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HP Forum Archive 14

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Woodstock capacitor values?
Message #1 Posted by John Garza on 8 May 2004, 3:20 p.m.

Does anyone have a list of the values for those pesky tantalum caps in woodstock series machines? (A real schematic would be excellent, but I've never seen one with all the component values listed.) I'm working on a hp-27 and a hp-25c and I'd like to replace them.

-John

      
Re: Woodstock capacitor values?
Message #2 Posted by Tony Duell on 8 May 2004, 3:59 p.m.,
in response to message #1 by John Garza

For the one across the switched battery line : 60uF (68uF is fine). For the ones in the Vss supply : 47uF (some machines use 22uF, but 47uF is fine in all cases). For the little ones in the Vgg supply (and elsewhere) : 2.2uF. Rmemember that the HP27 uses NMOS ICs, so the power converter is different to that in other machines. BTW, HPCC has reverse-engineerd schematics of all the Woodstocks...

            
Re: Woodstock capacitor values?
Message #3 Posted by Albert on 9 May 2004, 6:52 p.m.,
in response to message #2 by Tony Duell

Hi Tony,

How can I download these Woodstocks reverse-engineered schematics from the HPCC?

Thanks in advance, Albert

                  
Re: Woodstock capacitor values?
Message #4 Posted by Tony Duell on 9 May 2004, 10:20 p.m.,
in response to message #3 by Albert

I said we had reverse-engineered these machines, I said nothing about having scanned the schematics and put them on the web site. Most of them exist as hand-drawn diagrams on paper (sometimes people forget that not everythign has to involve a computer!)

                        
Re: Woodstock capacitor values?
Message #5 Posted by John Garza on 10 May 2004, 12:05 a.m.,
in response to message #4 by Tony Duell

Thanks for the values! Looks like I'll be going parts shopping tommorrow. And I thought I hadn't seen any schematics in Datafile... :)

I also noticed one of the diodes has that waxy "glazed" appearance (overheated?). Looks like it says 20642 and I can make out the Motorola logo. It's located below the wound toroid, next to a 680 Ohm resistor. And by the way, under magnification that resistor has faint hairline cracks throughout it's body. It looks sort of the way an antique china dinner plate does. Ah, my babies are showing their age!

-J

                              
Re: Woodstock capacitor values?
Message #6 Posted by Tony Duell on 10 May 2004, 2:37 p.m.,
in response to message #5 by John Garza

Be careful. If that's the black diode (all the others being white with coloured bands), it's a zener used as part of the PSU regulation circuitry. I don't have the schematics in front of me, but basically, one transistor is an oscillator, the other removes the base voltage from it if the output voltage gets too high. Now, Vss is around 6.2V, you want about 0.6V be-drop across the base-emitter junction of the regulator transistor, so I guess it's a 5.V zener. But I don't know. What I do know is that it's not just a normal diode!

                                    
Re: Woodstock capacitor values?
Message #7 Posted by John Garza on 11 May 2004, 12:32 a.m.,
in response to message #6 by Tony Duell

Yep. That's the one. One lead traces right to the Base lead of a transistor marked "4-071". Thanks for the heads up warning.

A funny thing: I've been reading in the archives and found a good article entitled "A Common Woodstock Problem and Repair Suggestions". In the last paragraph, mention is made of an uninstalled U5 chip on the HP27. My 27's board doesn't have this. I don't know if the author (Katie) was writing about another HP, or perhaps there were several "versions" of the HP27 circuit boards produced.

cheers,

-J

            
HPCC?
Message #8 Posted by Wayne Stephens on 9 May 2004, 7:45 p.m.,
in response to message #2 by Tony Duell

Sorry to display my ignorance, but what is an "HPCC"?

Take care.

Wayne.

                  
HPCC, yeap!
Message #9 Posted by Vieira, Luiz C. (Brazil) on 9 May 2004, 7:52 p.m.,
in response to message #8 by Wayne Stephens

Hi;

it's the Handheld and Portable Computer Club, formerly PPC. It's in United Kingdom.

Cheers.

Luiz (Brazil)

                        
Thanks!!
Message #10 Posted by Wayne Stephens on 10 May 2004, 7:07 a.m.,
in response to message #9 by Vieira, Luiz C. (Brazil)

Thank you. I will check it out.

Wayne.

                  
Re: HPCC?
Message #11 Posted by Tony Duell on 9 May 2004, 10:18 p.m.,
in response to message #8 by Wayne Stephens

HPCC == Handheld and Portable Computer Club. It's the UK HP calculator user club, now 22 years old and still (very) active.

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