HP Forum Archive 17

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hp 28s repair Message #1 Posted by Adrian on 22 May 2007, 8:07 a.m. My HP28s is dead. There are a few discrete components on the main board. A resistor marked 102K, is that a 100k resistor? Next to that a capacitor or resistor. It is 0 ohms. Next a glass circular component, possibly a diode. Then a three terminal device with two ports joined, probably a diode. A tantalum capacitor then a 1000uF electro. Can someone verify these observations. The case was easy to open and then repair if you have the right tools.

Re: hp 28s repair Message #2 Posted by DaveJ on 22 May 2007, 8:36 a.m., in response to message #1 by Adrian Quote: My HP28s is dead. There are a few discrete components on the main board. A resistor marked 102K, is that a 100k resistor? Next to that a capacitor or resistor. It is 0 ohms. Next a glass circular component, possibly a diode. Then a three terminal device with two ports joined, probably a diode. A tantalum capacitor then a 1000uF electro. Can someone verify these observations. The case was easy to open and then repair if you have the right tools. 102K is 1Kohm, the "2" means "2 zero's". That zero ohm component is probably an inductor. If it does not look like the other resistor and it's not marked then it's more likely an inductor than a shorted capacitor. The three terminal device is most likely a diode. Glass circular components are almost always diodes. I have never opened my 28S, so this is only an educated guess from an electronics designer. Dave.

Re: hp 28s repair Message #3 Posted by Ron on 22 May 2007, 9:00 a.m., in response to message #1 by Adrian Quote: The case was easy to open and then repair if you have the right tools. Really?! What tools? I've taken one apart, but how do you get it back together?

Re: hp 28s repair Message #4 Posted by Randy on 22 May 2007, 9:18 a.m., in response to message #1 by Adrian The 1000uf cap is the supply bypass/memory backup. 102k is a capacitor, the component next to it, an inductor. Those two form the clock tank circuit. The others are part of a voltage doubler for the LCD display driver. The usual failure is not electrical but rather mechanical in nature - a loss in contact between the logic board and keyboard due to failed heat stakes around the keyboard connector area. Quote: The case was easy to open and then repair if you have the right tools. What tools would those be?

Re: hp 28s repair Message #5 Posted by Adrian on 22 May 2007, 4:28 p.m., in response to message #4 by Randy Remove the decals, then using a drill bit slightly larger than the plastic pin diameter, drill the tops only of the plastic pins . I have a high speed drill press as used in drilling PCBs. I did not drill very deep just enough to weaken the plastic. The top simply pops off with most of the stakes remaining.

Re: hp 28s repair Message #6 Posted by Adrian on 22 May 2007, 5:18 p.m., in response to message #4 by Randy The 102K is 0.1 uF and the inductor ?????mH. Any suggestions as to its value. Next step remove the 2 active components and test.

Re: hp 28s repair Message #7 Posted by DaveJ on 22 May 2007, 5:47 p.m., in response to message #6 by Adrian Quote: The 102K is 0.1 uF and the inductor ?????mH. Any suggestions as to its value. 102K for a capacitor would make it 1nF (0.001uF). 0.1uF is 104. K means +/-10% tolerance Dave.

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