Mikes C1 build

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MP76
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Re: Mikes C1 build

Post by MP76 »

Just to add, the MONO jumper and the insertion of the mono monitor cable asserts PIN 32 on the MFP to LOW, jumper out and no cable gives a HIGH which is what I'm expecting.

So is PIN 30 on the shifter the MONO output?
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exxos
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Re: Mikes C1 build

Post by exxos »

MP76 wrote: Mon Aug 02, 2021 9:09 pm So is PIN 30 on the shifter the MONO output?
I can't check the schematic right now but the shifter outputs MONO on the pin which is called MONO. Those are feed directly to the RGB buffers when the jumper is forced to mono mode or you plug in a mono monitor.

I'm not sure I'm following what your actual problem is ?
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MP76
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Re: Mikes C1 build

Post by MP76 »

Thanks for the info, I'm just trying to go through the methods in the manual for fault finding - probably misunderstanding a fair bit of it, so bear with me :D

So as per the previous message, checked the 3.3v and it is indeed around 3.5v, I only had a small selection of resistors so i settled on 330R as it was the closest to 1k I had.

resistor mod.jpg
resistor mod.jpg (143.07 KiB) Viewed 2393 times

So after my handiwork I've still got 3.44v which I assume is still a bit on the high side measured off the regulator)? Is this higher voltage on this supply holding up part of the reset circuit? How low dare I go with the loading resistor?

thanks
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Re: Mikes C1 build

Post by exxos »

Odd.. Anyway 330R is about 10mA which should be enough. 1K is about 3mA and that seemed fine. You could try 100R for 33mA but I think that's just going to heat up the regulator more than anything,
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MP76
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Re: Mikes C1 build

Post by MP76 »

@exxos loading up the regulator too much - that's what I was worried about, to check the idea is just to load up at the regulator so my resistor placement is correct?

Regarding this comment "I have found something odd here. I'm using a mono monitor with the mono to VGA adapter cable that forces mono, and I've installed the force mono jumper and seen pin 30 of the shifter be low as expected, pulling out the jumper and putting in a standard RGB lead (or indeed NO video lead at all) pin 30 is still LOW, I'm expecting this to go HIGH"

I see what I've done here, assumed MONO is an input... BUT as it's an output it also seems odd that I've got a low output, only checked with the meter so far. But by comparison the adjacent RGB pins on 21-29 are all showing 5v or there about.

I'll set up the oscilloscope tomorrow and give the MONO output a proper look, might have a duff shifter chip?
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Re: Mikes C1 build

Post by exxos »

MP76 wrote: Mon Aug 02, 2021 10:44 pm @exxos loading up the regulator too much - that's what I was worried about, to check the idea is just to load up at the regulator so my resistor placement is correct?
Looks correct. I think the regulator can push 2 or 3 amps, can't remember offhand.
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MP76
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Re: Mikes C1 build

Post by MP76 »

Ok, putting the feelers out for Help, got my diagnostics cartridge today and I'm getting nothing from the ST monitor. It all works on my other ST's

Running the ST in MONO mode with a known working monitor, replaced MMU/GLUE/DMA with chips checked working in another ST

Followed this set of faultfinding steps

ST manual.JPG
ST manual.JPG (405.25 KiB) Viewed 2341 times

Jumping to 2/3 down the page having done the earlier steps

I have set the MONO jumper and confirm that this and/or the mono monitor lead toggle pin 32 (29 on the DIL version) on the MFP PLCC, that was an easy test so the unit should be set to MONO?

The test immediately before the above talks about "BLANK" going high, I found this on pin 36 on the PLCC GLUE and am measuring just 3.7v rather than the expected 5v.

Tracing the PCB tracks this appears to be connected to the BLANK jumpers near to the SMD PLCC DAC, belling from these jumpers back to Pin 36 on GLUE measures 1.3ohms rather than getting the "bell", this seems like a high resistance, could this be why I've only got 3.7v on BLANK on the GLUE chip? Could this be why I've got no video output?

Could do with some thoughts. thanks :)
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Re: Mikes C1 build

Post by exxos »

Do you get sync at all ? even ST medium low.. GLUE generates syncs by itself.. have you tested V & H sync from the GLUE ? I think you're focusing on the wrong end of the circuit here.. trying to get MONO before checking if there is any syncs at all being generated from GLUE.
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viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
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MP76
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Re: Mikes C1 build

Post by MP76 »

@exxos all help and pointers gratefully received..:)

HSYNC and VSYNC "look" ok to me roughly 50Hz and 15.67

HSYNC

GLUE HSYNC P37_01.png
GLUE HSYNC P37_01.png (51.88 KiB) Viewed 2310 times

VSYNC

GLUE VSNC P38_02.png
GLUE VSNC P38_02.png (49.85 KiB) Viewed 2310 times

Composite output - Pin 2 on the D13 connector. I am concerned this has a low output voltage of 3.7v peak, should I be worried by this?

D13 P2 composite_01.png
D13 P2 composite_01.png (54.3 KiB) Viewed 2310 times

Lastly, _BLANK from GLUE P36. Not sure what I'm looking for but it looks clean

GLUE _BLANK P36_01.png
GLUE _BLANK P36_01.png (52.1 KiB) Viewed 2310 times
MP76
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Re: Mikes C1 build

Post by MP76 »

Some observations on the Clocks going in and out of GLUE

8MHz on GLUE P34 in is a lovely sinewave, although only 4.51v at 8.025MHz

20210806-0003_01.png
20210806-0003_01.png (59.6 KiB) Viewed 2307 times

2MHz on GLUE P54 is ok frequency wise but the waveform looks dog-rough, should I be concerned by this or am I just worried about the transitions? 5.22v and 2.007MHz

GLUE P54 2Mhz_01.png
GLUE P54 2Mhz_01.png (54.53 KiB) Viewed 2307 times

Again, 500KHz signal looks rough, anything to be concerned about? GLUE P43 5.13v 501.3kHz

GLUE P43 500kHz_01.png
GLUE P43 500kHz_01.png (51.28 KiB) Viewed 2307 times
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