HP85B awakens

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Re: HP85B awakens

Postby kaimaiguy on Thu Jul 21, 2011 9:04 am

OOH kie dokie then

Well I retract my last report on the capacitors. I believe these are a foil axial lead type and what appears to be a blown out end on one side just may be their normal construction. There are four of these on the Logic board. Also what I thought was discolouration from blowout was just a little tarnish on the leads, there are no black or burn spots on or near these capacitors anywhere and their soldered joints are clean.

I think the only next thing to try are the removable DIP ROMs themselves and to try and obtain the missing one. Anyone have an ideas where these may be found? In the manual all four are noted to have some control over the CRT.

Thank you so much

Rick
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Re: HP85B awakens

Postby tezza on Thu Jul 21, 2011 10:30 am

Hi Rick,

Some ROM sockets in old machines can sometimes be empty and are used for “optional extras”. The Commodore PET has sockets like this. So just make sure they are really necessary. I guess the manual can tell you that.

If it is a necessary ROM, there are three options. Either seek a junk machine you can extract it from, seek a replacement ROM someone has as a spare part or seek/get/burn a replacement EPROM for the ROM. Given these machines are quite rare, neither of these things are easy. If you haven’t already, I’d engage with the HP85 community through that Google group. Someone might have a spare or they might be able to burn an EPROM for you. Often an EPROM adaptor also needs to be made as easy-to-get EPROMS are not directly compatible. I have an EPROM programmer and might be able to help but you'd need to find a the ROM binary image and information on what type of EPROM is required as a replacement for the ROM (and details of an adaptor if needed)

However, the erratic symptoms you describe don’t seem to be a ROM issue. Even if a required ROM is missing, the buzzing and flaky nature of the display would suggest some other issue also. If it was just a ROM the symptoms would be steady. My approach would be to sort these other issues out first (a steady consistant display, no buzzing and correct voltages). Ensuring the voltages on the board and the VDU are correct are an essential first step in diagnosis. Otherwise it's like someone trying to improve the steering on a car by tinkering with the alignment when the car has four flat tyres.
Tez (Terry Stewart) (Administrator)
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Re: HP85B awakens

Postby kaimaiguy on Thu Jul 21, 2011 3:09 pm

Thank you for your advice!

I think I'll take the sobering part of it which says to leave it alone. Sometimes we enter an endeavor with unrealistic attitudes and hopes. We so much want to achieve and see a result for our efforts. At my age and health state I don't have the time , resources or energy at hand to engage in something that may take years to effect a satisfactory end.

I do not have the knoweldge neccessary to approach this project in sane fashion as I am not familiar with measuring voltages etc and so will regretably reoffer the unit on TM and hope that it will not be too much of another investment lark.

I truly thank you all for your kind time and attention to this now defunked project and wish you all the very best.

Abundant blessings

Rick
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Re: HP85B awakens

Postby tezza on Thu Jul 21, 2011 5:13 pm

Hi Rick,

One thing I certainly don't want to do is pour cold water on someone's enthusiasm. However, if you're unfamiliar with measuring voltage I would suggest it would unproductive (and maybe dangerous to both you and the machine) to delve any deeper into the diagnosis. Not at least until you've got some more basic electronic knowledge anyway.

This book might help. "Electronics for dummies". It's aimed at the hobbiest who is just starting on this stuff. I THINK you can download it for free here:

http://www.ebook3000.com/Electronics-Fo ... 22624.html

Either way, given its rarity (hence scarcity of spare parts), non-standard CPU and lack of a wide body of people familiar with the machine, a HP85b is likely to be challenging to fix even for the experienced technician.
Tez (Terry Stewart) (Administrator)
Collection: http://www.classic-computers.org.nz/col ... /index.htm
Projects and Articles: http://www.classic-computers.org.nz/blog/index.htm
Twitter: @classiccomputNZ | YouTube: Terry Stewart
Trade Me: tezza5
tezza
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Posts: 2291
Joined: Thu Jan 01, 1970 12:00 pm
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