Decca DR31. Line standards switch.

[Geordie McBoyne]

This topic is on the UKVRRR about my Decca DR101: https://www.vintage-radio.net/forum/show...p?t=147000

Introduced a year earlier the DR31 employs a similar chassis as the DR101 except for the method of actuating the line standard switch lever.
Both sets employ two smoothing chokes which also function as solenoids for the switch lever.
The DR101 circuit is straight forward enough but that of the DR31 does need some explanation.
Here my own explanation how the circuit possibly works:

"The other circuit is the one in the DR31. No short circuits across the chokes. Instead, that 0.25microfarad capacitor is connected across CH2 on 405 and on 625 across CH1.
So how does that work? Is it some kind of resonant circuit introducing a high energy pulse across the appropriate choke when the user changes line standards. Enough energy induced into a choke in order to move the switch lever."


Geordie McBoyne.

[Murphyv310]

The 0.25 cap is effectively shorted out when on either standard, on VHF you can go from 1 to 13 without it changing. When switching to UHF or back to VHF there is instantaneous damping of the current flow across either CH1 or CH2 dependent what way you go as the sustem switch is moved so the cap is allowed to charge until SW3 & 6 have completed the cycle and discharge the cap. Whenever switch 13 is open it is used to trigger an imbalance in the chokes and the stronger magnetic field on one choke will fire the system switch, then of course SW3 & 6 come into play to reverse the discharge so the system switch fires the correct way. Basically the cap is always shorted until you wish to change system.
Thats my take on it.
Perhaps not the best explanation but i'm sure you'll get the gist

[Murphyv310]

Forget my last post I'm talking wrong.
I see where I messed up but have to go.
Back in an hour or so.

[Geordie McBoyne]

Hi Trevor,
Info from the Trader sheet 1632/T248.From the Trader service sheet, a description on how the solenoids operate the switch lever:

"S3 and S6 form part of the changeover assembly, and accordingly they are b and a to indicate that they close on 625 and 405 respectively as are all the other system switches. It follows therefore that with the selector at a VHF channel, as shown in the diagram, S6a will be closed and S3b open.
The system switch assembly is operated by solenoids, forming a magnetic relay, which pulls the armature in opposing directions and the solenoid coils GH1 and CH2 are used as smoothing chokes on both systems, but one of them is bypassed during the actual changeover operation.
If the selector is turned from the VHF channels to the UHF stud, with S6a still closed, CH2 will be short-circuited, HT current will by-pass it, and it will cease to be energised. The "pull" on CH1 will the cause an armature linked to the system switch assembly to move over to the UHF position, closing all the "b" switches. It can be seen correspondingly that if the selector subsequently turned to any VHF channel S3b closed at the time, CH1 will be shot-circuited. It's "pull" will cease to hold the armature and as CH1 is now energised it will pull the armature in the opposite way and move the system switch to 405
The control switch unit is connected in series with S6a on the other. But when it is set to UHF S6a automatically opens and when it is set to a VHF channel S3b automatically opens.
As a result, although one choke is short-circuited during the changeover, both are energised as soon as changeover is effected and assist in smoothing the HT current. The armature remains passive while both chokes are energised; it only moves when the magnetic of one choke collapses."

No mention of the function of the capacitor C65.

Ill post up an extract from the circuit diagram as it appears in the Trader sheet.

Geordie McBoyne.

[Murphyv310]

Hi.
Thanks David for the explanation. I had just suffered a major PC breakdown and I wasn't really concentrating at the diagram.
The 0.25uf cap would seem to have the opposite charge applied to either choke on system changeover, I did think this was like a tuning cap in a conventional points and coil ignition system now I'm not so sure. The use of a wax cap and its normal ageing and leakage would lower its effectiveness anyway.
The crunch is how well it works in practice?

[ppppenguin]

The designers must have argued that a relatively big and expensive capacitor was essential. Otherwise it wouldn't have got past the accountants.

[Geordie McBoyne]

I'm having second thoughts about the theory of operation of the DR31 system switch.
Consider rotary switch SW13: When the VHF channel selector is turned the connection between the junction of the chokes and the sliders of SW3 and SW6 is well and truly broken. This would leave the capacitor C65 isolated until contact with either solenoid is established again when a VHF channel is selected.
So it follows is the 0.25microfarad capacitor an active component in the operation of the system switch?

Perhaps what we read in the Trader service sheet is the author's interpretation on how the circuit works.

C65 is now shown as 0.25uF and not 0.25mF. Can you imagine a 0.25millifarad capacitor in a TV set of 1963? My apologies for using the archaic term mF for microfarad.

Anyway, Decca abandoned that system switch circuit and replaced it with a simpler arrangement in which only one of the two solenoids is activated to move the armature.

Geordie McBoyne.

[Murphyv310]

That cap must give the solenoid a spike to "help" the switch to move as its polarity reverses at switch over.
Why not disconnect it and see what happens.

[Geordie McBoyne]

Hi Trevor,
             The system switch in my DR101 does need that "kick" so that switching between line standards can be achieved more efficiently.  But the solenoids are wired in to the circuit in a different manner.

System switch problems came back. OK changing from 625 to 405 but 405 to 625 had once again become unreliable.
In an attempt to improve the switching over between line standards a spring has been fitted between the switch lever and the front edge of the chassis.
Let's see how long this bodge repair lasts?

Comment from Pieter H  UKVRRR:

I think the issue with the DR31/101 construction is the lack of a spring construction, it is a symmetrical arrangement, which is very tricky. Normal coil/solenoid-operated relays always have some form of a spring that pulls it to a neutral position in the absence of current through the solenoid. That's clearly missing in this arrangement and it will thus exhibit some form of sticking, also because it has to overcome the mechanical friction forces of the large switch assembly.  
  I therefore still think, like I described in my earlier post, that the capacitor C65 that is temporarily put in parallel to the "target" solenoid is there to provide some resonance in an effort to overcome the "sticking".

The attachment shows the spring fitted to improve switch over from 625 to 405.

Geordie McBoyne