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DIY 75Ω Matching Pads

Recently I found the need to do some measurements on 75Ω CATV equipment, only having 50Ω test equipment to use. For this, matching networks exist to convert 50Ω to 75Ω, but they’re fairly simple, so building them was a viable option.

Matching Pad Schematic

Matching Pad Schematic

Above is the very simple schematic to create the 75Ω match. To help keep any parasitics down, this circuit will be built directly onto the back of BNC connectors, that are soldered back-to-back, before being covered in shielding tape.

Resistors Soldered

Resistors Soldered

Here’s the first 50Ω BNC connector, with the resistor network soldered on. I’ve used 4x 360Ω resistors in parallel to create the 90Ω to ground, and a single 43Ω series resistor on the centre pin.

End View

End View

This end view of the arrangement shows the 4 resistors evenly spaced around the centre pin & soldered to the shell.

BNCs Soldered

BNCs Soldered

The centre pin of the 75Ω BNC connector is trimmed down to match the length needed to touch the end of the series resistor, and it’s soldered in place. It’s a bit tricky, soldering within the gap between 2 of the ground pins!

Completed Matching Pads

Completed Matching Pads

Finally, the internals are shielded with copper tape, soldered at the seams.

HP 8753C Display LCD Replacement

Completed Install

Completed Install

Since I inherited an old HP 8753C Network Analyser from work, I figured updating a few things to relatively modern standards would be good. The factory CRT, being 28 years old, is definitely getting a little tired, not to mention being slow to warm up. I read over on the EEVBlog forums about a DIY modification to integrate an LCD display into place instead. There was also the option of a ready-made kit for these instruments which would integrate an LCD, but the cost at over £300 was very prohibitive!

CRT Pinout

CRT Pinout

The CRT display unit is a self-contained Sony unit, taking RGBHV signalling from the graphics control card of the analyser. Power is 65v DC which will definitely come in handy for powering the new LCD & control gear, after some conversion.

Test Wiring

Test Wiring

Doing a quick test with some wiring stuck into the video connector from the graphics controller, proved that I could get a decent video signal out of the unit! The only signals used here are RGB, along with the vertical & horizontal sync.

GBS-8200 Converter Board

GBS-8200 Converter Board

The video is converted to VGA by way of a GBS-8200 arcade machine video conversion board, which will take many different video formats & spit out standard VGA signalling. The power supply to the left is a standard 100-240v to 12v PSU, which is happy to run at 6t5v DC input voltage, albeit with a ~5 second delay on output startup when power is applied. This is due to the massive 6.6MΩ resistance of the startup resistor chain, which I did reduce by 50% to 3.3MΩ with no effect. Since it does start OK even with the delay, I think I’ll not tinker with it any further. I doubt I could pull the full rated power from it with such a low input voltage, but all included, this mod draws less than 600mA at 12v.
A custom 20-pin IDC cable was made up to connect to the analyser’s graphics board, and this was then broken out into the required RGB & sync signals. Quite a few of the grounds are unused, I’ve not yet noticed any issues with EMC or instability.

Sync Combiner

Sync Combiner

There is a quad-XOR gate deadbugged to the PCB, which is taking the separate sync signals & combining them into a composite sync. The conversion board does have separate sync inputs, but for some reason doesn’t sync when they’re applied separately. This gate IC is powered from the 3.3v rail of the converter board, with the power lines tacked across one of the decoupling caps for the DRAM IC.

LCD Control PCBs

LCD Control PCBs

The donor 8.4″ LCD came from eBay in the form of a POS auxiliary display. I pulled the panel from the plastic casing, along with the control boards, and attached them all to the back. This LCD also had a sheet of toughened glass attached to the front, no doubt to protect against the Great Unwashed while in use! This was also removed.

Control Boards Mounted

Control Boards Mounted

A cut piece of plexiglas allows the boards to be mounted in the cavernous space the CRT once occupied, with some brass standoffs. 12v power & VGA are routed down to the LCD on the front of the analyser.

LCD Wiring

LCD Wiring

The LCD itself is tacked in place with cyanoacrylate glue to the securing clips for the glass front panel, which is more than enough to hold things in place. The input board which just has the VGA connector & power connector is glued edge-on to the metal back panel of the LCD, and is under little strain so this joint should survive OK.

Covid-19 Lockdown Projects – Extracting Diesel Heater Exhaust Waste Heat

Since I fitted the new “8kW” diesel heater to the camping power trolley, it has occurred to me that there is a lot of energy in the exhaust gas stream that ordinarily would be wasted into the atmosphere. Since we’re all still on lockdown here in the UK, I figured it would be good to run an experiment to see if it was worth recovering this energy – in the form of heating water.

Heat Exchanger

Heat Exchanger

Some time ago, I stripped an old gas combi boiler, and recovered some parts – most important here the HDW plate heat exchanger. This large chunk of stainless steel is a stack of formed plates, brazed together, that usually would heat Domestic Hot Water. In this instance it’s being repurposed to transfer heat from exhaust gas to water.

Brazed Connections

Brazed Connections

These heat exchangers are mounted in the boiler via a plate with O-Ring seals on, so they don’t really have fittings – just holes in the end plate. Solving this problem was simple – braze on some copper fittings with 55% silver brazing rod. The 22mm is the exhaust side, while the 15mm is the water side.

First Test

First Test

Cobbling together some random hose & fittings, along with a small water pump allowed me to run a first test. At this point there is no lagging at all on the exhaust system from the heater, so it’s going to shed a lot of exhaust heat into the air before it even gets to the heat exchanger. However I was able to get around 600W of heat into 15L of water, heating it up nicely. The heat exchanger is plumbed contra-flow here – exhaust comes in via the stainless tube on the bottom right, and water comes in through the speedfit elbow on the top left.

Lagged Heat Exchanger

Lagged Heat Exchanger

After the temperature of the water tank hit a plateau at around 45°C, I decided to insulate everything the best I could with what I currently have. I’ve wrapped the heat exchanger with some recycled PET insulation here, just to hold the heat inside. I’m not concerned about the exhaust outlet being in contact with the fluff – this system is so effective at pulling the heat out of the exhaust that the gas exiting the far end is totally cold!

Unlagged Exhaust

Unlagged Exhaust

Now it was time to get the exhaust system under the trolley insulated. This is the system removed from the unit entirely. This is constructed from copper pipe, brazed onto standard silencers. Deadening the sound from the unit is important, as this gets used on campsites!

Fibreglass Tape Insulation

Fibreglass Tape Insulation

An hour & some itchiness later, the exhaust is completely covered in fibreglass insulation, secured in place with stainless steel ties.

Exhaust Hanger

Exhaust Hanger

The exhaust originally passed through a close-fitting hole in the frame rail which would obviously not work now due to the thickness of the insulation layer, so this was modified with a grinder. Since there was now no support for this end of the exhaust, a pair of drilled holes & some stainless steel wire form a nice hanger!

With all this insulation in place (including around the tank & pump), the rig is now able to easily hit 65°C within a short time, so there has definitely been an improvement. At this point, it’s clear that waste heat recovery is worthwhile, so I’ll be building a proper rig to capture this energy for reuse!