First time posting- need help with Coco carts and no color output on RF

[Staggerwing]

Hi everyone!

First time posting here. Back in the 80's I owned a Vic20, then a coco2 16, and later a C-64. In the early 90's I picked up a used amiga 500 from a friend which I used until my first PC (a Leading Edge 486). I've gotten into retro computing but had to reacquire the hardware since I lost all my originals to a surprise post-divorce garage cleanup. I've picked up (among other things) a Coco2 and more recently a Coco3. The Coco2 will only output black and white over the RF out. I've tried both channel 3 and 4 and also switched TVs but no luck. does anyone have a suggestion as to where to start diagnosing? I'm an electronics newbie but willing to get my hands dirty. Also, the Coco3 came with a bunch of cartridges of which only 2 worked. The rest either didn't boot at all or booted to a blank screen. I tried both Cocos with the exact same results. My suspicion is that the contacts are corroded but it's almost impossible to keep the cart open to clean the contacts. They are a terrible design, hard to insert into the machines as well. Does anyone know how to open them up safely to check them and clean the contacts?

Thanks all, any help is appreciated.

-Erik

[JamesD]

Have you used this TV with any 8 bit computers before?  Some have issues with the frequency of the output.
Try a different RF cable. 
Try reseating the chips in the sockets.

[Staggerwing]

 Thanks for replying JamesD.

I’ve tried the TV out with the Coco3 and it works fine on either RF or composite. I used the same cable as with the coco2.

Could the modulator on the 2 not be getting the Chroma signal? Where does that enter the modulator? If anyone has a diagram understandable by someone who doesn’t know circuit symbols I’d be grateful for a peak. What should the signal strength be on the relevant input?

[MarkO]

4 hours ago, Staggerwing said:

 Thanks for replying JamesD.

I’ve tried the TV out with the Coco3 and it works fine on either RF or composite. I used the same cable as with the coco2.

Could the modulator on the 2 not be getting the Chroma signal? Where does that enter the modulator? If anyone has a diagram understandable by someone who doesn’t know circuit symbols I’d be grateful for a peak. What should the signal strength be on the relevant input?

The RF Modulators have been know to fail, while the rest of the CoCo is perfectly fine...

 

In this, Day and Age, with TVs with Analog inputs becoming less and less common, replacing the RF Modulator with a Composite Output is a Good Direction to Go..  Or even a VGA Output...

 

Check Out Zippster's, ( Ed Snider ) CoCo 2 A/V Modulator:

https://thezippsterzone.com/2018/10/28/coco2-composite-a-v-modulator-replacements/

 

Or Brendan Donahe's CoCoVGA:

http://www.cocovga.com/

 

Zippster doesn't have any of the CoCo A/V Modulators in his store, but Brendan will have CoCoVGAs available..

 

 

MarkO

[ChildOfCv]

Color generation is pretty straightforward on that system.  The MC6847's pin 9 is the color output, and it goes straight to the RF modulator.  You can check that pin for a positive voltage just to verify that the video generator is working.  There is one input to the modulator for the color modulation clock, which the service manual calls "Pin A" which should have an AC voltage reading.  It's connected to a 10K resistor and a capacitor, both of which "never" go bad, but you never know.  But most likely the problem is in the modulator itself.

[Staggerwing]

Thanks again guys. I'll see if I can get the problem sorted out. If the RF modulator is indeed bad I might consider the VGA mod. Luckily, My Coco3 seems to be working fine and I just got an RGB out cable for it- very sharp pic!

 

Meanwhile, does anyone know how to safely open the game carts for cleaning/repair? I don't see any screws. Are they glued shut or are there hidden clips? I don't want to break any, though if I get desperate enough I might have to sacrifice one.

[jhd]

21 hours ago, Staggerwing said:

Meanwhile, does anyone know how to safely open the game carts for cleaning/repair? I don't see any screws. Are they glued shut or are there hidden clips? I don't want to break any, though if I get desperate enough I might have to sacrifice one.

 

If memory serves, the shells are glued together. I am unaware of any non-destructive way to get a cartridge apart. I suppose you could try to soften the glue with a heat gun...

 

That said, you should be able to slide back the dust flap enough to access/clean the contacts. 

 

 

[JamesD]

I'm not sure if they were all the same, but mine all had a screw on the top under the label

 

[bluejay]

My CoCo 2 also outputs b&w when it first boots, but when I switch to channel 4, the color comes back for a second and displays jumble. When I switch back to channel 3, most of the time, the color comes back after a second.