RGD1800 Line Drive Question

[boxdoctor]

Hi- If you have low width, the E.H.T. will probably be low, and thus the electron beam is more easily deflected. This will result in excess height, and the frame circuit itself may be perfectly O.K. When you get the line circuit right, then have another look at the frame circuitry (if necessary).
Tony.

[IJK2008]

Hello

An update - the sound problem was a bad 6F13 valve found using voltage checks. When I swapped 6F13s between the sound and video circuits I got a reduced quality picture and sound appeared.

The other NOS valves I am waiting on have arrived today so by tomorrow I might have this all sorted (now there's a brave statement!)

Ian

[IJK2008]

NOS line output valve fitted and thyratrons courtesy of heatercathodeshort for which many thanks.

The volume won't reduce to zero so I need to check the control. The picture seems unbalanced - wider on the left hand extremity of the test card and has pronounced white line down that side. Still I'm getting there.

Any thought on where to look to fix up the test card display?

Cheers

Ian

[oldticktock]

Line linearity? possibly needs some work i think, if the control does not sort it then the components around line-linearity circuit should be checked, but i'm still learning so the gurus will know for sure, but looking better. I also wonder if line form is employed, I watch with interest

[Heatercathodeshort]

I am confused here. Have you tried the line linearity adjustment taps and pot? J

[IJK2008]

John

Yes I was being stupid - I did find the tapping points on the resistors today (by myself - honest!) and improved things.

Ian

[FERNSEH]

Hi Ian,
You mention that there is a white line down the left side of the picture. This has to be in the line linearity arrangement. The scan-coil damping is very crude, nothing more a variable resistor and a capacitor to by-pass the HF ring. The waste energy is dissipated as heat.

With regard to the line timebase. The drive waveform will be in a sawtooth form, and will be in the order of 60volts P - P. I doubt if the EL38 is pulse driven as in the Bush TV22. Also, bear in mind that a typical line timebase must operate upto 100Kc/s.

Here's an idea. The bypass capacitor across R59 is 0.02Mfd. Try increasing the value to 0.047Mfd, this will increase the gain of the line amplifier at the lower frequencies.
The width control sets the current though the EL38, which will be about 80 to 90Ma at max width. It follows with these "simple" line timbases that they can cause all sort of problems. One of the most difficult sets to sort out for me was the pre-war Pye 815 which employed a similar circuit as in your RGD.

DFWB.

[IJK2008]

Hello David

Thanks for your wisdom - I'll give it a try.

Unfortunately the volume pot is no good - I took it out tonight so further progress is hampered for the time being.

Regards

Ian

[FERNSEH]

Hi Ian,
The RGD 1800 is a decendant of the pre-war models 391 and 393.
The post-war model was the 2347T and was similar and employed the same valve line up. All Mazda octals.
That set was followed by the 2350 which used many Mazda B8A base valves instead of the Mazda octal types. The T41 gas thyratrons in the timebase were retained in this model. All beautifully over engineered sets.
The 1700 and 1800 models are similar to the 2350 except for the simplified power supply unit and the flyback EHT.

Visit the valvepage website to see pictures of the earlier RGD TV sets.

DFWB.

[murphyv310]

Indeed a closer look at the circuit reveals that the EL38 is running class "A", although the grid does bottom and go well negative in respect to the cathode.
I would agree with Fernseh here that these at first glance line stages that seem simple are in fact working on a limb and prayer and stretching the available technology of the time to the limit, any problem with scan coils, LOPT or off value R or C will cause irregularities that will show up as low width, EHT, or distortion of the scan.

[Heatercathodeshort]

I think we tend to go a bit deep with these simple circuits. We all do it and I still do it!
Most faults on these old receivers are usually basic breakdowns fortunately! The problems started with flywheel sync and complicated AGC configurations that take a bit of head scratching to work out what is going on.
Great work Ian and that looks a very nice black and white picture. I reckon you have done pretty well there. remember that modern components, capacitors usually have infinite resistance and tend to work better than the originals requiring adjustment/realignment.Regards, John.

[IJK2008]

Hello Again

Tried David's trick with the extra capacitance, replaced C33, fiddled with the line linearity tappings. Results as shown. Still not there yet - have a 'frame line' to the left as if the picture is not filling out properly. Any ideas?

Ian

[Heatercathodeshort]

You may well have replaced it but the H.T. metal rectifier may well be down giving low H.T. It can be replaced with a silicon diode and a 25-50 ohm 10 watt wire wound resisitor wired to the AC input. J.

[IJK2008]

Hello John

I had to fit a diode as the original rectifier was gone when I got the set. I had a 47 ohm in series and HT of 243 volts. The spec calls for 270 so I doubled up the 47 ohm with another to give about 24 ohms (is that a bit low) and pushed the HT up to about 262V. As commented by boxdoctor that has pushed out the width. The RF gain pot was found to be kaput and only working at the limit of its setting so I've changed that. Also the sound was lost again and that proved to be a bad connector pin in the IF valve base. Then I had a picture rolling all over the place but that was sorted by cleaning a socket in the timebase circuit. Result is the best test card yet! I think I'm nearly there. I'll play about a bit more with the settings and try to post another picture of the screen tomorrow. Thanks all for the invaluable assistance.

Ian

[Heatercathodeshort]

The surge limiter can go down to about 10 ohms with safety providing the H.T. voltage is not excesive. You may have to wire a 1000pf 400v A.C. working capacitor across the diode to prevent switch on spikes. These can destroy the diode at the instant of switch on due to the inductive nature of the primary of the mains transformer from which I believe it is fed. Fortunately the silicon diodes only cost pennies so not a financial disaster. Other than that it should be fine. J.

[IJK2008]

Hi All

Improved Test Card. I think I have taken this as far as I can with my limited experience. I'm going to do the cabinet next and I'll post a picture when it's all done.

Ian

[BGmidsUK]

That looks pretty good. And it's nice to see it set up without overscanning - I see so many sets with the periphery of the picture cut off!


Brian

[IJK2008]

After the holidays I have managed to do a bit more and its all back together.

Reducing the surge resistor boosted the available width.

I think other than cabinet tidying it is more or less there.

I think it's quite a classy set, apparently expensive in its day. I just need to find a place in the house to keep it!

Many thanks for all the help given - much appreciated.

Ian