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Old 12th Aug 2017, 9:19 pm   #21
matspar
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Default Re: And the next one! a GEC BT2147

Yes the 9" also has a flat faceplate David. Are these tubes meant to have an ion trap magnet? there isn't one fitted to mine.
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Old 12th Aug 2017, 9:29 pm   #22
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Default Re: And the next one! a GEC BT2147

Hi Matt,

Something wrong there. Has it had a turret tuner fitted (ITV modification)?

It seems that the first RF stage is mostly missing...
I used to have a spare brummie RF unit, I'm really not sur what bacame of it. It will probably be Christmas before I can have a good look for it. Maybe someone else has one to spare? Anyway, don't that stop you from continuing, you can still get the timebases working and check the tube.

Hi John,
I did have a go at a capacitor. This one had only faint markings on it unfortunately. I was surprised to find it actually had quite a strong cardbard tube hidden under all that treacle. It turned out to be one of the easier capacitors to restuff. You never know, I might end up putting them back in my set

Cheers
Andy
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Old 12th Aug 2017, 9:34 pm   #23
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Default Re: And the next one! a GEC BT2147

Hi Matt,
That's correct, no ion trap magnet on these tubes.

Cheers
Andy
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Old 12th Aug 2017, 9:43 pm   #24
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Default Re: And the next one! a GEC BT2147

Hi Andy,

That is weird! no sign or evidence of a turret tuner having ever been fitted. What on earth could have happened here then?!
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Old 12th Aug 2017, 9:48 pm   #25
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Default Re: And the next one! a GEC BT2147

Hmm,
Maybe the phantom bodger. There was a lot of it about.

It is worth having a look at top of the RF chassis to check if it has its full compliment of beehive trimmers. Anyway there seem to be tell tale marks where the aerial socket used to be...

Cheers
Andy
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Old 13th Aug 2017, 7:03 am   #26
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Default Re: And the next one! a GEC BT2147

The original TRF unit was replaced by an IF strip when ITA conversion took place. GEC supplied an external tuner together with a service exchange panel thus converting the receiver to a superhet.
They did not consider universal converters [Band 3 to Band 1 aerial converters] satisfactory. This made a very high quality and cheap conversion. I think GEC only charged around 2 for the exchange unit plus the cost of the tuner unit that was mounted in a Bakelite box. The original returned units would have been converted to an IF strip making the RF section [that was in the external converter] redundant.
This may well explain the removal of the RF amp in yours. The loose converter was probably lost some decades past. John.

Last edited by Heatercathodeshort; 13th Aug 2017 at 7:20 am. Reason: Added info.
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Old 13th Aug 2017, 8:23 am   #27
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Default Re: And the next one! a GEC BT2147

That would explain a lot John! may also explain the absence of the pair of N77's. Did this external tuner plug into the pair of valve bases by any chance? Think this is going to present a bit of a problem if I'm successful with the rest of the set
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Old 13th Aug 2017, 4:28 pm   #28
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Default Re: And the next one! a GEC BT2147

The standard GEC band 3 converter had two B7G adaptors that plugged into the RF position to gain power, heaters and H.T. and the second to feed the IF signal to the mixer now acting as an additional IF amp.

It's my guess that when GEC modified the RF plates, converting them to IF plates, they used a similar arrangement. This would have been very convenient as no modifications would have been needed to the heater ballast resistor.

I think the IF frequency is 34/38 mc/s so virtually any tuner from 1956 could be wired in. J.
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Old 14th Aug 2017, 4:14 pm   #29
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Default Re: And the next one! a GEC BT2147

Quote:
Originally Posted by D Cassidy View Post
Hello DFWB.
I also was watching your thread 're the "green" cabinet, I'd like to have seen how you would have tackled that one.
The "Green" cabinet has now reappeared on another forum if you wish to follow it's rebuild.
https://www.radios-tv.co.uk/forum/bl...ation-project/
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Old 19th Aug 2017, 5:44 pm   #30
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Default Re: And the next one! a GEC BT2147

A free afternoon so time for an update!

The lopt and the crt are both good!!!!!!

I've replaced all the missing valves and fitted a new EHT rectifier, and replaced 2 sections of the mains dropper which were o/c. One 100R and one 87R which form part of the heater chain.
Gently ran the old girl up on the variac. Plenty of dust soon started to burn off but the HT only got up to a dismal 67v. This was due to the metal rectifier which had reached retirement age.
I hung a new 1N4007 diode in its place and tried again, and after a while the lopt started to whistle and a raster appeared!! the tube is quite bright so it looks like fate was on my side.
Lots of nasty treacle toffee caps oozing everywhere so I didn't leave it on for long. These will all be replaced and I'll probably restuff them as Andy recommends. Should be fun!
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Old 19th Aug 2017, 7:15 pm   #31
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Default Re: And the next one! a GEC BT2147

I was just contemplating sending you a message saying 'does it work yet?!' Clearly, I don't need to now, because it does! That tube looks nice and bright, looks like you found a good set there!

Regards,
Lloyd
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Old 19th Aug 2017, 7:24 pm   #32
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Default Re: And the next one! a GEC BT2147

Yes I'm quite pleased about that Lloyd, for once I've been lucky!!! The frame has since collapsed but I'm not too concerned. I think it will come back to life with all the big waxies replaced.

Matt
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Old 20th Aug 2017, 11:11 pm   #33
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Default Re: And the next one! a GEC BT2147

Hi Matt,
That's great news.
It looks like a really good tube. The getter on the GEC CRT from my set looks like it was never even fired, so of course a bright purple glow at the back was all I got from it...

If we can't find a spare RF unit, it might be possible to copy the RF amp and osclillator/mixer circuit from a TV22!!
However I will have a good look for the RF unit I used to have when I next pop back to my parents' place. I doubt I still have it though.
I do have an EF80 type TV22 RF unit with a very rusty band III converter. The converter could be stripped and changed to a band I circuit, leaving the rest of the TV22 RF unit intact.

Cheers
Andy
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Old 21st Aug 2017, 7:31 am   #34
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Default Re: And the next one! a GEC BT2147

Any 34/38 mc/s tuner can very easily be wired into the IF plate. Just HT Heaters. Earth and IF coax are all that is required. John.
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Old 21st Aug 2017, 7:47 pm   #35
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Default Re: And the next one! a GEC BT2147

Thanks chaps looks like there are a few possibilities here then! ideally I'd like to return the GEC to original spec so if you do happen to find that RF unit Andy I'll happily buy it from you.

I fall down a bit of RF and IF stages so this is going to be a challenge for me. I don't think I have anything suitable here that I can use. John in theory could a typical mid/late fifties turret tuner be used on channel 1 if I can suss out the connections to the two B7G bases? I do think I'll need guidance on this

Matt
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Old 26th Aug 2017, 8:52 pm   #36
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Default Re: And the next one! a GEC BT2147

I only managed an hour on the old girl today, had too many interruptions. Not getting anywhere fast at the moment. I can't seem to correct the frame collapse. I've replaced all the dodgy looking caps nearby, been through most of the resistors. The height slider pot is ok, has volts either side of the control. The scan coils and frame oscillator tx all have continuity. I was stopped in my tracks by crackling and a plume of smoke from R73 (V15 cathode decoupler?) which collapsed the eht and blew one of the mains fuses. Looks like a short which I can't trace. With R73 disconnected I'm back where I was with the frame collapse. The other thing which doesn't seem right is the HT rectifier. I've fitted a 1N4007 diode but it won't let me fit a series resistor. I've tried a few between 100 and 300R but they just burn up on switch on. Can anyone advise what sort of wattage resistor do I need? I've only ever done this on record players before (successfully)
In all not a great session today but I'm not giving up yet!
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Old 26th Aug 2017, 9:32 pm   #37
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Default Re: And the next one! a GEC BT2147

Hi Matt,
I'm following your progress on this project with interest!! There are some interesting challenges especially, on the RF side of things.
With regard to the HT rectifier, I've just replaced the metal rectifier on my Pilot with a silicon diode. I did some digging around various threads and concluded that for TV's a series resistor in the order of 25 ohms or so is about right. 100 -300R is more suited to the lower current stuff like radio's etc so will cook with the HT current the TV will take.
I used a BY133 rather than an 1N4007, partly because I had some and partly because they are more TV related. The BY127 was used in many 70's sets. The BY133 looks like it has much the same spec and works fine with a 25R series resistor, the HT is spot on. It also seemed to be good practice to put a 1000pf 1000V cap in parallel with the diode to reduce the spikes produced when the diode turns off.
Hope that helps
Cheers
Nick
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Old 26th Aug 2017, 9:35 pm   #38
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Default Re: And the next one! a GEC BT2147

Hi Matt,
R73 frying is a sign that the frame output valve is overdriven with DC.
This could be due to resistive leakage through C55 or C57, the height control pot wiper not making contact or R71 being open circuit. Less likely is that the frame output valve has an internal short. Another posibilty is a short causing a low resistance in the primary of the frame output transformer, but this is also unlikely.

Cheers
Andy
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Old 26th Aug 2017, 9:50 pm   #39
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Default Re: And the next one! a GEC BT2147

From the pages of the 1955/56 Radio and Television servicing book, 371 to 374.
A survey of the various adaptors available to facilitate the reception of Band III transmissions. For the TRF models such as the BT2147 and BT5144 a special Band III adaptor was made. The Band III signals are converted to the BBC frequency and Band I BBC signals pass though the new tuner without the intervention of the local oscillator.
Converted superhet receivers use a three position Band I/III tuner which is similar to the one fitted in the BT1746 series.

DFWB.
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Old 26th Aug 2017, 9:53 pm   #40
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Default Re: And the next one! a GEC BT2147

Hi Matt,
I've just measured the HT current in the Pilot which was 440ma with the set displaying a normal picture.
This makes the dissipation in the 25ohm resistor 4.84W.
You will probably have to tweak the value of the series resistor to get the correct HT for your set.
Cheers
Nick
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