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APF MP-1000 AV video mod?


lazzeri
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Allow me to say it upfront: I know NOTHING about electronics. Zero. I´m trying to find some info so a friend of mine who´s a very competent technician might do the trick for me. So forgive me if I say something stupid. :)

 

Since APF and CoCo use the same video chip (Motorola MC6847), is it possible that a CoCo mod might work on the APF?

 

MP-1000 schematics shows that CoCo and APF also share its video modulator: https://console5.com/wiki/APF_MP1000

 

There´s a few CoCo mods out there, including one (apparently) using the video modulator quoted above: http://sparksandflames.com/g35.html

Edited by lazzeri
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Not that I have any knowledge in particular, but if I were trying to develop a mod, I'd start by studying the output of the "Video Display Generator" that I see in the schematic:

https://en.wikipedia.org/wiki/Motorola_6847

 

Page 12 of this datasheet shows a circuit that may be what you're looking for, at least it has a point labeled "color composite video out". Seems like a clue. Lots of helpful looking stuff in here.

http://www.colorcomputerarchive.com/coco/Documents/Datasheets/MC6847%20MOS%20Video%20Display%20Generator%20(Motorola).pdf

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I did that very mod awhile back and it does indeed work. I used the parts that already existed on the unit to do the mod and it worked OK. I was going to add this system to my FPGA stuff but haven't gotten a chance yet to do it, but did get the video modded so I could do testing. The one I had the buttons on the controllers fell apart inside, but this is an easy fix with some packing tape.

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I did that very mod awhile back and it does indeed work. I used the parts that already existed on the unit to do the mod and it worked OK. I was going to add this system to my FPGA stuff but haven't gotten a chance yet to do it, but did get the video modded so I could do testing. The one I had the buttons on the controllers fell apart inside, but this is an easy fix with some packing tape.

 

Thank you for your answer Kevtris!

 

Just to be sure, you used the one called "6847 Composite Video Schematic", right?

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You mean which part number of an operational amplifier you're supposed to use? I found one for the Colecovision using a LM318 but perhaps it depends on the input signals which one to use.

 

Yes! Apparently there´s a lot of options out there. Wikipedia also showed this LM318 as example, I guess it´ll work.

 

Thank you!

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Here is one more link that repeats some of the TRS-80 CoCo mods you already found:

http://www.cocopedia.com/wiki/index.php/CoCo-1_composite_video

 

The mod by Martin H. Goodman, 1983 seems to use pins 4 (ground), 11 (+5V) and 12 (RF modulator output).

The mod by Other Bob uses pins 4 (ground), 9 (luminance), 12 (RF modulator output) and 13 (RF Tank??)

 

The one you obtained uses pins 4 (ground), 9 (luminance), 10 (chrominance) and 11 (+5V). I am far from an expert on this, but it would seem to me that you could obtain some S-Video like signal from those inputs, perhaps not with standard timings.

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  • 1 year later...
On 9/3/2017 at 1:33 PM, kevtris said:

I did that very mod awhile back and it does indeed work. I used the parts that already existed on the unit to do the mod and it worked OK. I was going to add this system to my FPGA stuff but haven't gotten a chance yet to do it, but did get the video modded so I could do testing. The one I had the buttons on the controllers fell apart inside, but this is an easy fix with some packing tape.

Kevtris, you mention you used the very parts within the system to do the mod.  Does this mean you traced each pin of the MC1372 and the 6821 to the component on the board and ran a single wire for composite out and a single wire for audio out?

Do you happen to have any pics of where these are located or how you went about doing this?  I can't quite decipher what OP Amp U1 is required for this.  The pins in the schematic don't match up with any datasheets or my limited knowledge of any 8 pin op amp work, for example, the schematic a few posts back show pin 1 as the + but it's generally Null in the op amp's i've looked at purchasing.

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28 minutes ago, mooshue said:

Kevtris, you mention you used the very parts within the system to do the mod.  Does this mean you traced each pin of the MC1372 and the 6821 to the component on the board and ran a single wire for composite out and a single wire for audio out?

Do you happen to have any pics of where these are located or how you went about doing this?  I can't quite decipher what OP Amp U1 is required for this.  The pins in the schematic don't match up with any datasheets or my limited knowledge of any 8 pin op amp work, for example, the schematic a few posts back show pin 1 as the + but it's generally Null in the op amp's i've looked at purchasing.

I traced the pins of the MC1372 (which mainly conformed to the datasheet circuit, AFAIR) and switched the circuitry around to match the circuit above in the PDF.  I didn't really keep notes because it was relatively simple.  I didn't use an op-amp.

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  • 5 months later...

Hey Kevtris I built the circuit you linked to earlier. I wasn’t able to find trim resistors close to the values shown so I went with the recommended values as fixed resistors. 
 

When I measured voltages, they’re bang on. 
 

Except I have 1 strange issue. When first turned on, the picture is grainy and in black and while. If I connect my DMM to pin 12 (video out pin) magically color appears. Even in the DMM is off!! The IC is in a new socket and I made sure to remove all components in that “RF block”. 
 

Thoughts on what it might be? Did you use any variable trim pots?
 

 

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1 hour ago, leonk said:

Hey Kevtris I built the circuit you linked to earlier. I wasn’t able to find trim resistors close to the values shown so I went with the recommended values as fixed resistors. 
 

When I measured voltages, they’re bang on. 
 

Except I have 1 strange issue. When first turned on, the picture is grainy and in black and while. If I connect my DMM to pin 12 (video out pin) magically color appears. Even in the DMM is off!! The IC is in a new socket and I made sure to remove all components in that “RF block”. 
 

Thoughts on what it might be? Did you use any variable trim pots?
 

 

do you have a capacitor in series with the video out?  try throwing a 220uf cap in series if you have one.  It could be the TV doesn't like a DC bias on the video signal.

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Just tried that. 220uF 10V capacitor. +ve leg on 75ohm out, -ve to RCA port. No change. 
 

I did notice that if I remove the diode from the RF tank (1N4148 between pin 13 and 14) there is no change in behavior. Could the mc1372 be damaged? Stuck in RF mode?

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I think I figured it out. For some reason the luminance pin 9 was exhibiting the same behavior. Metering it resulted in color coming back. So I traced that pin back to a trim can. When I started adjusting it, color came back. 
 

The only schematics on the Internet are different on the internet so not sure what this is. 
 

A metal can with 2 pins on one side connected to ground 3 pins on the other for input/output. Black flat plastic screw in middle deep inside can. 

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that's an adjustable inductor/transformer.  Looking at the schematic, that's for the audio carrier- you will probably want to disable that.  It generates a 4.5MHz FM carrier that's injected into the video that's sent to the modulator.  This is an issue on 2600 video mods too, and for a similar reason.  Looks like you can disable it by lifting one side of C9.  This will disconnect it from the modulator chip.  That might make the waviness in the video go away, too.  Going by the schematic, C9 should feed into pin 10 of the MC1372.  Looks to be 15pf?  the schematic is really hard to read.

 

 

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  • 11 months later...

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