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Old 26th Aug 2017, 11:34 pm   #41
FERNSEH
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Default Re: And the next one! a GEC BT2147

The circuit diagram of the frame timebase. The sawtooth waveform is developed across C50 and is negative going so a leak in C52 would tend to over bias V17.
The frame linearity feed back capacitor should be checked or replaced. It is marked as C57 in the circuit diagram shown in post #36 and as C58 in the official GEC BT2147 and BT5144 circuit diagram.
DFWB.
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Old 28th Aug 2017, 3:13 pm   #42
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Default Re: And the next one! a GEC BT2147

Thanks Nick I've had a play with the values today and it seems quite happy with an 18R 5W resistor. HT just about right and no overheating. Andy - after a bit of head scratching I discovered that V14 N37 frame output valve has an internal short. I swapped it for V15 which also uses an N37 as the audio output, as I don't have a spare to hand. It doesn't seem to object so far but I will source a new pair. Still no luck with the frame collapse. been through every cap and resistor in the frame circuit and all were ok except R38 680k which had shot up to 1.7M. So I'm thinking valve trouble now. I did suspect the height slider but this seems to read fine when cold and voltages are quite normal when on. I don't appear to be getting the correct HT on V7 N77 or V8 B36 (now a 12SN7) The trader sheet says there should be 160v and 158v respectively but I'm getting around 80v at best. What am I missing here.
Thanks for the tuner info David makes very interesting reading - will be the next challenge no doubt if I get that far.

Thankyou all
Matt
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Old 28th Aug 2017, 7:19 pm   #43
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Default Re: And the next one! a GEC BT2147

Hi Matt,
Were you using a modern digital meter to take your measurments? The readings given in the trader sheets are measured on an AVO 8. The readings on the B36 maybe inconsistant since there is a large AC signal present which will be interpreted differently by different meters. An AVO might even stop the oscillator from running. I was not sure by the way which half of the B36 you were measuring...
However the voltage on V7 should be consistant with the readings from the trader sheet, so something is not right there.

Cheers
Andy
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Old 28th Aug 2017, 9:30 pm   #44
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Default Re: And the next one! a GEC BT2147

An advertisement from the September 1960 Practical Television magazine.
The plinth type Band 1/III converter for certain superhet band I only GEC TVs

DFWB.
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Old 28th Aug 2017, 9:39 pm   #45
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Default Re: And the next one! a GEC BT2147

Hi Matt,
Are you saying that a short in the frame o/p valve V15 is what caused the cathode resistor to fry? If so, are you sure that the primary of the frame o/p transformer is still intact?
Can you measure the anode, cathode and grid volts on both V15 and the frame osc, V8b ? If the oscillator is running, I would expect the grid of V15 to be slightly negative. It would be nice to see what was happening around V8b with a 'scope, but I'm guessing you don't have one to hand?
I presume you have checked the continuity of the frame osc transformer as well?
It would be nice to determine if the fault is in the osc or the output stage.
Good luck
Cheers
Nick
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Old 28th Aug 2017, 9:48 pm   #46
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Default Re: And the next one! a GEC BT2147

Wow! That's what it must have had fitted back in the day! I wonder if one still exists anywhere that would be quite a find. I Wish I could send off the 32/6 and have one delivered!!

Andy I'm using my modern meter, I've got a nice old AVO 8 which was my Dad's but now solely used as an ornament! I won't be able to do any more work on the GEC until the weekend now as the reluctant return to work looms tomorrow, but watch this space

Matt
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Old 28th Aug 2017, 10:00 pm   #47
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Default Re: And the next one! a GEC BT2147

Quote:
Originally Posted by 1100 man View Post
Are you saying that a short in the frame o/p valve V15 is what caused the cathode resistor to fry? If so, are you sure that the primary of the frame o/p transformer is still intact?
Hi Nick,

Thanks for your help

It was indeed a short in V15, to prove a point I swapped the pair again and switched on. The new 1.1k resistor went o/c instantly.
I haven't checked the transformer properly yet but will report back on this along with the other voltages. I don't have a scope sadly.

Is it likely that the frame transformer is now scrap? When it worked briefly there was a distorted raster but interestingly had a horizontal line on the screen also.

Matt
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Old 28th Aug 2017, 11:39 pm   #48
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Default Re: And the next one! a GEC BT2147

Hi Matt,
Worst case, assuming all the current went through the primary of the transformer via the anode of the shorted valve and through the 1.1K cathode resistor, If the HT is about 200V, then the current through the resistor and primary would be roughly 160ma. I would imagine the transformer would probably handle that OK for a short time so hopefully hasn't suffered. The poor resistor would be trying to dissipate some 30W or so and would fry instantly!!
Measure the primary resistance and make sure it's something sensible (probably 150-200 ohms or so) rather than dead short!
And throw that valve away before it destroys anything else!!
It's also worth disconnecting the frame coils and making sure there is continuity through them. While they are disconnected, measure the resistance of the frame transformer secondary- should be a couple of ohms or thereabouts.
Hope that helps,
Cheers
Nick
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Old 29th Aug 2017, 3:59 pm   #49
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Default Re: And the next one! a GEC BT2147

The transformer must have survived the first resistor failure for it to happen again!

The 80V on V8b anode sounds low enough to indicate the oscillator is not running. Do you have a scope? If not then put your meter on AC volts and connect via a 0.1uF capacitor to the oscillator anode.
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Old 29th Aug 2017, 4:17 pm   #50
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Default Re: And the next one! a GEC BT2147

The blocking oscillator valve V8B only conducts during the flyback stoke. As it's name "blocking" implies the valve is cut-off during scan period because of the negative charge on the ramp forming capacitor. It's most likely a low anode voltage will be the result of cessation of oscillation.

DFWB.
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Old 2nd Sep 2017, 11:29 am   #51
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Default Re: And the next one! a GEC BT2147

Bad news! the primary of the frame output transformer (T5 on trader sheet) is open circuit. With the transformer out of circuit I can't get any reading on the meter. The secondary is intact and measuring 17R.
So I'd say it's a pretty safe bet that this is the culprit. I'm off to search for something suitable to replace it with.

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Old 2nd Sep 2017, 3:26 pm   #52
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Default Re: And the next one! a GEC BT2147

I found a similar frame output transformer, can't remember what it was out of but the secondary resistance was close to the original. Fitted it temporarily but no improvements sadly. C50 0.02uf then went bang and shot red hot wax up my arm and the kitchen wall so I'm going to put this one on hold for the time being until I'm in a better frame of mind. Its the oldest set I've ever worked on and have to admit I am struggling with it. I've had two successful restorations this year but the last two have beaten me thus far. Thanks for your help everyone.
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Old 2nd Sep 2017, 5:29 pm   #53
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Default Re: And the next one! a GEC BT2147

Bring it with you Matt.......J.
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Old 18th Sep 2017, 7:41 pm   #54
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Default Re: And the next one! a GEC BT2147

A couple of weeks ago Matt brought over his Pye VT2 and a GEC BT2147 9" table model I will call the 'Bakelite Bitch'.

Matt was experiencing some problems with the VT2 and after a lot of testing and head scratching, it was returned to good working order and was signed off [we hope]

The second candidate proved much more challenging. the Bakelite Bitch had been stored if you could call it that in appalling damp conditions. At a guess I would say a simple garden shed or maybe an old leaky garage.

The chassis was corroded and it looked to be in a bad state. Matt had cleaned most of the grot from the chassis and successfully obtained first light by replacing the old selenium rectifier with a silicon diode and surge limiting resistor.
This gave a very good indication of the serviceability of the LOPT and CRT both in 100% condition. This was only the start of the epic as can be seen in the earlier posts.

Matt had done a good job recapping the line timebase but the original frame timebase was in a sorry state. It was decided to remove all tatty bits from repairs decades ago and rebuild the circuit as per the original GEC manual.
Matt discovered that the output transformer primary was O/C requiring a visit to a spider infested garage loft to see if I could find a transformer with a reasonable match. One particular one caught my eye from a Thorn 850 dual standard chassis 14 years it's junior, and it was certainly worth a try.
The frame timebase was completely rebuilt and tidied up. All solder splashes were removed and the H.T. was checked for shorts.

With a pair of EF91's plugged into the two vacant valve holders on the I.F. deck [to maintain heater continuity] the mains was applied. To my utter surprise what looked like a very promising shaped raster appeared on the tube face. It was out of focus slightly but with the seized focus magnet removed it was soon cleaned up and lubricated. It was now possible to focus and center the raster correctly.

It was now time to shove a signal down the I.F strip. Surprisingly the I.F. is 34/38 mc/s making it possible to wire in almost any tuner from the mid 1950's/60's. An old Cyldon Band 3 converter was dug up from the stores and given the once over. It had coil 'biscuits for channels 1 & 9 together with it's valves PCC84 and PCF80.

Power for the tuner was obtained from the I.F. unit. The heaters were wired to the B7G sockets originally intended for the external Band 3 tuner. H.T. was picked up from a tag strip and the I.F. output connected via a 1000pf capacitor to the I.F. input coil of the common sound and vision amplifier.

It was time to test the 'lash up' and upon powering up with the Cyldon switched to channel 1 I was presented with a test card. Both line and frame were off frequency due to useless slider presets. Temporary conventional pots were soon wired into place and after a few simple adjustments an incredibly linear picture was produced. That Thorn FOPT must be a perfect match!

The chassis is very stable and soak testing for over an hour proved that nothing drifted.

There is still a lot to do. I have not checked the sound but the circuit is so simple I might leave it to 'Interlace' my pet Orangutan to sort it if it's silent. He insists the tube needs an ion trap despite me pointing out that it is a straight gun triode with an aluminised screen hence no trap required.

The question is what to do about the lack of a front end? It might be possible to construct an RF stage followed by a mixer/oscillator using EF91 valves but for the time being I think the donor tuner will have to remain.

If anyone has one of these GEC receiver units either TRF or Superhet going spare I'm sure Matt will make very good use of it.

A replacement preset control panel will have to be constructed with a small sheet of Paxolin drilled to take conventional rotary pots.

Pictures show: Under chassis before start. Tuner lash up. Early raster. Picture on mirror. Dropper.
Regards, John.
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Old 18th Sep 2017, 8:22 pm   #55
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Default Re: And the next one! a GEC BT2147

HKS wrote: " If anyone has one of these GEC receiver units either TRF or Superhet going spare I'm sure Matt will make very good use of it" Or construct a plinth to accommodate a 35-38Mc/s tuner unit. The tuner unit will have to be small and best one for this application is the Pye incrementment tuner as fitted in late production VT4 models and the CW17.

DFWB.
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Old 18th Sep 2017, 9:57 pm   #56
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Default Re: And the next one! a GEC BT2147

John!! this is incredible stuff!!!!!!!!!!

Hat's off to you Sir, I am more than humbled by the work you've put into this! Can't stop looking at the pictures. Test card C looks magnificent and I can't wait to see it in the flesh!! That cat is going to get his backside roasted if he gets any closer to those Polo Mint droppers. Winston looks resplendent as he inspects the upper deck!

I'm quite happy with the donor Cyldon tuner if you don't mind parting with it. I'm sure I can construct a little enclosure for it to live in and attach it low down on the back out of sight. I expect another rf chassis or a scrap set will turn up eventually, or even one of the Pye tuners that David mentions, although I imagine this would be a super rare thing now.

Did you ever discover what was causing that weird dead short? either way it looks like it's no longer present!
Wonderful wonderful stuff John. I'm thrilled to bits. Give me a shout when you're ready and I'll come up one weekend and see you (already looking forward to it as I write this) The bakelite bitch will soon become a bakelite beauty!

The VT2 is still behaving itself and is playing old Pathe newsreels at the moment!

Best regards
Matt

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Old 21st Sep 2017, 5:33 pm   #57
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Default Re: And the next one! a GEC BT2147

This afternoon I spent a little time in my workshop. Having obtained such encouraging results with the Bakelite Bitch I thought it was time to reconstruct the old slider preset plate, replacing it with conventional rotary pots.

The original sliders made by a number of firms were always the source of intermittent faults and frame 'jitter'. The types employed on the GEC are manufactured by Welwyn, a quality company,one of the best available.
Almost 70 years have passed since this bank of four were produced, at least 50 years past their design life!

An adapter plate was made up from Paxolin sheet using the old mounting plate as a jig. I had to offset the pots slightly so as to allow room for refitting. This sounds simple but is a fiddly job but it's worth taking your time to reach a practical solution.

Three new NOS pots were exhumed from the stores together with a high quality Colvern wire wound for the line hold control.
I will fit this ASAP!

'Interlace' is still puzzled by the lack of an ion trap. He loves to twiddle them round to obtain maximum brightness. Another spanner in the works is that GEC did fit what looked like trap magnets to some of their triode tubes. These were adjusted to eliminate slight corner cutting that was apparent on a few of their straight gun tubes. I must dig out the GEC notes for him to read up on. He may have to take over when I retire. Ha ha!

The pictures show the rebuilt frame output stage together with the original [O/C primary] transformer. The Thorn 850 tx can be seen on the right, and the reconstructed preset panel. Regards, John.
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Old 21st Sep 2017, 8:03 pm   #58
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Default Re: And the next one! a GEC BT2147

Hi John,
that's an excellent solution to the problem of finding a replacement for those troublesome slider controls. The preset controls are fixed once and for all. I was very lucky with the BT5144 when one consideres the bashed up state of the chassis, the CRT survived and so did the slider preset controls.

DFWB.
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Old 22nd Sep 2017, 9:40 am   #59
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Default Re: And the next one! a GEC BT2147

It's the only practical solution David. It is possible to utilize the wire wound elements of the square 11w ceramic resistors but this little baby uses three 100k carbon sliders and a 20K wirewound.

To add difficulties the spring 'wipers' have rotted somewhat and have lost all tension.
You never know, if the stores manager of Welwyn Electric reads this he may just find a spare panel in a faded cardboard box on a dusty shelf..

Those days are over I'm afraid. Old 'Stan' the stores manager retired many years ago and all his treasures went with him.

I must admit, bringing these old receivers back to life is a little like walking in dead mans [and women's] shoes. Everyone involved with this receiver, built at the Coventry works of GEC is unlikely to be still with us. The design office, assembly line, testing, packing, transport and finally the agent that sold it to his very lucky customer, now just a memory as are most of the shops and the factory that built it!

Mentioning the assembly line. The construction is to a very high standard. All connections are wrapped around the tags, pinched and then soldered. It takes a while to unpick the slick work of those ladies on the line.

I'm busy this weekend but am looking forward to finalizing Matt's project. I will have a look through my tuners and find something more suitable that can be contained in a simple exterior box.

Just to add. Thanks for your kind comments Matt. To be honest there is little skill involved with a receiver of this type. Being able to follow a circuit diagram and component layout is 60% of the task.

I am very fortunate in having a warm comfortable period workshop very much like a workshop that would have been around in the 1950s. This certainly sets the right mood to tackle these projects. Also having a reasonable stock of vintage spares helps!
Thanks again.
Regards, John.
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Old 22nd Sep 2017, 12:22 pm   #60
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Default Re: And the next one! a GEC BT2147

I don't know about old 'Stan' but Welwyn are very much still in business. They were helpful a couple of years ago when I needed to specify some resistors capable of handling huge short-term overloads. The answer? The traditional green vitreous enamel wirewound resistors, still unsurpassed at handling abuse!

Chris
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