Colecovision Control Problems; replacing the SN74LS541N chips didn't help

[guy767]

I have a Colecovision that has severe control issues. Games will auto start (Can't select options) and the right input is always activated. (IE Donkey Kong will auto start and Mario will move to the right on his own) There's no response from 1 or 2 player inputs either; the ports seem to be completely dead.

 

I purchased a few SN74LS541N chips hoping to make the repair but there's no change after installing them.

 

Being a newb I detached a few pads trying to desolder the old ICs on the top side of the board. I needed to connect some traces to the pins of the new ICs with solder. However the connections seem stable and I get continuity for every pin on the ICs along their respected traces plus there aren’t any bridged connections.

 

I'm not sure what I should try next. Did I purchase the right part? Perhaps I ruined the board by detaching some pads? One thing of note is that Colecovision has a Factory Serviced sticker on the box indicating that the console had trouble in the past. I'm attaching a few photos of the board to help others diagnose the problem. Any advice would greatly be appreciated; thank you.

[+Stephen Moss]

What made you change those devices other them appear to belong to the inputs, i.e. did you make any electrical tests to make you think the problem was with them?

 

Assuming that you were using two different controllers when testing both player inputs (thus avoid the controller being the problem) I would think that baring a freak power surge it is unlikely both 74LS541N would die at the same time or that U25 and U26 (which appear to be pull up resistors on the controller inputs) would both loose their power supply at the same time but check the latter anyway (should be 5V DC at the end connection indicated with a dot).

 

Q1) Are the 74LS541N replacing 74LS541N's for 74541N's?

If the latter then it is possible that although operating correctly the 74LS541N's do not give enough current out to drive then next stage.

 

The most logical thing that would effect both controllers is that one or both of the tri-state control pins (G1 & G2) are not getting the 0V signal they need to enable the 541's outputs and as a consequence the outputs are in their tri-state (high impedance) state. So check that those pins are getting a voltage of 0V to 0.4V DC.

 

I don't really know anything about this system so doubt I can help any further but from what you say the system appears to be otherwise functioning normally so it probably is the input stage which is at fault.

[guy767]

Thank you for the reply. After reading several posts about control issues on the Colecovision I'm assuming it's the SN74LS541N chips to blame.

 

Apparently it's a very common problem and it seemed like an easy fix so I decided to replace them. Here's a few discussions I found about the issue...

 

auto-select a level, and player #2 automatically runs to the left

 

Auto Start Problem

 

gameplay is always moving left

 

Also here's a few pertinent facts about the dilemma I'm having...

 

1 - Games will auto start (Can't select options) and the right input is always activated. (IE Donkey Kong will auto start and Mario will move to the right on his own)

 

2 - The problem appears regardless if the controls are inserted or not.

 

3 - Tried 2 pairs of controllers that are both verified working on a different console.

 

4 - No response from Player 1 or 2 from controller input when they're connected; ports appear completely dead.

 

5 - There's continuity for all connections along their respected traces.

 

6 - Besides the control issue the system functions normally and loads games great. In fact, It will read carts very easily without any graphic corruption whatsoever; a very treasured thing indeed. (Owners of the console know what I'm talking about )

 

7 - The system has been Factory Serviced (Huge sticker on the box stating this) in the past possibly indicating that the system has had this problem before.

 

8 - Serial numbers for the replacement ICs match the old so I'm assuming I got the right part. Here's the lot that I bought on Ebay; can anyone confirm if the part is compatible with the Colecovision?

 

I hate to admit this but I'm a newb when it comes to electronic repair; this is my fist time replacing IC chips in fact. I can do basic fixes like replacing caps, repairing traces and checking for continuity but that's about it. I was hoping someone who has had experience with Colecovision repair would see an obvious problem in the pics I provided...

 

Well, I'll try checking for voltage; I don't know what to do if the values are incorrect though. Perhaps I bit off more than I can chew here; I need my trusty The Half-Assed Approach to Colecovision Repair guide for this one

[guy767]

What made you change those devices other them appear to belong to the inputs, i.e. did you make any electrical tests to make you think the problem was with them?

 

Assuming that you were using two different controllers when testing both player inputs (thus avoid the controller being the problem) I would think that baring a freak power surge it is unlikely both 74LS541N would die at the same time or that U25 and U26 (which appear to be pull up resistors on the controller inputs) would both loose their power supply at the same time but check the latter anyway (should be 5V DC at the end connection indicated with a dot).

 

Q1) Are the 74LS541N replacing 74LS541N's for 74541N's?

If the latter then it is possible that although operating correctly the 74LS541N's do not give enough current out to drive then next stage.

 

The most logical thing that would effect both controllers is that one or both of the tri-state control pins (G1 & G2) are not getting the 0V signal they need to enable the 541's outputs and as a consequence the outputs are in their tri-state (high impedance) state. So check that those pins are getting a voltage of 0V to 0.4V DC.

 

I don't really know anything about this system so doubt I can help any further but from what you say the system appears to be otherwise functioning normally so it probably is the input stage which is at fault.

 

Well, I've checked the voltage for U26 and U25 and I get 2.534 for both.

 

Also I checked U18 and U19 (The infamous SN74LS541N chips) and I get 1.64 each.

 

Er, just to confirm; I check voltage properly by placing the negative tip of the Multimeter on the ground of the motherboard and the positive tip at the pin where the dot is located correct? (Such a newb ).

 

I'm not sure where the G1 and G2 pins are located though. Are you referring to one of the 9 pins on the joystick port? Or perhaps one of the pins on the SN74LS541N chip?

Edited January 28, 2014 by guy767

[guy767]

I'm getting 5v for U18 and U19 on the pins indicated in the picture attached to this message; is that normal? The other pins on the SN74LS541N chips have varying voltages...

 

U25 & U26 I get 2.5v on all pins; is that good or does it indicate the coming of the Pasta Apocalypse and The Great Noodling is at hand???

 

Seriously though, I have no idea what to do next. I've replaced the SN74LS541N chips, checked continuity in all traces and scrubbed down the motherboard. I've even removed both joystick ports on the assumption that they may be defective but no joy. I still get auto start and constant right input entered on player 1 even with the joystick ports unsoldered and detached...

 

I suspect this Colecovision has some seriously funky issues that are beyond my comprehension; the huge Factory Serviced sticker on the box was a warning I shouldn't have ignored. Damn you Colecovision! You win this round but there will be a reckoning!

Edited January 29, 2014 by guy767

[+Stephen Moss]

One of the discussions you referenced had a link to a circuit diagram ( http://www.atarihq.com/danb/files/colecovision.pdf ) which could be useful (if accurate) and another suggested you that U25 and U26 should be changed as well as the 74L541's although I would not think that necessary.

 

Although too late now it as it appears to be a common problem it would have been wise to solder turn pin sockets (allow soldering both sides) to the PCB and to put your new 75LS541's in there to avoid further damage to the PCB should they need replacing again.

 

Yes to measure voltage you should place the to lead connected to the COM socket of you meter to ground on the motherboard (or the 0V/GND pin of any IC) and the lead connected to the V socket to the point being measured.

 

I presume that with no controller input that after turn on the game will either sit at the intro screen or run in demo mode.

 

From the circuit diagram I found the voltage you measured at U25 and U26 is correct although why they found it necessary to half the supply voltage so that the input voltage to the 74LS541's is only 0.5V above the logic high input level is beyond me.

 

I was referring to the G1 and G2 pins of the 75LS541's (pins 1 and 19 respectively), Pin 1 is indicated by the dot on the PCB (to left of the notch in the IC) and the other pins are numbered sequentially in an anticlockwise direction (see http://www.ti.com/lit/gpn/sn74ls541 ) .

 

With a cartridge but no controllers inserted could you measure the voltages of the pins on the 74LS541's and and then repeat with neither a controller or cartridge inserted.

 

I would expect...

Pins 1, 2 & 19 need to know what this is as it could be anything from 0 to 5 depending on if and how fast they are being switched

Pins 3 to 9 = about 2.5V

Pin 10 = 0V

Pins 11 to 18 = about 5V (although that may depend on the switching)

Pin 20 = 5V

 

Might also be useful to check the voltages at pins 3 and 6 of U24.

 

Hopefully this will help show where the problem is otherwise you have to start overriding control signals putting parts of the circuit in to know states which is not something that should be done by someone inexperienced in soldering and PCB repair and would be difficult to guide someone through.

[guy767]

Although too late now it as it appears to be a common problem it would have been wise to solder turn pin sockets (allow soldering both sides) to the PCB and to put your new 75LS541's in there to avoid further damage to the PCB should they need replacing again.

Yeah; I shiver to think how many pads I might detach trying to remove those SN74LS541N chips again. Not installing sockets when I had the chance was a big mistake for sure. It's my first time removing/installing ICs though so that's my lame excuse

 

With a cartridge but no controllers inserted could you measure the voltages of the pins on the 74LS541's and and then repeat with neither a controller or cartridge inserted.

Will do and thanks for your time and for the assistance; it's appreciated. I'm going to check the voltages on my confirmed working Colecovision as well to compare the readings; I'm a bit embarrassed that I just thought of doing this. I plan to post pictures of both consoles with the pins labeled for voltage.

 

It's looking however that this problem is going to be beyond me. If my luck was good replacing the SN74LS541N chips should have done the trick and my half-ars repair skills would have been adequate.

 

I suspect there's a more serious issue at cause here; perhaps a corroded trace that's hidden behind the solder mask or something more devious that would make my head explode learning the horrible truth. The Factory Serviced sticker on the box should have been my clue to run away...

 

Ugh, truly the Colecovision is an evil and cunning beast. Just opening the darn thing is an exercise in frustration that has to be done in a very awkward manner. Colecovision you're a SOB!