BRC 1500 blank raster and weak audio. Help please

[yampy187]

Hello.

I have a 24" Ferguson 1500 chassis with a fault that's driving me nuts.

After replacing the EHT tray and the 148R dropper section. I now have a bright raster and some sound.

This set looks like Billy bodge job has worked on it quite often (poor soldering etc.)

I have also replaced a few capacitors around the LOPT stage.

I have gone through some stock faults. Changed C38 with regards to no sound or picture. No difference.

So I have a couple of questions. The first one is does anybody happen to have a scan of the 1500 service manual I could have a look at? As I'm currently working blind.

And would anyone know if their are modern substitutes for the BF196 /197/194 lockfit transistors? Would general purpose NPN types work?

Thanks in advance.

Jan

[Cobaltblue]

Hi Jan

The F in the part number suggests a high frequency part.

Most GP transistors will be C

You just need to look at the data sheet there were always non lokfit versions of these transistors.

Cheers

Mike T

[yampy187]

Hi. Thanks Mike. A valuable piece of information right there. Many thanks

[Duke_Nukem]

Be sure to check the HT rail for the IF section. Whilst getting a 1500 working, I resorted to a beefy zener across the supply after the second transistor popped.

TTFN,
Jon

[Lloyd 1985]

Hi, if no one else gets there first, I've got an original thorn BRC manual for the 1500. I'll scan it for you when I get home later. I've had a similar fault on mine when I got it, I was missing some volts to the tuner caused by a large resistor just under the hole for the CRT neck on the main board. Can't remember any numbers without looking at the diagram though!

Regards,
Lloyd

[Hybrid tellies]

If the heater dropper rectifier diode goes short circuit this will remove the IF LT supply giving a blank raster and no sound. The LT supply for the IF stage and tuner being fed off the end of the heater supply.
The idea was that if the heater dropper diode goes short circuit the customer would switch the set off before the valves and tube were damaged.

[yampy187]

Originally Posted by Lloyd 1985

Hi, if no one else gets there first, I've got an original thorn BRC manual for the 1500. I'll scan it for you when I get home later.

Thanks Lloyd. That would be a massive help!

Regards, Jan

[yampy187]

Originally Posted by Hybrid tellies

If the heater dropper rectifier diode goes short circuit this will remove the IF LT supply giving a blank raster and no sound. The LT supply for the IF stage and tuner being fed off the end of the heater supply.

Hi

The heater dropper diode seems to check out ok.

Many Thanks.

Jan

[yampy187]

Originally Posted by Duke_Nukem

Be sure to check the HT rail for the IF section.

Thanks Jon. Will check this when i get hold of a service manual.

many thanks.

jan

[jayceebee]

As Duke_Nukem says check the transistor HT supply, if it's excessive this was usually caused by the 317Ω section of the dropper going o/c. The most likely cause of your loss of signals if not man made is most probably due to either VT7 or VT8 being o/c.

It would be a good idea to the the voltage across the base and emitter of each of the transistors in the IF strip, anything wide of 0.6V usually means the transistor is faulty. Metal can types BF198 could be used as a replacement for a BF196 and BF199 for BF197 but I recommend obtaining the BF196/7 types for best performance. Another thought, misadjustment of the preset contrast pot in the emitter of VT8 can also be responsible for no signals. Set it to mid point if you're not sure, you can set it correctly as per the manual later.

Good luck,
John.

[yampy187]

Hi. The 317 ohm section is O/C.

Closest I've got was 2x 150 ohm 3w. Tacked that across and I now have good loud sound and snow on the screen.

Pattern generator tunes in perfect with a very good loud test tone and Crosshatch pattern.

For some reason nothing else will tune into it (VCR, sky box or PlayStation)? These sources all work on another TV.?

Many thanks

Jan

[jayceebee]

When the 317Ω goes o/c apart from high transistor HT you also lose base bias to VT8 the video driver hence the blank raster. For the tuning issue is the bar moving when changing channel or tuning? As these sets are around 45 years old now it's common for the plastic cams on the selectors to turn to dust. See here on how to deal with this problem.

If the bar is moving OK then it's possible the metal strips that earth the rotor of the tuning capacitor may need cleaning and retensioning of the fingers. Not difficult but needs to be done with care, use the very smallest amount of solder when refitting.

John.

[yampy187]

Hi

Yes the tuner has succumbed to the plastic rot. Also re-tensioning the spring clips seems to have worked. I can now tune in a VCR clearly. I just need to get some washers to rebuild the channel selector mechanism.

The tube was also very low and has responded well to a little tickle with a CRT tester.

Seems to have a very good picture now

Many thanks for all the help everyone

Jan

[jayceebee]

Hi Jan, that looks very good indeed. Great to see another Thorn saved, tough as old boots is the 1500 chassis.

Good work.
John.

[Hybrid tellies]

That does look very good especially for a 24" tube. So glad another 1500 has been saved.

[yampy187]

Hi.

Thanks mate. Not entirely sure what make of tube it has got as there's no label on it. The emission hasn't dropped off considering I boosted the it. It was really that flat that you couldn't see it without the lights off so I had nothing to lose really.

Regards. Jan

[Focus Diode]

Excellent results indeed. I recognise this model. There was also a 1400 version with the VHF turret tuner at the top above the Ferguson badge. Think that model was a 3670.

Great to see another example saved.

[Duke_Nukem]

Originally Posted by Focus Diode

I recognise this model.

Ditto, a 3810. I did have one which was a stand-in until (after many years looking) I managed to get it's smaller screen 3809.

Dropper was fine, but the IF HT was still just a tad too close for comfort and after replacing one of those BF transistors another would pop.

Tube was a replacement that - given it's "performance" - was probably a Mazda reject

TTFN,
Jon

[Welsh Anorak]

Hi
I have the almost identical 1400 version of this which gives a very good picture for a Mazda tube.
I'd use a bigger wattage for the dropper section, though. I might have a 'polo mint' or two that would do the trick unless you can find a NOS dropper.
PM me if you need one.
Glyn

[Lloyd 1985]

Hi Jan,

The set is looking good now!

I've just finished scanning the service manuals, quite a sizeable folder, so I'll drop you a PM.

If anyone else would like a copy, just drop me a PM too!

Regards,
Lloyd