Revisiting My First Restoration (Ekco A277)

[OldTechFan96]

Hello,

Yesterday I was having a tune about the FM band on my Ekco A277 which I restored (paper capacitors and electrolytics replaced) in 2015. I was not satisfied with the performance on various stations using the internal dipole aerial so I decided to make an external FM dipole as outlined in Gordon J King's 'Practical Aerial Handbook'.

I centred the dipole around 98MHz as it is in the middle of the VHF-FM band. The span of the dipole is 1.47m, very close to the 5' specified in the above book.

The new aerial did improve things but I was not completely satisfied.

BBC stations come in very strong while commercial stations are weak by comparison. For exapmle: BBC Radio 1 booms in at 98.1 while Smooth Radio at 97.5MHz is so much weaker. I checked Smooth Radio on a early '70s Panasonic transistor portable and it comes in good and strong. I am definitely in a good signal area.

Also, Radio Tyneside was unlistenable on the Ekco and picked up satisfactorily with the Panasonic.

At the moment I recon that there could be a fault with the AGC circuit of the Ekco. I'll do some voltage checks and come back.

Any ideas welcome!

[paulsherwin]

Where is the dipole physically located?

Signal strengths do vary a lot, and this can be more apparent with 1950s sets. The A277 is a decent enough set, but isn't especially sensitive.

There is a Hunts Mouldseal hidden away in the tuner head. Did you change it?

[OldTechFan96]

The dipole is located on the loft wall, the highest I can get it. C1 was changed originally. I remember because it was a bit of a pain to do.

[paulsherwin]

Loft aerials are always a bit of a black art, but I notice you have it horizontally polarised. You may find it works better with the commercial stations if slanted or vertical. It's worth experimenting before you fix it permanently.

Personally I wouldn't just clip it to the wall but use some sort of movable structure - a bamboo garden cane will do.

[OldTechFan96]

Thanks for the advice. I will take some signal strength measurements from the grid of the TI valve with the aerial horizontally polarised then repeat the measurements with the aerial vertically polarised.

[Herald1360]

The Ekco is probably just less sensitive than the Panasonic. A signal strong enough to be fully limited on the Panasonic may not even begin to limit or even hit the AGC on the Ekco.

On the Panasonic there would appear to be no difference in apparent signal level between the BBC and the local stations since its AGC or limiting action will mean no audible difference between a signal that is just hitting limiting and one that is fully limited.

[Sideband]

I had to mess about with the best position for my loft dipole. Not just height but angle as well. Local radio comes from Crystal Palace and Croydon and BBC R2, 3 and 4 come from Wrotham....approximately a 30 degree difference. There are also some community stations dotted about in different areas. In the end I found that slant mounting aimed at Wrotham gave best results from the main BBC stations (not surprising) but also gave very acceptable results from Croydon/C Palace (geographically from here the Croydon and C Palace masts are about 10 degrees apart) but the signals are so strong it overcomes the loss caused by the aerial being about 30 degrees from optimum for the locals. For best results I'd need two aerials and a diplexer!

Remember, the wanted signals are at right-angles to the aerial.

Another point to remember is that transistor tuners are much more sensitive than valve tuners so even if your Panasonic radio can receive weaker stations very well, it doesn't mean the Ekco will even with a first-class aerial.

I have a situation here where a popular community station on 107.8 is received very well on the car radio and my transistor sets even though strictly I'm out of their service area up here on the outskirts of Croydon (they mainly cover SW London). Only one valve set can manage to pick up a noisy signal even with my loft aerial connected.

[OldTechFan96]

I do agree with the comments regarding the sensitivity of the Ekco. The best FM aerial in the world would only marginally change things.

Here are some signal strength measurements taken from the grid of the TI valve using an Avometer 8 MKI on the 2.5VDC range. I've also included where each station is transmitting from and its power.

Radio 1: -0.45V. Pontop Pike 250KW
Radio 2: -0.65V. Pontop Pike 250KW
Radio 3: -0.5V. Pontop Pike 250KW
Radio 4: -0.55V. Pontop Pike 250KW
Radio Newcastle: -0.25V. Pontop Pike 10KW

Metro: -0.1V. Burnhope 10KW
Smooth Radio: -0.1V. Burnhope 9KW
Classic FM: -0.45V. Pontop Pike 65KW

It is easy to see why I'm having trouble with the commercial stations when they are transmitted at such a lower power than the BBC stations.

This has been a good practical lesson in receiver sensitivity!

I have measured valve voltages and all of the resistors and will post them later.

[vinrads]

I purchased a pre amp off E Bay cheep , it works very well . Mick.

[paulsherwin]

An external preamp should help in these circumstances. You can build one with just a single transistor powered by a wall wart.

[OldTechFan96]

I would be interested in building a simple pre-amp. What circuits would members recommend?

[Nuvistor]

Pinning the aerial to the wall will detune it, it won’t help with weak signals.

[OldTechFan96]

I found a circuit based around a BF199. I have all the parts but no veroboard. I might have a go at using a wooden board and some pins. Good enough for a test. This will be a good lesson in transistor theory.

[Ed_Dinning]

Hi Vero board is not the best for RF circuits, all those tracks add capacitance !

Ed

[paulsherwin]

You don't need an RF transistor. A jellybean 2N3904/BC548 class has enough gain at 100MHz.

You don't even need any tuned circuits. 4k7 in the collector, 1M between the collector and base, 1nF connecting the aerial to the base, 1nF connecting the output from the collector to the radio. 6-12V power between the collector resistor and the emitter.

Needless to say, more sophisticated designs are available

[OldTechFan96]

I constructed the circuit today. It was so nice that I sat in the garden to do it!

I will test it soon with my signal generator and oscilloscope.

The photograph was taken before I soldered everything.

It was quite fun constructing a circuit 'the old fashioned way'! This was the first circuit I've built from scratch.

[Ed_Dinning]

Hi OTF, glad you enjoyed the experience, some of my engineers at Sevon on the valley used those techniques with brassed steel pins to build the analogue parts of the inverter circuits in the 70's.

Ed

[OldTechFan96]

Hello Ed,

Figuring out the best way to organise all of the pins and twisting everything together was great fun.

Unfortunately, the circuit has not worked and will need a bit of fault-finding. I think I will start a new thread for this.

[Silicon]

Modern buildings and refurbished older houses can have a lot of metal foil attached to the foam insulation and plasterboard.

Even some of the roofing membranes may have metal foil as part of their construction.

I imaging this would go some way towards shielding indoor aerials from VHF transmitter signals.