BRC 1500 television.

[chompy1]

thanks Mike. I will do some prodding around.

Paul

[Welsh Anorak]

Do check the wiring to the CRT base - this can become brittle and intermittent, also the connections to the brightness control and the soldering of the video output transistor.

[chompy1]

thanks Glyn. Whenever the raster comes into full brightness, it only lasts for 2 or 3 seconds.

Paul

[chompy1]

Hi Everyone,

I have checked the CRT base wiring, the connections to the brightness control pot and the soldering of the video output transistor. All is good. There doesn't seem to be any cracks on the board and EHT will come and go even when the board is secured in its proper position.

Paul

[Welsh Anorak]

So it's the EHT that comes and goes?

I'd hang a meter on the screen grid of the PL504 and see if the voltage vanishes when the fault is present. You could even rig up some sort of LED arrangement to catch it!

If it remains then off to the control grid and see if that goes. That's more likely IMHO as the voltage has to travel around the CRT opening and uses small tracks.

I'd be tempted to solder link wires as a temporary measure to see if you can isolate the missing voltage. Do check the wiring of the LOPT to the main PCB and double check the tripler is firmly attached to the LOPT.

[chompy1]

Hi Everyone,

I'm still having trouble finding the fault. Is it possible that one of the L.O.P.T coils is breaking down? When the fault is present, the screen grid voltage of the PL504 goes from -70 volts to -95 volts .

Thanks, Paul

[Welsh Anorak]

It would be a first if it is!

It is of course possible one of the leadout wires is fractured at the pin.

However I'm not exactly sure what the fault is. Are you saying the EHT vanishes completely or drops to a low level when the fault appears?

I assume it's the control grid of the PL that's increasing (well, strictly speaking decreasing)? That would suggest the valve is not working either at all or only slightly when the fault's present.

Check the HT to the screen grid and see if that increases or vanishes- that will be the key place to check.

Considering all the other faults you've had on this chassis I think there's certainly a crack somewhere.

[chompy1]

thanks Glyn. When the fault appears, the EHT will either drop to a low level or disappear completely. I've tried a new PL504 and a new PY801, but no difference. I will have another search for cracks.

Paul

[its ur aerial]

As Glyn says, never ever had a 1500 LOPT go, are you getting the grids of the PL mixed up ? the Screen Grid G2 should have HT on it, a common failure back in the day was its feed resister and the Control Grid G1, should have a -ve, ie the drive from the Line osc, cant recall the exact figure now, but -30v or if you can check it with a Scope and have a look.
Ken G6HZG.

[Welsh Anorak]

When the fault appears does the PL504 get hot or cool slightly?

If the screen grid voltage (HT) is dropping it may be due to the feed resistor (R127) as Ken says. This feeds pins 6 and 7 of the PL504 and should be around 225v. The screen grid voltage of -70v is correct. As the line osillator is fed from HT3 (derived from HT1) and that's working fine then the fault has to be in the output stage and can't be far away.
My money's on a crack on the route HT1 takes to the line stage.

[chompy1]

yes, I think I was getting the grids of the PL504 mixed up. I've now lost EHT completely. R127 is good and on pins 6 and 7 of the PL504 there is 185v. R124 keeps springing open and I think the PY801 is running much hotter than it should be. I've had another search for cracks on the board, but there doesn't seem to be any cracks.

Thanks, Paul

[frsimen]

That article in Television magazine linked to in post 5 above describes these exact symptoms. It suggests C95 (17/20") or C113(24") will be faulty. It's mounted directly on the pins of the line output transformer.

If it isn't that, check the boost capacitor, C92.

Paula

[Welsh Anorak]

A quick word of warning - be very careful replacing the capacitor on the LOPT. Best to snip and solder very quickly as the pin can become detached - the only cause of failure in a 1500 LOPT in my experience.

[Chris55000]

. . .I've heard of this and I've actually had a pin come out of a Thorn Jellypot LOPT myself – I seem to remember one of Thorn's Service Bulletins advised cutting external connections as close to the pin and (very quickly!) making an open loop in the wire or replacement component lead and soldering it to the pin/existing joint as quickly as possible!

Chris Williams

[chompy1]

I've replaced C95 in the way suggested and C92 is good. I've also checked the resistance of each LOPT coil - on my multimeter G to H reads 1.5 ohms, H to B reads 10 ohms, B to A reads 6 ohms, A to F reads 8 ohms and D to E reads 0.6 ohms. Could one of the coils be failing while under load?

Thanks, Paul

[Welsh Anorak]

I suppose anything's possible, but there's lots to rule out first.
Have you tried a spare rectifier tray?

[chompy1]

unfortunately, I don't have a spare 1500 rectifier tray, but I do have a spare 1400 rectifier tray which I know is good. Would that be worth trying?

Paul

[Welsh Anorak]

Yes, that would be fine - the picture width might be a bit out but good to test with.

[Tazman1966]

Hi Paul.

Provided it's the right number of rectifier sticks - either 3 for a doubler or 5 for a tripler, electrically it'll work. The only problem is whether it'll fit spatially and the length of the EHT lead. I cannot remember if 1400 ones will fit physically 1500 or vice versa.

[chompy1]

thanks guys. I've tried the 1400 tray, but no difference. Prior to R124 springing open, connection F is lifeless and I can't find what is causing R124 to spring open .

Paul