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Old 9th Oct 2019, 9:02 am   #61
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Default Re: Moreton Cheyney

I suppose, back in 1946 the Westector was seen as a jolly good modern diode and they weren't clairvoyant so wouldn't know it may degrade over 50 years or so.
The DH63 was used for their TRF receiver circuit so they were aware of these, using the diodes for AVC, but the circuitry around the normal detector and AVC may have made the DH63 not a suitable choice. I haven't traced this out yet.
An EB34 would entail more admin, chassis bashing, wiring, extra heater consumption and even a couple of EA50s would need admin, more wiring and heater current so the new miniature Westector must have seemed a perfect solution in 1946.
I haven't noticed an advert for surplus Westectors. Maybe they were dirt cheap compared with a valveholder plus valve? Didn't AVOs use something similar in bridge form?
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Old 10th Oct 2019, 10:07 am   #62
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I spent a few minutes considering the adverts for the Silver Dragon receiver. I suppose the key point in the description of the Silver Dragon is that it has "automatic frequency control". I've seen this in the ARR2 VHF FM receiver but never in an AM receiver. The local oscillator could be tweaked in some way but I see no circuitry around this for that sort of purpose. Another way may be to somehow use the built in TRF receiver or change the IF response in some way?
Could the action of AVC applied to the mixer tweak the oscillator (pulling effect)? If so the tuning would become very rubbery in feel unless the switch marked QAVC off is used for normal tuning then switched back once a station was tuned in?
It's new to me... any ideas?
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Old 10th Oct 2019, 10:08 pm   #63
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Default Re: Moreton Cheyney

AFC was a feature of some 1930s AM receivers, and in fact the Foster-Seeley discriminator was originally developed for AM AFC purposes, and later used for FM. Sturley covered AM AFC in quite some detail, see Part II, chapter 13, p.224ff. AFC does not seem to have been used much in post-WWII domestic AM receiver practice, although of course, quite sophisticated AFC systems were used in commercial point-to-point SSB/ISB receivers. And it was common in valve-era FM receivers. In later years it did turn up occasionally on AM receivers. For example the Eddystone 1570 and 1590 solid-state single-conversion HF receivers of the late 1970s had (AM) AFC.

For an AM AFC system in the valve era, one would expect to find both a discriminator, usually a diode pair, and a reactance valve, usually a triode or pentode (The use of diodes as reactance elements appears to date from the mid/late 1950s, for TV and FM AFC systems.) Depending upon the discriminator circuit details, the diode pair could have a common cathode or they could be independent. Sturley showed a combined AM detector and discriminator circuit, and also a circuit in which a triode did double duty as the AFC reactance valve and an AF voltage amplifier. If MC had used one or both of these techniques, then the AFC system could be hard to find at first glance. Given the multiple features that MC endeavoured to include in its receiver, use of such combined circuitry does seem plausible in order to keep the valve count (not to mention chassis area and heat generation) within bounds and still leave room for the kitchen sink!

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As to the effects of AFC on tuning operations, to some extent that would depend upon the AFC range. Some FM receivers had quite strong AFC that effectively did the fine tuning, and quite often this could be switched out for initial tuning. Or the AFC could be made variable (as was done by Jason for some of its tuners.) On the other hand, sometimes very mild AFC was used, whose purpose was to hold against drift the correct tuning point that was initially set using some form of centre-channel indicator; in this case it might not have been switched out for initial tuning. I think that one would need to look at some 1930s examples of AM AFC to see how that was done in practice.


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Old 11th Oct 2019, 1:04 am   #64
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Default Re: Moreton Cheyney

For an actual example of an AM receiver with AFC, the Murphy A40 circuit is available at: https://www.service-data.com/section.php/5075/1/a40c. In this case the AFC was referred to as the automatic tuning control, ATC. The AFC discriminator was driven by a sidechain IF amplifier stage that also fed the AGC system, providing a form of amplified AGC.


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Last edited by AC/HL; 11th Oct 2019 at 1:11 pm. Reason: Forum rule B8
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Old 11th Oct 2019, 3:31 am   #65
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Default Re: Moreton Cheyney

Another aspect of the entwinement of the AFC function with other circuit parts is that sometimes a limiter was used before the AFC discriminator in order to make the AFC range less variable with signal strength. It is possible that in such cases the limiter grid might also have been used as an AM detector, which at first glance would make for a rather confusing stage. The use of a limiter grid as AM detector was not unknown in British practice, but probably quite rare, whereas it was found to some extent in American FM-AM receiver practice.


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Old 11th Oct 2019, 9:42 am   #66
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Default Re: Moreton Cheyney

You're certainly right that the AFC circuitry is hard to find..
One critical area is around IFT4 which has a very odd arrangement of parts. A resistor tagboard is bolted in place over the underside of the transformer and the use of 18SWG wire makes for peering into the tangle of connections quite hard. I'm gradually tracing parts of the circuit though. The IF strip is not what I imagined as one transformer has two valve top cap leads. I'm guessing one is for AFC and the other for signals. Up to now I cannot find any link to the local oscillator but this may be because someone cut away the wiring. I'm updating my drawings as I go and I'm certain there are some further changes in the pipeline.
I found an excellent publication about AFC which I downloaded and to which I provided a link.
My ARR2 receiver used a Foster Seeley discriminator and a reactance valve and it worked very well. I fitted a pot and used this for fine tuning in the good old days when police used the top end of the band
My latest result is to discover what I believe was a 6A8 valve (rather than a DH63/6R7) in the rear corner of the chassis. That valve was missing and it appears that there isn't an alternative to the 6A8. What it's used for is not obvious as it has a decoupled anode. I assume it's a cathode follower and possibly an AVC amplifier but I'm coming across wiring changes here and there which don't help.
I suspect you may be right about the use of a limiter. The possibility that KTW63 and KTZ63 (and one 6K7) valves may have been swapped around or fitted randomly is also feasible so I need to check their grid circuits to check on the designer's choice. The 0D3 is not connected to the local oscillator. It's wiring goes off to the jungle at the rear of the chassis.. so what's it for?
Those Westectors appear to be part of a discriminator. One seems to connect to V7 anode via 500pF and the other to the secondary coil.. I think??
Because of the wide passband I wonder if the AFC is used in some way within the IF strip?
I'll upload the IF circuitry later for perusal... today maybe.
I'll look at the A40 circuit next to see if there's a clue in that...
A customer is collecting his McMichael 368 today so I'll need to tidy it up.
http://www.radiomuseum.co.uk/mm368.html
Fingers and toes crossed that it works when I turn it on....
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Old 11th Oct 2019, 11:46 pm   #67
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Default Re: Moreton Cheyney

The Rider book on AFC is very good – I hadn’t seen it before.

The IF strip does look rather complex. The Quiet AGC (interstation muting) system would have added further complexity. Various ways of doing this were shown by Sturley (*), one here that includes amplified AGC:

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It would appear that Q-AGC could be integrated with demodulation, and that AFC could be integrated with demodulation and AGC rectification. But putting them all together in one circuit subgroup probably would have been a challenge.

* Sturley is available on-line at several places, including:

Part I: http://www.nj7p.org/Manuals/PDFs/Books/Sturley_1.pdf

Part II: http://www.nj7p.org/Manuals/PDFs/Books/Sturley_2.pdf

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Old 12th Oct 2019, 7:41 am   #68
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Quote:
Originally Posted by allan View Post
My latest result is to discover what I believe was a 6A8 valve (rather than a DH63/6R7) in the rear corner of the chassis. That valve was missing and it appears that there isn't an alternative to the 6A8. What it's used for is not obvious as it has a decoupled anode. I assume it's a cathode follower and possibly an AVC amplifier but I'm coming across wiring changes here and there which don't help.
A couple of possibilities for a 6A8 heptode late in the signal chain come to mind. More likely I think is that it was part of the AF volume expansion system (mentioned in the literature), where it would be used as a dual-control amplifier stage, with the AF on the signal grid and the control bias (typically derived from a diode perhaps driven by its own side-chain AF amplifier) on the “oscillator” grid. The other (remote) possibility is that it was used as a noise-blanked gated IF stage, with gating of the “oscillator” grid from the noise pulse extraction and rectification circuit. However, as nothing was said about noise blanking (or even noise limiting) in the literature, and I think that had some such circuitry been fitted, it was more likely to be a noise limiter than a noise blanker.

More generally, I imagine that most circuit elements of the MC receiver were drawn from established, perhaps in some cases very recently established practice, as published in literature of the time. The ingenuity may have been more in choosing which circuits to use and getting them to work well together.


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Old 12th Oct 2019, 9:59 am   #69
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Default Re: Moreton Cheyney

To me, this thread is vintage radio at its absolute best. A jolly good puzzle of a set being analysed and unravelled, its manufacturer being similarly analysed and unravelled, all leaving a few unsolved mysteries to keep the plot seasoned.

I'm really enjoying this!

AM Receivers with 16-18kHz bandwidth are still being made. Not for high fidelity, rather the reverse. Some aviation channels are declared as '25kHz spacing' and require wide receiver bandwidths. Say you are flying along a big valley, longer than a single ground transmitter would cover, but needing to be on a single frequency for air traffic control. Along the route would be several transmitters some on the channel centre, some offset above some offset below and with two different amounts of offset.

Your AM receiver will hear whichever are strong enough. As you pass from one to another audio fades across, but there will be a godawful strong heterodyne tone at a few kHz (the offset spacing) and your receiver is required to have an audio cutoff which drops like a stone above 2.8kHz. This way you can fly along talking with the same controller, blissfully unaware that you're passing from ground station to ground station. You always transmit on channel centre, all ground receivers are tuned to channel centre and are narrowband.

Would you believe they even cram channel centre and two offset frequencies into the channels designed for the new 8.33 kHz spaced channels?

Hifi it is not, but it does mean I have to design IFs for AM reception whose wide bandwidth seems like something out of the past. I wonder how many people have designed AM transmitters in the 21st century and put them into production? You definitely get the feeling you're paddling against the flow of technology!

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Old 12th Oct 2019, 10:07 am   #70
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Default Re: Moreton Cheyney

That Sturley is an interesting book. I'll add it to my Amazon Fire for bedtime reading.
QAVC.. I must admit that interstation quieting hadn't come to mind but it sounds very likely.
The 6A8 for some sort of audio processing is a possibility because it's used in the R1132 for dealing with extremely large variations in signal strength at WW2 and later air traffic control rooms, but sorry to say a 6B8 is what it is. A senior moment or brain decay is maybe to blame
I'm tracing the difficult area at present. It seems to be mainly dealing with AVC so I'll construct a drawing showing the interconnections of this. I've found four diodes. Two in the 6B8 which are wired together (I haven't figured out whether these are for audio or AVC), and two Westectors which are a puzzle because I still haven't found an AFC link to the local oscillator so maybe one for quieting and one for either normal AVC or audio. There are wiring changes so it may be possible that part of the AFC circuitry has been removed?
Last night the McMichael 368 worked well on medium waves but failed on long waves. It was fine a few weeks ago when it was warm and sunny so I'm wondering if a slightly low emission TP22 is the culprit. My PSU has a pot for tweaking the 2 volt supply so I may suggest the owner carefully adds 0.25 volt. Could low ambient temperature be to blame when emission is marginal I wonder?
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Old 12th Oct 2019, 10:23 am   #71
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Default Re: Moreton Cheyney

David...I have a few SDRs. My favourite being an Andrus which is perfect for medium and long waves. With SDR Console you can see exactly the width taken by broadcast stations and alter the receive passband to hear best fidelity. Reception can be really excellent. One or two broadcasts are +/- 8KHz and others are restricted to +/- 6KHz (eg Radio 4 LW) or less and therefore not as good.

I have a DST100 receiver with a wide IF passband but that's another story (I have written it up so it's there to find). It was an intercept receiver where an operator could sit back without fear of missing anything slightly off-channel, then once heard the receiver could be tweaked to hear it better.

Using a VHF SDR I always set the bandwidth to +/- 20KHz for aircraft as not everyone's transmitter is accurate.


Quote:
Originally Posted by Radio Wrangler View Post
To me, this thread is vintage radio at its absolute best. A jolly good puzzle of a set being analysed and unravelled, its manufacturer being similarly analysed and unravelled, all leaving a few unsolved mysteries to keep the plot seasoned.

I'm really enjoying this!

AM Receivers with 16-18kHz bandwidth are still being made. Not for high fidelity, rather the reverse. Some aviation channels are declared as '25kHz spacing' and require wide receiver bandwidths. Say you are flying along a big valley, longer than a single ground transmitter would cover, but needing to be on a single frequency for air traffic control. Along the route would be several transmitters some on the channel centre, some offset above some offset below and with two different amounts of offset.

Your AM receiver will hear whichever are strong enough. As you pass from one to another audio fades across, but there will be a godawful strong heterodyne tone at a few kHz (the offset spacing) and your receiver is required to have an audio cutoff which drops like a stone above 2.8kHz. This way you can fly along talking with the same controller, blissfully unaware that you're passing from ground station to ground station. You always transmit on channel centre, all ground receivers are tuned to channel centre and are narrowband.

Would you believe they even cram channel centre and two offset frequencies into the channels designed for the new 8.33 kHz spaced channels?

Hifi it is not, but it does mean I have to design IFs for AM reception whose wide bandwidth seems like something out of the past. I wonder how many people have designed AM transmitters in the 21st century and put them into production? You definitely get the feeling you're paddling against the flow of technology!

David
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Old 13th Oct 2019, 12:07 pm   #72
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Default Re: Moreton Cheyney

After more time peering into the wiring I've updated my circuit of the IF strip. I seem to have detached one of the valves which I've called V9 so I'll need to trace its circuit and see how it fits in.
I think I've worked out the AVC circuitry but I can't guarantee my analysis.
Scroll down to the last paragraph.
http://www.radiomuseum.co.uk/Morton.html
If your browser pulls up an old copy try using https, instead of http, which will pull down the most recent edits.
Comments are welcome
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Quote:
Originally Posted by Synchrodyne View Post
Another aspect of the entwinement of the AFC function with other circuit parts is that sometimes a limiter was used before the AFC discriminator in order to make the AFC range less variable with signal strength. It is possible that in such cases the limiter grid might also have been used as an AM detector, which at first glance would make for a rather confusing stage. The use of a limiter grid as AM detector was not unknown in British practice, but probably quite rare, whereas it was found to some extent in American FM-AM receiver practice.


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Old 15th Oct 2019, 5:46 pm   #73
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If anyone's still following this thread I've changed the webpages as I found the original was too long for my scroll wheel to work in PageMill.
The circuits are now on a second page
http://www.radiomuseum.co.uk/Moreton%20circuits.html
I started trying to trace the whole audio circuit but ran into trouble when trying to fit a KTZ63 into the wiring of its socket and decided it may be a second 6B8.
See the end of the page for the circuit so far..
The stages are most peculiar (to me) but maybe some valve audio experts can shed some light on my findings (assuming I haven't made any mistakes). Some of the valves are operated as cathode followers and seem to operate at very low anode currents. The fact that V10 top cap is grounded looks odd and the wiring in that area does not look entirely original.
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Old 16th Oct 2019, 3:30 am   #74
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Default Re: Moreton Cheyney

Still following - you are making very good progress in mapping out the MC circuitry!

As you say, there is no evidence of an AFC system. Perhaps it got too hard, too costly or there was just not enough chassis real estate. Or maybe it was found not to be needed. Sometimes done in the quest for oscillator stability and freedom from pulling was using a separate oscillator valve, but not in this case. Another approach was to use fractional or no AGC bias on the HF bands. That could be more difficult to trace, as it would be bound up with the bandswitching. Fractional AGC bias appears to have been used for the RF stage; this I think was seen as a simpler alternative to having an additional delay or to having a separate wideband RF AGC system (as outlined by Sturley).

The IF strip turned out to have two signal stages, with a 3rd stage as a sidechain AGC IF amplifier, but fed from the 1st stage not from the 2nd stage. (Marconi did it that way on some of its HF receivers.) There is evidence that two IF signal stages probably provided more than enough gain for HF broadcast listening purposes. The separate AGC IF stage obviated differential distortion and modulation rise issues. QAGC added quite a bit of complexity, and I think had become a rare feature by the mid-1950s.

The TRF sidechain may have been used more as a “marketing” feature, given that there was probably still a body of opinion in favour of TRFs and infinite impedance detectors for quality AM reception, although by the mid-1950s I think that that idea had died. I suppose though that having the TRF meant that the IF strip did not need to be extended all the way to 24 kHz bandwidth. And the infinite impedance detector, even if chosen for marketing reasons, was technically beneficial in situ because it did not load the preceding RF tuned circuit.


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Old 16th Oct 2019, 8:06 am   #75
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Default Re: Moreton Cheyney

Still following with interest but not clever enough to have anything useful to add at this stage
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Old 16th Oct 2019, 10:05 am   #76
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I'm at the stage where getting the receiver working is not too far off. From what I've traced I should be able to pick up demodulated AM signals early on in the circuit without initially bothering with the audio stages. Looking at the electrolytics it looks possible that many (the yellow ones) are not original and, being physically very big, are masking lots of connections so swapping these for modern parts is a good idea.
AM is rectified by the diodes in V8 so that will still be much as the designers intended, except for resistors that have drifted high, but the two Westectors may be u/s. Swapping these for modern diodes is tricky because I recall that copper oxide rectifiers have some resistance which will have been dealt with in the choice of components, for example X2 (delayed AVC) has a reverse bias of about 3% of the HT (say 7.5 volts at 250V HT). Fitting a silicon diode in its place will perhaps require the bias voltage to be changed from 3%?? If a WX6 has a Vf of 2 volts and a modern diode 0.6 volts the bias will need to be 1.4 volts less.
X1 (QAVC) however has the benefit of the preset bias control VR5 on the rear of the chassis so a silicon diode swap should be OK.
For a start I might remove all the audio valves before checking to see if it can receive signals. Tests on the wax condensers indicate those used in the MC get slowly less leaky while HT is applied so putting 100 volts or so on the HT line via a limiting resistor for an hour or so (less valves) is perhaps the next test?
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Old 18th Oct 2019, 10:32 am   #77
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The old MC has had some changes made to it in the long distant past and I'm at the stage where it's possible that there was one too many changes and the guy put the thing on one side and just forgot (or died of frustration or old age).
I've traced out the "last" modification and revealed some problems.
1. there isn't a connection for audio to pass beteeen two of the amplifier valves
2. The second of those valves has its top cap wired to chassis.
3. The bandwidth switch shorts together V9 and V10 anodes.
Am I missing something? Suggestions invited.
http://www.radiomuseum.co.uk/Moreton...tml#anchor1424
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Old 18th Oct 2019, 10:58 am   #78
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Assuming that the current circuit possibly never worked, all you can do is try to work out what the pre-modified circuit looked like. It might be worth marking up on the circuit the components/connections that look non-original.

Some amplifier designers (Decola for example) adopted the approach of differential design from source to output transformer using a matching transformer for the pickup.
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Old 19th Oct 2019, 1:29 pm   #79
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I just spent another session on tracing the circuit and found some hidden wires plus an error or two in my previous attempts.
I can just see the body of a 3.3K resistor under one of the tagboards near to V9 but I can't see the wire ends so don't know to where it connects so the circuit remains slightly incomplete.
The audio circuit is probably 95% of what currently exists so can anyone suggest what VR1, 2, 3 and 4 are supposed to do.
Rumour has it they are volume, treble and bass (VR3 and VR4 are ganged together). Scroll down slightly to the full audio circuit dated 19/10/19 from here.
https://www.radiomuseum.co.uk/Moreto...ml#anchor48024
And how about V9/V10 interconnections and what are V10s diodes for?
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Old 19th Oct 2019, 3:12 pm   #80
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Default Re: Moreton Cheyney

VR3 and VR4 ganged together - could this be some sort of early version of a 'loudness' control where there's different amounts of [frequency-conscious] signal-processing applied depending on the volume setting?


I'm wondering of the strapping/wiring of V10's diodes is to act as some sort of AGC "clamp" to prevent the AGC line going positive under some circumstances?

Looking at your drawing of V12 and V13, I can't understand what V12 is doing as it seems to have an input (from VR2) but there's nothing sane connected to its anode or cathode!
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