Homebrew telly - good news

[renato_enca]

That cable grounded the HT to the LOP stage (the most powerful of a TV set) by melting by making contact to the wirewound resistor that drops the HT voltage to that stage (320VDC to 240VDC). Due to the large drop and the current that passes through it, it got very hot and melted the insulating layer of "rubber" of the cable, exposing the conducting wire of the cable. Eventually that wire made contact with the wire of the resistor by flashover, effectively shorting the 240VDC supply to ground. The resistor acted as a fuse and blew right away (no, the set hasn't got any fuse...I need to complete the circuit to measure the power consumption and calculate the safety margin of the fuse), preventing anything more catastrophic.

I'm not working with minimum resources, but as the project grew up, the number of wires travelling back and forth increased exponencially. May be due to the reason that I builded the set's circuitry by modules: one for the LOP stage, one for the frame stage, one for the video and IF amp and so on....there are wires that carry the sync signals, 600VDC to the CRT's G2, etc...

[matthewhouse]

Hello, I am absolutely amazed by how far you have got with this project, especially as you are only 17! I'm very very impressed! I look forward to seeing the finished result, you should be very proud.

[renato_enca]

Thank you Matthew! This project really looks insane, in fact the results impress me, as I weren't even waiting to get a nice picture. The scanning coils, for example, were picked from a scrap (albeit being almost new) Sony set, which technology doesn´t have anything similar to my project. I soon felt dismotivated by just thinking about the terrible mismatch between the pl36 scanning circuit and that scanning coil, I've even started to study the workings of the line output stage (with help of this excellent forum, of course) and ended up winding a LOPT by hand. Yes, eventually my LOPT worked, but would last only a couple of minutes due to breakdown between the insulating layers. So I quitted, until a pack of 4 old (but unused, so they were new!) LOPT's from a VC208 chassis were kindly given to me by a friend of mine, which clicked me to re-attack this thing again...and I came to this point.

Now...I thing I need your help again: The picture always displays some bars running vertically and they are stationary. I first thought they were Barkhausen oscillations, but my efforts to solve the problem failed. I will post some photos for you to see...they're really annoying.

[renato_enca]

Hmm...bad news: while making a final inspection in the dark to make sure there was no valve anode glowing red, I discovered that the PCL805 anode was in fact glowing. That may have also explained the slight reduction on height over time as the set warms up and gets going. In fact, I had to adjust the height pot between every 30 minutes to correct the slight height decrease. Of course, when I turn of the set and let the valves cool down, when I start it up again the height is too high.
After a close inspection of the valve, when I shake it next to my ear, I can hear something loose inside it, but can't figure out what is. Also it appears to have some evaporation marks next to the anode holes, "printed" on the valve's glass. I'm starting to think this was a used (and faulty) tube, explaining my dificulties in the frame stage when I first builded it. I will see if I can buy a new PCL805 next month. Is there anything I can do to get going with this valve or should I dump it for good and see if I can buy one? They're very expensive here in Lisbon (about 11 euros, and I haven't got no money ), but the shop's stock is still plenty of them.

[dave walsh]

Keep going renato. I don't understand most of this but I can see that it is original! You might even get valve donations. Dave W

[renato_enca]

Hi Gents!

The set is now almost complete. Although the aluminium foil used to make the chassis was inpressingly cheap, manufacturing the actual chassis took me almost 2 weeks! Mainly because I haven't any craftmanship skills concerning to the metalwork...
Anyway, I could afford what I've expected from my sketches about the chassis: modules and parts positioning, heat distribution, etc...all things that warm up like a boiler (valves and high power resistors) are spread throughout the chassis.
The electrical connections were, now that I've got the things simplified, easy stuff. One point of view: I'm using the live chassis technique...may sound a blasphemy in terms of safety, but as I use an insulating transformer, everything is all right...when the set comes up to real use, I will identify (mark with a pen) the live pole of the mains plug, in order to maintain safety (and to don't blow a fuse, hopefully).

Here are some photos.

PS: The EHT cable became dislodged from his socket during some stage of construction. It has been fixed as seen in the last picture. The coil in series to the line coupling capacitor, above the scanning coils, has been also removed. It was an effort of mine to reduce the width.

[Colin]

Amazing!

Very well done indeed ... but be careful

[matthewhouse]

Fantastic! Do you have any photos of the picture?

[Tazman1966]

Hi Renato.

This is facinating stuff! Well done to you.

Cheers,

[renato_enca]

Thank you for the comments!
I've had to finish the tuner circuit before I could take some photos of the set working. Now that the set is almost complete, here they are. Notice that there is cramping on the upper side of the picture, which I suspect that the PCL805 is playing around...it seems used and also seems so have some loose parts inside; also the parts nearby the valve that comprise the frame output stage are OK.

[BGmidsUK]

Hi, what was the cause of the vertical bars you were having trouble with? I've seen it on other sets too .. by the looks of the latest pictures you seem to have cured it



BG

[renato_enca]

Sorry for the delay. The vertical bars were indeed ringing in the otput stage. Doing some severe decoupling on the RF and video stages attenuated the problem a bit, but the bars continue to show up sometimes when the set is mistuned or in low contrast scenes. Anyone know what may I do to stop the ringing?

[Top Cap]

CRT Aquadag earthing?
Ah! and who remembers wrapping an earthed tinned copper wire ring around the line output valve..happy memories.
Les

[Jimmyhaflinger]

great! wish i could be able to make my own telly like you did... i have plenty of parts, LOPTs, CRTs and valves, but i just lack the necessary skills (and i'm quite lazy too)... well, at best i could try to build a cabinet around a working salvaged chassis

[renato_enca]

Hello folks!
The school has begun two months ago, making the progress on this set even more slow. But in the last weekend I was able to get a new PCL805, repairing the cramped picture problem, and now it can give a very solid picture even after several hours of work. The line timebase drift was also been sorted: as the set warmed up, the line oscillator would drift so much that the set would lose is H.Sync at anytime. I will now post some photos of the chassis of the set while working, and only God knows when I will be able to make a cabinet for it...
BTW, many thanks for your help throughout those many hours of work! Without You, I wouldn't be able to do this project!

PS: It is amasing how the PCL805 can have so many ways to fail...is it really pushed to his limits?

[Steve_P]

Depends on the set.

Well done!! What more can I say.

Cheers,

Steve P.

[RickIvo]

Hello Renato,

I am an electrical engineer from Porto, Portugal working in the after sales area of consumer electronics. Just want to congratulate you regarding your fantastic and didactic project “Homebrew telly”.

Cumprimentos (Best Regards) ,
Ricardo Faro.
Porto - Portugal

[Ed_Dinning]

Hi Renato, congratulations on a great result.
The PCL85 and the "improved" PCL805 were well known in valve days as having short lives and causing all sorts of linearity problems. It was worked pretty hard.

Ed

[batterymaker1]

I am so impressed.

How many people can say they've built their own television--from scratch?!

[dominicbeesley]

Well, done, excellent picture now you've got it all sorted!

Dom