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CoCo2 troubleshooting video problems


KeithM

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I owned a CoCo2 as a kid. I was probably about 10 when my mom, who worked at a local radio shack, bought me a 16K CoCo 2. I used that for years, especially with the Radio Shack direct connect 300 baud modem. So as a fun loving middle-aged Dad, reliving my youth through nostalgia is one of my favorite pasttimes. Enough of that!

 

I bought a Korean 64K CoCo 2 from ebay, PCB 20261058, with the XC80652P VDG (I think this is the 6847T1?). RF channel selector is to the left of the RF RCA connector.

 

I attached it, via an analog TV Tuner that takes RF in, and spits out composite video, and stereo audio. The item can be seen here. https://www.contemporaryace.com/contemporary-research/232-mta/product.cfm

 

I'm getting absolutely no video. Of course tried channel 3/4 with no improvement. Tried swapping a couple cables, still no good.

 

I put my o'scope on the RF RCA connector, and I'm absolutely not seeing any waveforms whatsoever. So I traced that back and popped the lid off the RF can, and started probing the MC1372. It's getting +5.21v, outputting a 3.584mhz signal on pin 1, but there's no output on pin 12, RF Modulator Output. That's gotta be bad!

 

So I went back to the 6847, looked to make sure its data pins are being driven, which they are, and 6847 is generating Color A, Color B, Y, and Luminance. Those signals are seen at the inputs to MC1372. Which has power. But nothing is happening on pin 12, or at the RCA connector.

 

Is this safe to say that the MC1372 is bad? I know very little about this stuff, and don't really know how bad passive components, or other environment things may cause that chip to not perform.

 

For general purpose schematics, troubleshooting guides, sources for MC1372's(!!!), and so on. Where's the best place to find these things? Google is working ok, but there are mixed sources, and with the multiple variations of models/PCBs/etc I'm all over the place hoping that I'm probing the right points on the circuit board.

 

Appreciate the help. Fwiw, I may be attending CoCo Fest in a couple weeks, and I'm excited about that!

 

Thanks!

Keith from Pittsburgh

 

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Pin 28 of your vdg. Should give something like this .. yes you have the t1.. i may have a extra 1372 if you need. But check for bad connections especially the rf connector..look for shorts..clean everthing..  try any adjustments in the rf can.. there are schematics and service manuals on the color computer archive..

Screenshot_20211021-104413_Gallery.jpg

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Thanks for the reply!

 

I've attached my pin 28. Looks pretty close, forgive the lousy scope probe grounding

 

 

mypin_28.thumb.png.c898173c40a078379fe3e0082737574c.png

 

Cleaned the board today with IPA and a toothbrush.

Neither the center pin of the RF connector or pin 12 have any output.....

 

I'll send you a PM regarding the spare 1372 you have.

 

Thanks

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When you have a coco not showing image.. a quick way to test if its working is to tyoe MOTOR ON

then hit enter..you should hear a little click of the casette relay..

Also looking at your scope.. you can type CLS 0

Watch for a change in the y signal..

I will look for a 1372..

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21 hours ago, KeithM said:

I put my o'scope on the RF RCA connector, and I'm absolutely not seeing any waveforms whatsoever. So I traced that back and popped the lid off the RF can, and started probing the MC1372. It's getting +5.21v, outputting a 3.584mhz signal on pin 1, but there's no output on pin 12, RF Modulator Output. That's gotta be bad!

Not necessarily.  RF output is intentionally small because it's supposed to be near the same strength as antenna signals.  It will register in millivolts.  You don't want to destroy the TV's tuner input by overwhelming it with thousands of times more power than it was made for.

 

 

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Yeah, and you can also build the circuit for test circuit 2 if you feel adventurous.  That ignores the RF modulator and only outputs a composite signal on pin 12.

 

Otherwise if you don't see any output on that pin, one possibility is the RF tank circuit on pins 13 and 14.  They should be a few volts each on a DC setting, and should be oscillating at 60MHz+ otherwise.  Use the 10x setting on your probe if you attempt to visualize it.

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Some progress made. Pulled the VDG and reinserted it.

 

I now have black on gray text on an ECB 1.1 screen. As long as a metal screwdriver is inserted into TC1 to adjust, the screen turns blurry, but green. Twist it, alternates between black and gray and black on green. Remove the screwdriver, simply does back to black on gray.

 

No amount of adjusting TC1 or L1 has any permanent noticeable effect. L2 has some silastic/glue purposely placed over top of it to prevent adjusting.

 

One other thing --- I went to use a DIP test clip on the 3172 and the mere presence of the clip causes the screen to go blurry+green. Remove the clip, back to black on green. I hate analog things.

 

 

 

Edited by KeithM
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  • 2 weeks later...

As @ChildOfCv mentioned, and after watching the Coco-2 RF Modulator fix video, I quickly removed the glue on top of the L1 inductor, which is part of the RF Tank Modulator circuit, rotated it clockwise about 90 degrees my color returned. The RF tank circuit is a "A tuned circuit connected between these pins [13 and 14 on the MC1372] determines the RF oscillator frequency."

 

You guys were, of course, 100% correct in terms of visualizing that RF signal. I didn't really know what to expect, and still can't really see anything. But obviously it's there because it's working.

 

I did remove the TC1 near the main crystal, and replaced it with a 10ppm crystal ABL-1431818MHz-B1U from mouser. This didn't help with the original problem, but I'm sure it couldn't hurt.

 

The video, now in color, still looked pretty crappy and my sound didn't work at all. So I decided to remove the RF modulator from the board entirely, which is not anything I suggest for the meek. I'm ill-equipped both in knowledge, skills, and equipment for this to go smoothly. I used a 140watt soldering gun for the large tabs, which worked wonders. And then used solder wick, a digital soldering station, and a lot of patience to get the main (8) signal pins.

 

I replaced the RF modulator with an 8-pin male header, which allows me to plug-n-play RF modulators with a 6-inch custom dupont-connectorized cable. Coupled that with a fat ground wire from the original pads.

 

My video is still not where I want it, but at least sound works now, and this gives me a reliable platform to connect logic analyzer/scope from without problematic probing.

 

I have some plans about rolling my own VGA solutions, since no options are in stock anywhere. No cocovga, no zippster composite board, no rgbtohdmi solution.

 

Thanks for the help!

 

 

Edited by KeithM
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7 hours ago, KeithM said:

The video, now in color, still looked pretty crappy and my sound didn't work at all.

Hmmm.  The sound has to go into a 6MHz FM modulator before being mixed with the output signal.  So there should also be another tuning device for that.

 

Unfortunately, the RF modulator is a black box even on the CoCo2 schematic, so I can't be of much more help than that.  Might be fun to reverse-engineer one though.

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The same guy who posted the modulator "fix" video reverse engineered the schematics for both the early and late model RF modulators. They are here

 

https://drive.google.com/folderview?id=1iawKo8rM3PNL5g4nWRu-KFlnn0n_WJ4P

 

The schematics are a bit on the low-res side, but are perfectly usable.

 

12 hours ago, MrDave said:

You can use a cheap comp / svideo to vga converter ..they are 20$ amazon..

 

The problem isn't really composite to VGA. I also found that $15-$20 model present in that linked video above. Your video shows using the Luminance Y channel, which is great for single-color video. The hard part is how to read those 3-state color signals and get those combined.

 

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2 hours ago, KeithM said:

The same guy who posted the modulator "fix" video reverse engineered the schematics for both the early and late model RF modulators. They are here

 

https://drive.google.com/folderview?id=1iawKo8rM3PNL5g4nWRu-KFlnn0n_WJ4P

 

The schematics are a bit on the low-res side, but are perfectly usable.

Ah, okay.  So in early models, C1 and T2 tune for the TV channel, and T1 tunes the audio subchannel.  In later models, L1 tunes the TV channel and L2 tunes the audio.

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I'm not really familiar enough to know how to replace those transistors with the op amp. I saw your rough hookup diagram moving around on the youtube video. Is there any particular reason why the LM6361N is required for this application? Will other more common ones work? Which one?

 

I don't know enough about opamp/transistor applications/configurations to even know what to research to learn.

 

 

 

 

 

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