Trying to make a vision IF and Detector - fun!

[AdrianPH]

The tuner is the Philips AT7650 and was fitted many UK made TVs ....snipped....
Geordie McBoyne.

Yep! unfortunately it is a bit off, I would guess with age as some of the channels will not actually tune on frequency, but others are fine, if all were off by the same amount then I could have said drift in a common component, but not the way this is.  But it does work enough for this exercise.  The old Channel 4 is OK.

Jeffrey, yes that may be the way I have to go, I used the CRT previously as a scope, so was trying to stick to what I already had working.  But with a scope you do not need video input, now to try and source some higher voltage caps and potentiometers that can also handle the voltages and make up some form of tripler.

Adrian

[ppppenguin]

Many scopes have a Z axis input. In most, this is a simple AC coupled feed to the cathode or grid. Some may be more sophisticated, but you've still got to get the signal to sit on -ve EHT.

[AdrianPH]

My HP scope has Z in but unfortunately requires it to be negative going video and not positive.

I have done a change to the diagram adding a few resistors and a diode to provide a DC clamp on the video amplifier input.  The idea is to use the other half of the EB91, but it's physical location on my development makes that a bit difficult at the moment, the anode of the EF91 video amp sits at around 90 Volts with video in the positive direction and peak levels of up to 60 to 70 Volts available.

Time for a break from the soldering iron over the weekend, but thoughts still welcome.

Adrian

[Geordie McBoyne]

Absent in the vision IF amplifier in many 405 line line receivers was the 33.15Mc/s trap.
Sometimes called the adjacent channel rejector it also defined the limits of the IF amplifier at the carrier end of the response curve.
A 41.5Mc/s trap was sometimes used although mainly confined to dual-standard receivers.

Geordie McBoyne.

[AdrianPH]

I did think about that, but then as I am only going to drive it with a modulator I was wondering if required on this little project.
Many of the 1950's DIY circuits did not consider much filtering only having sound rejection. I guess back in the day there was only BBC so adjacent channel rejection was not considered.
On the circuit L2 and L5 are providing me with some sound IF rejection, not sure if enough yet. I could add others if my coil formers were big enough. I have some better ones now thanks to Ed, but trying to save them for a final build.

Adrian

[Geordie McBoyne]

The second generation post-war HMV and Marconi TV sets had TRF circuits, HMV models 1805 and 1806 and Marconi VRC52.  The design of the receivers differed from the previous range of models which were superhets with low IFs, most common models were the HMV 1804 and Marconi VT50.
The HMV 1806 was an expensive set and was considered well worth conversion to band 3 reception. In order to install the EMI fourteen position tuner the factory solution was to convert the TRF receiver to superhet operation.  The sound retuning was easy from 41.5Mc/s to 38Mc/s. To retune the vision TRF circuits from 45Mc/s to 34.5Mc/s small value capacitors were connected across the coils. Doing this did peak up the response somewhat.

Birmingham models were introduced in 1949 had superhet circuits with IFs of 34.5Mc/s vision and 38Mc/s sound, almost BREMA values.

Geordie McBoyne.

[AdrianPH]

Well I had a trip out today, 355 mile round trip on some Ebay collections, I am absolutely knackered found the weather grim this afternoon. Basically saving a Murphy TV chassis and some old radar tubes to see if I could use or play with.

Back to the project, I have built a tripler to add to the mains transformer, it outputs around 1KV, just awaiting some potentiometers to arrive for the focus and  brightness control, need to source a small 6.3 Volt transformer for the CRT heater  separate from the rest as that will be up at around 120 to 140 Volts

I did find one white screen tube, but I doubt I would be able to use it with such a low EHT.
   

Adrian

[peter scott]

That sounds far enough to buy the radar tubes in this lot that you posted on Facebook.

Was it those?

Peter

[AdrianPH]

No it was not them, it was in the opposite direction. I got 19 tubes for £38 from Ashbourne area and then the TV from Towcester, the Murphy I posted a link to on here.

Adrian

[peter scott]

£2 per tube! Can't get much cheaper than that. All radar or a mix of TV and radar?

Peter