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Old 22nd Dec 2019, 12:15 pm   #1
Pyehead
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Default GEC BT5144 tv restoration

This might be a daft question but should this type of rectifier like in the attached read like a normal diode? This one is open circuit and looks burnt on the positive side. If so what would be an ideal substitute diode to replace it? I don't think I will find anything like this off the shelf

Thanks John
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Old 22nd Dec 2019, 1:29 pm   #2
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Default Re: GEC BT5144 tv rectifier

The replacement for the old metal rectifier will have to be a silicon diode and a series resistor of 25ohms - or higher if the HT voltage is too high.
I was lucky with my BT5144. The service replacement RM4 rectifier is OK.
Link to my BT5144 topic: https://vintage-radio.net/forum/showthread.php?t=138936

DFWB.
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Old 22nd Dec 2019, 3:37 pm   #3
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Default Re: GEC BT5144 tv rectifier

Thanks very much for the advice. Will try the 25ohm as you say first and see if the HT voltage is too high

Fronts looking a bit cracked and perished but still in one piece. Just hope the tube and line output transformer is ok.

John
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Old 22nd Dec 2019, 4:41 pm   #4
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Default Re: GEC BT5144 tv rectifier

Hi John,
I'm sure the CRT and line output transformer will be OK. But to be on the safe side it is a good idea to warm up the transformer windings. The favoured method is to pass a current of 20 to 30mA through the EHT overwind and the primary. Power source to top cap of the KT36 and the anode of the U37 EHT rectifier.

DFWB.
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Old 27th Dec 2019, 11:22 am   #5
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Default Re: GEC BT5144 tv rectifier

Hi All

Hope you had a great Christmas !!

I have my rectifier which I found on Ebay RM4 which i have fitted

Now all the caps have been replaced electrolytic and wax.

On switch on there was quite a flash and crack which at first I had thought was coming from the top of the rectifier. I now think it might have been the scanning coils underneath?

Anyway no fuses blew and the sound output came up fine receiving a strong signal tone from my standards converter.

Now I have no Line drive at all not even a faint whistle. Would anyone be able to send me a screen shot of the line drive/line output stage of this set. Or can anyone pint me in the right direction of a service sheet. I cant seem to find one on the internet for this set.

I have a horrible feeling the scanning coil winding might be o/c

Many Thanks

John
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Old 27th Dec 2019, 12:07 pm   #6
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Default Re: GEC BT5144 tv rectifier

There is a copy of the service data on your service CDs if you joined the BVWS no 976
If not you should be able to download a copy here, https://elektrotanya.com/gec_bt2147_.../download.html

Good luck with the restoration.
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Old 27th Dec 2019, 1:09 pm   #7
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Default Re: GEC BT5144 tv rectifier

Thanks for your help Guys so far
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Old 27th Dec 2019, 2:21 pm   #8
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Default Re: GEC BT5144 tv rectifier

Very unlikely that the scan coils are faulty. I'm surprised that the RM4 is good. They have not been manufactured since the early 1960's when the BY100 silicon rectifier became available.
You will have to check voltages to see where the breakdown has occurred. It's just old fashioned detective work with your meter [preferably an AVO 8] but as you are checking, at least for this basic fault, a low impedance circuit, your digital meter will suffice.
Scan of service booklet attached. A good time back I submitted a write up on the BT2147 which is the 9" Bakelite version. It should be in the search archive on this Forum.It may be useful. John.
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Old 27th Dec 2019, 2:57 pm   #9
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Default Re: GEC BT5144 tv rectifier

Something to keep you busy The voltage chart for the BT5144. Allow a good tolerance when taking readings. A 10 to 15% difference may apply to certain parts of the circuit but this may not indicate an actual fault. Regards, John.
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Old 27th Dec 2019, 3:58 pm   #10
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Default Re: GEC BT5144 tv rectifier

I found my line oscillator fault which turned out to be a wire broken off the cathode of the valve base of the line oscillator valve. Didn't look obvious however I must have disturbed it.
I now have a frame fault which I am looking into however the CRT looks really flat. I have run it for a while obviously with the brightness down so I don't burn the tube just to see if it recovers. It doesn't sadly.

Any equivalent tubes to the GEC one I could look out for and fit?
It looks like the 12 inch 6704A ?

Thanks

John
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Old 27th Dec 2019, 3:59 pm   #11
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Default Re: GEC BT5144 tv rectifier

Forgot to attach photo so far
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Old 27th Dec 2019, 4:33 pm   #12
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Default Re: GEC BT5144 tv rectifier

Don't be too hasty. The EHT is probably much reduced due to possibly a damp LOPT. A lot has been explained about this in various posts recently.
Get the raster correct first and you will probably obtain a picture. You can then assess the condition of the tube more accurately.
I recently switched on my BT5146 after a number of years at rest to discover the L63 frame blocking oscillator valve had expired giving of course a frame collapse. A new one soon had it working as new. A very basic scope and your meter should soon sort it out. Good luck with it. I very much like GEC receivers. John.
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Old 27th Dec 2019, 9:17 pm   #13
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Default Re: GEC BT5144 tv rectifier

Circuit diagram of the frame timbase. Repetition rate determined by C50 and the frame hold control R83 and R84. C54 is a decoupler. A leak in C52 will allow the negative going DC component to pass through and over bias the output valve.

DFWB.
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Old 28th Dec 2019, 11:57 am   #14
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Default Re: GEC BT5144 tv rectifier

David's frame timebase circuit should reveal the problem.
You mentioned that you had replaced ALL the capacitors other than those of silver mica construction. Be aware that you may have either made a mistake I.E. connecting a lead to an incorrect tag or fitting a component that is of incorrect value. It's all too easy to do!
The frame blocking oscillator should be ok with these particular chassis but a resistance check on both windings should prove this. John.
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Old 28th Dec 2019, 3:05 pm   #15
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Default Re: GEC BT5144 tv rectifier

Thanks a lot Guys

I will have another crack at it tomorrow. This will give me more to go on.

John
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Old 29th Dec 2019, 2:54 pm   #16
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Default Re: GEC BT5144 tv rectifier

Hi Guys

I have gone through the circuit in detail with the diagram. Everything seems to read ok. All caps I have checked are correct values and in the correct places. There are no old caps left in this stage now.

What I have noticed is if I adjust the frame height to max it as no effect except when I hold the actual control knob the frame does open to about and inch. I assume I am giving the frame output an input possibly?

I have checked the resistance on all the controls. They appear to read ok.

Voltage checks on the Frame output valve all appear correct however the B36 valve (V13) the service sheet says there should be no volts on the grids of this valve. I appear to have -60v on G1(pin 4) is this correct?

I am starting to think maybe the B36 valve is possibly faulty.

Thanks

John
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Old 30th Dec 2019, 12:03 am   #17
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Default Re: GEC BT5144 tv rectifier

What meter are you using, if a DVM it may be giving erroneous reading, use a moving coil type.
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Old 30th Dec 2019, 12:30 am   #18
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Default Re: GEC BT5144 tv rectifier

Quote:
Originally Posted by Pyehead View Post
What I have noticed is if I adjust the frame height to max it as no effect except when I hold the actual control knob the frame does open to about and inch. I assume I am giving the frame output an input possibly?
Hi John,
It sounds like the output stage is working. By holding the knob of the height control, you will introduce some hum into it. As it's directly connected to the output valve grid, you will see some deflection.

Either the oscillator (V13b) is not oscillating, or if it is, it's output is not getting as far as the height control.

My best guess would be an open circuit winding of the oscillator transformer connected to pin 4. Check to see if you get any resistance reading between the bottom end of the frame hold control and pin 4 of the valve.

What voltage do you get on the anode? That will also give further clues.

All the best
Nick
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Old 3rd Jan 2020, 6:41 pm   #19
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Default Re: GEC BT5144 tv restoration

Hi Guys

Thank you for all the advice. V13 the B36 valve now replaced and has given me full frame. Only issue I have is it must be locking at the wrong speed as I have two halves of the picture with a bar through the middle. it does lock at least
Going to check the resistors and the frame hold control next.
Wish I could show a picture however the CRT is very poor and can only really be viewed in the dark. There is no blooming etc so believe its not lack of EHT.
What is frustrating is the set found in a loft came with a brand new still in the box 9" tube. My set is a 12" sadly. I would have thought the owners may have purchased the wrong tube to fit in it!!
Below is a picture of the tube with it
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Old 3rd Jan 2020, 10:00 pm   #20
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Default Re: GEC BT5144 tv restoration

Refer to the circuit diagram on post No.13. Check the value of the 470Kohm resistor in series with the frame hold control. It's R64 in the diagram.

DFWB.
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