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Old 4th Jul 2018, 2:04 pm   #61
ms660
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Default Re: Decca DR101 dual standard TV

At the moment I'm thinking it might be about inertia and how far the moving contacts of the main system switch have moved over by the time the pull of the "opposition" solenoid starts to take its full effect, so maybe the charged capacitor dumps energy into the solenoid that was initially selected to pull in order to overcome that, eg: to make sure the system switch has moved fully over?

Lawrence.
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Old 4th Jul 2018, 2:28 pm   #62
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Default Re: Decca DR101 dual standard TV

What we need is sight of the Decca service manual for the DR31. I'm sure there will be a description how the system switch circuit operates.


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Old 4th Jul 2018, 4:12 pm   #63
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Default Re: Decca DR101 dual standard TV

The chassis is out from the cabinet and connected to the control panel so now it is possible to observe what's going on with the system switch. In fact the solenoids are working, when one is energised it is difficult to pull the armature away from the iron core. I think the best plan is to remove the solenoid assembly and examine it on the bench.

When the switch is in the 625 position, turning the channel selector to a 405 channel will most times result in the lever moving the standards switch to the 405 position, but not vice-versa.

DFWB.
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Old 4th Jul 2018, 5:25 pm   #64
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Default Re: Decca DR101 dual standard TV

News Flash! The system switch is working. Changing over between the two systems. Occasionally, it doesn't catch and the channel selector has to be turned a few times to select 625.

DFWB.
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Old 4th Jul 2018, 8:50 pm   #65
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Default Re: Decca DR101 dual standard TV

Fault: reduced picture height and cramping at the top. The 330ohm cathode bias resistor of the PCL85 frame output valve had gone high value, reading 418ohms. Replacing it has restored the frame timebase to working order, plenty height and perfect linearity.

DFWB.
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Old 6th Jul 2018, 6:38 pm   #66
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Default Re: Decca DR101 dual standard TV

System switch problems came back. OK changing from 625 to 405 but 405 to 625 had 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?


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Old 6th Jul 2018, 8:13 pm   #67
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Default Re: Decca DR101 dual standard TV

The DR404 that we had at home around 1965 would not default to 625. If switched on with 625 selected it would warm up in the 625 position. Likewise with 405. A distinct clunk would be heard as the system switch was pulled into place.

It did tend to stick after a couple of years and it was sluggish when changing. What an expensive way of doing it! Thorn got it right with their solenoid changeover 950/1400.
John.
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Old 6th Jul 2018, 8:53 pm   #68
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Default Re: Decca DR101 dual standard TV

The Decca DR1 series including the posh DR123 employ chassis assemblies similar in appearance to the BRC 850 dual standard chassis. The system switches in the DR1 receivers are actuated by Bowden cables, not very nice.

The twin solenoid system employed in the DR31 and DR101 models was not chosen by the designer for cost cutting reasons, in fact the DR31/101 receivers are rather over-engineered, just look at all the metalwork in the things. Way back in 1963 when the sets were designed the dual solenoids were probably considered a good idea

DFWB..
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Old 6th Jul 2018, 9:51 pm   #69
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Default Re: Decca DR101 dual standard TV

Purpose built solenoids yes David but that rather ingenious arrangement like other similar clever ideas often fail in the long run. Is the system switch itself 'tight'? John.
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Old 6th Jul 2018, 10:04 pm   #70
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Default Re: Decca DR101 dual standard TV

Hi John,
compared with other system switches it does takes a bit more effort to move between systems but there is far more switch sections than say, a BRC850..

DFWB

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Old 7th Jul 2018, 10:53 am   #71
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Default Re: Decca DR101 dual standard TV

I thin k 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".

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Old 12th Jul 2018, 11:55 am   #72
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Default Re: Decca DR101 dual standard TV

The chassis is installed in the cabinet. The system switch can now be considered totally reliable. The spring has done the trick. See post No.66.

DFWB.
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Old 12th Jul 2018, 12:16 pm   #73
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Default Re: Decca DR101 dual standard TV

Excellent work again David.

And lots of fun figuring out how it works and how to get it working again.

Jac
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Old 12th Jul 2018, 1:37 pm   #74
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Default Re: Decca DR101 dual standard TV

Hello Jac,
Despite the use of the very modest video amplifier valve, a PCF80 the contrast ratio is excellent on both line standards.
Decca addressed the TV system demodulator switching demands by employing separate diodes and associated components for each transmission standard.

The output from the demodulator is AC coupled and then DC restored and supplied to the grid of the PCF80 V9a. As the DC restorer is of the sync tip type this might result in slight differences in the black levels between 625 and 405, assuming that is the 405 sync height is 30% of peak white level and 625 is 25%.

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