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Old 19th Oct 2019, 4:34 pm   #81
allan
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Default Re: Moreton Cheyney

I think V12 might have been used once but no longer om. I thought V12 and V13 were originally push pull drivers for the external power amp.
VR1 has an on/off switch so may be a basic volume control.
I think you are right about VR3 and VR4 being a loudness control.
VR4 appears to be able to turn off the input to V8 which presumably feeds in via C26 as that circuit can be the Gram input. I might have misdrawn the RHS of the source switch inputs as the ground shouldn't short the QAVC/Radio input except when Gram is selected.
Loudness is only selected at the source switch for Radio and QAVC.
It looks like VR3/VR4 is a combined Loudness + volume because of the grounding of V8 grid at the min setting of VR4.
VR1 also grounds its valve grid so must be a second volume control.
Presumably you juggle VR3/VR4 and VR1 to get the best fidelity?
VR2 seems to sit in the centre element of a high pass filter which would make sense if it has a value in megohms? A few calculations may show its effect (if V13 were to be re-connected)??
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Old 19th Oct 2019, 5:16 pm   #82
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Default Re: Moreton Cheyney

Quote:
Originally Posted by allan View Post
I think V12 might have been used once but no longer om. I thought V12 and V13 were originally push pull drivers for the external power amp.
Yes, that makes sense. Methinks this particular radio has been seriously 'reworked' at least once!
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Old 21st Oct 2019, 12:15 pm   #83
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Default Re: Moreton Cheyney

I've now added the full circuit diagram of the Moreton Cheyney but there are a few things to check out (re the note on the drawing).
http://www.radiomuseum.co.uk/New/fulcct.jpg
and of course the switchery around the front end isn't shown.
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Old 22nd Oct 2019, 1:12 am   #84
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Default Re: Moreton Cheyney

Thanks for posting the full circuit.

Looking at the AF side, and starting at the output end:

Click image for larger version

Name:	Morton Cheney Tone Control Section.jpg
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V10 is a cathode follower feeding the tone control.

VR3 might be a preset gain control.

The tone control itself, VR1 treble and VR2 bass, is a Volkoff/James passive type of the era (see: https://www.vintage-radio.net/forum/...1&postcount=62), although with an unusual feature.

At first glance one would say that instead of combining the outputs of the two tone control potentiometers in the usual passive way, MC has used electronic recombination, the slider of each potentiometer feeding its own triode stage with recombination thereafter.

But that idea fails because the output of the bass triode, V12, goes nowhere except to ground. As its stands. R51, at the anode of V12, with the 4700 pF capacitor, probably forms a low pass filter to stop any treble going via this route, bearing in mind that whilst the bass tone control network around VR2 ensures that this potentiometer has no effect on treble, it does not actually block treble. One might expect the top end of R51 to be connected to a load that is common with that of V13. The likely candidate, R59 fails, because its bottom end is bypassed to ground via C29.

The treble alone gets out, from the anode of the treble triode V13. In this case a high pass filter is not needed, as the network around the treble potentiometer blocks the bass.

It is difficult to imagine that this was the original circuitry. Rather, as Tanuki has said, it has been seriously reworked. In that case, knowledgeable intent seems unlikely, or at least if so, the purpose. Is very obscure. Accidental mis-rewiring seems more likely.


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Old 22nd Oct 2019, 2:23 am   #85
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Default Re: Moreton Cheyney

Moving back to the gain control section, V9 looks as if it is part of a feedback volume control using VR4. Except that the feedback loop is broken by switch S1/3 when it is in the “gram” position. Then VR1 appears to act as a current-dividing control. It certainly looks odd.

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Name:	Morton Cheney Volume Control and Expansion Section.jpg
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ID:	192223

The selectivity switch wafer connected to the anode of V9 switches in additional load resistors (in AC parallel with the main V9 anode load). It would appear to adjust the AF gain according to selectivity position, quite reasonable given that IF gain is likely to be higher in the narrower bandwidth positions.

My best guess as to the purpose of V10 is that it is the centre of the volume expansion circuit. Its output is in parallel with that of V9, in that its HT pathway from its anode is ultimately via V9’s anode load resistor. Its input is fed back from its output via an R-C network that involves the V10 diodes (setting the grid bias according to signal level) and probably sets the time constants and frequency response of the expansion process. Its immediate anode load, R46, appears to be bypassed in some positions of the selectivity switch, suggesting that the amount of expansion varies with the bandwidth setting.


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Old 22nd Oct 2019, 10:27 am   #86
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Default Re: Moreton Cheyney

OK on the purpose of VR1 & VR2.
I've just added a note on the full circuit to the effect that VR3 is ganged with VR4.
VR5 is some sort of preset gain or AVC depth?
I suspect the anodes of both V12 and V13 originally went to the rear connector via 2700pF capacitors, one of which is now grounded.
I had assumed that the outputs were in push pull but maybe the external amplifier has the combining stage followed by a phase splitter. There are two EF39s and two KT66s on the amp chassis.
There may be an odd error or two in my circuit because of the difficulty in accessing and even seeing some parts. A 3.3K resistor is hidden away under the rear tagboard such that you can't even see to where it's connected.
Whoever made the modifications didn't possess a decent soldering iron as many wires fell off during the circuit tracing and wafer 3 of the bandwidth switch may have had its solder tags mis-identified. See Bw3/2 which shorts out the anode resistor. Can this be right?
I'd have expected a ground connection in the circuitry around the cathode of V10 but the resistance to ground is too high for one to be (now) present.
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Old 22nd Oct 2019, 11:27 pm   #87
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Default Re: Moreton Cheyney

Quote:
Originally Posted by allan View Post
I've just added a note on the full circuit to the effect that VR3 is ganged with VR4.
OK, so the volume control was done by tandem potentiometers at two points in the circuit. Sometimes this was done to steer clear of overload on the one hand and poor signal-to-noise ratio on the other.

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Originally Posted by allan View Post
VR5 is some sort of preset gain or AVC depth?
It might also be to set the muting threshold for the QAVC system.

Quote:
Originally Posted by allan View Post
I suspect the anodes of both V12 and V13 originally went to the rear connector via 2700pF capacitors, one of which is now grounded.
Possibly the two outputs were simply combined after the DC blocking capacitors.

Then maybe the 4700 pF capacitor at V12 anode was not part of a low-pass filter. It could be that V12 and its load served in place of the standoff resistor that was normally used in this kind of tone control in order to avoid the bass control having any material effect on the treble. V12 and V13 would in part have made up the loss of 14 dB or so incurred in the passive tone control networks.

Quote:
Originally Posted by allan View Post
I had assumed that the outputs were in push pull but maybe the external amplifier has the combining stage followed by a phase splitter. There are two EF39s and two KT66s on the amp chassis.
Possibly one EF39 as driver and the other as phase splitter?

Quote:
Originally Posted by allan View Post
Whoever made the modifications didn't possess a decent soldering iron as many wires fell off during the circuit tracing and wafer 3 of the bandwidth switch may have had its solder tags mis-identified. See Bw3/2 which shorts out the anode resistor. Can this be right?
With R46 shorted out, V10 anode is still looking into R48, the anode resistor of V9, which adds the outputs of the two valves. The purpose of R46 appears to be to reduce the amount of the V10 output that is added to the V9 output at certain selectivity settings. I am not sure about C32 – with the anode resistances it would seem to form a low-pass filter, but presumably with a sufficiently high turnover point not to adversely affect the signal.

Quote:
Originally Posted by allan View Post
I'd have expected a ground connection in the circuitry around the cathode of V10 but the resistance to ground is too high for one to be (now) present.
V10 cathode looks to be connected directly to ground. But there is no apparent DC return path for the grid.

I have the impression that the MC designers may have had more RF than AF expertise, perhaps not unexpected if their background was WWII military radio. With AF they may have used circuits from various available sources. For example, treble and bass tone controls with electronic summing had been introduced by Sound Sales by the beginning of 1946. And I think volume expansion circuits had been published in Wireless World.

Be that as it may, it looks as if whoever modified the circuit might have gotten lost in the complex wiring. It would be difficult to develop a sound rationale for taking the output from V13 alone.


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Old 23rd Oct 2019, 12:01 am   #88
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Default Re: Moreton Cheyney

Yes..om I meant ground return for the grid leak.
Away for a couple of weeks so the Moreton Cheyney is on hold
It's not easy to see connections to the parts on the rear tagboard so I may have missed something, but I noted there is nothing connected to the junction between R65 and R76 which is odd from the resistor values. Why add 15k in series with 1M? Maybe that junction should be grounded or connect somehow to V9?
Also C32 at 0.5uF looks out of place as it has a low impedance to AF. I can see it decoupling the hot end of R46 but not decoupling V9 anode.
Maybe V9 anode via C27 and Bw4/2 should connect somewhere in the resistor chain R77 to R67 to reach the grid of V10. There are a couple of rough solder joints in this area so perhaps someone made a mistake?
Another possibility is C41 should go to V9 anode and the connection between C41 and R64 is wrong? I'll double check this area of the circuit as it doesn't make sense to me. As I say there's evidence of rewiring in this area.
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Old 23rd Oct 2019, 12:02 am   #89
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Default Re: Moreton Cheyney

Not making much sense to me as all audio will be wiped out by C32 (0.5uF) from anode of the 1st audio stage to earth.
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Old 23rd Oct 2019, 9:23 am   #90
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Default Re: Moreton Cheyney

I suspect the last owner made a mess of modifying the audio stages.
There was a twisted flex sticking out a hole in the front panel which I think took audio from an early stage. He probably gave up on the complicated bits.
Hopefully I can work out how to fix things. There are a few clues.. as you say that 0.5uF shouldn't be there and he must have also removed the feed into the following stage as there isn't a connection to the grid and there isn't a grid leak to ground.
I suspect trouble started when perhaps he plugged in a 6K7 or 6J7 into the socket which is clearly wired for a 6B8. As the socket wiring is completely shielded by the two ganged volume controls he probably didn't realise his mistake so started messing around with the wiring and got into deep water and finally gave up and consigned the receiver (and amp) into a damp shed. Possibly he was the second owner so wasn't familiar with the receiver. The 6K8 was missing so he probably pinched that for another project.
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Old 17th Nov 2019, 1:11 pm   #91
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Default Re: Moreton Cheyney

Quote:
Originally Posted by PJL View Post
Not making much sense to me as all audio will be wiped out by C32 (0.5uF) from anode of the 1st audio stage to earth.
I checked the circuitry again and found a few errors in my previous tracing.
I've edited the full circuit accessed at the bottom of this page
http://www.radiomuseum.co.uk/Moreton%20circuits.html

I'm not too far from powering up the RF and IF sections subject to other work.
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Old Today, 12:20 am   #92
allan
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Default Re: Moreton Cheyney

I'm now working on the Moreton Cheyney amplifier that accompanied the receiver, and powering up sections of the latter if anyone was following this
http://www.radiomuseum.co.uk/mca.html
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