Salvage: HP 5243L/5245M/5426L Frequency Counter Plug-in Modules

Time’s always been an issue – but salvage doesn’t wait for a blog post! A month ago, while visiting the Electrical Engineering (is not a) dumping ground at UNSW, I came across four modules, each inside its own cardboard sleeve which caught my eye. It took a little bit of looking around to find out what they were …

Lets just jump to the answer first – they were plug-in modules for an HP Frequency Counter – likely to be models 5243L, 5245M or 5246L based upon the codes – click for the very dirty scans of the manual. Interestingly, the manual makes it clear that Agilent has bought out HP, and they haven’t changed the branding in the manual – not that it matters. From the manual, the equipment would have been dated 1968-1970, which is about 45 years old!

Interestingly, Ralph Klimek at Monash University picked up a 5246L counter in 2009. He managed to get it plugged in and it was working – the look of those nixie tubes are so sweet! He’s got many good photos of the unit – well worth a visit if you’re interested. But I didn’t have the main unit – only the plug-ins. Oh well, can’t have it all!

From the two available manuals, the available plug-in modules are:

• 5251A Frequency Converter
• 5252A Prescaler
• 5253B Frequency Converter
• 5254A Frequency Converter (5246L only)
• 5254B Frequency Converter (5246L only)
• 5254C Frequency Converter (5245M only)
• 5255A Frequency Converter
• 5256A Frequency Converter (5245M only)
• 5258A Sensitive Prescaler
• 5261A Video Amplifier
• 5262A Time Interval Unit
• 5264A Preset Unit
• 5265A Digital Voltmeter

As you can see, most of them are common to both the 5245M and the 5246L, and most of them are frequency converters (to scale frequencies outside the baseband of the counter into its range). As these would have been optional, it would be uncommon to find a cache of all of them – I only managed to salvage four – a 5251A, 5253A, 5261A and 5265A. Hold on, the 5253A wasn’t part of the plug-ins for the 5245M or the 5246L. Were these used in something else?

A bit of searching later, I turned up this preserved HP catalog which mentions the 5253A being a plug in for a different frequency counter – a 5243L, likely an even earlier model of frequency counter. This was probably what it came from.

The wise choice of HP’s modular design becomes apparent when one considers the expense of the solid state radio frequency components back in 1960’s – 1970’s.

HP 5251A Frequency Converter

There’s the frequency converter – it has a stated range of 20 – 100Mhz (or Mega-Cycles-per-Second – MCS). There is a knob with a nice click detent at each 10Mhz step, and a BNC input with 2V RMS maximum. The front panel also has a meter which is used to indicate signal level – a classical D’arsonval Movement! (Remember that from HSC Physics?)

All of the modules are open-frame construction – this allows for easy alignment adjustments, but it also means damage if the modules aren’t handled with care. You can see the front panel knob connects to a long shaft which runs through the whole module. This drives some rotary switches which have potentiometers soldered right to it! A very brute force approach to changing circuit parameters on command.

The top side houses a PCB on a paper-like substrate with components soldered on the same side as the foil – suggesting a double sided board, but since the board is opaque, we can’t see through it. There’s a few transistors in cans – black coloured ones. I’ve never seen them before! Maybe that’s to improve heat dissipation because of the emissivity. The resistors are also decidedly old fashioned, being perfect cylinders, instead of the more bean-like shape of the modern ones. I like the fact there are modules inside your modules, and they have the HP logo on them. The slotted screw shows its age …

The rear of all modules uses a Centronics 50 pin connector mounted on the bottom to communicate with the main unit. This one also has a PCB mounted outside with some silver transistors on it, and a resistor mounted externally to the shield on two standoffs. A strange arrangement. The QA stamp can be seen faintly against the aluminium.

HP 5253A Frequency Converter

This module is heavy. It’s more than 1kg from my estimate – the other modules are about half as heavy. The module itself is a 100 – 500Mhz frequency converter, with a BNC input tolerating 2v RMS maximum. The front is adorned with a precision dial with a tuning knob that spins many times over to move the scale from end to end. Again, an analog signal meter is provided.

From the side view, the reason for its heft is clear. A large metal cavity forms the main part of the module. I’m not quite sure how it works, but it appears to be a resonant filter cavity – I suppose both resonance (to get harmonics) and filtering (to remove unwanted harmonics) can serve to downconvert frequencies in the opposite sense to a frequency doubler? Anyhow, it’s an intricate mechanical design, as the front knob drives a set of gears which drives a screw, which moves a cam which slides in the slot in the bronze coloured module, thus rotating it around (which presumably alters the frequency selected). A clear warning is placed not to disassemble the cavity – likely because its resonance is reliant on a very small mechanical tolerance.

Here’s a close-up of the gearing and the drive screw which is black in colour. The aluminium coloured screw in front is a limit-stop screw, which slams into the bulkhead of the cavity when you reach the end of the tuning range. A similar screw is attached to the other side to slam into the other end when we reach maximum frequency.

Doesn’t this look familiar? It looks like a virtually identical module to the one in the Frequency Converter above. Same quirks and all …

A closer look at all the black coloured cans reveals they are Philco 2N2048 transistors. Philco’s been long gone … but it’s a sign of how old this thing is. Strangely enough Philco was first bought out by Ford (yes, the car company), which then passed it to GTE (a US phone company) who then sold it to Philips (who wanted it due to trademark issues).

HP 5261A Video Amplifier

By now, you’re probably spotting a pattern – but this one’s a little different. This is a video amplifier supposedly, with the front dial for sensitivity, adjustable in discrete steps of 1, 3, 10, 30, 100mV RMS. There’s an input, as well as an output on this module – the only one to have this. It’s also a pretty light module.

There’s no side view of this module because most of it is inside the large metal can. There’s a PCB external to it, with a few caps, a transistor with a heatsink, and some without. You can see how signals are routed through the can, and bypassed with capacitors.

That’s the top of the shielding can that has been removed. It might look boring but it’s interesting to note the hole in it – for reaching an adjustment pot, as well as the fact that to ensure the whole metal can functions at RF – there is a rolled steel-wool-like substance used to ensure both the top and bottom segments of the cans are highly conductive to RF. So that’s what they did before they invented that foam-self-adhesive-metallized-strip which is so commonly used in laptops today.

Inside the can – not much circuitry – you can see the switch soming in and the video input connected by rather thin wires. But ho-ho-hold on a sec – what’s that in the metal can? Is it what I think it is?

Uh, yes it is! It’s a RCA Nuvistor 8056 tube! Fancy seeing one of these around. I never thought I would, but then I remembered that for high frequency work at the time, solid state semiconductors were still a bit behind and colour TVs still used tubes at the time – although it was the last hurrah for them.

You might be wondering – why doesn’t it look like a glass bulb, like the normal tubes? That’s because the RCA Nuvistor was different – it was specially manufactured in a vacuum, so it could be made in a metal can! And it had exemplary performance characteristics.

This also explains why the whole circuit board is mounted to a heavy block of metal, which is glued to a piece of foam and glued to the can. Microphonics. Tubes can pick up vibrations from the surrounding environment and cause those to be induced as an electrical signal. This can also happen with other electromechanical devices like transformers, inductors, and is part of the reason why phone phreaks could hear network switching operations, and plant equipment squeaks through the phone line. The foam and metal block serve to damp out any vibrations which may come from the outside environment or even the cooling fan inside the base unit.

HP 5265A Digital Voltmeter

This is our last module – and it’s the one I know least about. But at least some manuals can be found for some of these from the BAMA. It’s a voltmeter, apparently – with its banana plug inputs. I wonder what it actually does – is it just a voltage controlled oscillator (so the base frequency counter unit will display the frequency which will coincidentally be the voltage)?

The module shows the same inspired module-in-module design with stacked cards. Even with that, some adjustment pots are mounted right on the edge for direct access. Life must have been a chore using these old pieces of test equipment – they probably needed a large multi-step calibration every six months.

Well, we see this board is a double sided board with a translucent substrate – which looks like modern FR4. There’s tinned copper – no fancy silkscreening. Their logo is clearly visible in the middle – and there’s some precision pots as well. Interestingly is the thing right-most – it’s either multiple inductors wrapped around a substrate, or some loosely coupled transformer … hmm.

The second board is the ground board and has suffered some corrosion – I’m not sure why, but maybe it was stored near the fumes of a Ni-Cd battery while it was being overcharged or something. Some really strangely shaped resistors here …

This is the third board, known as the input board. Interestingly, it seems where several resistors join together, there are these big joints made with heaps of solder in a plastic sleeve of some sort. I wonder why?

And this is the bottom board – mainly just capacitors and two precision trimpots. The red wire that is floating plugs into the board above. Interestingly, the large silver can is a high voltage polystyrene capacitor by Sprague, which is now owned by Vishay Components. Interestingly, the brand itself seems to have some following – which is why I’ve heard of them, but have never seen one in person …

Conclusion

It’s a bit of a hollow ending I suppose. I’ve seen a few things I have heard of, but never seen with my own eyes. But at the end of the days, these modules are in unknown condition – and aren’t exactly useful as they are without the frequency counter base unit. That being said, the performance of modern frequency counters is much better with much less complexity, power usage, and with higher accuracy and reliability – so I’m not going to bust my gut looking for one of these 45 year old HP ones …