Colecovision Black Screen Repair Help

[dafa_123]

12 hours ago, ChildOfCv said:

Just wondering:  Where are you clipping your ground lead?  We've already seen that the clock signal is good, but your measurement above (pin 6) shows it weak and with a 0.5V offset.  If the clock is really that weak, then no wonder nothing works.  But as I say, I'm sure we've seen better.

 

To see anything on pin 16, you need a spinner hooked up -- Super Action Controller, trackball, or steering wheel.  And you have to be rolling it around.

 

Pin 17 (NMI) is holding at 3.5V which also seems weird.  It ought to be closer to 5V when not pulled low.

 

If pin 18 can be trusted, then the processor is in a halt state.  But then if IORQ can be trusted, then it's also asking for I/O.  I don't think it can be both.

 

Pin 24 (wait) also looks weak.  Pin 27 (M1) is being held low.

 

If all of this can be trusted, it would seem that the CPU is not being driven by the clock.  Well, many of the signals are consistent with that.  Get a scope on U8 pin 14.  And also verify U8 pins 8 and 9.  Finally, see if the signal improves after removing U1.

I am connected directly on the ground of the PSU via an alligator clip. According to my meter, the connection between the ground and the probe is good. I even tried connecting the probe's ground to U1:29, which is the CPU ground and I get the same results.

 

All the clock signal appear weak with or without the CPU. I even tried removing U2, U3, U4, U6 and U9 just to be sure and I get the same low signal. Any idea which component(s) could be doing this?

 

 

 

[ChildOfCv]

The clock output is from U8, the 74ls74.  If it has good power and ground (pin 7 is the same potential as CPU ground, and pin 14 is same potential as CPU power) but it can't clock strongly, check pin 11.  If there's a good signal there, replace U8.

 

Edit:  Your reading on pin 14 shows 4.6V which is below the minimum.  What's the voltage at the power plug?

Edited January 14, 2020 by ChildOfCv

[dafa_123]

1 hour ago, ChildOfCv said:

The clock output is from U8, the 74ls74.  If it has good power and ground (pin 7 is the same potential as CPU ground, and pin 14 is same potential as CPU power) but it can't clock strongly, check pin 11.  If there's a good signal there, replace U8.

 

Edit:  Your reading on pin 14 shows 4.6V which is below the minimum.  What's the voltage at the power plug?

U8:7 looks good for ground and U8:14 gets 4.95v same as U1. I get 4.8v on pin 11. 

 

At the plug, I get 5.1v, -4.9v and 12.12v.

 

I do not have a 74LS74 on hand. I will try and get one at the store tomorrow. Hopefully, they have it in stock. In the mean time, I will remove the old one and put a socket.

 

Thanks.

[ChildOfCv]

Hmmm.  How is the oscilloscope's reading so different from the volt meter's?  I guess it could be oxidation on the pins.  Maybe get a can of De-Ox-It while you're out.

[ChildOfCv]

BTW, a useful feature for the scope software is to go to the Measure menu and turn on various outputs for the channels.  In this case, Vp on channel 1 would be useful.

[dafa_123]

11 minutes ago, ChildOfCv said:

Hmmm.  How is the oscilloscope's reading so different from the volt meter's?  I guess it could be oxidation on the pins.  Maybe get a can of De-Ox-It while you're out.

I will do that. Thanks.

[dafa_123]

11 minutes ago, ChildOfCv said:

BTW, a useful feature for the scope software is to go to the Measure menu and turn on various outputs for the channels.  In this case, Vp on channel 1 would be useful.

Not sure where to look for that Vp option. I am not very familiar with oscilloscope. Sorry!

[dafa_123]

14 minutes ago, dafa_123 said:

I will do that. Thanks.

I actually found a 74LS74 in some stuff that I ordered before Christmas. I replaced it and now the Clock signal is gone !?!? I am going nuts I am either doing something wrong when measuring or there is something weird going on with this board.

 

[ChildOfCv]

36 minutes ago, dafa_123 said:

Not sure where to look for that Vp option. I am not very familiar with oscilloscope. Sorry!

No, it would be under the "Measure" menu.  I think it has them separated out by "horizontal" measurements and "vertical" measurements.  Vertical would be the voltage-related ones.

 

Looks like the useful measurement would be "Maximum"

 

Did you scope U8 pin 11 after making the switch?  (Scope because it's a 7.16MHz clock input, not a DC voltage).

Edited January 14, 2020 by ChildOfCv

[dafa_123]

18 minutes ago, ChildOfCv said:

No, it would be under the "Measure" menu.  I think it has them separated out by "horizontal" measurements and "vertical" measurements.  Vertical would be the voltage-related ones.

 

Looks like the useful measurement would be "Maximum"

 

Did you scope U8 pin 11 after making the switch?  (Scope because it's a 7.16MHz clock input, not a DC voltage).

Found the Vpp. Thanks.

 

I put the old 74LS74 and still getting a flat line. It's possible that I damaged one of the traces when I removed the old IC. Do you have by any chance the schematic showing where each pin of U8 goes? I see some of the pins on the schematics you sent me for U8 but not all of them.

 

Otherwise, I will try and figure it out by following the traces if possible.

 

[ChildOfCv]

4 minutes ago, dafa_123 said:

Found the Vpp. Thanks.

 

I put the old 74LS74 and still getting a flat line. It's possible that I damaged one of the traces when I removed the old IC. Do you have by any chance the schematic showing where each pin of U8 goes? I see some of the pins on the schematics you sent me for U8 but not all of them.

 

Otherwise, I will try and figure it out by following the traces if possible.

 

U8 has 2 flip-flops.  One feeds the "WAIT" signal for U1.  You'll find that on the CPU page.  So for now, you only need to be concerned with pins 7, 14, and the ones on the clock flip-flop (the clock page).  The input must be oscillating at around 7MHz with at least 3V, and the outputs must be oscillating at 3.58MHz.  The R and S both need to be at +5V.

 

Also, the PDF is text-searchable with ctrl-F

Edited January 15, 2020 by ChildOfCv

[dafa_123]

16 minutes ago, ChildOfCv said:

U8 has 2 flip-flops.  One feeds the "WAIT" signal for U1 while the other is the clock.  You'll find those on the CPU page.  So for now, you only need to be concerned with pins 7, 14, and the ones on the clock flip-flop (the clock page).  The input must be oscillating at around 7MHz with at least 3V, and the outputs must be oscillating at 3.58MHz.  The R and S both need to be at +5V.

 

Also, the PDF is text-searchable with ctrl-F

I checked the traces and they are all good. I took some new measurement of U8 using the original 74LS74 and 2 other 74LS74 that I got before Christmas and I get the same results.

 

I checked with the calibration hooks on the oscilloscope to make sure that it was working properly and it is.

 

Cam the 7Mhz, I thinks it's a crystal, on the board be bad?

 

Here the readings that I am now getting!

 

[dafa_123]

This what I get when I measure directly on each pins of that silver 7Mhz "crystal".

 

Not sure if it's supposed to be like that.

[ChildOfCv]

First, turn on Maximum on the Measurement|Vertical menu too.  You can turn on multiple items, and it will just show them as a list.

 

Could you try those U8 measurements again with the probe on 10x?  You'll also want the scope software to show 10x to match the probe setting.  I'm wondering because probes have a maximum effective frequency on 1x that is vastly expanded at 10x.  You could be seeing an inaccurate reading of the signal if it's a higher frequency than what the probes like.  If it's about the same, don't bother posting the pics

 

The clock voltage on pin 11 of U8 must fall below 0.8V and then rise above 2V to reliably toggle the clock.

 

I do know that the crystal component of an oscillator doesn't typically get a full swing, but I'll need to take measurements tonight on my system to see if what you're showing is too weak.

 

[dafa_123]

5 minutes ago, ChildOfCv said:

First, turn on Maximum on the Measurement|Vertical menu too.  You can turn on multiple items, and it will just show them as a list.

 

Could you try those U8 measurements again with the probe on 10x?  You'll also want the scope software to show 10x to match the probe setting.  I'm wondering because probes have a maximum effective frequency on 1x that is vastly expanded at 10x.  You could be seeing an inaccurate reading of the signal if it's a higher frequency than what the probes like.  If it's about the same, don't bother posting the pics

 

The clock voltage on pin 11 of U8 must fall below 0.8V and then rise above 2V to reliably toggle the clock.

 

I do know that the crystal component of an oscillator doesn't typically get a full swing, but I'll need to take measurements tonight on my system to see if what you're showing is too weak.

 

I put the probe x10 and put the 10x in the Ch1 Probe in the software. I added Vmin and Vmax and Frequency on screen.

 

When I measure U8:11 I get Vmin -1.37v, Vmax 3v and Vpp of 4.38v and the frequency goes rapidly from *** to 7.14Mhz and sometime I see quickly 3.58Mhz and it oscillate rapidly between the 3 measurements.

[ChildOfCv]

So, here are my waveforms.  For mine, it definitely made a difference between 1x and 10x.  Here's the crystal at 1x:

Notice how weak it is.

 

Now at 10x:

Not the textbook sine wave, but it at least has the necessary highs and lows.

 

Here's U8 pin 11:

 

And pin 8:

 

You'll notice that my signals aren't what you'd find in the textbook either, but they do have the needed voltage range.  There's a lot of overshoot, which may adjust out if I took the time to calibrate it.  But at least it gets the point across.

[dafa_123]

16 hours ago, ChildOfCv said:

So, here are my waveforms.  For mine, it definitely made a difference between 1x and 10x.  Here's the crystal at 1x:

Notice how weak it is.

 

Now at 10x:

Not the textbook sine wave, but it at least has the necessary highs and lows.

 

Here's U8 pin 11:

 

And pin 8:

 

You'll notice that my signals aren't what you'd find in the textbook either, but they do have the needed voltage range.  There's a lot of overshoot, which may adjust out if I took the time to calibrate it.  But at least it gets the point across.

There is something wrong with U8. I got some Deoxit and used it. I also got a brand new 74LS74, I get a similar clock on Pin 11 but my Vmax is a lot lower than yours. The signal that I get on pin 3, 8, 9 are unstable.

 

I get a Vmax of 1.88v on Pin 3.

I get a Vmax of 1.88v on pin 8.

I get a Vmax of 2.8v on Pin 9.

I get a Vmax of 1.66v on pin 6 of U1.

 

The frequencies are all completely erratic on those pins. No stable signal!?

 

Here is my clock on Pin 11

 

[ChildOfCv]

32 minutes ago, dafa_123 said:

There is something wrong with U8. I got some Deoxit and used it. I also got a brand new 74LS74, I get a similar clock on Pin 11 but my Vmax is a lot lower than yours. The signal that I get on pin 3, 8, 9 are unstable.

 

I get a Vmax of 1.88v on Pin 3.

I get a Vmax of 1.88v on pin 8.

I get a Vmax of 2.8v on Pin 9.

I get a Vmax of 1.66v on pin 6 of U1.

 

The frequencies are all completely erratic on those pins. No stable signal!?

 

Here is my clock on Pin 11

 

One thing to make sure of is that your trigger is at signal level.  When it's trying to count frequency, it works best when it has a consistent start.

 

But it still sounds like you have a power/ground issue.  A brand new 74LS74 with +5V VCC and good ground should be able to output a strong Q and Q as long as its R and S inputs are >2V.  Pin 14 should be very close to 5V and pin 7 should be very close to 0.  But also, reading power and ground can be a trap too.  If power is not connected, the chip can still be "powered" by inputs.  It will just be doing a lot of stupid things (like this).  In the same way, ground, if not connected, will only have its outputs for a ground outlet and will also act weird.  To check this, remove all power but keep the switch on, and then check resistance between pin 14 and the +5V at the plug, and do the same between pin 7 and the ground at the plug.  And make sure you use the top of the pin because if you are looking at the bottom, you may either just be reading the socket, or you may even be completing the circuit through your probe, which will troll you.  Also, check resistance between pin 14 and pin 7.

Edited January 15, 2020 by ChildOfCv

[dafa_123]

resistance between pin 14 and the +5V at the plug = Don't get any reading.

between pin 7 and the ground at the plug  = 0.35 ohm

 

I found something strange when I was testing the pins on U8.

Pins 3, 8 , 9 and 12 are connected together. Is that suppose to be like this? From the datasheet, I understand that 8 and 12 are supposed to be connected but I don't think that 9 should be there??

 

 

[ChildOfCv]

31 minutes ago, dafa_123 said:

resistance between pin 14 and the +5V at the plug = Don't get any reading.

between pin 7 and the ground at the plug  = 0.35 ohm

 

I found something strange when I was testing the pins on U8.

Pins 3, 8 , 9 and 12 are connected together. Is that suppose to be like this? From the datasheet, I understand that 8 and 12 are supposed to be connected but I don't think that 9 should be there??

 

 

No, 8 and 9 should not be shorted.  They output opposite voltage states.  But you also need to find out where pin 14 lost its power.  You should be following the traces looking for its path back to the inductor next to the switch.

[dafa_123]

2 hours ago, ChildOfCv said:

No, 8 and 9 should not be shorted.  They output opposite voltage states.  But you also need to find out where pin 14 lost its power.  You should be following the traces looking for its path back to the inductor next to the switch.

Eureuka, got an image and sound. The image is constantly flickering. I am thinking that I could adjust it with the little screws in the RF module. The weird thing is that the sound and game play are a little slower than they should. Any idea why?

 

Check my video

 

[ChildOfCv]

Yeah adjusting the middle hole on the RF box may fix the snow and vertical hold issues.  That's assuming that the metal slug within the tuner isn't disintegrated.  You'll probably need the correct size hex wrench.  I'm thinking something around 1/8" but not sure.  I have a plastic alignment set that I use, and I just use the right hex wrench from that set.

 

As for speed, is this a PAL or NTSC unit?  The PAL TMS9929 has a 50Hz refresh rate, while the NTSC TMS9928 has a 60Hz refresh rate.  Since most games use the refresh period to determine game speed, NTSC games will expect the 60Hz rate and only run at 5/6 the speed on a PAL system.

[dafa_123]

20 hours ago, ChildOfCv said:

Yeah adjusting the middle hole on the RF box may fix the snow and vertical hold issues.  That's assuming that the metal slug within the tuner isn't disintegrated.  You'll probably need the correct size hex wrench.  I'm thinking something around 1/8" but not sure.  I have a plastic alignment set that I use, and I just use the right hex wrench from that set.

 

As for speed, is this a PAL or NTSC unit?  The PAL TMS9929 has a 50Hz refresh rate, while the NTSC TMS9928 has a 60Hz refresh rate.  Since most games use the refresh period to determine game speed, NTSC games will expect the 60Hz rate and only run at 5/6 the speed on a PAL system.

IT'S ALIVE!!! Yeah

 

The GPU I got from Ebay was a TMS9929. I installed the old TMS9928 and got the image with the proper speed. I got some thermal glue and I am going to put the heat sink back on tonight.

 

Thank you for your time and patience on this project with me  

 

If you ever finish that diagnostic kit, let me know. I would be happy to buy one.

 

Next step, AV MOD.

 

I got a schematic that seems straight forward. Any thoughts on this mod?

Coleco AV MOD.docx

[ChildOfCv]

First of all, that output capacitor should be 470uF, not 47uF.  As he mentions, you could replace the 2 150-ohm resistors with one 75-ohm resistor.

 

You can buy a schematic-exact build from retrofixes.com, but I'd also buy a 470uF electrolytic capacitor and a 10uF ceramic capacitor to replace what they send since they probably use the schematic version.  The 10uF electrolytic capacitor is supposed to act as a power filter, but in general, ceramics are better for that purpose.

 

[nick3092]

A friend of mine actually designed that mod many years ago. At the time I don't think he knew that was the best place to pull the audio from. Although back then, I don't think it mattered because it mainly affects the SGM, and it didn't exist back then.

 

I believe the preferred spot now days is to pull audio from the leg of C88 closest to the modulator. As I understand, pulling it from the chip directly causes low volume on anything from the expansion port (module 1 or SGM - but since module 1 doesn't mix with the Coleco sound the way SGM does, it doesn't matter as much for that). Not sure if pulling it from the RF box pin header/connector causes the same issue.