HP 67 with some dim leds

[juan demin]

Hi, I’ve just bought an HP 67 in really good condition…it is working properly but the leds are not as bright as those of my HP 65.
I know some of you have also both calculators….can you tell me if this is normal?

[Eric Smith]

Do the segments have uniform brightness, or are some significantly dimmer than others?

[David Ramsey]

If the brightness is even across the LEDs, it's fine. There is some brightness variation among the LED calculators, sometimes even among different examples of the same model.

The display may be brighter if you're running the calculator off its AC adapter. However, with this model, make sure you have a battery pack installed if you're using the card reader, regardless of whether you're using the AC adapter or not.

[juan demin]

Thanks for your answers!!!! All the segments have uniform brightness...just dimmer than the hp67 and yhis is the same using it with or without charger. I was told to use always the charger with the battery pack instaled. Thanks again

[Geoff Quickfall]

Yes it is normal,

I have 5 HP-67's including a New Old Stock model and they all vary in brightness (evenly) between each other.

The two HP-65's I have are both brighter then the HP-67's.

Since the HP-67 is related to the HP-21 through HP-29C series; i.e. it is a Woodstock, one should ALWAYS have good batteries in it when using the charger. In fact, the best thing to do is to purchase an external reserve power pack 82004A and never use the charger for the HP-67.

Also, modify any Woodstock battery packs (remove the centre spine and this will allow access to the batteries and the ability to swap out dead ones for recharged versions) to accept regular NiCd or NiMh batteries and charge them with a generic rechargeable supply. NEVER NEVER use the chargers with the Woodstocks UNLESS you are certain there is NOTHING wrong with the battery pack. A good rule is to never use the charger/adapters with the Woodstocks including the HP-67.

Cheers, Geoff

[juan demin]

Thanks Geoff...!! I wonder if these rules of "battery / Charger" are well known among Woodstock users... As I said, I was once told about not connecting the charger without the battery pack inside, but it was a kind of soft warning... I appreciate the advise very much (unlike you, I have only one!!!!)

[Mike Morrow]

I don't believe that the power supply arrangement of the HP-67 is anything similar to the disasterous design of real Woodstock units. The Classic battery charger is used, which uses a three-conductor cable to deliver a feed to the electronics that comes from a regulated supply, and a separate circuit section for the battery charging function. The battery is separated from the electronics when the charger plug is pushed in.

[Eric Smith]

In the 67, if you use the charger without a good battery pack, it won't damage the processor or memory chips, but it apparently is possible for it to damage the custom read/write amplifier chip for the card reader. I'd recommend following the same rules as for the Woodstock series.

[Dan W]

Quote:

---

I don't believe that the power supply arrangement of the HP-67 is anything similar to the disasterous design of real Woodstock units. The Classic battery charger is used, which uses a three-conductor cable to deliver a feed to the electronics that comes from a regulated supply, and a separate circuit section for the battery charging function. The battery is separated from the electronics when the charger plug is pushed in.

---

True for all Classics except for the 67 and 65, in which the card reader is powered by 2 additional wires that are connected directly across the battery terminals. When the charger is plugged in, they are then connected to the charging circuit, not the logic circuit.

The charging circuit provides a constant 50 mA current to trickle charge the battery. But the voltage drop can be as high as 16 volts. As I understand it, the battery in effect forces a voltage drop of about 4V give or take. Without a battery, what would happen?

I suppose this was done because the charger logic supply cannot deliver the current needed by the card reader, but the battery can. Is this correct?

[Geoff Quickfall]

Hello Mike,

Eric scooped my response by a few hours! The power I am concerned with in the HP-67 is directed to the card reader circuitry.

But in a more general sense, why subject your 30 year old collectable to any internal charging? Regardless of the family, I would charge all batteries outside the calculator and not stress the aging circuit at all, or subject it to a failing charger.

Cheers, Geoff

[Mike Morrow]

I appreciate these several responses which clear up something I've wondered about for more than 32 years, when I got my new HP-67. The manual cautions about damage if the battery pack is not installed when the external charger is being used. But back then I opened and traced the circuit of the charger, which seemed well-designed for calculator operation without a battery installed. I never knew the real basis for the warning in the manual until now.

I suppose the designers wanted the card drive motor current surges to come direct from the battery. But at least the charging design which generates DC in the charger (constant current, IIRC) is better than the real Woodstock's cheap AC-output charger and simple dangerous diode/resistor charge circuit in the calculator. There is no justification for such an incompetent design.

I have always followed the manual's recommendation, and I've always kept a reasonably decent condition battery pack in the HP-67.

Thanks all!