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Calculator blog

Musings and comments about our common interest


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HP 41c battery bay repair kit


One of the most frequent requirements we're getting is to provide assembled repair kits for HP41C. The problem is that we depend on getting used, destroyed HP41c calculators to peel off the old flexible circuit, clean it and file off the rivet remains, and adhere the new flexible circuit to the plastic base. The issue is the first part - getting the part to perform the work.

Once you have used the part, your calculator cannot work - so you can only get it from damaged calculators that you're going to use as part donors. This is the reason that we don't get more of these assembled repair kits. 















On the other hand, it really saves a lot of time and patience to avoid folding and collating the sticky flexible circuit - I always wonder whether it is going to work properly once finished. The assembled part takes the doubt off the process!

I will post here a message once I get more assembled repair kit parts.

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HP54501a repaired

I spent some quality time during this weekend to get the oscilloscope working. As I said in previous installments of the blog, the most likely culprit of the problem was the VRAM chip. These were built with a battery inside, and were used to keep the calibration variables. If it is not working, it not work properly until calibrated - but the calibration would then last only until powered off - and you'd have to recalibrate each time! Mine was even worse - it wouldn't show a trace. Running the test routines it failed on the VRAM (as expected) but also in the AD and DA tests.

I disassembled the machine, which was extrmely clean inside. The disassembly procedure was well explained in the service manual, and I managed to get the beautiful main board, equipped with a complete computer of the time - starting with the Morotola 68000 microprocessor (way more powerful than the 80X88 that equipped the primitive IBM PCs. The 68000 processor was also used in the Apple Macintosh)

The problem was that the integrated circuits were directly soldered to the board - no sockets in sight. Then it was risky to apply too much heat to solder or unsolder the chips. 

After all the effort, we switched on the machine. It worked. Then we proceed to the recalibration as per the user manual (the machine is self-calibrating, which saves a lot of time getting the proper equipment to calibrate it. You only need a cinch-to-cinch connector to run from one of the calibrating outputs in the back of the oscilloscope to one of the four channels)

After the recalibration, we powered off and on again, and run the self-tests. This is what we got:

IMG_1060This meant that we had succeeded. This unit is a little bit battered on the outside, but excellent inside - I hadn't seen so clean a machine since a long time ago. Built like a tank, too.














I then proceeded to see the time response of the filters of the new Meridian Explorer 2 DAC, with MQA. I could then see the ringing of the filters, which is supposed to be all after the pulse (as opposed to Linear phase filters, which "ring" before and after the event.). These filters are supposed to be nicer on the ear. Lets see them applied to a sqare wave:





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More information on the HP54501a oscilloscope

I already commented in a previous blog that I was repairing an 54501a oscilloscope. I was impressed by the quality of its circuit and general build;  but also its logic was miles ahead of what I was used with analog oscilloscopes. Moreover, you could print out a screen through HP-IB to a suitable printer (that we also have).

The 100 MHz bandwidth may sound limited nowadays, with a lot of equipment running in the gigahertz range, from calculators to computers; on the other hand, it is plenty enough for any digital or analog audio applications, and this is what I am planning to use it for. Nowadays, with the advent of the MQA audio codecs, it will be priceless to perform all kind of analysis on how different waveforms are encoded.

As I have let the unit in the mountains' laboratory, I am not able to post any pictures - but i will during this weekend. The problem this unit seems to have is that it has lost the capacity to store a calibration. It used a vram memory chip that included its own battery, and it has run dry after all these years. The solution is to unsolder the old chip and solder in a replacement. This is a 5 euro chip, so it is a quite safe investment. Apparently, once this problem is sorted, it can be self-calibrated and the new values stored. Then -fingers crossed - all self tests will pass again.

I'll keep you posted. Please also email me if you have a similar machine to repair, or are looking for similar equipment.

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HP 8591E Spectrum Analyzer

I was always surprised to see the wide price differences between HP vintage oscilloscopes and HP spectrum analyzers from the same period. So it was with excitement that I got a HP 8591E spectrum analyzer for our shop.
















Nowadays most of the current oscilloscopes have some kind of spectrum analysis tools - always performed as FFT (fast fourier transforms) of the waveform readings. These machines use a completely different method. I will copy from wikipedia so as to be able to explain it better:

A swept-tuned analyzer uses a superheterodyne receiver to down-convert a portion of the input signal spectrum to the center frequency of a narrow band-pass filter, whose instantaneous output power is recorded or displayed as a function of time. By sweeping the receiver's center-frequency (using a voltage-controlled oscillator) through a range of frequencies, the output is also a function of frequency. But while the sweep centers on any particular frequency, it may be missing short-duration events at other frequencies.

In future installments I will discuss this wonderful machine - complementary but completely different to your oscilloscope.

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Más sobre: Hp8591E
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HP 54501a oscilloscope

During this week end I have been beginning to repair our unit of HP54501a oscilloscope from the early nineties.

What a design! The main circuit and all the other circuits (keyboard, power supply, CRT driving circuits) are of exceptional design. The main circuit is built around a Motorola 68000 microprocessor, and the layout and the PCB itself are incredibly beautiful and well done. This is the quality of HP of that era, second to none. 

Also the clarity of the manuals and the accompanying documents is exceptional. You get a course on oscilloscope basics for free when you read them!

I will post updates as the repair go. Suffice to say that the self test information was extremely clear and conducive to an easy diagnostic. 

More on next issues...

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Time module in HP41CL


So far the hp41cl doesn't provide a clock to be used as time module, and it requires a working one to be able to function as stopwatch or alarm clock (or launching programs at a defined time)

However, the hp41cl "raw" provides all the time-related calculating functions of the time module: days difference between two dates, date calculations, etc. So, if you don't want the above clock features, you can live without it (and why would you want them when you have a smartphone?)

There are always those that would like to have the full Monty (never better said when referring to Monte’s development), and then you may want to have the time module too. Or, you have interfaced your hp41c with other items, and then you need to launch programs at a determined time intervals. 

I once did that, when I connected a HP3468A HP-IL multimeter to the HP41CL to check what was the voltage in my weekend cabin during an extended period of a week. Then it was grabbing data every half hour for the whole Monday to Friday we’re not there. 

Other uses that we’ve seen are: 

- use it in conjunction with astronomical programs, to better direct your telescope.
- Use it during flight to calculate fuel consumption rates, direction drift on side winds during time periods, etc. 

As always, here is no reason this couldn't be done with a smartphone - but if you don't have the program that does what you want, then the easily programmable HP 41CL can come to the rescue nicely.

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New connectivity and virtual Prime calculator software!

New Prime software issued in December.

You remember probably that we announced a beta version of new connectivity software, and also virtual calculators, with the advantage of introducing Mac OS versions of both. So, for those of you that were using Windows virtual machines to be able to make it work, you need to run to the ftp site and retrieve the new versions to have MAC OS native Prime apps. And they do work well!

The FTP sites you can download it from are:

Quoting from Tim Wessman:

The connectivity kit and emulator are now supported on both windows and mac. The iOS/Android/Winstore versions will be downloaded through your normal phone update mechanism as they are approved by the various stores.

HP Prime Connectivity Kit Installer - Windows

HP Prime Virtual Calculator Installer - Windows 

HP Prime Connectivity Kit DMG - macOS

HP Prime Virtual Calculator DMG - macOS

This release is focused around getting macOS support, and all the store applications synchronized. There were some changes in the Core calculator code, but mostly this will be identical to the last release.

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HP instruments coming

There are two items of the measuring area pending testing and cleaning:

A HP 54501A Oscilloscope. One of the first Digital Oscilloscopes by HP, with the quality of the past HP;

A HP 8591E Spectrum Analyzer. This is a very interesting machine, which many amateurs don’t have due to its high price. We’ll try to make a special price since we got a decent deal ourselves.

Also, there are many other items coming our way - including boxed, near mint calculators  - stay tuned!

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New HP41CV units!

We now have again two HP41CV units left, with all back ports and battery side cover. It is still a wonderful machine, that can be enhanced with peripheral and modules to a still competitive machine - not in speed but in functionality. And with the legendary HP keyboard click!

One unit is a full nut and the other is a half-nut: the newer design which is more reliable since it has fewer inner parts.

Price is 130 € plus shipment.

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New articles for this Christmas tree

We have received several calculators in good physical state that we have thoroughly revised and -when needed- repair.

Let’s list them here:

  • A HP41CV converted to HP41CL. Very nice physical status and completely new battery/back ports module
  • Same, but with a tall keys HP41C. Probably the best click we have  seen in a long time. Good tall keys are always better than normal keys in my opinion. Please email for pictures or more information
  • Two HP41cv machines. One of them is a full nut; the other is a half nut. Both are in very nice shape, with all back and side ports available.
  • Repair assemblies: nicely collated flex circuits onto original plastic inserts for battery-port contacts.
  • We also have memory modules, extended memory modules, math and stat modules (boxed!), a rare Home Management module, the powerful Advantage module, and many more goodies.
  • We also have a HP3478A multimeter that works well and still keeps the battery strong. 

We’re also preparing a HP 54201A Oscilloscope, for which the data battery died and we’re soldering a new one, good for 30 year more, and a HP 8591E Spectrum Analyzer, for which we’re doing a thorough testing - so far it seems to be working well. It is an amazing machine, that you can interface through HP-IB to your (old) computers. Please drop a line to to get more information about our measurement items.

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