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The Mr. Video

Laser 128 won't turn on

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Posted (edited)

Hi, I tried to use my laser 128 today, but when I turned it on, I heard something pop inside and the computer smelled like something was burning inside. When I opened up the computer, I checked the power regulator section and noticed that the small chip there had a chunk blown off the top with its insides exposed. Does anybody know what chip it is so I can get a replacement?

 

20210503_210019.jpg

Edited by The Mr. Video
Scene of the crime

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That big cap in the foreground there looks swollen, as well.  Could be what caused the circuit to fail.

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2 hours ago, Baldrick said:

According to the schematic, that's U1, which is a 74HCT244

 

https://ia800709.us.archive.org/12/items/Laser_128_Technical_Reference_Manual_alt/Laser_128_Technical_Reference_Manual_alt.pdf

See page E-12 and F-1

 

And do replace that swollen capacitor.

Does the brand of the chip matter? Or is it just a generic ic?

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Posted (edited)

Generic.  And they cost a dollar new at Digikey.

You can order that replacement capacitor while you're there.  Mind the voltage and temperature rating of the capacitor.

(and the size).  If you can get a similarly sized cap with a slightly higher voltage and temperature rating it will guarantee longevity. Especially since it is sitting right next to a 5 watt resistor which can emit a LOT of heat.

Edited by Baldrick
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Posted (edited)

I looked through Digikey's selection and they did not have any that would fit the laser. The current chip has 7 pins on each side and all the ones I saw on Digikey have 10 on each side. I can't even figure out how to get the power board out of its shielding so I can solder.

Edited by The Mr. Video

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On 5/15/2021 at 5:11 PM, The Mr. Video said:

I looked through Digikey's selection and they did not have any that would fit the laser. The current chip has 7 pins on each side and all the ones I saw on Digikey have 10 on each side. I can't even figure out how to get the power board out of its shielding so I can solder.

You will not find the dip-14 version being sold new. They are hard to find. This seller has 20+ at a low price. I’d buy a few just in case.

 

https://www.ebay.com/itm/201866333408

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Sorry, ripguy9907, there's no such thing as a 14 pin version of a 74HCT244.  That eBay listing is showing a generic photo of an unlabeled chip.

 

There's got to be an error in that Laser128 schematic.  Definitely.

The 74HCT244 is an octal buffer line driver, which belongs in a logic circuit, not a power supply.

 

The schematic clearly depicts a voltage regulator of some sort, and the photo of the part number starts with a 7, with what appears to be a 2 as the next digit.

 

I believe that part is an LM723CN voltage regulator (which would be right at home in a power supply).  The pinouts seem to jibe with the datasheet shown here:

https://www.jameco.com/Jameco/Products/ProdDS/24467.pdf

 

And Jameco sells them for $1.19 

https://www.jameco.com/z/LM723CN-Major-Brands-IC-LM723CN-2-to-37V-150mA-Voltage-Regulator_24467.html

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Yup that's right. The JAMECO part should work just fine.

 

It's a uA723CN, designed by Micro Analog (spinoff from or formerly Fairchild, and bought by Texas Instruments).

 

If you get sent an LM, all that means is the part was designed by National Semiconductor and they used the term LM for Linear Monolithic. LM means multiple analog parts on a single piece of Silicon. Groundbreaking back in the day. Some of these designs were made in the early 70's! They were so good that multiple mfgs second and third sourced them.

 

C is consumer temperature range 0-70C, and N is plastic molded DIP.

 

5 hours ago, Baldrick said:

There's got to be an error in that Laser128 schematic.  Definitely.

That schematic may be for a different version of the Laser128/128EX, OR, the parts list says it's for the computer - not necessarily the power supply. IIRC the power supply is in two parts, an external brick and an internal daughterboard/module.

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Ok, I bought both the power regulator and the capacitor to replace the bulging one. In the mean time, anyone know how to disconnect the power board from the rf shielding so I can desolder the old parts and solder in the new ones?

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