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mthompson

Advice needed on repairing a motherboard

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I've got an Intellivision I with a faulty Rev. B motherboard, and I've made it a project to try to fix it. This console was taken good care of, and one day in the 90's it powered on with a black screen.

 

When I press Reset with a game cartridge inserted, I usually get momentary vertical white bars. Sometimes I get a flash of solid color screen. Never sound or anything else. I managed to snap a photo of the bars:

 

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Interestingly, when I insert my LTO Flash! cartridge and power on, the white bars flash in regular pulses. Maybe that's a clue as to which component has failed. Here's a short video of how that looks:

 

 

Disconnecting and then reconnecting the blue wire from the power board while the console is on can result in solid color screens for a few seconds, but I don't know if that's a valid indicator of anything.

 

 

This board doesn't work in other consoles, but other motherboards work in this console, so it's not the power board. I've done most of the typical things to try to bring it back to life:

 

1. Swapped in new chips from a working board for the 8 socketed original chips. I've also put the original chips on a working board, and they seem fine.

2. Used Deoxit on the chip pins and cartridge port.

3. New ribbon cable (thanks Humblejack!!)

4. Replaced the 5 capacitors.

 

I checked voltages with the service manual awhile back, and power seemed to be correct.

 

At this point, I'm looking to replace other likely components that could be bad. I don't have the technical skills to actually test transistors and other parts. Here's where I'm hoping some of you will have advice for me.

 

• HunterZero has previously suggested replacing the two 2N3906 transistors near the CPU. I could certainly do that.

• Intvnut mentioned elsewhere that the HD74LS86P Logic Gate can sometimes go bad.

 

Is it reasonable to replace the GROM chips (GTE 3539 x 3) and others if they are available? Which parts should I start with, given what I'm seeing on the TV screen?

 

I've made notes on a high res photo of my board to show what I've already replaced and what I haven't tried yet. Hopefully the photo lets you experts take a good look at the unit.

 

I realize that getting to the bottom of one of these board failures is tricky. Thank you for reading this far and for any technical advice and direction anyone can give me. I truly appreciate it, and I'll report any success or failure.

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Is the board pictured the actual faulty board?

 

Certainly one or both of those transistors Q1/Q2 could be bad. Cheap to replace, may as well just shotgun them. It's possible to check a transistor out of circuit using a multimeter with continuity check, or you can get one of those cheap component tester boards from China that do a reasonable job (check eBay).

 

Check the sockets for the RAM, STIC and CPU chips are good. Easy to check with a multimeter. No need to remove the chips, just touch one probe lead to the top of the leg, and the other probe lead to the other side of the board, or follow the trace to some easy to get at location. Check the cartridge port has no faulty pins as well.

 

A logic probe or oscilloscope will help to diagnose faulty logic chips, and check clocks/crystals. You can check to see if any data/signal/clock legs that should be pulsing are stuck, and trace back through the circuit diagram until you find the source. Multimeter can verify that chips are getting correct power on voltage lines.

 

The fact it flashes says that the video system is running, so it's possibly a component issue around the CPU, or its bus lines to Scratch RAM/Exec ROM. Eg, it could be the U12 scratchpad RAM, which you've marked "3", but I haven't seen one of those go bad before (first time for everything?).

 

Otherwise you are going to have to start checking the diodes and non-polarised capacitors on the board.

 

- James

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Is the board pictured the actual faulty board?

 

Thanks so much for the reply. Yes, the photo is my faulty board. I've already replaced the 5 capacitors and verified the 8 socketed chips.

 

I will get a couple of the 2N3906 transistors to install. I can also check the continuity on all of the areas you mentioned.

 

I'm not able to test chips, so I'll look for some replacement parts. Are the 3 GTE 3539 chips (U7, U8, U12) all the same, even though one serves as scratchpad RAM?

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is the GTE 35391 RAM chip a suitable substitute for the 3539 that's used for U7, U8, & U12?

 

post-39531-0-92373200-1526663605_thumb.jpg

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I have to ask, sincerely, is it worth repairing these old consoles? Why not just buy another one on e-Bay?

 

I'm not trying to be flippant, I just want to know. Of course, I have absolutely no electronics skills, so repairing would never be an option to me...

 

-dZ.

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Troubleshooting and repairing these things is a game itself. Fixing one is highly satisfying. Learning new skills is fun too.

 

Edit:

Replacing old capacitors is not a bad idea but there should be a way to test them as well as the transistors to rule them out as the problem here. Would an Intellivision show some graphics even with bad scratch ram or bad gram? Maybe test the crystal oscillator.

Edited by mr_me
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is the GTE 35391 RAM chip a suitable substitute for the 3539 that's used for U7, U8, & U12?

 

Yes, the GTE 35391CP is a direct replacement for the GTE 3539UCP. From the datasheet, the only difference between the 1CP and the UCP is that the 1CP is 400ns, the UCP is slower at 650ns.

 

http://www.datasheet4u.com/datasheet-pdf/GTEMicrocircuits/35391CP/pdf.php?id=536339

 

Might be an idea to socket these if you replace them.

 

- J

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Troubleshooting and repairing these things is a game itself. Fixing one is highly satisfying. Learning new skills is fun too.

 

Edit:

Replacing old capacitors is not a bad idea but there should be a way to test them as well as the transistors to rule them out as the problem here. Would an Intellivision show some graphics even with bad scratch ram or bad gram?

 

The polarised electrolytic capacitors in these things are typically a limited life device, they can degrade with age to a point where they are out of tolerance and cause problems. Intellivisions are more than 30 to nearly 40 years old. You need a capacitance and ESR meter to check them accurately (some multimeters check capacitance, but not ESR). These are expensive, relative to the capacitors themselves which are dirt cheap. For this reason it's quite common to do preventative maintenance on old electrolytics and "shotgun" replace them, as they are cheap (except for the larger ones on the power supply) and easy to replace. That said, the SHOEI branded capacitors that are standard on most Intellivision main boards are quite good quality, and stand up well to age.

 

The transistors can easily be checked out of circuit with a multimeter or cheap Chinese component tester, but again the cost of the parts is so small that they are easy and cheap enough to just replace.

 

Logic chips are harder to troubleshoot, they require information about the pinout of the part, knowledge of the overall circuit schematic and corrrect operation of the IC being tested, and specialised equipment to test, such as at the very least a logic probe, preferably an oscilloscope, or best of all a logic analyzer. These aren't tools a hobbyist typically has in his arsenal, although a logic probe is quite cheap, and there are inexpensive (if less than optimal) options these days even for oscilloscopes. Even the Intellivision service manual recommends just swapping logic chips with known good ones to test them, as doing this is often more reliable and faster than setting up and testing with any of the IC analysis tools.

 

For example, checking for clock pulse at pins 37 and 38 of the CPU will verify transistors Q1 and Q2 are good.

 

A certain amount of experience shows what the common faults are with the Intellivision logic board. From experience (ignoring the ribbon cable, power supply/transformer or cartridge slot connectors), the RA-3-9600 RAM is the most common failure, then the STIC chip, then the transistors Q1/Q2, the sound generator chip and CP1610.

 

Edit - does the logic board still do the same thing with no controllers connected?

 

- J

Edited by HunterZero
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The 16-bit ram, stic, sound, colour, and cpu chips have been tested working. I could be wrong but I don't think gram can cause this problem, so I'd replace the logic gate chips before gram. The wires/cables, sockets and solder should be closely inspected on this mainboard if it hasn't already. I'm not efficient at soldering so I'd prefer to minimise that if possible but that's me.

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Edit - does the logic board still do the same thing with no controllers connected?

 

I've been testing without the controllers connected, so they can't be the problem.

 

Adding sockets for any chips I remove is a good idea. I'll order some 14 and 22 pin sockets. They're cheap.

 

I've got some transistors coming and will try replacing those first. If that doesn't do the trick, I'll move on to the smaller IC's.

 

Trying to repair this board is mainly just an interesting project, and I'm learning things along the way. If it can't be revived, I'm only out a few bucks, and if someone else benefits from this thread, all the better.

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I could be wrong but I don't think gram can cause this problem, so I'd replace the logic gate chips before gram.

Yes and no... A bad GRAM issue could result in the system running with graphics corruption in custom background cards and/or sprites.

 

However all the RAM and ROM ICs in the Intellivision are on a common bus, so it's possible a short in the GRAM could halt the whole system.

 

Do any of the RAM or logic chips get hot to touch? Eg U11, U17? Or U13 U14 U15 U16? (4 5 6 7 8 9 on your diagram!)

Edited by HunterZero
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Trying to repair this board is mainly just an interesting project, and I'm learning things along the way.

 

Repairing for fun and learning is GREAT! Sure, you can buy another one. But saving THE ONE is GREAT FUN and INCREDIBLY satisfying.

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Do any of the RAM or logic chips get hot to touch? Eg U11, U17? Or U13 U14 U15 U16? (4 5 6 7 8 9 on your diagram!)

 

I left the console on for about an hour, and all of the chips were warm but none were hot. The three with heat sinks were warmer, but you'd expect that.

 

Sorry about the numbering on my photo. I should have tagged them with the official designations, but for some reason I didn't. This diagram (I think it's yours) is very helpful, BTW.

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In checking the continuity on the chip sockets, I found two pins on the AY-3-8915 Color Processor chip were connected. A closer look showed me this:

 

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Cleaning that spot didn't help. After de-soldering the two pins, here's how it looks:

 

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So, are these two pins supposed to be bridged? If not, what's the best method of breaking that solder bridge? And could this be the problem?

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I think those are pins 16 and 17 on the chip description sheet (if pin 1 is at the top of your photo). I think they are supposed to be connected and could be a problem if they aren't.

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I had just reached the same conclusion after taking another close look. It looks like an intentional connection. Darn, I had hoped I'd found something.

 

post-39531-0-16971100-1526776419.jpg

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The datasheet has pins 16 and 18 of the color chip marked "N.C.", or no connection. They are unused.

 

Check the CPU is getting clocked first.

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Success!

 

I replaced the two transistors Q1 and Q2 and everything works. I don't know whether one or both were faulty, as I replaced both before testing, but in the end the problem turned out to be two small parts that cost less than a gumball.

 

Yes, this console was "the one" I bought myself after graduating from college and getting a few paychecks. And it looks like a museum piece, so the darned thing needed to work. And now it does, so all is good. Plus it also has new capacitors from my first try at fixing it. It passes all of the tests on the MTE 201 cartridge as well as a few other test programs that are out there. Actually, searching for information about fixing this thing is what led me to AtariAge four years ago, so lots of good things have resulted from me wanting to get it working.

 

Here is the relevant information for anyone who comes looking for help in fixing this same problem:

 

The transistors are 2N3906, available just about anywhere. I reused the ferrite beads that were on the originals; don't forget to save those when you remove the old transistors. Here is a close-up of the originals before removal.

 

post-39531-0-32752800-1526942583.jpg

 

I was prepared to replace the 2N3904 transistor (Q3) but left it alone. It's also a cheap part. I sourced all of the other ICs in case I needed to go further. Jameco has chips for U11, U13, U16, and U17. The GTE 35391 mentioned above for U7, U8, and U12 are on eBay, as were NOS samples of the chips for U14 and U15.

 

Finally, I updated the photo of my motherboard with the correct chip labels and notes about which parts I replaced to fix the board. Thank you all for chiming in here with thoughts and advice. I couldn't have done it without you.

 

MT

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So glad you got it going!

 

For future reference, those transistors Q1 and Q2 are part of the circuit that clock pins 37 and 38 of the CPU. So the CPU was running, basically. If you have a logic probe or oscilloscope, you can check those pins to verify that the CPU is actually being clocked. You can then see activity on the data pins to check it's actually running.

 

- J

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I have to ask, sincerely, is it worth repairing these old consoles? Why not just buy another one on e-Bay?

 

I'm not trying to be flippant, I just want to know. Of course, I have absolutely no electronics skills, so repairing would never be an option to me...

 

-dZ.

 

In this case, the cost of a couple of replacement transistors, the learning experience during the repair, and the reward of getting an Intellivision is nice shape going again, was definitely worth the effort.

 

In Australia, tested working Intellivision units now go for well over $100, and are coming up for sale less and less, so not easy to get a cost effective replacement.

 

*edit from previous post* So the CPU was NOT running, basically.

 

- J

Edited by HunterZero

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Congrats Mark, a job well done. Wish I had thought of that.

 

This was a group effort. Your fix for my ribbon cable was no small contribution (and you saw my half-assed patch job on it!).

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Thanks for posting this thread. I just got a flaky Intellivision that works sometimes, black screen others. Thought it was the cartridge slot but it looks like the 2N3906 area once I got the RF shielding off as seen under that area on the pictures. The motherboard is discolored there. Just ordered replacement parts for this from Arrow. Will see if it fixes it.

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Edited by MMarcoux66

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Replacing the 2 transistors fixed the issue. Machine now works perfect and even the picture seems clearer! Thanks so much for this thread!!!!

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