HP 8711A: modifications, tweaks, refurbishing

I recently acquired a fairly well preserved HP 8711A network analyser. As sold, this is an economy model, offering only scalar measurements (although vector information can be acquired through the HP-IB bus).

Conversion

What makes this unit interesting, is that the hardware is the same as the never released 8712A, which has vector capabilities. Some changes in the bootrom allow the user to “convert” the unit, making it behave like a full fledged VNA.

Credits for this rom modifcation go to Massimo Porzio (IK1IZA). As far as I know, he is the one who took the time to figure this out. I also found a lot of information on the site of Michal Lewczuk (SP2XDM).

Before doing anything, a backup of the correction constants was made to disk.

Unit boot screen as received:

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I used the TL866 USB High Performance Programmer which can be bought online for around €40. Download the software while you wait for the package to arrive, download speed reminded me of the late nineties…

The bootrom with attached label (on the A1 CPU board, removal of the CRT/PSU module is necessary):

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Contents of the ROM at address 0x1FFA0 prior to modifications:

eeprom-before

After modifications:

eeprom

New bootscreen (SRL and fault location enabled as well):

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Smith chart as proof of vector capabilities:

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CRT realignment and brightness setting

Some adjustments on the CRT were done:

  • When looking from a normal viewing angle with the unit placed on the desk, the alignment of the options on the right of the screen seems “off” when compared to the location of soft buttons next to them.
  • The screen was rather dim.
  • Sizing of the display compared to the cutout in the front panel could be enlarged.

I marked the locations of the front panel cutout and the top and bottom buttons on the CRT:

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After disassembling the CRT/PSU module and attaching the PSU to the back, followed by some creative cabling, the unit was powered on while leaving access to the alignment potmeters:

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The positions of the potmeters before adjustment were marked:

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After adjusting the height/width and brightness, the screen looked much better.

Power supply patch

According to a ECN from HP, units with serials between A00000 and 3325A00941 need a 3W, 680Ω resistor between pin 9 and 16 on J5 in the power supply.

Before:

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Resistor added and pcb cleaned:

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Speaker “modification”

The A models of this network analyser had an issue where noise would get into the audio circuit, causing a high pitched, squeeling noise. When used in a silent environment this was really bothersome, so I first tried replacing the speaker by a version with a slightly different resonant frequency.

This didn’t help enough, so I ended up with this ugly (but fully working) solution:

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Cleaning and restoring the unit

A lot of time was spend cleaning and restoring the unit, as to give it as much of its original appearance as possible.

  • The inside of the case was cleaned, connectors on the backplane (which are notorious for causing problems with this model) were cleaned using a combination of MEK, IPA, soda, warm water, patience and lots of love.
  • Front panel and keypad were cleaned, and the yellowing of the plastic due to age and UV was reversed through the use of high concentration hydrogen peroxide. Dents, holes and cuts were fixed.
  • The vinyl was cleaned, restored and treated.
  • Internals of the disk drive were cleaned.

Documenting this is worth a separate post.

Dive light rebuilding

About two years ago I bought a Hartenberger “Maxi Compact” dive light for about €60.

As this was a 10 year old dive light with a halogen bulb and a well-used battery pack I think that was a fair price. Anyway, the plan was to use the housing for a modded LED light.

At first I just made a second battery pack, the first one was just (not) enough for one dive with the original bulb, so I needed a second one and charge one pack while diving with the other. Also bought a Graupner “Ultramat 14 Plus” battery charger as this was an affordable charger that can be used for a variety of different chemistries, but best of all, it has a port that allows for cell balancing. As I was using a 6S NiMH battery pack, that was a feature I wanted to have.

A couple of weeks ago I started making the LED module with driver, heatsink and connections to the battery pack. The original idea was to hack into the driver circuit that delivers the PWM to the original bulb as I could use the original on/off/dim switch at the back of the light, but I didn’t get there yet…

So. Starting with the heatsink. I couldn’t find a 62mm aluminium rod that I could use to turn the heat sink from (inner diameter from the housing), but I did find a scrap 5mm plate. Decided to try with two layers of that.

First drilled out a circle with a drill press, trying to keep it as round as possible.

Dive Light 1

Drilled a hole in the middle to allow a 8mm bolt (started with 5mm but clearly wasn’t strong enough for the coming abuse) to put the piece in the chuck of the lathe and two extra holes for 4mm bolts to make sure the two parts wouldn’t rotate but stay together nicely .

With the piece in place fired up the lathe and started grinding the rough parts with a metal file. When the piece was round enough -about 1.5 hours later- started off with the lathe, turning off really small pieces at a time. Never thought using a lathe was so much fun. I’ll come up with other ideas just to use the lathe again.

Dive Light 2

Ended up with this, rather happy with it:

Dive Light 3

Designed a drawing in Autocad for the driver, led modules, optics and wire holes and drilled them. Countersunk the bolts where necessary.

Dive Light 4

Rough positioning to check if everything fits.

Dive Light 6

With only a limited time before leaving on a dive trip I bought the driver circuit from Led-Tech as they could get me the parts quick enough. I wanted the warm CREE’s as their colour reproduction is better: CRI for CREE XM-L cool white is 65, CRI for neutral white is 75. There’s also a special warm white XM-L with a CRI of 90. Obviously hat would be the one to go for.

The battery pack delivers 4200mAh’s at 7,8 volts max, and the original idea was to drive 3 XM-L’s at 1200 mAh’s as that seems a sweet spot between power consumption, heat production and light output. To do so I’d need a boost converter (the leds have a 3.05V drop at 1200 mAh, so 9.15V nominal) but Led-Tech only had buck converters. New plan: drive two leds at 900mAh, use the light in a real-life situation for a number of times and see what the light output is like.

Dive Light 5

Dive Light 0

As I didn’t manage to hack into the PWM driving circuit (which had a feedback loop that relied on the halogen bulb and didn’t play nice with the constant current driver circuit), I just added a reed switch that I found in my parts bin. No holes that can cause leaks under water, just a magnet outside the housing to switch the light on/off.

Dive Light 7 Dive Light 8 Dive Light 9 Dive Light 10

Comparison with a 1000 lumen dive light:

Dive Light 11 Dive Light 12

And finally:

Dive Light 13

After about twenty dives with the light I realised that 3 XM-L’s at more than 1000 mAh would be overkill. Also, the optics that I used now are to wide. Nice in clear water, but as soon as there’s a bit of sediment there’s too much reflection. I might keep these optics on one of the XM-L’s, and give the other two a narrower beam. I’d need some kind of control to choose between the two and also a dimming circuit but that’s for later.

Really happy with the power consumption. Where I couldn’t do a single dive without the light getting dimmer at the end of the dive when using the halogen, I can now easily do 3 long dives without charging.

I’ll have to work on the connectors as well. Cramming an 7-pin connection on the battery packs, a connection between the front and the back (where the switch is), a connection between battery pack and the driver while keeping it all easy to open up for charging will take some thinking.

So far, I’m happy with what I have right now and *really* loved the lathe.

Recent projects

So.. After a diving trip it’s about time to write up on two recent projects:

  1. Home server: As I wanted to get into Linux for a long time and also wanted to set up a home server for VPN/SSH/FTPES/firewall/my own Exchange replacement and sharing of music and movies in my own network (and remote, of course), this seemed like a good time to dive in. Starting with a generic Ubuntu server package without GUI I’ve started setting up a low power system that I can leave on permanently without having to worry about it too much. A good opportunity to read up on current computer hardware as well.
  2. Dive light: a while ago I bought a halogen dive light with the idea of using the housing for a diy LED dive light. Benefits: light output, can choose my own beam characteristics, longer runtime. Drawbacks: colour reproduction. Pretty much still a work in progress, but I’ve done about 20 dives with a system with 2 XM-L’s driven at about 900 mA’s and easily lights up the beachfront after a night dive. Also: first time working with a lathe.

More about these two soon.

Mares Dragonfly BCD cleaning/modding

Warning: salt, corrosion and dirt up ahead:

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Since I began diving about two years ago I’ve mostly rented gear whenever I needed it.
What I like about renting things is that you get to try different brands and models, and little by little start finding out what you like/don’t like/need/want/hate about certain gear.  You also get some time to figure out how things work and what kind of preferences you have.
Diving equipment isn’t cheap, but taking time to shop around can get you a significantly cheaper set of gear.

A while ago I got hold of a second-hand Mares Dragonfly semi-wing jacket for €80.
Even though it seemed in good shape, I wasn’t sure how well the previous owner took care of it so I wanted to thoroughly clean it. And of course, in all honesty, just wanted to take it apart to see how it was made and how things work.
A service manual would have been nice, but I couldn’t find it for this jacket. I did find some exploded views in the “01 revision of the Mares service manual that’s floating around on the internet, but no guidelines. Well then… Off we go.
First I took a look at the dump valves on the right shoulder and the lower right side on the hip. They came off easily just by turning them counter-clockwise, as one would expect.
There’s just the outer plastic cap, a spring, a plastic piece that holds the rubber and the rubber seal that seals off the port. Pull the cord and the seal moves away a bit from the port. Overpressure inside the jacket puts enough force on the rubber from the inside to overcome the pressure from the spring and open until the pressure is below the threshold.
The rubbers are not glued onto the plastic caps, there’s just some kind of grease. I left that on, but cleaned the underside of the rubber seals from dust and patches of salty build-up. I didn’t take any pictures before the cleaning, only afterwards.
While they were open, I gave everything a rub, and got out the spray paint. Why not touch them up a bit again and make my jacket more recognizable?
Also replaced the dull black cord with some cord from my rock-climbing stash of cords.

This jacket has one big downside and that’s the weight. Close to 4 kg it’s really heavy when travelling by plane. I have been looking where I could lighten things without touching the main functionality of the jacket. I’ve tried replacing one of handles/weight on one of the dump cords with a ping-pong ball. My only concern is (ironically) the lack of weight. While it can fill up with water, it has no weight in itself so I’m not sure if it won’t float around too much while diving. We’ll see.

While, supposedly, you need a special tool to open the plastic ring that seals the airtrim thing on your left hip to the jacket (same for the valves itself) I opened up the console. An exploded view can be found in the Mares manual p. 102, drawing J83.
The console opens up after unscrewing the three screws on the outside. Mind you, these are machine screws without bolts. They’re just screwed into the plastic so treat them accordingly.

Jacket Mares Dragonfly pimping_26aug2012_0025_resize

The system is a white plastic thing that comes off after unscrewing the two visible screws. There’s one big O-ring inside to seal it off. That’s all there is to it. The connector to the hose that feeds the two valves stays on the jacket, it has an O-ring and just slides into the white plastic. (pictures are after cleaning)

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The connection to the low pressure feeding hose screws out of the plastic (14mm key), and so do  the two buttons (both 17mm key). They both have another thread, so there’s no mistake possible while assembling them again. Jacket Mares Dragonfly pimping_26aug2012_0020_resize

Inside the hole where the low pressure feeding hose connection goes into is a small plastic filter to keep contaminants out of the system. Make sure it stays in there. The inflate button assembly comes apart into its separate pieces, but for the dump button construction to come apart you need a 6mm and a 5mm key. The centre piston is in two parts, and must be unscrewed. The upper part comes out from the top, the lower one slides out to the bottom.

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To me, the O-rings all seemed in good shape, so I didn’t search for replacements. If that would have been necessary I’d probably just would have taken the jacket to a shop for maintenance.
I cleaned everything with a lots love, warm water and some vinegar. I’m not sure if that’s the best way, but it’s the best one I found.

Jacket Mares Dragonfly pimping_26aug2012_0023_resize

I gently tried to open the “special” bolt that needs special tools. An owner of a dive shop showed me this “special” tool and it seemed to just be a cup-like mould that slides over the bolt to get more grip on it. The thing didn’t move however and I really didn’t want to break anything. It would only need to come apart to satisfy my curiosity so it isn’t worth breaking it.

Everything was reassembled and put back on the vest. The plastic parts outside got a scrub and some layers of paint.

The jacket isn’t any better now, but at least I’ve seen the inside of the system, it got cleaned out and the visible parts got a cosmetic touch-up. I also cleaned and did some small changes to the manual inflator hose, but that’s for an other time.