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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.

Click image for larger version

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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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.

Quote:
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.

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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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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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Quote:
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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 20th Nov 2019, 12:20 am   #92
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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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Old 20th Nov 2019, 8:14 pm   #93
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Default Re: Moreton Cheyney

Most impressed with your patient and diligent work and documentation Allan, I wonder how long this sophisticated tuner had been dormant for? It's just a shame that the hay-day of LF/MF/HF AM broadcasting is now past and that it will have less opportunity to prove what it would have been capable of,

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Old 22nd Nov 2019, 1:56 am   #94
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Assuming that the input side of the power amplifier is as originally intended, then it does look as if each half of the push-pull output had its own driver stage (EF37A), and that the phase-splitter should then have been in the tuner/control unit section.

But the latter shows no apparent signs of housing a phase-splitter stage, which logically would have been the last stage in that unit. Rather that circuit appears to end with the bass and treble controls, each with its own output buffer stage, V12 for bass and V13 for treble. One would expect that the two buffer outputs would be mixed, but as it stands, they’re not. Only the treble side makes it to the apparent output, the bass side going nowhere.

I wonder if originally, both tone controls fed into V12, and V13 was the phase-splitter. Or V12, following both tone controls, fed one output directly and the other via V13, which did the inversion. The latter would explain why V13 has a cathode resistor bypass capacitor, assuming that it is original.

Is there any evidence that the tuner/control unit originally had two audio outputs? One would have come from the anode of V13, the other from either the cathode of V13, or perhaps more likely, the anode of V12.


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Old 22nd Nov 2019, 10:00 am   #95
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Default Re: Moreton Cheyney

The phase splitter is earlier in the circuit at V10 om.
If you look carefully at the full circuit you'll see one of the outputs (P2/4) but it's fed via a 240K resistor which seems mighty high and a 0.001uF.
I suspect the outputs from two 6J5 anodes should be combined at their capacitors C81/C74 (P2/3) unless somehow the two valves interact?
http://www.radiomuseum.co.uk/New/fulcct.jpg
The two outputs from the receiver match exactly when you examine the pair of 5-pin plugs & sockets (P1 and P2).. P1 is for power and P2 audio.
There's still some uncertainty around the cathode area of V10 because of the jumble of components hiding connections.
I revised the ground connection of C32 and this now almost makes sense but I still haven't found a ground return for V10 grid leak. There's a junction between two resistors R65/R76 that doesn't have a connection to it and the resistor values 15K & 1M do not look like their total was intended to be added together.
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Old 22nd Nov 2019, 9:53 pm   #96
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Default Re: Moreton Cheyney

Thanks for that clarification.

The output from the anode of V10 goes to two places. One is the output P2 pin 4. The other is via V11 cathode follower (non-inverting) to the second volume control, thence to the treble control and V13 (inverting) and on to the output P2 pin 3.

The outputs P2 pin 4 and P2 pin 3 are in antiphase, and so nominally at least suitable for feeding a push-pull output amplifier. But they are not necessarily “equal but opposite”, as one is unmodified in frequency response and fixed in level, whereas the other is modified in frequency response (according to the treble control setting) and level (according to the volume control setting).

Asymmetric inputs to a push-pull stage would I think, be very unusual. It was not unknown because it happens in orthogonal amplifiers (which idea I think post-dated the MC unit). But it is difficult to fathom why it might have been done that way. If it was the original form, then presumably MC had a reason for it, obscure as it may be.

It does seem highly likely that the output of V12 originally went to P2 pin 3. If not, then this output would have had diminished bass. The treble tone control network (C80, R14 and VR1) would act as a first order high-pass filter. (It would have to, to ensure that VR1 adjusted the treble level with but minor effect on the bass.)


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Old 22nd Nov 2019, 11:32 pm   #97
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I am still very sceptical of the ganged VR3/VR4 pots. The shaft is running at an angle and no longer fits in the intended slots. Given the hand-built nature, I guess this could have been done by Moreton Cheyney, but it would be worth checking the mounting plate to see if it has been added/moved. It also seems highly unlikely anyone would design it with the phase-splitter on a separate chassis to the power output.

This is a small manufacturer with a standard design who may have made modified 'specials' to order, and then it appears this one has had post manufacturing alterations. This leaves you with a dilemma, do you try to get it working with minimal number of 'fixes' in the hope of returning it to the state it left the 'factory', or try to work out the original standard circuit.

Is it possible to identify soldering that has occurred post manufacturing?

Last edited by PJL; 22nd Nov 2019 at 11:38 pm.
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Old 23rd Nov 2019, 11:27 am   #98
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Default Re: Moreton Cheyney

Synchrodyne & PJL
It is puzzling that the push-pull amplifier would be driven by totally different audio signals especially one via an 0.001uF in series with 240Kohm, although I guess the frequency response of the capacitor would be swamped somewhat by the resistor and might be used to provide a sort of loudness effect. It's an area in which I have no experience.
The two capacitors C81/C74 at the anodes of V12/V13 are exactly the same and a bit unusual so this tends to imply they were used for the same purpose and C81 at V12 should not therefore be grounded, but connected to C74.


The double gang pot shaft isn't off-line or if it is, within manufacturing tolerances. The wiring around the two pots looks a bit scruffy as does wiring in other areas. I suspect the last owner may have been chasing a problem and we're looking at the end results... failure!
Understandable, because faced with a misbehaving receiver and the lack of a circuit diagram the last user may have got into a real mess.

Thinking laterally.. there seems now to be not one but two 6B8 valves. One position was certainly occupied by a KTZ63 and this would cause no end of trouble. I also noted that KTZ63 and KTW63 valves were in the wrong positions. A fixed u valve in a position wired for a variable u valve would pose a problem.. maybe even severe distortion from overloading?
If the guy tackled the audio stages rather than RF stages he may have ended up in a muddle and of course a KTZ63 without the diodes would also result in audio problems. The resulting audio may have been developed not by 6B8 diodes, but by anode bend detection in the audio amplifier?

The plan is to work out the most likely original circuit.
The possibility is that there were three individuals. The original purchaser.
The guy that swapped to a pair of KT66s and swapped to some RS parts, and the guy that messed up the equipment when it started to go wrong.

I've begun to get the receiver going. I've determined the IF is almost certainly 465KHz and the Sprague condensers are in excellent shape.
It's going to take ages because I keep getting interupted by customers demanding my attention.. the usual stuff.. "there's a 93 year old stuck on the 3rd floor because the lift isn't working.. it's just a few bad relays" and "here's 3 faulty drive units.. please make one good one from the three"
I should retire and concentrate on important stuff.. my hobbies.
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Old 23rd Nov 2019, 4:32 pm   #99
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It would take very little to rewire the EF37's as a common cathode phase splitter and that may even be how it started off?
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Old 23rd Nov 2019, 5:47 pm   #100
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The only way to proceed with the amp/psu is to remove everything and refurbish the chassis so anything is possible. The wiring is a hotchpotch of wires, many with rubber insulation brittle and falling to bits, and oddments of plastic coverings, and soldering extremely bad with bits falling off.

The deal-breaker is the mains transformer. If it's suffered from damp and developed shorted turns the amp will be relegated to display only. I do have a spare KT66 p/p output transformer and I can maybe find a spare choke or two.

Before I do anything with this I'd iike to get the receiver going. Fortunately, much to my surprise, my valve collection yielded two new 6B8 valves.
I've already discovered alignment will be a pain because the tuning has only one setting... very very slow motion.
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