Friday, 31 January 2014

Loosening my load

I was wondering why my little analogue multimeter wouldn't read any resistance values. It seems I'd forgotten to put a battery in it!

Well, I don't have any suitably rated 200 ohm resistors to make the load with, but I do have thousands of 1k 1/4w resistors. Now, 20 1k resistors in parallel gives exactly 50 ohms. It also, by lucky chance, gives a combined power rating of 5w. This is more than the meter is rated for, but that's no matter for the convenience.

The first difficulty then, was cutting the two holes needed in the tin. The BNC wasn't so hard, the step drill took care of the hole and the edge of the lid, and tin shears did the rest. The cutout for the meter was trickier. The mass of metal was removed by drilling numerous small holes and snipping it out, but then it was a case of slow, careful work with needle files.

A few test fittings of the meter showed up the tight spots, and soon a good fit was established

A great feature of working with  tinplate is that you can solder direct to it. The first ten resistors were installed between a copper bus and the case, taking care to ensure they all fitted neatly in the space available without being too near the meter

The other side was then done, and before proceeding  the load bank was tested to ensure correct resistance. It metered out at 51 ohm, which is pretty close for 5% tolerance resistors.  The remaining components installed point to point, with the electrolytic capacitor doing double duty as a stand-off post!

On test, I can prove that the circuit works. However, my test method was to feed the PSK31 idle signal from the DX-70TH into it, via the SWR bridge in the ATU. At FSD, the ATU's meter was showing just 1W. FSD on this should be 2W. I suspect the problem is the inaccuracy of the ATU's meter at these low powers. I'll have to find someone with a calibrated power source to calibrate it against. I could try it against a VHF handheld, calibrated to 2W against my Marconi 2955, but im not sure how well it would perform at VHF. Apart from calibration, and a new meter scale, the only thing left to do with this project is label it up and put some rubber feet on it!

Just prior to moving onto my next task, the modification of the digimode interface, I was looking at the antenna that came with the little DVB-T2 USB stick. Now, no matter what I do I cant get a peep out of the stick, that might be due to the antenna, but i'm really not too fussed. The antenna is a little one inch magmount. I started wondering if its fitting was the same as the little dual band magmount that a certain Richard had conned me into swapping a decent sized magmount base for! And indeed it is! So, Ive swapped the elements over. All that's needed now is to change the little plug on it for a BNC, and it should be a usable little dual band 2m/70cm magmount.

Modifying the digimode interface proved a bit tricky, simply due to the amount of wiring in there. Unfortunately, once modified, I found that the radio would complain bitterly in bypass mode even with very little volume set on the PC. Even in 'level control' mode, ie with the internal level pot switched in, only the first 5-10% of the track has any affect, above that the radio complains about being over driven. That 5-10% is from the 'cold' end, and so represents most of the resistance in series. The pot is a 10k, which should be fine, but I have a feeling its track may be damaged, either that or I need a log taper pot in there. It also seems that the speaker out levels of the PC are way too strong even when turned right down. So I don't yet dare feed the PC audio to the PSK-20 to test it. I think that might have to await its own interface unit. I have a 22k log pot I could try, but im inclined to go higher, perhaps 100k

Anyway, its raining now, and a bit cold for being in the workshop. So radio work has been put on hold in favour of model painting and tea drinking. I might revisit the power meter later if the rain stops, and try a few tricks with it. I also intend seeing if I can add another soundcard into this computer, so I can dedicate the settings to feeding the radios, that means a trip into the loft to dismantle the old PC.

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