dafa_123

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

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 VID_20200115_205714[1].mp4

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??

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

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.

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.

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!

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.

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.

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

I will do that. Thanks.

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.

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?

Here are the new Oscilloscope results with Donkey Kong inserted. I did not do all the test since many of them look wrong from the start. Let me know if you want me to test something more specific. U1:17 NMI Signal looks wrong. Sometimes it stays near 0v when I switch the Coleco ON. U1:18 stays neay 0v. U1:22 does not have the same wave. U1:24 does not look right. U1:27 appears dead. U1:19 seems wrong. Sometimes it stays near 0v when I switch the Coleco ON and I have to cycle ON/OFF a few time for it to go up. U1:20 seems dead. U1:28 is wrong. U1:35 is wrong. Sometimes it stays near 0v when I switch the Coleco ON and I have to cycle ON/OFF a few time for it to go up. U6:5 looks Ok. U7:3 Not sure, lokks Ok U8:1 looks wrong. U8:3 and U8:8 look wrong U8:9 looks wrong

I have an Ultimate SD Wafer Drive with a LED on the front. When I plug it into a working Coleco, it blinks several time when loading the menu. With the faulty Coleco I only get a quick blink at power up and then nothing. I will redo the oscilloscope test tomorrow and post my findings. Thanks.

Can I have a bad capacitor that is leaking power or something? Is there a way for me to test the capacitors without removing them from the board?

There is something weird with the power. I have repopulated all the sockets and connected the Coleco to 110v with the power switch off. I was preparing to redo the checks with the oscilloscope and I noticed that, even with the power switch off, I get some voltage on some of the IC. For example: U3:8, U4:8 are getting 2.8v and some other pins are getting ±0.1v when the power switch is off. U6 is getting some voltage on most of it's pins between -0.21v up to 0.42v U1 is also getting some power between 0.1v to 0.3v on some pins. I would expect with the power switch off that everything should be at 0v. Could this be a power switch issue?

Sorry again for the long delay in my response. I was swamped at work and home. I have managed to find some free time to work on the console. When all IC are present, High values are OK but Low values I get: U1:31 --> ±0.82v U1:37 --> 0.77v I removed U2, U3, U4 and U19 and I get the same results. If I rermove U6 I still get 0.76v on U1:31 when pulled low but U1:37 is now 0v. When I replace U6 with a new 74LS138, I get: U1:31 --> 0.6v U1:36 --> 0.6v U1:35 --> 0.6v U1:31 --> ±0.76v I have not found a replacement for U19 at my electronic store. but I am note sure that it's bad. I will have to order one from the Internet if I want to replace it. To recap, I now have U2, U3, U4, U6 and U19 removed and I still get ±0.76v on U1:31. When I follow the diagram, I see U1:31 going to U19:19 and U7:1. The IC U7:1 is an inverter, so a low signal on U7:1 becomes high on U7:2 and vice versa. If I check the diagram, I see U7:2 connected to U18:19 and to a 5v source through a 3.3k resistor. When I do a high/low test, I get the proper voltage on U7:1, U7:2 and U18:19. I do find the low value of U1:31 quite high. I an note sure where this can be coming from and if it might be an issue? From the values that I am getting, should I replace U6 with a new 74LS138? Thanks.

Sorry for the delay in my response. I've been quite busy. I managed to redo the IC test. Here are the results: U5:7 --> 6.15v U5:9 --> 4.8v U5:10 --> 4.7v U5:11 --> 4.65v U5:12 --> 4.8v U5:13 --> 4.8v U5:14 --> 4.7v U5:15 --> 3.45v U6:7 --> 4.7v U6:9 --> 4.95v U6:10 --> 4.95v U6:11 --> 5v U6:12 --> 4.6v U6:13 --> 4.6v U6:14 --> 4.6v U6:15 --> 5.8v 5 4 3 U5 --------+--- G G G | 15 --> 0.1v G G 5 | 14 --> 0.1v G 5 G | 13 --> 0.1v G 5 5 | 12 --> 0.1v 5 G G | 11 --> 0.1v 5 G 5 | 10 --> 0.1v 5 5 G | 9 --> 0.1v 5 5 5 | 7 --> 0.1v The other pins will vary between 4.85v and 6.2v 22 35 36 U6 ---------+--- G G G | 15 --> 0.12v G G 5 | 11 --> 0.13v G 5 G | 13 --> 0.11v G 5 5 | 9 --> 0.13v 5 G G | 14 --> 0.13v 5 G 5 | 10 --> 0.13v 5 5 G | 12 --> 0.13v 5 5 5 | 7 --> 0.13v The other pins will vary between 3.45v and 4.75v I've noticed that the HIGH value on U6 is a lot lower then on U5. Could there be a power regulation issue? Thanks.

Yes, the RAM chips are in sockets. Basically, I put everything I replace in socket. Not sure what you want me to do? Removed U1, U2, U3 and U4 and check the lines with my meter? I checked the all the lines with my meter between U1:31 to U19 (input), U2 (ROM), U3 and U4 (RAM), and U7. It's all good. Same for U1 pin 37. Thanks.

I got the RAM and replaced it. No luck. Same symptoms. I double checked the lines between U1, U3, U4 and they are all good. Not sure what to look for next.

That would be very cool. BTW, I've found 2 SRAM module with the exact same P/N on Ebay. Got them from Taiwan for $3.50USD each with free shipping. It will take a few weeks for me to get them. I've already removed the old SRAM module and I also ordered some sockets. This way I can swap them easily when I test them. Hopefully, it will bring back to life this old console. If not, I'll keep digging and I will have some spare SRAM module for a future project or something. At this point, it's more of a personal challenge for me to repair this console. Plus, I really enjoy understanding at a more basic level how these old console worked. I will keep you posted once I receive and install the new SRAM modules. Again, thank you for your time and patience. Daniel

Nice. Are you going to sell kits? I have ordered the SRAM. Should be getting them in a few weeks. I will let you know once I have replaced them. Thanks.

Unfortunately, I don't know anyone with a chip tester. Found some TMM314APL-3 on Ebay for around 5$ each. Would they be compatible? From what I gather, the only difference is that the TMM314APL are rated at 450ns and the TMM314APL-3 are rated at 300ns. Some people seem to build there own memory tester with an Arduino. I wonder if it would be worth the time to get one and try to find a program to test the SRAM?

I rigorously tested the pins. Here are my results: Check that U22, pin 6 is the opposite of U1 pin 27 for both states. --> yes it is in both states. Check that U22 pin 12 is the opposite of U1 pin 28. --> yes it is in both states. U7 pin 2 should be the opposite of U1 pin 31. --> yes it is in both states. U7 pin 4 should oscillate at half of the main clock speed when U1 pin 27 is grounded, but should be high when it's +5V. --> it appears so, see attachment. U7 pin 6 should read low. --> It's low. U7 pin 8 should read high. --> It's high. U7 pin 10 should read high when you're not pressing reset, or low when you are. --> That is correct. U7 pin 12 should read the opposite of 10 in both cases. --> That is also correct. I tested the address and data lines. I used a 5.1k resistor and I get 4.68v ± 0.1v on high and between 0v and 0.1v on low except for pins 31 and 37 where I get a low of ±0.8v. For now everything looks Ok except maybe pins 31 and 37 on the low state. Note sure if 0.8v is significant? Thanks.