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Old 11th Dec 2018, 10:22 am   #1
Ian - G4JQT
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Default Ekco A22 circuit changes

A question for the Ekco experts!

I'm restoring an A22 and have noticed a few circuit differences between real sets and the circuit. S/N 017772

There's a 1nF capacitor fitted across the input side of the LW antenna coil on my A22 which is not on either the Trader sheet 768 or Ekco service data.

The V2 anode coupling capacitor is given 50pF on Trader and Ekco sheets, but mine is marked and measures 15pF.

V2 suppressor grid is shown on Trader and Ekco sheets connected to V2 cathode (and its biassing arrangement), but mine is connected directly to the chassis.

Not sure if these are likely to be deliberate production changes or errors.

Can anyone throw any light on this?

Thanks.

Ian
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Old 16th Dec 2018, 11:25 am   #2
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Default Re: Ekco A22 circuit changes

Incidentally, the second IF can has one coil and capacitor as per the circuit, but the other a smaller coil and a larger capacitor - 220pF rather than 100pF.

A circuit improvement, or running out of parts towards the end of the run, but makes no noticeable difference?
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Old 16th Dec 2018, 8:59 pm   #3
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Default Re: Ekco A22 circuit changes

This has been noticed before on A22 restorations see post #19 which is talking about the same capacitor.
https://www.vintage-radio.net/forum/...d.php?t=110919
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Old 18th Dec 2018, 10:16 am   #4
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Default Re: Ekco A22 circuit changes

Thanks Stephen. I missed that. Yes, it's obviously not a bodge. Either they tweaked things to account for parts supply or tweaked things for performance improvements. Who knows now...?

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Old 13th Mar 2019, 10:49 am   #5
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Default Re: Ekco A22 circuit changes

Compared to the published circuit, this change appears to give a wider IF bandwidth with a gentle double hump.

With a wobbulator on both this became obvious.

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Old 28th Mar 2019, 3:44 pm   #6
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Default Re: Ekco A22 circuit changes

I've done plenty of playing around recently with two Ekco A22s which I've completely restored and have noted minor differences in their performance.

The first is a brown example with the standard (as published) IF capacitors; i.e. all four IF tuning capacitors 100pF with a 50pF capacitor from V2 anode.

My most recent A22 is a black and chrome version with the final IF capacitor being 220p with a smaller matching inductor - noted in post #3 above. The other difference is the V2 anode capacitor is 15pF, not 50pF.

As described previously, with this version it is easy to select a slight double-hump IF response centred on 465 kHz. I was not able to do this with the other A22.

Performance:

Assuming all the valves are within tolerance (which as far as I can tell on the AT160 tester they are) there are slight performance differences. On SW performance is as far as I can tell more or less the same on both. A 30% modulated AM signal of 1uV at the antenna inputs can just about be detected above the receiver noise on both sets across the band.

On MW the brown one is more sensitive at the HF end with sensitivity of 1 uV, falling to 3uV at the LF end. The black one it was exactly the other way around.

On LW both sets were between 1 and 2 uV lowest discernible signal.

I feel these differences are well within manufacturing tolerances, and nothing to do with circuit differences, so probably doesn't mean very much. However, a more significant difference on the black set (with the IF divergences) was that on very strong signals (my 'pantry' transmitter for example) the set would motorboat when the volume was turned right up. This didn't seem to be a valve problem, cable dressing or anything else I could pin down. But adding another 22uF capacitor across the already new and tested smoothing capacitor completely cured it. Is this an occasional known problem I wonder? I couldn't get similar effect on the other A22. Slightly odd...

(This extra capacitor might exceed the surge current of the AZ31, but as it's after the choke I'm going to assume it won't be a problem...)

In order to take advantage of the wider IF on the second set, I increased the tone corrector resistor from 5.6 k to 22k. I will mostly listening to my wide bandwidth 'pantry' transmitter, and this increased audio bandwidth sounds quite nice.

Incidentally, other five-valve sets I have measured of similar vintage have a sensitivity of between 0.5 and 1uV. This is academic really since natural and man-made noise is way over that, it just shows the A22 'short' four-valve superhet is very slightly less sensitive.

Note: The sensitivity tests are from a signal generator with a 50 ohm output impedance. The receiver antenna tuned circuits are set using a dummy antenna intended to imitate a high-ish impedance random antenna wire. Since I made no attempt to match the impedances (which probably vary significantly on the A22 between ranges and frequencies) there will be a significant impedance mismatching, so these cannot be taken as definitive measurements.

Last edited by Ian - G4JQT; 28th Mar 2019 at 3:50 pm.
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