Ekco A274 - Project No 2

[paulsherwin]

The most likely explanation is that the EZ80 is getting tired. They can soldier on for years like that. The reduced HT will reduce performance but is unlikely to produce obvious effects.

[Barnjet]

Originally Posted by paulsherwin

The most likely explanation is that the EZ80 is getting tired. They can soldier on for years like that. The reduced HT will reduce performance but is unlikely to produce obvious effects.

Ha! I know the feeling!

Are the two near-zero anode voltages and one high anode voltages I measured on V4 a concern? - see post no 39 repeated below

"I then went on to measure V4 (EABC80)
Pin 1 Anode b -0.17V (35V) (-100%)
Pin 2 Anode a 0.0V (35V) (-100%)
Pin 9 Anode c 47.5V (35V) (+ 35%)
All other pins showed negligible values expect Pin 8 which had -0.5V and incurred a loud hum when touched with the probe."

I will be measuring the remaining valves tomorrow

[paulsherwin]

The readings suggest that there's something wrong in that part of the circuit, but I don't have the diagram to hand to check in detail.

Does the radio actually work?

[Barnjet]

The radio works .....sort of. It's difficult to describe. Using the newly fixed internal foil aerial, I was able to get some faint reception on R2/R4 when on the limiter. On full power for the first time, I got some very loud, distorted reception, but it was never constant.

Leaving the tuner set on a known station, and changing nothing else, it would periodically fade in for 10 - 30 secs before fading out for 1 to 5 minutes. Basically, it was unstable and I don't believe environmental factors were the cause.

I will do some more measurements tomorrow

Thanks for your patience

[Nuvistor]

V4, I think you are misunderstanding the voltage chart which could be clearer.
Pin 1 and 2 will not have 35 volts on them, those voltages are probably ok.

[Barnjet]

Originally Posted by Nuvistor

V4, I think you are misunderstanding the voltage chart which could be clearer.
Pin 1 and 2 will not have 35 volts on them, those voltages are probably ok.

Thanks Frank, this is indeed exactly what I have done. See photo extract of valve data. Another own goal, my apologies all round.

Max

[Barnjet]

It took me a while to identify the V1 pin numbers in the FM tuner area, as it's very snug in there, with wires everywhere - see photo

Anywhere, after identifying the pin1 position by removal and reseating V1 this morning, I got the following values (specified values in brackets)

Pin 1 Anode b 180V to 164V (195V)
Pin 3 Cathode Nil (2.1V)
Pin 6 Anode a 189V to 175V (227V)

BUT, more importantly, while measuring, I got a strong, clear, reasonably stable signal. I wonder if my poking around in the tuner did something? For example, when I touch the probe on Pin 1 Anode b, the signal cuts off, only to return when I remove the probe. There is still occasional intermittent signal loss.

I shall monitor for a while..... but it's definite progress, even if I'm not exactly sure why

[ms660]

Originally Posted by Barnjet

It took me a while to identify the V1 pin numbers in the FM tuner area, as it's very snug in there, with wires everywhere - see photo

Anywhere, after identifying the pin1 position by removal and reseating V1 this morning, I got the following values (specified values in brackets)

Pin 1 Anode b 180V to 164V (195V)
Pin 3 Cathode Nil (2.1V)
Pin 6 Anode a 189V to 175V (227V)

According to the schematic, pin 3 is the oscillator/mixer cathode, it shouldn't be 2.1V as you've shown in brackets, so no voltage on pin 3 is ok because the oscillator/mixer cathode is connected directly to chassis, it's pin 8 that the 2.1V is attributed to which is the cathode of the RF amplifier.

Originally Posted by Barnjet

when I touch the probe on Pin 1 Anode b, the signal cuts off, only to return when I remove the probe.

That's to be expected, remember that all circuits are potential dividers, that includes your meter when in circuit, remember that potential dividers apply to AC as well as DC no matter what the frequency is.

Lawrence.

[Barnjet]

Thanks Lawrence, I didn't spot the difference between cathodes "a" and "b" in the diagram. And your reminder about potential dividers is appreciated.

All part of the learning curve!!

[paulsherwin]

I suspect you just don't have an adequate aerial. The built in VHF aerials in 1950s sets are usually pretty hopeless, and that is especially true of the foil sheet aerial used in this set. Try making a simple dipole aerial - just two 75cm lengths of wire connected to the two aerial sockets. Move the two wires around to get the best signal.

[Sideband]

Originally Posted by paulsherwin

I suspect you just don't have an adequate aerial. The built in VHF aerials in 1950s sets are usually pretty hopeless, and that is especially true of the foil sheet aerial used in this set.

Valve FM tuners are nowhere near as sensitive as transistor types so you need a dipole at least like Paul suggested. My location is Croydon.....some 20 - 25 miles as the crow flies (and radio waves travel) from Wrotham where the national BBC channels are broadcast. I think it's a 100kW transmitter but I stand to be corrected on that. So despite the fact that it is pumping out a fair old signal, I don't have a good signal on any internal aerial...at least not stable. One of those cheap 'T' tape aerials gives a much better signal and I get acceptable results from a simple dipole that Paul suggested. The best signal is obtained from my newly installed FM dipole from CPC in the loft which I have optimised for Wrotham.

Basically, for good FM reception, you need more than a piece of wire in the aerial socket (although that might do for a quick test).

[Barnjet]

Well, the set is going from strength to strength. It just gets better every time I turn it on! I now have very acceptable signal strength and clarity right across the FM tuning band, both national and local stations, from R2 to Classic FM.

I tried a simple dipole aerial but I continue to get superior results from the internal foil. Perhaps it's because we have a national VHF relay tx in Chard, some 20 miles away?

Anyway I would like to replace the TI valve (EM80) as it no longer functions. Any tips on where best to source a replacement?

thanks

[paulsherwin]

If you have a very strong signal the internal aerial will indeed give acceptable results, but I'm surprised even a very basic homemade dipole doesn't give better results.

All magic eyes are scarce and expensive, because they have short lives and are largely cosmetic, so owners didn't replace them when they went dim. This in turn means that only small replacement stocks existed at the end of the valve era, and these rapidly ran out as soon as people started refurbishing old radios.

Your best bet initially is to ask on the forum if anybody can supply you with a replacement. If that fails there's the usual valve dealers and eBay. I believe Nick tracked down a Russian equivalent for his A277 which worked well.

[Barnjet]

Originally Posted by paulsherwin

All magic eyes are scarce and expensive, because they have short lives and are largely cosmetic, so owners didn't replace them when they went dim. This in turn means that only small replacement stocks existed at the end of the valve era, and these rapidly ran out as soon as people started refurbishing old radios.

Your best bet initially is to ask on the forum if anybody can supply you with a replacement. If that fails there's the usual valve dealers and eBay. I believe Nick tracked down a Russian equivalent for his A277 which worked well.

Thanks, Paul. I tried a Russian equivalent site www.rutubes.com
But sadly all out of stock. I shall try the forum, and search Ebay. The Radio Museum suggest that an EM81 might be a suitable substitute, but I would need confirmation before I tried something like that.

[paulsherwin]

Don't spend a lot of money on it without thinking it through. The radio will work perfectly well without a magic eye. The official advice in the 60s when EM80s became obsolete was to rewire the base for an EM84 and change the aperture with black tape, but that is rarely done as it looks awful.

[Nickthedentist]

Awful, but better than a gaping hole.

A non working but glowing replica might be another option.

I think mine was one of these: https://www.tubes-store.com/product_...?products_id=5

[Barnjet]

Originally Posted by paulsherwin

Don't spend a lot of money on it without thinking it through. The radio will work perfectly well without a magic eye.

Good point Paul, I don't want to over-gild the lily.

Originally Posted by Nickthedentist

A non working but glowing replica might be another option.
I think mine was one of these: https://www.tubes-store.com/product_...?products_id=5

Thanks, Nick, a possible alternative, but still the best part of £20. Under consideration. I need to focus on the cabinet first, as a fair amount of the veneer edge trim is missing.

BTW - I notice you did a "soak" test on your set. What does that entail and why do it?

thanks

[paulsherwin]

It's just slang for leaving it running for several hours to see if anything goes wrong.

[Barnjet]

What a voyage of discovery, learning and satisfaction!

Having replaced all the white paper Hunts' caps much earlier, there was still fading and signal instability. But the simple act of cleaning the pins on V1 with WD40, using a cotton bud, and reseating it a few times cured this problem dramatically. So much so, that there was quite a bit of distortion on the stronger stations.

A bit of research on this site and elsewhere suggested that the AF coupling caps might be the culprits, so I replaced C30, C31 and C26 one at a time, testing the set after each change.

Some improvement after swapping out the Metalmides, C30 and C31, which were off-spec, but a drastic improvement after replacing C26, the black Hunts. Quite astonishing audio clarity now!

Even the wife said, that sounds good. Happy Days!

[Nickthedentist]

Well done!

Those Hunts capacitors are invariably faulty nowadays.