Defiant 1403 progress

[jim_beacon]

I thought that I'd detail the progress I make (or otherwise) on the forum.

As a start, I have managed to get the chassis out of the case - this is not as easy as you think! After removing the knobs, then the back (the controls exit through the card back), the carrying handle must be removed from teh top of the cabinet, as this doubles as the top chassis mount. You will also need to remove the cabinet bottom cover, and the cabinet earth screw. The final two chassis screws are Nylon screws which also hold the rear feet on (this is a live chassis set, so the chassis is bolted to the outer case with Nylon screws at the bottom, and the steel handle screws go into a piece of chipboard. The metal outer caes has layer of card around its interior for insulation, and is connected to the mains earth).

For preliminary checks, I tested the continuity of the heater chain (good ) and the line output transformer (also good ), as if the tube heater or the LOPT were faulty, there would be little point in carrying on.

I then disconnected the HT line from the smoothing network, and applied mains, via an isolation transformer and a Variac, in the hope of reforming the main electrolytic. After a while, bringing the voltage up slowly, the valves glowed, and the HT reached an unloaded level of around 290V, which seems about right, without the cap getting hot, so at the moment, it seems to be successful!

The rest of the set is full of wax caps, so before I re-connect the HT, a lot will have to be changed (several have obviously failed). Unfortunatly, to do this, the tube has to be removed, and this will involve removing the ion trap magnet - what is the procedure for setting this up afterwards?

As the work progesses, I'll keep you all informed

Jim.

[dazzlevision]

Hello,

Before you remove the ion trap magnet, make a note of its position and (if provided) the direction of the arrow stamped onto the magnet holder.

When refitting, place it the same way round (arrowhead or possibly some other form of marking) and "clock position". Don't tighten the clamp (if provided) fully - you need to be able to move it.

With the brightness kept as low as possible, rotate the ion trap and slide it back and forth until you get maximum brightnes. Do not use it to centre the picture (there is another adjustment for this).

Hope this helps,

Dazzlevision

[jim_beacon]

Dazzlevision,

thanks for that! I won't have chance to do anymore until Monday, so I'll take notes and dismantle it then.

Jim.

[jim_beacon]

Well, time for a quick update....

I've changed all the wax caps, plus the three cathode bypass electrolytics, and had a good look for other damaged bits!

I put the tube back into the chassis today, and gingerly powered it up. After a couple of minutes (slow to warm up...), I was rewarded with crackles from the speaker, and a whistling line oscillator.

However, there was no sign of raster on the screen. The heater of the U25 EHT rectifier is glowing, so I don't think there is mch wrong there, and the tube voltages seem about right, so I think I must have put the ion trap and centering magnets back in the wrong places.

I'll have another go tomorrow, and see what happens.

Jim.

[Ed_Dinning]

Hi Jim, do you have any EHT?
Try the old well insulated screwdriver test IF YOU KNOW HOW TO DO IT. If not leave well alone.
You should have a purple arc on the anode of the EHT rect (AC), a blue white thinner arc on the cathode (DC).

HTH Ed

[jim_beacon]

Well a quick check with the EHT probe showed lots of volts (14Kv!), so after adjusting the ion trap magnet properly, I got a ratser.

Coupling up the pattern generator provided a distorted set of bars, but locked in both vertical and horizontal. Tweaking the relevant controls produced a reasonable picture.

There are still a few problems......

1) The picture jitters - I suspect some ripple is getting in (I have 2V ripple on the HT line - is this excessive?)

2) The leading edges of white verticals are brighter than the rest - a scope on the CRT cathode shows ringing on the leading edges of the video waveform. Is this IF alignment, or later in the set?

3) There are odd sparkles from, I suspect corona discharge from somwehere (I can hear "pips" in the line osc frequency that corespond to the bright-up on the screen). I can't see anything, and am worried it may be internal to the transformer (the wax on it has melted at some point), is there any way of testing the transformer?

4) The interlace is terrible!

Otherwise, I'm very pleased witht the first results, the tube is bright, and appears to have plenty of life left, and I got pictures with remarkable ease!

Jim.

[jim_beacon]

Well, after a long gap, I've done a bit more.

The set now produces a good contrasty picture, and the sound is OK, however, the picture "balloons" in the vertical direction, in a pulsing manner. Has anybody got any ideas as to what might cause this? Looking at the frame synch waveform, the synch pulses are good, but the whole waveform seems to move up and down by around 0.5V over about 1 second - would this cause the variation in height?

Thanks

Jim.

[Kat Manton]

Hi Jim,

Not even looked at the schematic for this but...

...that sounds like mains hum beating with the field timebase, have a look at the supply rails.

Regards, Kat

[jim_beacon]

Originally Posted by Kat Manton

...that sounds like mains hum beating with the field timebase, have a look at the supply rails.

Thanks Kat,

I have a feeling you're right - I've previously found about 2V ripple on the HT, but I'm not sure what an acceptable value is, though thinking about it, as the synch pulse on the frame osc grid is about 7V, I think 2V of ripple in that area could cause a lot of trouble (especially as the frame synch pulse shaping diode - MR1 if anyone has the RTVS page handy - is biased from the HT rail).

Jim.

[Mike Phelan]

Jim
2v sounds like much too much!
In sets like this, the interlace tends to vary when you adjust the vertical hold within the lock range, but any 50Hz ripple on the HT will make it critical to lock and cause the beat that you have.
The video ringing/overshoot - did you cure it? Fine tuning will sometimes affect that.

[jim_beacon]

A little more progress (work keeps getting in the way).

I've cured the bloom on the picture - a new line ouput valve (30P4), the old one was gassy, and drawing too much current. I know also have the correct EHT, and a correct size and position picture.

Still a couple of faults, there is a lot of frame jitter - this may be down to HT ripple, but new caps didn't fix the ripple, there is something else causing that.

I also have a "binary" contrast control - depending on the position, I either get a reasonable picture, or a blank screen - there is no gradual change of contrast. The voltage on the wiper of the control (pot is connected between HT and earth) does not vary betwen the two extremes. It goes from 200V down to about 110V, and then back up to 200V - I suspect another gassy valve.......

Jim.

[jim_beacon]

Meant to say - a loose IF core was the cause of the video ringing - a quick tweak and a new rubber band cured it