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Old 18th Dec 2019, 1:06 am   #141
allan
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Default Re: Moreton Cheyney

Not really sure but the receiver originally had a miniature Westector for delayed AVC, another for QAVC (inter-station quieting) two diodes in one 6B8 for detecting audio and a further two diodes in a second 6B8 which I suspect are for producing a limited max audio voltage (a bit like AVC). IFT4 is solely for QAVC and I suspect this gives a relatively sharp IF response compared with audio which uses IFT1, 2 and 3. Normal AVC for RF and IF stages use IFT1, IFT2 and IFT4.
http://www.radiomuseum.co.uk/New/fulcct.jpg
I've shown the QAVC response curves here (scroll down to the end of the page)
http://www.radiomuseum.co.uk/Moreton%20circuits.html
I'm wondering if there's a problem using OA91s in place of Westectors in these specific applications ie the latter may have inherent delay due to a high forward voltage whilst the OA91 is acting too easily and cutting off the RF and IF valves rather than just backing them off slightly. The AVC voltage is around -6 volts.
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Old 18th Dec 2019, 11:08 am   #142
Mr 1936
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Default Re: Moreton Cheyney

Hi Allan

I believe you said earlier that the MC uses WX6 westectors. Assuming they have 6 series elements at about 0.2 V forward voltage each, that tots up to 1.2 volts. To replicate, maybe a couple of series silicon diodes would be worth trying.

You could even add some parallel capacitance and/or series resistance to replicate the Westector's rather non-ideal characteristic.

Cheers, John (Mr 1936).
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Old 18th Dec 2019, 1:00 pm   #143
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Default Re: Moreton Cheyney

That's the approach I was thinking John
The numerous valves in the set apart from the 6J5s are basically unknown for sure.
The two 6B8s I only guessed at because of their wiring and there are not really any options. The KTZ63/KTW63 (or 61?) valves I'm assuming are variable mu KTW63 (6K7) if there's an AVC voltage on their grids and KTZ63 (6J7) if not.
I fitted a 6K8 for the frequency changer and this seems happy enough on the lowest 3 bands.
All this means they are very dependent on their grid bias. The KTZ63 may be cut off if it sees less than minus 6 volts.
I've just done a run through of voltages and the first two audio amplifiers are coupled together with their anodes sharing a 51K resistor. The voltage at their anodes is only 30 volts which seems far too low.
I have a suspicion there's an earth connection or wire link missing in V10 (6B8) wiring because there are two resistors in series 15K + 1M with no connection at their junction which seems strange.
V10 diodes are at minus 4.7 volts so they're well cut off which is a bit odd.
I'm floundering a bit..
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Old 18th Dec 2019, 11:57 pm   #144
PJL
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Default Re: Moreton Cheyney

The response curves you have shown show a fairly consistent bandwidth (-3dB point) of +/-25KHz? Is this with the two IFT cam mechanisms working? Can you adjust the instrumentation to 1dB/division and 10KHz/division? Did you disable the AGC?

Aligning the IF to get the bandwidth working properly could prove quite challenging. The Decca Decola Tuner has mechanical bandwidth control on all IFT's and the alignment instructions set the position to less than narrow and peak each IF individually.

The Decola also has an AGC amplifier but it is nice & simple!
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Old 19th Dec 2019, 1:12 am   #145
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Default Re: Moreton Cheyney

Yes, all IFTs are working as they should with slugs smoothly pushing into place.
I reckon I can possibly tweak the shape of the responses once the thing is stable.
Today I unplugged V10 a 6B8 and it made not the slightest difference which seems to imply there's something amiss with a component or a missing wire, or I'm even using a wrong valve type (my latest idea.. could it be an EK32/VR57 as this is used in the R1132 for audio control).
I also discovered the second (tandem) volume control pot VR3 was wired back-to-front.
Now I can hear the effect of the bass and treble controls which I'd describe as bass-boost and treble-boost.
I can now see a volt or two of audio output. As the output is very high impedance I need an amplifier to hear it properly as low impedance phones kill the output voltage.
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Old 20th Dec 2019, 2:49 pm   #146
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Default Re: Moreton Cheyney

I've now adjusted the IFTs to give me a fair representation of the Moreton Cheyney receiver bandwidths (5/8/11/15/18KHz). Not perfect but results sound pretty good when listening to Radio 4 and the Irish Radio 1 broadcasts.
A lot cleaner sounding than FM radio and way better than digital.
http://www.radiomuseum.co.uk/Moreton...tml#anchor1510
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