chairsmack:

HigashiJun wrote: 19 Jan 2024 07:33

rubber_jonnie wrote: 18 Jan 2024 16:52 Only trouble is, it doesn't power up, so I'm looking at the service manual, as it has some oddities I'm told that you need to do before troubleshooting. I can say that 5v does appear to be everywhere it should be, and I don't see a clock at the CPU.

The thing you don't want to happen once your build is completely finished...

:chairsmack:

Definitely getting a clock at the crystal, but not at the socket, so I need to work back through from the CPU socket to the clock circuit.

:dizzy:

In some ways I'm guessing I'm experiencing exactly what may builders experienced back in the day!

rubber_jonnie wrote: 19 Jan 2024 09:25 In some ways I'm guessing I'm experiencing exactly what may builders experienced back in the day!

Indeed, but you have better diagnostic tools and outside support than those building it in the late 70s.

I guess that there's a 74LS04 somewhere in the clock circuit squaring it up, maybe a flip-flop acting as a clock divider too.

stephen_usher wrote: 19 Jan 2024 09:38

rubber_jonnie wrote: 19 Jan 2024 09:25 In some ways I'm guessing I'm experiencing exactly what may builders experienced back in the day!

Indeed, but you have better diagnostic tools and outside support than those building it in the late 70s.

I guess that there's a 74LS04 somewhere in the clock circuit squaring it up, maybe a flip-flop acting as a clock divider too.

Indeed, and for that I am eternally grateful :)

What's bothering me a bit right now is the schematics. They are poor quality and not very clear. The ones in the service manual are slightly better so I am going to print it off and make one big schematic so I can trace stuff properly.

The service manual also shows oscilloscope images, and looking at those, I do get what looks like an expected waveform at the right pins, and the frequency is correct, but the manual is a bit vague, and best I can tell is the voltages should be 3v or 4v peak to peak depending on which pin you measure at on U58 (74S00) which is what the oscillator feeds into. They are more like 1.6v peak to peak. I am using 10x on my probes BTW as recommended in the service guide.

Also, the video output circuit should output something, even with the CPU removed, and that is dead too. Could still be as a result of the clock circuit though.

Well, if the clock isn't getting up to TTL transition voltages then you'll not get anything out of the other side of the 74LS chip.