No Green on Thorn 3000

[pastyboy]

Hi all any suggestions would be most appreciated.

I had this set working with a fair picture 2 weeks ago, whilst setting convergence up a nasty cap let go (it hissed and spilled c518 on the LTB set was still running fine). Now I have replaced it after a very kind user sent me one, there is now no green at all.

Set has not been moved from the bench

Emission on tester shows all guns good
A1 as measured on tube pins around 400v and the preset a1 varies the voltage
Drive as measured again on tube pins is around 100v per gun...

I have tapped and wiggled as usual esp the beam switches

Grid voltage as measured on the pins are 8v

What is rather worrying is if I swap the drive wires on the 3 pin plug we get red or blue bars if either are connected on the green output from the board but no green if its connected to red or blue

Hope its not an internal tube failure.... a63 200x it was fine and not been moved off bench although I had collected the set from a long distance but it was OK to start with.

I have scoped each gun and with a colour bar gen there seems a healthy waveform which disappears off the scope if I switch off the colour on the gen...

[HamishBoxer]

Check your A1 voltages.I have had the A1 switches go "leaky" in the past.Think there is a cap as well on each feed plus Spark Gaps fail on crt base panel.

[pastyboy]

thanks, yes the a1 switches are prone. All the a1's are around 400v on the tube pins themselves

I have swapped the drive wires around and the green drive cct produces red or blue if connected to that gun, might be worth trying swapping the red and greed a1 wires where they go onto the tube base but they read OK on the meter...

Stuart

[MALC SCOTT]

s/c the green cathode to deck, if bright green raster the crt should be ok, Malc.

[pastyboy]

thanks for you reply Malc, yes bright grn raster with fb lines

I have swapped the drives and also the rd and gn A1 leads on the tube base but still the "green" colour bars are missing and the a1 or drive lead on another gun causes the gun to produce its bar...

Its promising the tube may be OK, as I say I have popped it on the tester.

Not done any repairs as such for 25 years and I was only 20 then so very rusty.

I had also tapped the neck in case there was an intermittent connection

[Hybrid tellies]

Interesting fault. It looks like the voltages on all three guns are ok and the green gun proves ok when you short the green cathode to ground. So I take it you are left with a mauve or purple picture.
I would gently ease the tube base off and check the state of the pin/socket connections.

[HamishBoxer]

Just a thought the solid film res on the video panel used to fail or go dry jointed? Built in heatsink that one.

[FERNSEH]

In the event of failure of the thick film unit, this device can be replaced by six resistors. The PCB artwork is designed to permit this. Three 12Kohm 5watt collector load resistors, R229, R250 and R264. Three 56Kohm resistors, R249, R263 and R276.
Gun switch failure was common. Measure the voltage on the green output transistor and compare it with the red and blue OP transistors. If it is too high it's possible the green OP transistor is faulty. Another common fault that comes to mind, there is 2.2microfarad electrolytic capacitor in each of the video output the DC restoration feedback circuits. C227 in the green amplifier.

DFWB.

[pastyboy]

thanks guys for the replies, but what is baffling me is that if I put the cathode drive plug obviously not in correctly so that I am driving the green gun from the red or blue I get nothing but if I drive the red or blue guns from the green video output then I get the appropriate colour bar.

Also if I swap the a1 wires that go to the tube base over so I am driving the green gun with red a1 I still get nothing

I would think unless I am missing something so obvious the the 2 above steps would rule out video drive and a1 problems

I have had the base off and given it a very good look at, the voltage measurements were made at the tube pins themselves with the base on ie grid 8v a1 400v cathode 100v

tester shoed emissions all level but green comes alive when tube base cathode is decked..

[FERNSEH]

Reading through the service notes for the BRC 3000 the voltage at the collector of the green output transistor is 160V.
CRT grid bias adjustment: R450: Operate the Set White switch to remove the frame scan and turn off the beam switches. Set the AVOmeter to the 100 volt DC range (a lower range must not be used) and connect to the CRT grid, pins 3,7 and 12on the tube base. Adjust R450 for zero volts on the Meter.

It seems that the green cathode volts are on the low side. According to the circuit diagram 160 volts should be present on the collector of the green OP transistor VT212. Low cathode volts should result in increased brightness.
check the voltage on the cathodes of the red and blue guns.

The service manual informs us that the A1 voltage is adjustable between 400 to 900 volts. On the CRT base A1 red is pin 4 and A1 blue is pin13.
DFWB.

[pastyboy]

Thanks DFWB will try tom, oddly all the volts are very much the same on all 3 guns would I be confusing the issue by swapping drive and a1 wires ?

[HamishBoxer]

That is how we set them up with the set white switch.Turn the A1s up till the line could just be seen.

[pastyboy]

Hi all, I have had the video board out and clocked all the usual suspects, the drive transistor volts do seem low but all 3 are low 80 to 140v depending on contrast setting.

The grid bias was showing (on a digi though don't have an avo) as 12v setting up as fernseh suggested it made no difference, although with the SW on and beams off the collector volts were around 160v

Shorting the cathode pins gives a bright raster of about the same intensity on both red and green

A point to note is when the set white was operated on its own ie with beams on (I note the caution for burning the crt) a green line was produced ??

The brightness control doesn't have as much effect as I would expect so I will have a look there too.

A1 's will only go upto about 450v by advancing the presets on the conv pnl

I can't help thinking that having the LTB out and apart I have disturbed something but can't envisage what !

hmmmmmmmmm

[HamishBoxer]

The A1 diode and a cap on the line panel did fail on occasions.

Setting up with the Set White switch,you only turn up the A1s till a line can just be seen.Do each gun one at a time then turn off till you have done the last.Turn all back on and you should see a faint white line (RGB together).

[MALC SCOTT]

I would remove the beam switches and short them out. They gave problems 40 years ago. Also check the 1.5 ohm resistor on the beam current limiter pcb. Malc.

[pastyboy]

beam switches removed and shorted, checked the caps that hang off them, it does seem the a1 volts are low, checked the cap on the ltb that smoothes the supply. The most the a1 will go up to is 700 ish. Looking at the screen again it does seem to be a lack of brightness all over - it certainly wont white out so maybe the green is "hiding". The preset bright is right on one end (like that when I took the set on) volts across beam limiter r907 seem OK at 1.3 and the zener w906 reads at 4v The 59 and 30v rails are OK.

Will go back and look at the brightness cct and known faults

[Philips210]

Hi.

There's a very interesting and informative guide to fault finding on the Thorn 3000/3500 here http://www.radios-tv.co.uk/?page_id=288.

The section on Video Troubleshooting could be very relevant, it's about 3/4 of the way down the article.

Regards
Symon.

[FERNSEH]

From the 3000 set-up notes: Pre-set brightness R906. Turn Beam limiter R903 fully anticlockwise, brightness control mid position, colour and contrast to minimum. Set Avometer to 250V DC range and connect via a 15Kohm resistor to the Green cathode on CRT base (Pin 6 on tube base) Operate the the set white switch and note the meter reading, return the switch to normal and adjust the pre-set Brightness R906 for the same reading.

DFWB.

[Mikey405]

Hi Pastyboy.

Although I can't really offer much more than the others have suggested, there's quite a lot about missing colours on the 3000 in the old "J McCourt" TV faults manual. You can download a copy on the link below (choose selection 5) and the colour faults are on page 17.

http://www.oldtechnology.net/documentation.html

Thanks Pastyboy.

Kind regards.

From Mike.

[Philips210]

Hi Mike.

Thanks for the link to your documentation page. Unfortunately, none of the pages would open which I think is only due to a back slash after the word documents. I tried a forward slash and I was able to download the relevant zip file. I think each file is like this.

Great service information there and thanks again.

Regards
Symon