HP-35 Red Dot - Is this authentic?


There is a photo of an HP35 Red Dot on EBay. I know there is text about the serial number matching but what I wonder about is:

1) The picture on the HP Museum site shows the Red Dot hole about the same size as the "ON" text and centered. The one on EBay is about 50% larger and is not centered on the "ON" (offset low).

2) The switch is in the "ON" position and the dot is not visible, yet in the original photo, it is visible.

Is there a specified serial number range that positively identifies a Red Dot?

Does the 2501 positively mean it is a Red Dot?

Were there hole size variations?

Why would the dot not be visible in the ON position?


It was not me, but a one-time friend made a fake red-dot HP35 by carefully drilling a hole into the keyboard and monkeying around with the on/off switch.

he said it would be impossible to tell the difference.

However, he was in a different part of the country than the seller on ebay right now, so I have no knowledge that this is false in this instance.

There is, though, one fake HP-35 red dot out there...


But I don't know if the ebay one is one of these.


I am looking at a red dot HP35 right now that I know to be
authentic. The seller on eBay has posted a lot of additional extreme close-up photos on the auction. It is
very difficult to see much difference from the hole of the
eBay 35 and the one I am holding. It looks to me like the
OFF and ON letters are a little higher than the one on eBay,
but the difference seems extremely minor.

I will say that the serial number tag on the eBay 35 matches
pretty close to the red dot 35 I am holding. I have a
second edition 35 with the red dot missing but that still
has the raised bump on the '5' key. The serial number on
the red dot 35 I have has smaller serial number sizes than the second edition model, just
like the one on eBay. My second edition 35 has smaller text
size for the 3.75V, Hewlett-Packard, Made in USA, patent pending than my red dot 35 tag text and the one on


I am working on a very old HP-35 for a friend. This calculator has a similar serial number decal as the one on ebay. The # is 1143A 27162.(Ebay calc has serial # 1143A 02501). The interesting thing about this calculator is that, although it is not a HP red dot, the on/off slider switch has a red dot on one side of it! Of course the calculator must be taken apart to see this. The cpu board connection is also different than the standard classic boards. The cpu board (date code 3-72)on this calculator has sockets instead of holes for the keyboard pcb pins. Also the black backbone is in three parts/pieces, instead of one like all the other classics.


One curious thing I noticed between the calculator that I am working on and the HP-35 red dot on ebay is that the back label on the calculator I am working on ser. # 1143A 27162 has HP-35 INSTRUCTIONS at the top of the label. The red dot calculator on ebay has HEWLETT PACKARD HP-35 INSTRUCTIONS, like the later HP-35's. This makes me wonder if the calculator was serviced by HP maybe to replace the CPU board that had the ln (2.02) bug. It might be worth asking the red dot owner to check for the bug.
Take the natural log of 2.02 and then press e^x and you get 2. instead of 2.02.

I have another older HP-35 w/o red dot but with raised nipple on the 5 key. It also has the bug and the HP-35 INSTRUCTIONS label. Unfortunately, it is missing the serial#.


On the subject of serial numbers, I'm not sure how reliable they are to help determine if an HP35 is a red dot or not. I've had 3 red dots with the following S/Ns


(none have signs of entry)

But I also have these S/Ns for two of my HP35v2 calcs:


(both have been opened)

And further, I have an unopened HP35v3 with the following S/N:



1) It appears that the serial number alone cannot be used to identify the calc as a Red Dot. Tom has newer models that span that number.

2) The Red Dot switch is available on other models. All that is missing is a hole.

3) The label is one that shows up in later models.

4) The "N" on the one on EBay can entirely fit within the red dot hole area without touching the edge of the hole. The one on the HP site cannot come close. Use Image Processing s/w and see for yourself. Both photos are good enough to use for sizing purposes with the s/w. Clear size difference.

5) The hole does look larger, offset and closer to the "N" than the one on EBay.

The price is rising on that one on EBay. It would be nice if there was a way to positively identify it as a legitimate Red Dot.


Well, the internals are a bit different for the red dots (and maybe the early v2s?) if the seller wants to post a picture of the insides. Looks like the calc had been opened a number of times anyway.


anonymous states: "4) The "N" on the one on EBay can entirely fit within the red dot hole area without touching the edge of the hole. The one on the HP site cannot come close. Use Image Processing s/w and see for yourself. Both photos are good
enough to use for sizing purposes with the s/w. Clear size difference."

I think this is a very questionable claim. For one thing
a good portion of the 'N' is worn away and they only thing
you are going on is the pictures that are provided. Did you
correct for the angle that the image was captured at? I think a much better test would be to compare the hole with the size of the 'O' from 'OFF' or 'ON'. If you do this, I
suspect you'll find a much closer match to the red dot on


I don't want to start an argument here. But let me tell you something about physics (or optics). If you have two objects on the same plane then their relative distortions are the same as something is tilted. You don't get one distorting more than another.

I think you are looking at the wrong photo. The "Photo 2" is virtually straight on. If you look at that photo, you can hardly question that the hole is larger than ANY of the OFF or ON letters.

There is no angle to correct for, in this photo. In fact, you can draw a line from the bottom of the 'O' in OFF across to the ON and see that the hole is clearly larger than it should be.

You say you have a Red Dot. Does the top of the hole align with the top of the characters and have the bottom of the hole about 30 to 50% below the bottom of the characters?

In fact, you can take a straigh edge and line it up with the slot that the switch is in and see that the hole is way below the letters.


You just can't make the kind of comparisons you are attempting to make between the photos of the eBay 35 and
the on on the hpmuseum. Look at the drastic difference
in lightning source location. The eBay 35 has huge shadows
cast by the 35's keys. The lighting source is clearly
coming from above the 35's display. The hpmuseum image has
virtually no shadows at all. I can play around with lighting source location quite a bit and cause perceived
differences in the way the red dot hole looks (including
minor changes in where the hole edges appear to be) in the resulting image. In fact, the red dot in the hpmuseum
image looks strange cause it doesn't look all that much
like a hole.


You can vouche for the 35 as being legit and maybe it is. I'm just trying to get to the bottom of this.

But I can tell you with absolute certainty that the lighting , in this case, does not detract one little bit, in the ability to see the hole size comparred to the lettering.

It's plain as day. It's also plain as day that the hole extends far below the bottom of the lettering.

It's very easy to see the bottom edge of the on/off slot. This is NOT affected by lighting.

It's very easy to line up a straight edge and make these comparrisons. There are no distortions in these photos that preclude that. These points of interest are flat and have no shadows. They are NOT affected by lighting.

Does your legit 35 have a hole that is aligned at the top with the text and extends far below the lettering?

Yes or No.

Because it does in the Photo 2, shown on EBay.

Let's hear from other on the alignment and the size of the Red Dot hole, as shown in Photo 2, and see if they agree with you.


The red dot HP 35 that I have also has a back label title

I know that my red dot hp 35 is an authentic red dot cause
I know it's complete history.


Does anybody know why there are two versions of this back label?
Were the early calculators built at two different sites?
My guess was the HEWLETT PACKARD HP-35 INSTRUCTIONS label was maybe a later type and replaced the HP-35 INSTRUCTIONS type when the calcular was serviced at HP (probably to replace the bug ROM). Does your red dot have the bug?


you are probably right that 35s sent back for ROM bug fixes
had their labels replaced. I know for a fact that the
red dot I have was indeed sent in and it does NOT have the
ROM bugs. The label that is titled "HEWLETT-PACKARD HP-35
INSTRUCTIONS" probably replaced the original label at the


One other thing that could be very enlightning to check is
the piece of metal that makes a connection to the two outer
pins of the charger socket. Both the red dot that I have and the very early second edition have metal pieces that
do NOT over lap the plastic edge directly under the charger
socket and that is mostly covered by the battery compartment
door. Later models have metal pieces that actually overlap
this edge and fit into a slot in the same plastic surface
that the two case screws under the battery cover go into.
The very early models don't have a slot in this part of the
case for the metal piece at all.

Hope this explanation is clear. If you look at the metal
piece that connects to the two outer charger socket posts
I think you'll see what I mean.


slight correction on this. The metal piece does overlap
but the part of the metal that overlaps is black. But
there still is NO slot in the plastic that the two case
screws go into. The later revs have a slot that the metal
piece fits into.


It looks like photo 23 on the eBay red dot 35 auction
shows that at least the charger socket metal piece conforms
to the very early HP 35 revs. Notice how the metal piece
that overlaps the plastic edge is black and does NOT fit
into a slot in the case. Newer revs definitely had bare
metal overlapping the edge and fit into a slot in the case.
At least all the later 35s and 45s (and 80s) that I have
seen conform to this.


Yes, the older HP-35 I am working on has this same type connection scheme. It is a very early model, but not a red dot. The black back bone piece is also in two parts, instead of one like later classics. This is the black plastic piece that you see when you open up the battery door.


Turns out that I live near the seller. I am not an expert on the red dot but I have used virtually every HP calculator that has come down the pike. I have even opened quite a few.

If we could devise some test or come up with some inspection criteria, maybe the seller would let me stop by and take a look at the 35. I could then report back what I found.

Just a thought...


How about the seller's claim that there are only 12 of these in the US? Personally, I own 4 and 1/2 red dots (yes, a 1/2. I have a half finished one that came from the estate of a HP engineer that worked on these. It has defective electronics and doesn't appear that the back was ever installed.) I know know of at least five others in the US including one guy that has 3 and I suspect that there are a lot more than that. I read somewhere that a LOT of the red dots still survive. The author stated that the people that had them knew they had something special so they took care of them and many original owners still have them.


So he's off by 3 orders of magnitude. That's a relatively small exaggeration for ebay :-)


Personally I think the eBay seller deserves a bit of a break. He didn't even know he had a red dot till Dan B...
sent him mail. It was Dan B. that made the claim about
possibly only 12 in the U.S. I wonder who Dan B... is?


Since one of my red dots has a S/N of 1143A02852, one might assume that HP made at least 2,852 of them. But as to how many survive, that's anybody's guess.



>But as to how many survive, that's anybody's guess.

My guess would be to count the number of 35 red-dots that
you own and add 1 of the one on ebay and 1 for ever
other collector on this site that claims to have one.


-An envious red-dot-want-to-be-owner


They certainly aren't common, but in my personal experience, I've found that the 10A, 70 and 28S-100th are harder to locate. Good luck!


I just bought it and will report more, when I've received the calc and somebody's interested.

There aren't too much other red dots here in my surrounding (At least I don't know of an European collector owning one).
I'm pretty sure, there are way more red dots in the surrounding, but lots of them have surely been thrown away.

Can anybody confirm the 3000 produced units ?

I spoke with some people, who threw their calcs away just half a year ago. One of them was a HP35v2 with plastic box and some other goodies.

I fear, that this is not the right device for an ebay auction somehow (just like valuable pictures and other artwork).

I wouldn't take the hole as a measure (at least not from a photo).

The seller has a feedback rating of more than 3100 with only 2 negatives and I'm pretty sure such a guy wouldn't ruin his reputation that way.

Unfortunately, eBay is the only way for getting such a device in Europe.


I noticed in the pictures that the seller put out there, that the cover of the manual shows a RED DOT 35. From what I've seen, the later models came with a manual that showed a caclulator without the red dot. Of course the manual could have come from another calculator, but I took that as good evidence that this is a real RED DOT 35. (And hence bid high on the auction too.)


The manual that came with my late version 2 (says Hewlett Packard 35 on front edge) shows an early version 2 (says just Hewlett Packard). Definitely no red dot.


Good eye, Katie! 95%+ of the HP35 manuals I've seen have the standard v2 image on the cover, but I do have 3 with the Red Dot on the cover. The insides are quite a bit different, as well. I also have an early HP35 sales brochure that has pictures of the red dot throughout, so one may wonder if the Red Dot was only intended for internal HP use, as some claim.


My gut feel as to why the red dot was discontinued is that when you turn it on, you will see 3 fairly bright red LEDs that are turned on.

If you can't see the LEDs, you can't see the dot:-)


Katie said "I noticed in the pictures that the seller put out there, that the cover
of the manual shows a RED DOT 35. From what I've seen, the later models
came with a manual that showed a caclulator without the red dot. Of
course the manual could have come from another calculator, but I took
that as good evidence that this is a real RED DOT 35. (And hence bid
high on the auction too.) "

ALL of the early 35 manuals and possible ALL of them period showed the red dot in the picture on the manual so that's not a reliable indication.



There was a LOT of interest in that calc. I'm hoping you are able to positively verify its authenticity as a red-dot; I am pleased someone in our board's "family", with a lot of HP knowledge and experience has it. Congratulations and best wishes, RH.


Thanks. I noticed some people I didn't really know emailing me with congretulations.

I'm pretty sure, there's a lot of guys with more experience, but I simply had to have it. In Europe, there's no chance finding such a device, so eBay's the only way.

I'll take some holidays now and will use them to think about how I will re-fill my account (Donations welcome --- just kidding :-)

I'll be back wednesday and I hope, that the calculator arrives late next week. The seller agreed taking my payment with bidpay ($500 max.) and taking the rest in another way.
He said, with my feedback, he'd mail it out as soon as he has the bidpay confirmation.
Getting postal money orders from Austria -> U.S. is a real pain in another way (6 weeks).

Might be, that I'll answer him some questions about the calculator to recover it's history (at least the last owner), but I'm pretty sure I won't tell this info on the internet without an important reason.


Just an idea for you...

I just purchased an HP80 from Germany.
And had similar problem doing the payment (but in reverse).
The solution was to bank the funds directly in the account using a telegraphic transfer.
I have used this many times and it has worked well for me.
The payment gets there overnight as well.
The fee was $25 (Australian dollars).


My method was paying the majority with bidpay (which seems to be a very easy method) and sending the rest with the postal method. Normally this works well. Bidpay seems to be the fastest method, especially for international transfers.
Fortunately you can pay with bidpay, even when the items price is larger than $500 (You can pay $500, but not more).

International bank transfers are really expensive from Austria (seems, there's much more competition needed).

I've tried Western Union once and the agent was unable to say, how much the receiver would have to pay for getting the money. Finally, my seller ended up with $6 less than I expected and I had to send $6 in an envelope - a disaster.
The agent couldn't state, how much the receivers cost would be and so I couldn't pay these fees on my part. Additionally, Western Union is not cheap for international money transfers.

Money orders from Austria to U.S. take 6 (!!!) weeks. I asked at the main post office in Vienna and they described it: The money order is written on a list and this list is put into a letter. For some strange reason, the letter is sent SURFACE instead of air, which leads to the large delay. Emperor Franz Joseph (1848 - 1916) couldn't have done better. I wouldn't even try to force a seller accepting this . I needed a money order Austria -> U.S. for another reason abt 10 years ago and it took 6 weeks (!!!). A research for a lost money order took half a year.

Paypal isn't for non-U.S. citicens, so I'm not allowed using it. In fact it seems to be some kind of a bank, but without earning something, if you leave your money lying on their accounts.



Western Union usually doesn't cost anything to the recipient; I certainly never had to pay anything when I received a Western Union transfer. Just make sure that the amount is specified in the desired currency (e.g., even though I send sums from Canada, I do so in US dollars) otherwise the actual amount paid out may be less than expected due to their miserable exchange rates.

PAYPAL is supposed to become international real soon now. (Not sure if Austria is on the list of countries that they plan to show up in.) For now, I'm able to use it because I have a US bank account and a US mailing address.

Lastly, I believe what you call a money order in Austria is really a postal money transfer. A money order here is something you buy at your bank and then put in an envelope and mail yourself. The recipient takes it to his bank and deposits it, just like he'd deposit a personal check. I have not seen this form of payment when I was living in Europe.



I bought the HP-65 that the seller also had on EBay. Turns out that I live just a few miles from him. So, I decided to drive by and pick up the 65.

He still had the 35 so I asked to see it. Well, I was quite surprised. It looks better in person than it did in the photos.

The dot hole is offset, BUT it is centered on the slot as you would expect, since the red dot is also centered in the switch. The ON/OFF labels are slightly offset making it look odd.

The hole is also NOT oversized as was speculated in the photos.

I'm not an expert on these but it looks like it is a real red dot. I can't say this, as one who has seen these before, but nothing about the hole and the red dot look unusual. The hole looks manufacturered rather than added later.

The HP-35 looks good... The seller was a really nice guy also. If I had seen it before the auction ended, I probably would have bid on it myself.


I'm pretty sure, that it's a real 35 red dot after reading through this thread. I think there's hardly someone with another red dot machine in my surrounding, but I guess, that the effort falsifying a red dot machine is much larger than the earnings you can expect.

Just think, what you should have before even trying (the correct manual version, serial number, the red dot switch itself or a very early HP-35, ...).

The risk is rather high. If you fail, you have a v2 calculator with a hole in the front. Not, what I would wish, not to mention the time effort to make the machine look authentic.

It had also a very very good description. I've never seen anything that well-described on eBay. If one of the abt 30 photos would have shown a real hint to falsification, I'm sure two or three of the experts would have found it. Additionally, I guess, a false calculator would have had better cleaning (the falsifier would have wanted to make more money by making the calc look "sexy").

A guy, who wants to make money, wouldn't risk putting too much photos on the web. (he'd email some to a potential buyer, but posting them on a web, can open a broad discussion in the collectors community, which has already happened with this thread).

This together with the huge feedback, the seller owns, and now with Mikes statements, makes the machine and the seller very trustworthy...

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