HP70 project - cosmetc and electronic issues - Printable Version +- HP Forums (https://archived.hpcalc.org/museumforum) +-- Forum: HP Museum Forums (https://archived.hpcalc.org/museumforum/forum-1.html) +--- Forum: Old HP Forum Archives (https://archived.hpcalc.org/museumforum/forum-2.html) +--- Thread: HP70 project - cosmetc and electronic issues (/thread-204837.html) |
HP70 project - cosmetc and electronic issues - Alberto Fenini - 11-13-2011 Dear All, Re: HP70 project - cosmetc and electronic issues - David Ramsey - 11-13-2011 I have disassembled and cleaned some dozens of these "Classic" HP calculators, and I don't know any refinishing technique that would eliminate the scratches without radically changing the appearance of the keyboard. Frankly given the HP-70's well deserved status as the ugliest single calculator ever made in the entire history of the world (I gave my much nicer example away as a door prize at HHC 2006. Nelson got it.), I wouldn't sweat it.
I'll leave any electronics comments for someone else. After all, I don't even know a full wave rectifier from a bridge rectifier!
Re: HP70 project - cosmetc and electronic issues - John Robinson - 11-13-2011 Hi Alberto, I think you're lucky that the 70 has no lettering on the keyboard surround. I have had some success with this kind of issue by light sanding fine emery paper, then spray painting with Tamiya flat lacquer. The good thing is that I can sand and respray multiple times without issues. Just may sure you use to appropriate lacquer type, I use the "TS" lacquer. I have done this with a 65 which does have lettering on keyboard surround. On the electronics, I would try re-soldering, but there is always the risk they it will be worse after wards. I's start with by re-soldering all the pins on the bottom edge connector. Also make sure all the holes for the pins in the logic board are free from corrosion.
Cheers, Re: HP70 project - cosmetc and electronic issues - Eric Smith - 11-14-2011 Tony or Randy would probably be better sources of advice on the power supply circuitry, but if the transistors are getting hot to the touch, something is definitely very wrong. I doubt that it's just a bad solder joint. If it were my calculator, I wouldn't power it up again until I had come up with a theory explaining why the components are getting hot, and had attempted to repair it.
Re: HP70 project - cosmetc and electronic issues - Randy - 11-14-2011 Quote:I would not recommend re-soldering. You would be heat stressing almost forty year old components for no good reason. A good visual inspection should identify any possible bad solder connections and re-solder only those necessary.
Quote:If they work, why would you replace them? You achieve nothing, other than to say you replaced perfectly good components. As for the transistor and transformer getting hot - is this when the unit is functioning or when it is on the fritz? The intermittent behavior - Corrosion on the board or on the pins - is obviously the most common cause of trouble but if you find none: Remove the logic board and carefully inspect the 25 gold pins on the keyboard side - insure that the smaller, lower side extends outward enough to engage and make good contact with plate-through hole of the logic board. It only takes one compressed pin to cause major grief. When the pins look good, with magnification, inspect the plate-through holes of the logic board.
Re: HP70 project - cosmetic and electronic issues - Alberto Fenini - 11-14-2011 Thank you very much for all the replies I've got so far. Re: HP70 project - cosmetic and electronic issues - Randy - 11-14-2011 Quote:Yes, that is best for corrosion/battery leakage. Caig DeoxIT is also highly recommended for clean up and protection.
Quote:Sandpaper? One word: *NEVER* A clean "pink pearl" pencil eraser is most abrasive thing I'd every take to gold plated contacts. This is the best way to restore "shine" if there is any shine left to restore...
Re: HP70 project - cosmetic and electronic issues - Alberto Fenini - 11-14-2011 Thanks Randy, Re: HP70 project - cosmetic and electronic issues - Randy - 11-14-2011 D5 Aerosol is good for cleaning corrosion and over-all use on the classics.
Re: HP70 project - cosmetic and electronic issues - John Robinson - 11-14-2011 Alberto, I am not totally familiar with the DC-DC converter on the 70, but assuming it's the same as the 65 which I am familiar with, then the heating up of those components sounds like the calc is drawing too much current. which would indicate a short or near short on one of the three power lines, Vcc, Vss or Vgg. Unfortunately in the articles forum, Tony Duell's "inside the HPxx" doesn't cover the HP70, only the other classics, so I can't tell you where to measure Vcc, Vss and Vgg. The only other possibility for a hot transistor would be the 2nF capacitor (large capacitor next to coil) is on the way out. If that's the case, then the oscillator might be oscillating too fast. However, that would really only apply if one transistor is hot, not both. One transistor forms the oscillator, the other forms a kind of regulator via Vss line (I think !). On a HP65 the oscillator oscillates at around 670 Khz, do you may want to check that if you have access to an oscilloscope - try pin 1 of the "coil". The coil itself is extremely unlikely to be the problem. Personally, I'd replace the 2nF capacitor, and both transistors, and the diode in the feedback (connected to the zener diode). All other diodes are for rectification of the three output voltages.
Cheers,
Re: HP70 project - cosmetic and electronic issues - aurelio - 11-23-2011 Quote:Hello Alberto and compliments for your HP70 I read that it's not so easy to find that kind of calculator.......... expecially for a human price!
I'm curious to know where have you found the Caig DeoxIT here in Italy... If I've not well understood, may you suggest another equivalent product, I'll appreciate it, thank-you
Re: HP70 project - cosmetic and electronic issues - Joel Setton (France) - 11-24-2011 Alberto, |