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4th Jul 2007, 10:07 pm | #1 |
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GEC 2028 Thats in fact a 2040
Hi all
Tonight I decided to have a quick look at a GEC 2028 that's been fitted with a single standard chassis. The cabinet of this set is dated 4 June 1967, the chassis is dated 1969. After a quick look around the chassis power was applied, I then had sound, but no picture. With the workshop lights out, I could just make out a faint line that kept jumping into a small picture (frame collapse); another better look around this time over a cup of coffee then revealed a loose pin in the base of the PL508. With the valve firmly fitted, time to power up again - full picture now but very dull. I left it for a few minutes then decided to turn up the first anode controls on the convergence board: they are up full to get the picture bright. As in picture 2, if they're put back in their original position the picture looks like picture 1 - the convergence is slightly out and there is a purple patch at the bottom right. My main fault for now is no colour; I don't own a service manual for this set but I've just found an old scrap of paper in one of my filing cabinets from a GEC service manual that says "no colour - check chroma amp TR318 and TR319 both type BF194". I'll do that over the next few days, but if any member can shed some light on the no colour fault I would be very pleased for their kind help. Just thought I would mention there's a leaked cap C140 on the IF board -would that cap cause no colour? If a member could have a quick look in their service manual and tell me the value of the cap; the printed value on it is badly faded away. Cheers guys. Regards Marcus......... Last edited by Mike Phelan; 19th Jun 2008 at 8:55 am. Reason: S&P |
4th Jul 2007, 10:27 pm | #2 | |
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Re: GEC 2028 Thats in fact a 2040
Quote:
R &TV Servicing 1971-1972 gives the value of C140 for the GEC 2040 as 50uF. I couldn't say if it would cause no colour, but I'm sure you're going to change it anyway.... It looks like you are going to have to clean the PCB as well. |
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4th Jul 2007, 10:45 pm | #3 |
Octode
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Re: GEC 2028 Thats in fact a 2040
First you need to override the Colour Killer. Connect a 39K resistor between the junction of R385 and R386 and terminal of PC16-4.
If this results in running colour check TR324 (BF194), TR325 (BF173), C331 (0.005uF- nearest value today is 0.0047uF), C332 (0.1uF), C333 (0.005uF), D307 (OA91), D308 (OA91) asnd TR327 (BC148). If there's still no colour R416 (18K) is the main suspect. I had an interesting no colour fault on a 2028 once. O/R the CK resulted in running colour. Eventually I discovered the cause was low EHT which stopped the bistable from working. I always try and set the EHT as low as possible to preserve the LOPT, yet the colour would go if set to below 23KV! Great photos! I'm surprised the chassis is marked 1969 as some early 2040s employed a solid state boost diode in place of the PY500A. Hope that's some help. Cheers, Brian PS: If it's a 2040 inside a 2028 cabinet the colour beacon can be made to work as originally intended. Here's the circuit! Last edited by Focus Diode; 4th Jul 2007 at 10:56 pm. Reason: Added comment on photo and beacon information |
7th Jul 2007, 10:54 pm | #4 |
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Re: GEC 2028 Thats in fact a 2040
Hi all
Cheers guys for the info especially Brian This chassis, Brian, I think, has been built up from a box of spares; all the boards have different years the timebase and the scan coils are dated 1973 the rest 1969, 1971. The first job on this set was to replace the leaky cap on the IF board and give it a good clean, then on to the decoder. On this board I found 2 badly leaking caps so I swapped them all. I've taken note of the caps I've swapped, but have forgotten the list. I'll post that on Sunday after giving the decoder board a good clean, I powered it up, but still the same so this time I decided after checking all the transistors it would be best to override the colour killer: the results are posted in my next reply. |
7th Jul 2007, 10:58 pm | #5 |
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Re: GEC 2028 Thats in fact a 2040
The picture with the colour kiiler off is just green and purple any help would be much appreciated
Regards Marcus........ |
7th Jul 2007, 11:01 pm | #6 |
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Re: GEC 2028 Thats in fact a 2040
Decoders have 2 things that stop them working as well as the Transistors.
1: 4.43 Mhz Crystal. Look and see if that's there. 2: A feed of 7.9 Khz from the LOPT. Trace it through. There appears to be no red - or "Negative Colour." Check the things above, then the diodes. Cheers, Steve P Just had a thought - is this set yours and if not, where did it come from? Try realigning the decoder. Details in the manual.
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8th Jul 2007, 8:19 am | #7 |
Dekatron
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Re: GEC 2028 Thats in fact a 2040
Steve
Marcus' 4.43 oscillator is definitely working. Marcus Slightly difficult to see from the picture (colour bars would be better) but you appear to have the ident signal reversed or bistable stopped, or no B-Y. The latter would not cause the killer to operate, so that might not be the fault, unless there are two faults. Can you see any "blinds" on what should be red or green parts? If so, the bistable has stopped, probably because the resistor feeding line pulses from the LOP stage has burnt up (sorry, no cct). That might be R416 that Brian mentions. If there is no 7.8khz ident, the killer will operate, and you should have a 50% chance that the colour will be correct every time you swap channels - does this happen?
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9th Jul 2007, 10:04 am | #8 |
Octode
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Re: GEC 2028 Thats in fact a 2040
As the grey scale was correct before overriding the colour killer it looks like the bistable is in the wrong phase. Check the tuning of L308 (7.8KHZ amp) & bistable phase control circuit.
Hope that helps, Cheers, Brian |
9th Jul 2007, 11:58 pm | #9 |
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Re: GEC 2028 Thats in fact a 2040
Hi Mike
No, I can't see any blinds on the red or green parts and the picture just stays the same if you change channels or tune in any new channels. I have had the GEC on for over 4 hours and there's no improvement from cold or when it's up to running temperature the picture remains the same. Today I tried to find my colour bar generator. It's doing a very good job of hiding so I connected up a dvd recorder to my old motorised digital sat and recorded these - hope these pictures give a clue as to were the fault lies; green and red reversed. Regards Marcus.......... |
10th Jul 2007, 7:50 am | #10 |
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Re: GEC 2028 Thats in fact a 2040
Hi Marcus
The colour bars do give a much better indication. It's looking like what Brian says; the bistable is running, but permanently phase-reversed. The fault is around this part or the 7.8KHz ident. If there was no ident signal, there would be a 50:50 chance of the colour being correct every time you changed channels. A much less likely cause is a fault in the burst phase detector so the reference osc locks in the wrong phase; this happened on the Pye 691s when a Suflex went O/C in the BPD.
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10th Jul 2007, 10:40 am | #11 |
Heptode
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Re: GEC 2028 Thats in fact a 2040
Another one for you...
Is the degauss circuit working correctly ? This can cause some amusing colour faults ! Regards Dave W
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10th Jul 2007, 2:06 pm | #12 |
Dekatron
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Re: GEC 2028 Thats in fact a 2040
Hi there
This is an odd one. As you say, there was no colour before over-riding the killer, so there is a fault (as opposed to a swapping of wires or a purity fault). But looking at your off-screen pictures the colours look reasonably good - as Mike says the bistable is obviously running, albeit in the wrong phase, so there probably is an ident fault. There is a picture of exactly your fault in an old book I have - 'Colour Television Picture Faults'. It agrees with Mike - to quote: "misalignment of a 7.8 kHz ident amp coil can also give rise to this effect due to phase shift of the ident signal". Could there be any physical damage to the coil? It's usually obvious if someone's been twiddling.... Of course you could always swap the green and blue wires....only joking! Glyn |
10th Jul 2007, 5:14 pm | #13 |
Dekatron
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Re: GEC 2028 Thats in fact a 2040
That bit of advice was common to all TV enthusiasts, and engineers. All sorts of things get done to sets in the interests of 'The Phantom'.
All I can think of here is to check the tuning of L308. Check parts around it and then see if it can be tuned to the correct peak. Are there two? Because there seem to be no Hanover Blinds and the picture always locks to the wrong phase, it is obviously a phase error due to something stupid done by somebody else. Check D313. Note the position and give L308 a good spin about while watching the screen. Cheers, Steve P
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If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...? |
10th Jul 2007, 10:09 pm | #14 |
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Re: GEC 2028 Thats in fact a 2040
Hi all
Thank you everyone for all the advice Today I found an old tv mag dated September 1975, that covers the GEC hybrid single standard chassis, and after reading this decided to change TR327 the DC amp for the burst detector and reference oscillator (also as Brian suggested) but this didn't make much difference; faces were not as purple as before more reddish purple. I then did what Steve, Brian and Mike suggested and turned L308 7.8khz amp 2 turns clockwise and BINGO a much better pic, but on the colour bars there's not much blue and the purple bar is more pink in colour. After reading the tv mag again I thought it would be a good idea to swap over the PCL84 colour difference valves and try the set again on colour bars but this made no difference. I then removed the output board to check the clamp triode load resistors R417 (RED) reads open circuit, R425 (GREEN) reads 16 MEG, R434 (BLUE) reads 21 MEG they all should be 8.2 MEG. I've no resistors to hand so the next time I post details of this set it will, I hope have a spot-on picture - fingers crossed Regards Marcus.......... |
10th Jul 2007, 10:18 pm | #15 |
Nonode
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Re: GEC 2028 Thats in fact a 2040
Hi Marcus.
Nice to see some progress with your set. Judging by the colour bars, it looks like R417 is actually okay - maybe a slip of the meter lead? As a bit of a check and given that I'm pretty imparient, I would swap the green and the blue ones over and you may well be quite pleased with the result although not perfect yet! Cheers for now,
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10th Jul 2007, 10:28 pm | #16 |
Nonode
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Re: GEC 2028 Thats in fact a 2040
Me again.
Having just looked at the circuit, you may want to try the low value resistor (100R I think) to the grid of the B-Y amplifier secttion of the PCL84. Also seeing as the phantom twiddler has been about, try turing up the B-Y drive pot near the BF184 B-Y pre amplifier transistor. Good luck matey.
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10th Jul 2007, 10:45 pm | #17 |
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Re: GEC 2028 Thats in fact a 2040
Hi Tas
Will try what you have suggested after I've replaced the clamp triode load resistors; thanks, Tas Regards Marcus....... Last edited by Mike Phelan; 19th Jun 2008 at 9:06 am. Reason: Clean up |