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pmk222

Video Problems - Repair Questions.

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I recently acquired a TI99/4A in black/stainless. When I got it, I hooked it all up to my monitor (an old Panasonic tube TV) using the video modulator. The image seemed kind of blurry and fuzzy but I'm somewhat used to that having grown up with Atari 2600 games. So, I didn't think anything of it. As I was playing with my new toy, I noticed the screen was fading to black and white. After a while, my screen went to a "no signal" message. After looking around I found someone who has a similar problem and found that it was a capacitor in the video circuit going out of tolerance when it got hot. So, I looked around and ordered a pack of capacitors with the values the article mentioned (12pf). The capacitors I got looked vastly different and my meter was reading "0.000 nf" which didn't seem right. I took the original out and measured it with the same meter and got "OL", "0.001nf", and "56.5uf" all in super inconsistent amounts of time. I tried measuring a few other capacitors I had laying around and most seemed to read just fine.

 

I'll include pictures of the original capacito, I can't tell what value it is. I looked it up (as I'm not sure how to read capacitors with color bands) and was getting 120pf or 130pf from one source and 12nf or 13nf from another (depending of if I called the middle color orange or red). The schematic shown in the article I read said it was suppose to be 12pf

 

I'm wanting to know what value is the capacitor I have? What value should it be? What kind of capacitor is this? Where can I get a replacement (the original feels like it's starting to crumble)? And, what else should I check with the problems I'm having?

 

Thank you for taking your time to read through and answer this thread, much appreciated!

 

P.S. The capacitor was "C101" on the schematic

P.P.S. I intend on doing the resistor fix as well, I just don't have the right resistor on hand.

20201010_014320.jpg

20201010_014356.jpg

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I forgot to mention: I went to test an accessory I had just gotten for this computer (a speech synthesizer) but when I turned the computer on, I only got a "no signal" message.

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Hmm, the part you're showing is to my belief, a choke coil!

 

From comparing your picture with my schematic, I'm guessing that's L207.

 

I highly doubt that is the issue though ... looking at the small amount of heat-sync grease on the VDP I.C. itself, I'd start by replenishing or otherwise cold-testing that first.

 

Overheat problems are usually confined to electrolytic capacitors(cans).;-)

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6 minutes ago, HOME AUTOMATION said:

Hmm, the part you're showing is to my belief, a choke coil!

 

From comparing your picture with my schematic, I'm guessing that's L207.

 

I highly doubt that is the issue though ... looking at the small amount of heat-sync grease on the VDP I.C. itself, I'd start by replenishing or otherwise cold-testing that first.

 

Overheat problems are usually confined to electrolytic capacitors(cans).;-)

There is actually quite a bit of thermal grease on the shield part. I do intend on changing it out though. Out of curiosity: what do you mean by "cold test"?

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They sell refrigerant spray for testing purposes. Hitting the right component at the right time, can restore function momentarily, often revealing the problem component!

Edited by HOME AUTOMATION
spellin'

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25 minutes ago, HOME AUTOMATION said:

Hmm, the part you're showing is to my belief, a choke coil!

Definitely an inductor/choke.  ColecoVision uses the same kind of those epoxy-dipped coils.

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12 minutes ago, HOME AUTOMATION said:

They sell refrigerant spray for testing purposes. Hitting the right component at the right time, can restore function momentarily, often revealing the problem component!

I'll have to look into getting some! Definitely sounds like it could be useful for several applications. 

 

1 minute ago, ChildOfCv said:

Definitely an inductor/choke.  ColecoVision uses the same kind of those epoxy-dipped coils.

Glad to know! It does make me confused, even after rereading the article I had found on the fix. They even described it as "A large, fat ceramic capacitor that was going out of tolerance after it warmed up"

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4 minutes ago, pmk222 said:

A large, fat ceramic capacitor that was going out of tolerance after it warmed up

Those large, fat ceramic capacitors are more sensitive to temp changes, but not necessarily the cause of the issues.

 

I once built a rather sensitive, multi PLL tone decoder with them, had to re-tune the thing constantly. Finally, I replaced them all with tantalums.|:)

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14 minutes ago, pmk222 said:

I'll have to look into getting some! Definitely sounds like it could be useful for several applications. 

Curiously, I've had very poor luck finding overheating caps with the stuff! Dunno why.:roll:

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Well, I made a huge rookie mistake and found two problems I should have identified earlier. 

 

Number 1: I checked the power supply output voltages. I got 5.25 on the +5, 12.11 on the +12, and -17 something in the -5. Note: when I rechecked the -5 line, I got -5.00.

 

I checked this with the motherboard removed from the power supply. 

 

 

Number 2: I checked the input voltage to the computer. I got 9.29 and 29.66. The 9.29 seemed a little high until I saw that the 16v line was nearly double what it should be!

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1 hour ago, pmk222 said:

imageproxy.php?img=&key=f4dcc336a70d5c68this is a screenshot of the article I was looking at. 

20201010_231002.jpg

Looking at the diagram... C100 connects between VDP's pin 39 and GND., C101 connects between pin 40 and GND.. However, on the mainboard these appear to be the two resistors parallel to the XTAL, as these are the only components connecting these pins to GND! The third resistor(brown) being R101, between pins 39 and 40. TI=Very sneaky!:razz:

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1 hour ago, pmk222 said:

Number 2: I checked the input voltage to the computer. I got 9.29 and 29.66. The 9.29 seemed a little high until I saw that the 16v line was nearly double what it should be!

The outputs from the XFRMER are approx. 18vac, and 9vac, center tapped(off center), with both three and four pin configurations available. If read from the outer legs, you will see the summed voltages. So 29vac, sounds typical.:)

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I ordered some more thermal paste for the VDP. After letting it sit for several hours, I also tried the computer again with no display at all. The VDP got noticeably warm (barely) but definitely not hot during that time.

 

Is it possible that the video modulator is bad instead of something on the motherboard?

 

I was thinking about getting an rca style cable for it anyway as that would be more convenient for me.

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22 hours ago, pmk222 said:

... hooked it all up to my monitor (an old Panasonic tube TV) using the video modulator...

 

Does the TV work with another source, like a VCR? Wiggle the channel knob, try the fine-tuning. Switch the TI modulator to the other RF channel and the TV to match. Better?

 

Anyway, rugged as they are, I'd suspect the tube-type TV first and make sure it worked before disassembling your TI further.

 

I didn't know those old TVs would put up a "no signal" message on the screen. That makes it sound newer.

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I know the TV works. I use it for all my older systems and tried it with my atari 2600 today. It's really not THAT old compared to some. 

 

I tried both channel 3 and 4 with no luck.

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Dead modulator wasn't all that uncommon, I know a few people switched from RF to composite video rather than get a new one.

 

Also, dumb question, but you slid the tv/computer switch on the modulator, right? ;) In addition to switching the video, there's a power switch inside the box, so it won't do anything if it's not on.

 

 

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2 minutes ago, Tursi said:

Dead modulator wasn't all that uncommon, I know a few people switched from RF to composite video rather than get a new one.

 

Also, dumb question, but you slid the tv/computer switch on the modulator, right? ;) In addition to switching the video, there's a power switch inside the box, so it won't do anything if it's not on.

 

 

Funny you brought that up, I did. I try and stupid proof my testing. I switched the channel select and modulator switch while on both channel 3 and 4 at the TV side. Just to be sure.

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Apologies for the silence with this project. I was worried I was going to have to bench this project as I don't really have the skill or equipment for troubleshooting (I'm looking into getting an oscilloscope to change that). But, before I did I wanted to be sure that the computer was the problem and not the modulator. 

 

So, I got a cheap rca cable and made my own composite cable. The image was clear as day, so now I know my computer isn't bad.

 

Moving forward, I'm going to get some new thermal paste to put on the VDU and I'll likely recap both the computer and the power supply. Thanks for all the advice and time you all put into this for me, I really appreciate it.

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6 hours ago, pmk222 said:

But, before I did I wanted to be sure that the computer was the problem and not the modulator. 

 

So, I got a cheap rca cable and made my own composite cable. The image was clear as day, so now I know my computer isn't bad.

As I recall... The video modulator uses a rather unique output from the power supply(12v). So the issue still might not be the modulator.:ponder:

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I had measured the 12v out, it was right on. I wasn't sure if that would be issue but I tested it anyway. In hindsight, I guess the 12v line probably powers the modulator so it would be an issue of it was more or less than the 12v it's supposed to be. 

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I aimed my pointy finger at your monitor, but I gave your modulator a pass. When that turned out to be the problem, it made me recall the time I had a modulator go bad. In my case, I just swapped in one of several from my stash and I was off to the races again!

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