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Old 10th May 2020, 3:18 pm   #81
deepspacedaz
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Default Re: Fixing a GEC BT 330 Television

Hello, thank you for the great information I will get right on it today/ The book I'm learning from is this.(Television and radio repairing. by John Markus) 1953
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Old 10th May 2020, 3:56 pm   #82
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Default Re: Fixing a GEC BT 330 Television

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Originally Posted by deepspacedaz View Post
Hello, thank you for the great information I will get right on it today/ The book I'm learning from is this.(Television and radio repairing. by John Markus) 1953
Lots of good information in that book, remember though it was written for the USA market. Theory is the same but implementation can be quite different. Similar with the TV, the standard used in USA is different to the 405 UK system in quite a few ways, you need to be aware of the differences.

The Patchett series of books are much smaller and contain much relevant information about the circuits used in UK television sets.
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Old 10th May 2020, 4:31 pm   #83
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Default Re: Fixing a GEC BT 330 Television

Just a query. I cannot trace a GEC with the model number 330. Are you getting this confused with your Murphy V330? I have, or I thought I had all the GEC A5 size service manuals but the last manual is number 210 that covers the BT328 and BT329. The next manual covers the 19" and 23" BT336 series. I presume your sound amp/output valve is a PCL82. The circuits are all similar but the component numbers probably differ. John.
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Old 10th May 2020, 5:11 pm   #84
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Default Re: Fixing a GEC BT 330 Television

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Originally Posted by Heatercathodeshort View Post
Just a query. I cannot trace a GEC with the model number 330. Are you getting this confused with your Murphy V330? I have, or I thought I had all the GEC A5 size service manuals but the last manual is number 210 that covers the BT328 and BT329. The next manual covers the 19" and 23" BT336 series. I presume your sound amp/output valve is a PCL82. The circuits are all similar but the component numbers probably differ. John.
It may be difficult to be sure of the model number, as GEC used a small label (with the model number printed on it) stuck on the back cover - usually near the bottom. This will, more often than not, have fallen off at this stage in the set's life.

One way to tell is by looking at the EHT rectifier valve. In the first range of models (from BT302), it was a wired-in type (GEC's version of a Mazda U25), whereas in the later production models (starting at BT318 and using the same basic chassis), it is a 9-pin plug in valve (GEC's version of a Mazda U26), with a top cap.

If my memory is correct, later sets also used a 9-pin valve as the efficiency diode, rather than a (Mazda?) Octal 8-pin type.

I'm pretty sure I have all the GEC service manuals for the models of this range and they stopped at BT329. The next range (the last actually designed by GEC in Coventry) was from BT336 to BT342. These had either 19" or 23" squarer faceplate CRTs, whereas the BT302 to BT329 sets used 17" or 21" CRTs.

Last edited by dazzlevision; 10th May 2020 at 5:16 pm.
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Old 10th May 2020, 5:23 pm   #85
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Default Re: Fixing a GEC BT 330 Television

Yeah its wrong sorry its a GEC BT 302

It's on my photos

I have a Murphy 330 as well thats next to fix.
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Old 10th May 2020, 6:21 pm   #86
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Default Re: Fixing a GEC BT 330 Television

Forget the Murphy for now and concentrate on the GEC. The Murphy has a few pitfalls and is the last thing you need on your agenda.
The GEC comes first but you must follow the procedure. The BT302 is the easiest of the series. No FM radio to worry about. I have been working from the BT329 manual. There are a few differences but you really should be able to work out what is happening. This diagram should help lead the way.

Use the information with this diagram to follow the heater circuit. First check that the black mains lead is continuous through the neutral side of the switch to chassis. Then continue as I explained checking for continuity through the mains fuse 1.5amp to the live side of the mains switch S2. Carry on as I suggested to the dropper, through the dropper resistors to the thermistor bearing in mind that when cold it's resistance may be 4kohms. through the heater chain on the timebase panel [bottom] then on to the IF panel leading to the tuner and finally through the IF panel valves to the final item the CRT. Heater pins are 1 & 8.
O/C heaters usually occur in the first valves, the line output and boost diode but as mentioned the 30F5 valves do occasionally fail. As you can see it is a continuous loop from themains switch S2, dropper, thermistor, two panels of valves, the tuner and finally the CRT to the chassis.
Follow it through with care, take your time and if you get stuck, let us know. Regards, John.
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Old 10th May 2020, 8:57 pm   #87
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Default Re: Fixing a GEC BT 302 Television

Thanks for the diagram
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Old 10th May 2020, 9:17 pm   #88
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Default Re: Fixing a GEC BT 302 Television

Ok, I have continuity with black mains lead to chassis. I also have continuity with red wire tested with fuse each side of the fuse.
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Old 11th May 2020, 12:21 am   #89
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Default Re: Fixing a GEC BT 302 Television

Im having trouble finding R3 - R4
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Old 11th May 2020, 8:08 am   #90
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Default Re: Fixing a GEC BT 302 Television

I don't understand why you are looking for R3 R4?
The next step is to check for continuity through the live side of the on/off switch marked S2 on the GEC circuit [it's the combined double pole switch on the back of the volume control] and if OK move on to the center tag of the mains dropper at the junction of R79 66 ohms and R80 16 ohms. If you look at the circuit you will see the dropper with it's seven sections.
If you have continuity at this point check the remaining HEATER sections of the dropper, they are R79 66 ohms, R78 66 ohms, R77 33 ohms and R66 also 66 ohms. If you have contiuity move on to the thermistor shown as R75 checking you have continuity both sides of it bearing in mind what I mentioned earlier that it may read up to 4k [4000] ohms when cold.

Once you have established continuity to the thermistor you have completed the mains input side of the power supply. Lets get that far before bringing complications into the story. John.

PS the confusion over the resistor numbers was probably due to the wrong model number on your thread title. I was taking the data from the BT329 manual. Now we know it is a BT302, things may get easier.

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Old 11th May 2020, 9:39 am   #91
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Default Re: Fixing a GEC BT 302 Television

Ahh, I see ok thanks I look again.
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Old 11th May 2020, 12:33 pm   #92
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Default Re: Fixing a GEC BT 302 Television

Great! Check continuity to the dropper heater sections resistors, then on to the thermistor and let us know when you have done that. If done in small sections we just might get there.
So, you are checking continuity to the mains dropper, through the sections marked on the BT302 circuit [GEC book 203]as I have posted, R79 R78 R77 R76 not forgetting the thermistor shown as R75 I mentioned earlier as the burnt looking resistor. Do not continue further until you have confirmed continuity to both sides of the thermistor but if in doubt let us know.J.
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Old 11th May 2020, 1:03 pm   #93
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Default Re: Fixing a GEC BT 302 Television

Ok john thanks so much for this valuable information
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Old 11th May 2020, 3:55 pm   #94
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Default Re: Fixing a GEC BT 302 Television

I have a friend who has a valve tester taking them there now Im also buying some bits and bobs
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Old 11th May 2020, 4:32 pm   #95
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Default Re: Fixing a GEC BT 302 Television

DON'T fall into the habit of thinking that valves are likely to be faulty. It's a common misconception by beginners, probably not helped by all the old stories of changing valves to get things going. You MAY have a faulty valve or two but you can only tell by eventually getting power to the set and seeing what reults you get. It's also worth noting that valves that test low or poor on a valve tester can infact work perfectly OK in their circuit position. Valve testers don't always tell the full story! Remember that valves are a limited resource so no point in buying new ones just for the sake of it.
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Old 11th May 2020, 5:11 pm   #96
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Default Re: Fixing a GEC BT 302 Television

There is no need to do this but if you take them for check, test each one for O/C heater.
THAT IS THE PROBLEM WE ARE TRYING TO RECTIFY. Other than that they will be O.K. They will probably read low on the tester but valve testers are not a lot of use to be honest so don't take the results too seriously. John.
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Old 11th May 2020, 5:17 pm   #97
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Default Re: Fixing a GEC BT 302 Television

Just think of all the possibilities of putting valves back in the wrong sockets!
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Old 11th May 2020, 6:17 pm   #98
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Default Re: Fixing a GEC BT 302 Television

Hello, I've colour coordinated each valve so I know where they go. after the test, ill carry on with repairs. thanks again.
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Old 11th May 2020, 6:27 pm   #99
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Default Re: Fixing a GEC BT 302 Television

That is exactly what I thought but I was being kind. The other problem may be the 30F5 valves being used to working in say the sound IF but may not like to be a line oscillator.. We shall see. John.
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Old 11th May 2020, 8:57 pm   #100
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Default Re: Fixing a GEC BT 302 Television

Avoid using other RF pentodes than the correct GEC Z329 and Mazda 30F5 in the IF amplifier stages. Pin for pin compatible valves such as the EF80/Z719 and 6F23 have different inter-electrode capacitances which will have a bad effect on the alignment. OK to use plentiful EF80 type valves in the video amplifier, sync separator and line oscillator stages.

DFWB.
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