RetrogradeSecond attempt to source some #MC6847 ICs for my #homebrewcomputer.
Previous ones didn't behave as expected in the test rig, maybe these will...
FFS. None of these are good, and this one has obviously been sanded down and re-labelled... 😡 Tired of this hit and miss game (mostly misses these day, it seems.)
Linker3000@Retrograde We could do with a central registry of good / no-good suppliers, but I fear some of them are likely to receive the odd legit and non-legit batches even if they try to provide decent stock of pulls and NOS for discontinued parts.
@linker3000 Aye. For this batch, I tried to up my odds by avoiding any CN sellers and going with a UK seller with 99.9% positive rating and still this keeps happening... 😬 I'd pay some extra for sure for verified components, to avoid this hassle. If only there were a source that actually did that.
@Retrograde That's really annoying.
天 Oğuz Meteer 天@Retrograde How are you testing if the chip works? Do you have a simple test circuit?
I have one lying around that I'm not using, but I have no idea if it works. If I could test it easily and it works, then I can send it to you if you want 👍
@ometeer Testing it in the simple breadboard rig shown on the picture (rough schematic below), and then probing it with the oscilloscope.
As a first sanity check, the horizontal and frame sync signals should give sensible output signals (i.e. around 15.7 KHz/60 Hz).
Second level test is looking for address bus activity, as the IC is addressing display memory.
Third level is probing the Y (luma+sync) video output - it should be possible to recognize the general shape of a composite video signal on the scope.
Thanks for the offer, if it works and you want to part with it, I'd be happy to buy it off you. :)
@Retrograde Ah, a simple test circuit. I'll try to hook up my chip this evening after work. And you have it for free, I'm not going to use it anyway. However, I would appreciate status updates on your #homebrewcomputer :)
@Retrograde
Here is the setup. I don't have a 3.57 MHz crystal so I used a 3.072 MHz one. Image 2 is the H/V sync (13.5 kHz / 50 Hz), image 3 is D4 and image 4 is the Y signal. The last one might not be entirely correct, but the rest looks fine I think. You want to give this one a go?
Here is the setup. I don't have a 3.57 MHz crystal so I used a 3.072 MHz one. Image 2 is the H/V sync (13.5 kHz / 50 Hz), image 3 is D4 and image 4 is the Y signal. The last one might not be entirely correct, but the rest looks fine I think. You want to give this one a go?
@ometeer Well, that's the sanest MC6847 output I've seen on my quest so far. 😅
Syncs look good (slow down factor of about 3.072/3.57=0.86 so expected HS VS rates are 13.502 / 51.6 Hz). ✔️
The Y signal doesn't look half bad either - it's only about 1Vpp so hiding down in the noise floor on that vertical resolution. But you can make out a period of roughly 75 uS in the signal - the nominal rate is 64, and with the slow-down factor of 1/0.86=1.16 that becomes 74.375 ✔️
If you want to part with it I'd be much obliged - will reach out directly to sort the details. 👍
@Retrograde I would also like to make something with it, but realistically speaking, I'm never going to have time to do that. I'd rather have someone else play with it 👍