Sunday, 31 July 2011

Voltage Probe Antenna - Part 2

This weekend saw a family visit (wife, grown-up children, and in-laws) trekking across the Pennines to the Sunderland Air Show, and a Jolly Good Day Out was had by all.
As with the Windermere Air Show we attended a couple of years ago, the only flying Avro Vulcan XH558 once again stole the show.  What a stunning aircraft, and what an impressive noise!

A fly-past with the bomb bay open

Today, in between watching the Hungarian Grand Prix and listening to the Second Test against India at Trent Bridge, I managed to get the prototype Voltage Probe Antenna completed and fired up for a quick test.

It certainly works, and in the brief period of time I had to test it before I packed up for the weekend, I had received amateur signals at good strength on all bands from 160m to 15m ,with the exception of 60m.

First VPA lash-up
I also had broadcast signals on Long Wave and Medium Wave at decent strength.  Next week I hope to carry out a more extensive evaluation, and naturally I will probably report back here.

This first version doesn't have the required circuitry to be powered from the output coax cable, this will follow in due course, no doubt.

Sunday, 24 July 2011

A new project - the Voltage Probe Antenna

About this time last year I paid a visit to an old radio amateur friend whilst away on holiday, and he was enthusing at the time about a little "Active Aerial" he had built very quickly more or less from parts in his junk box.   This little aerial was about the size of a beer mat and was suspended from a piece of shelving, and was connected to his FT-817.   All kinds of stuff was being received, most impressive.

This was to a design by PA0RDT.

Moving forward to more recent times, in fact last weekend, I noticed from EMail traffic that our local radio club, the Eden Valley Radio Society were possibly embarking on another club project, and an "Active Aerial" was one of the contenders.   Ron, G4GXO (proprietor of Cumbria Designs) had come up with a circuit in part inspired by the PA0RDT design and it was mooted that this might be the basis of such a project.

Naturally my previous experience caused my ears to prick up, and after some correspondence with Ron I now have a new project - to build this little circuit and evaluate it.

In terms of complexity it is completely different from the "Eden9" project, in that an experienced constructor should be able to build it more or less as a "weekend project".   Looking at the modest parts requirement I realised that had all that was needed available and resolved to give it a go this weekend.

However, as is the way of the world, outside influences intervened, a major influence being the unexpectedly fine weather we were blessed with - cloudless blue skies and pleasant temperatures, so in the end I only got chance to get this project started, hopefully to be completed next weekend..

The aerial including the amplifier is all contained on the single piece of PCB - the photo below shows the project getting under way before I tidied the garden shed up to placate my XYL who insists on doing "things horticultural which I don't understand" in it whilst I am not around!

Saturday, 16 July 2011

TS-180S : Part 6 - Life Moves On ... (things looking Black!)

Since my last post more hours have been expended on this old radio.  To summarise :-
Numerous hours spent with the TS-180S connected to an aerial simply being a receiver.   The purpose of this was fourfold ;
1) Get some hours on the clock to see if any further "corrosion related faults" arose (they didn't)
2) To evaluate the intrinsic frequency stability of the transceiver (it proved to be acceptable for the technology of the day)
3)To evaluate its performance as a receiver (it lacks sensitivity - a lack of IF stage gain is prime suspect)
4) Allow me to ponder how to tackle the lack of transmit output (there is a very small amount of RF transmitted, but I think I need to tackle the suspected IF gain issue first as part of the IF is used in the transmit chain)
Anyhow with all this input to my overloaded brain I resolved that the time had come to put this project on the back burner therefore allowing me to move other projects on as I perceived I was becoming somewhat obsessed with this one!
Having made that decision I decided I would reunite the radio with its covers, albeit temporarily, and that is when I noticed something I had missed.  There on the bottom half of the case, on the inside was pretty conclusive evidence of some kind of "spill" which had pooled in the bottom of the case leaving a residue!
Ha!  I think my earlier suspicions have been proved correct.  Judging from the size of the marks inside the lid, it was clearly a sizable spill.
When I return to this project it will be armed with that knowledge, and I will need to bear in mind what other damage may have been done, and seriously consider what the chances are of a full restoration without a ridiculous amount of expense.

Saturday, 2 July 2011

TS-180S - Part 5 - Swimming Against the Tide

Well it sure feels like it!

Having restored receive functionality to this old set, and having then established that the transmit function was u/s, I set about figuring out a strategy to find out what the problem, or more likely problems were which prevented me from transmitting.

You always need a strategy of some sort, and I wanted to come up with one.   The most obvious thing I noticed at this point was that the ALC reading on the metering was "pinned" when I put the set to transmit.

This could be cause, or a symptom.  Anyway, some place to start, a little piece of evidence to build on.

Whilst all this was going on in my head, the radio was powered up and receiving lots of signals.  I began to notice an unusual tone to the received signals, and upon investigating this I realised that the frequency had developed a nasty "wobble", jumping about by small amounts.  I also noticed that the radio was pulling around 100mA more current from my  power supply than it used to, so something was clearly wrong.

I established fairly quickly that the 8V regulated rail from the AVR unit had increased to nearly 9V and was fluctuating.  It was possible to trim this back to 8V but the fluctuations continued.  This is when I noticed Q6 on this board running VERY hot.

Further investigation of voltages around the circuit established that the Q6/Q7 pair (the shunt voltage regulator error amplifier) was "up the creek" (Remember Q5 in this circuit was one of the first faults I found on this set).

Q6 was found to be short circuit from collector to base, and when Q7 was removed, I found ANOTHER case of "rotten transistor leg syndrome".

I am very suspicious now that at some point in the past this transceiver had some unpleasant, corrosive liquid spilt into it, or something of that nature way back in it's past.   That might explain a thing or two ...
Dead Q6 at the top (base lead looks "iffy too!) Q7 with collector lead (middle) almost eaten through.  The 180 ohm resistor R11 is absolutely fine.

Replacing both of these transistors has restored everything to normal - 2N3704s used to replace the 2SC945s - the base and collector terminals just need to be transposed.

Right, where were we? . . .