Bush Dual Standard Colour Restoration - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Welsh Anorak · 8th Nov 2021, 10:48 am

As far as I know the tuners were more or less identical.
A local private rental company used to swap out the complete timebase unit in the field which wouldn't have been too difficult - I was given a lot of dead Mk3s timebases for parts in the late Seventies - obviously the TVs had mostly been scrapped by then and the Mk1 versions had been seen off long before for obvious reasons!

Niechcial,Steve · 17th Nov 2021, 7:25 am

I've now been through the IF, decoder and colour/video output stages checking/replacing electrolytics and high value/high power resistors, quite a few of which were out of tolerance.
The result is that normal frame sync has returned, the luminance smears have gone, but verticals remain bent as does a colour fault whereby the top and bottom halves of the picture have a different tint (it's not purity- which is perfect). I will now get the scope out and do some fault finding

Niechcial,Steve · 1st Dec 2021, 3:04 pm

I have now come back to this for some detailed fault finding. Just to summarise, scan, EHT regulation, convergence and sync faults are sorted, and electros blanket tested and replaced as necessary throughout.

I next tried setting up the grey scale and found that despite having the tube show near perfect on the B and K tester, the grey scale would not track over the usual brightness range. The problem was no clamping pulse on the three PCL84 colour difference amplifiers. Diode 5D12 on the convergence board, which feeds line pulses in, had gone o/c. Replacing that resulted in a very good definition black and white picture with correct grey scale

I am now left with three faults:
1) Bizarrely wrong colours on the colour bars.
2) Smearing/overshoot on the left hand horizontal lines of the test card into the grey area of the circle beyond.
3) Green tinted broad horizontal band over the middle (black and white part) of a test card.
Taking these in reverse order, shorting out the U and V inputs to the colour output valves confirms that the green tint is coming in mainly on the V (R-Y) channel from the decoder.
I am not sure whether the smearing/overshoot is coming is on the luminance output stage (which is largely rebuilt), or coming in from the IF panel. I don’t suspect alignment, as the picture is otherwise good definition . I will feed in a video signal from another source into the output stage to test.
The wrong colours are the most interesting fault. For the colour killer to be switching the colour in (which it is), not only does the chroma derived bias have to be present but so do bistable pulses. The ident which syncs the bistable is there looking exactly as it should on the scope, but I guess there is the possibility of some strange phase error in there somewhere. I will start by viewing the individual blue and red channels on the screen to see if both are contributing to wrong colours. If so, I will start by checking out the sync of the reference oscillator and take it from there. I suspect and hope that the green tint will vanish once I have sorted out the more major decoder fault.

Niechcial,Steve · 10th Dec 2021, 9:20 pm

I've had a great chase on the colour faults due to two faults being present at the same time.
The first was due to the reference osc locking out of phase due to a mis-aligned burst channel- after having replaced the o/c diode mentioned above which feeds in line pulses to the burst gating circuit. Incidentally, smearing on luminance has corrected itself. Pulses from the burst gate are also fed into the brightness circuit, and I'm thinking the pulse problem may have affected the luminance as well as the chrominance. The burst gating is a bit weird in that the chrominance is sent to the first burst gate after passing through the colour (saturation) controls. So effectively, the burst amplitude will vary with saturation as set by the user. Not very sensible really.
Once I had got a good burst lock with correct primary red and blue, I had the cyan and yellow colour bars a bright green. Getting the colours right meant twiddling the b-y gain to be much lower than specified by the manual. So there is obviously gain misbalance for some reason in the colour difference output stages. That in itself is puzzling as they are largely rebuilt (including new valves). I guess I may have made an error in one of the resistors or somrthing. I will explore further

toshiba tony · 10th Dec 2021, 9:30 pm

I remember the logo "colour lock" wording on the sets. What exactly was locked? And were the decoders reliable, I came into the trade mid G6 days and worked on them a lot but don't remember many decoder faults, just psu and timebase problems. Thanks anyone.

Niechcial,Steve · 10th Dec 2021, 9:44 pm

I guess first generation discrete component decoders were always among the most reliable parts of a colour set, though the G6 and the Bairdes were amongst the few that had valves in their decoders with the usual valve attendant problems. I remember the odd rogue crystal or varicap diode, but not much else. When they get to this age it's often high power carbon resistors changed value or o/c transistors or detector diodes. Some of the early colour decoder ICs seemed to give more problems than discrete component jobs.

Graham G3ZVT · 10th Dec 2021, 10:44 pm

Quote:

Originally Posted by toshiba tony

I remember the logo "colour lock" wording on the sets. What exactly was locked? And were the decoders reliable, I came into the trade mid G6 days and worked on them a lot but don't remember many decoder faults, just psu and time-base problems. Thanks anyone.

The subcarrier crystal oscillator was phase-locked to the transmitted burst. No more and no less than every other PAL or NTSC set on the market. God bless the Marketing departments.

Niechcial,Steve · 10th Dec 2021, 11:41 pm

Yep that about sums it up, though some sets did have circuitry which claimed to compensate automatically for greyscale drift with ageing

Niechcial,Steve · 15th Dec 2021, 4:19 pm

Having aligned the decoder as per the book from beginning to end, I now have a good colour picture with all voltages as per the manual. I have one fault left which is causing me to pull my hair out. The fault is a horizontal band of slight tinting of magenta or green which moves up or down the picture with picture content. It looks very much like a decoupling or earthing fault but I'm running out of suspects. Anyone have any ideas?

FERNSEH · 16th Dec 2021, 12:56 pm

Or possibly the reference level clamps in the three colour difference amplifiers?
The triode sections of the PCL84s.

DFWB.

Welsh Anorak · 16th Dec 2021, 1:55 pm

Could it be H/K leakage in a PCL84?

ctc15 · 18th Dec 2021, 1:32 pm

Hi Check the 10M resistors and 4700pf capacitors in the triode anode of the PCL84s'
Keith

retroteck · 19th Dec 2021, 4:38 pm

Just a thought, I had a similar fault once on a Decca CTV25, turned out to be the tube.

Rod

Niechcial,Steve · 20th Dec 2021, 9:37 am

Thanks all. I had already had the three PCL84s as my main area of suspicion and changed everything around there including the 10M resistors and coupling Cs. I also changed the three A1 de-couplers. In the end it turned out to be something very stupid. This set has only internal contrast controls. I have been operating it at what seemed to me to be a good contrast setting. However, looking at grey scale bars showed me no distinction between the darkest two bars. In other words, slightly over- driven. Correcting that with the internal contrast control seems to have cured the fault. We will see.
I'm sorry I have no photos to offer at present. I am viewing in a mirror where I can't get a good angle. Before I turn it round for final convergence adjustments I want to see if the panels from the other CTV25 I have in the garage are redeemable. That set had been stored by its previous owner in very bad conditions, and I would not be at all surprised to find o/c coils etc. At the moment I've got the IF panel from that set in circuit. That works with an apparent agc fault (severe overloading). Putting a 12 db aerial attenuator in produces a reasonable UHF picture. I will go hunting for that one.

Welsh Anorak · 20th Dec 2021, 11:10 am

Having a good set to use as a 'mule' for the panels and units will be very helpful.
Looking forward to seing the pictures!

FERNSEH · 20th Dec 2021, 11:21 am

Well worth making sure the DC restoration of the luminance circuits is in order.
If the black level is wandering about this will have an effect on the colour reproduction. In the Bush CTV25, like other sets such as the Pye CT70 the final summation of the Y signal and the colour difference signals from the R G B amplifiers takes place in the CRT.

DFWB.

Niechcial,Steve · 21st Dec 2021, 2:40 pm

Thanks David. I did check out the heater/cathode idea, by changing all PCL 84s, the PL802 again. I also powered the CRT heater up from an isolated source. No change.
I have now sorted out the second IF panel (agc fault was a poor contact on tuner plug pin 1 which returns agc bias to the first IF transistor), and the fault remains with that panel in circuit. I am now going to overhaul my second decoder/CDA panel so that should be a further test. What I haven't done is scoped the beam limiter voltage from the LOPT to the brightness control network. I will give that a go.

Niechcial,Steve · 30th Dec 2021, 7:49 pm

I got so far with working on the second receiver/decoder chassis I have. Having done a blanket replacement of the usual suspects, plus several transistors with a rotted through leg, I finished up with a good monochrome picture and sound, with just a trace of the red colour bar. At this point I ran into a totally exasperating problem. The first chroma amp stage was showing very little gain, and the gain didn't change with the saturation or preset saturation controls. It's a very simple one transistor circuit which sits on a board of its own under the IF strip. Several of the resistors in the base bias circuit were well out and so replaced. Transistor (BC108) checked OK but replaced .I checked for signal at the base, DC voltages (ok) and for cracks in the board etc. The Collector load is just an air cored coil shunted by a 1K damping resistor. Presumably the coil is designed to resonate around 4.43 MHz. I'm wondering if the prolonged exposure to damp has damaged the response curve. Feels unlikely as it's wound with DCC rather than enamel wire. I can't think what else it could possibly be though. Anyway, I've had enough for the moment. All suggestions welcome.
I have re-assembled the set with the original chassis and the picture is excellent. I attach photos. The horizontal lines are entirely camera artefacts. That funny colour smearing on both b/w and colour pictures was entirely due to the preset contrast being set too high. I was able to replicate it on chassis number 2. Once set up as per the manual the streaking vanished.

Niechcial,Steve · 13th Feb 2022, 10:04 pm

Number 1 TV now has legs on it. I was very lucky to find a set which are a pretty close match for the originals (pic attached). The set has run faultlessly for several hours now. I would rank this design in 2nd place for picture quality among its contemporaries- excelled in first place by the G6, 2000 and hybrid Bairds, and about the same as the GEC hybrid. As Mikey points out in his gallery, the set up of the decoder is critical. This is because the burst take off is after the user and pre-set saturation controls. When you throttle back the saturation to a good level for watching, the burst is only just large enough to operate the colour killer. A very silly design feature really. In terms of reliability, looking at what had been replaced previously, I would guess it was pretty average. The carbon resistors are of a type where many more than usual were over 20% out of tolerance, and the LOPT is a very weak link as it was on most of these dual standard hybrids.
I have now finished rebuilding the timebase unit on number 2 TV. I am less optimistic about the LOPT rewind I did on this one because I had to go one winding more than on number 1 TV, and the winding concerned was a bifilar one. I shall plug it in, switch it on and beat a hasty retreat later this week

Niechcial,Steve · 4th Apr 2022, 7:01 pm

Just to finish the story, set number 2 lopt rewind did fail quite quickly by shorting between lead out wires where I had been rather careless and allowed at least one of the windings to overlap the lead out wires of another winding. So I rewound it again. This time it is fine. I now have everything I need to restore set number 2, but I'm a bit Bushed out so I will leave it a while. Set number one is going strong with lovely pictures.

8th Nov 2021, 10:48 am	#21
Welsh Anorak Dekatron Join Date: Apr 2005 Location: North Wales, UK. Posts: 6,921	Re: Bush Dual Standard Colour Restoration As far as I know the tuners were more or less identical. A local private rental company used to swap out the complete timebase unit in the field which wouldn't have been too difficult - I was given a lot of dead Mk3s timebases for parts in the late Seventies - obviously the TVs had mostly been scrapped by then and the Mk1 versions had been seen off long before for obvious reasons! __________________ Glyn www.gdelectronics.wales

17th Nov 2021, 7:25 am	#22
Niechcial,Steve Hexode Join Date: Jan 2007 Location: Beckenham, London, UK. Posts: 373	Re: Bush Dual Standard Colour Restoration I've now been through the IF, decoder and colour/video output stages checking/replacing electrolytics and high value/high power resistors, quite a few of which were out of tolerance. The result is that normal frame sync has returned, the luminance smears have gone, but verticals remain bent as does a colour fault whereby the top and bottom halves of the picture have a different tint (it's not purity- which is perfect). I will now get the scope out and do some fault finding

1st Dec 2021, 3:04 pm	#23
Niechcial,Steve Hexode Join Date: Jan 2007 Location: Beckenham, London, UK. Posts: 373	Re: Bush Dual Standard Colour Restoration I have now come back to this for some detailed fault finding. Just to summarise, scan, EHT regulation, convergence and sync faults are sorted, and electros blanket tested and replaced as necessary throughout. I next tried setting up the grey scale and found that despite having the tube show near perfect on the B and K tester, the grey scale would not track over the usual brightness range. The problem was no clamping pulse on the three PCL84 colour difference amplifiers. Diode 5D12 on the convergence board, which feeds line pulses in, had gone o/c. Replacing that resulted in a very good definition black and white picture with correct grey scale I am now left with three faults: 1) Bizarrely wrong colours on the colour bars. 2) Smearing/overshoot on the left hand horizontal lines of the test card into the grey area of the circle beyond. 3) Green tinted broad horizontal band over the middle (black and white part) of a test card. Taking these in reverse order, shorting out the U and V inputs to the colour output valves confirms that the green tint is coming in mainly on the V (R-Y) channel from the decoder. I am not sure whether the smearing/overshoot is coming is on the luminance output stage (which is largely rebuilt), or coming in from the IF panel. I don’t suspect alignment, as the picture is otherwise good definition . I will feed in a video signal from another source into the output stage to test. The wrong colours are the most interesting fault. For the colour killer to be switching the colour in (which it is), not only does the chroma derived bias have to be present but so do bistable pulses. The ident which syncs the bistable is there looking exactly as it should on the scope, but I guess there is the possibility of some strange phase error in there somewhere. I will start by viewing the individual blue and red channels on the screen to see if both are contributing to wrong colours. If so, I will start by checking out the sync of the reference oscillator and take it from there. I suspect and hope that the green tint will vanish once I have sorted out the more major decoder fault.

10th Dec 2021, 9:20 pm	#24
Niechcial,Steve Hexode Join Date: Jan 2007 Location: Beckenham, London, UK. Posts: 373	Re: Bush Dual Standard Colour Restoration I've had a great chase on the colour faults due to two faults being present at the same time. The first was due to the reference osc locking out of phase due to a mis-aligned burst channel- after having replaced the o/c diode mentioned above which feeds in line pulses to the burst gating circuit. Incidentally, smearing on luminance has corrected itself. Pulses from the burst gate are also fed into the brightness circuit, and I'm thinking the pulse problem may have affected the luminance as well as the chrominance. The burst gating is a bit weird in that the chrominance is sent to the first burst gate after passing through the colour (saturation) controls. So effectively, the burst amplitude will vary with saturation as set by the user. Not very sensible really. Once I had got a good burst lock with correct primary red and blue, I had the cyan and yellow colour bars a bright green. Getting the colours right meant twiddling the b-y gain to be much lower than specified by the manual. So there is obviously gain misbalance for some reason in the colour difference output stages. That in itself is puzzling as they are largely rebuilt (including new valves). I guess I may have made an error in one of the resistors or somrthing. I will explore further

10th Dec 2021, 9:30 pm	#25
toshiba tony Heptode Join Date: May 2006 Location: Accrington, Lancashire, UK. Posts: 978	Re: Bush Dual Standard Colour Restoration I remember the logo "colour lock" wording on the sets. What exactly was locked? And were the decoders reliable, I came into the trade mid G6 days and worked on them a lot but don't remember many decoder faults, just psu and timebase problems. Thanks anyone.

10th Dec 2021, 9:44 pm	#26
Niechcial,Steve Hexode Join Date: Jan 2007 Location: Beckenham, London, UK. Posts: 373	Re: Bush Dual Standard Colour Restoration I guess first generation discrete component decoders were always among the most reliable parts of a colour set, though the G6 and the Bairdes were amongst the few that had valves in their decoders with the usual valve attendant problems. I remember the odd rogue crystal or varicap diode, but not much else. When they get to this age it's often high power carbon resistors changed value or o/c transistors or detector diodes. Some of the early colour decoder ICs seemed to give more problems than discrete component jobs.

10th Dec 2021, 11:41 pm	#28
Niechcial,Steve Hexode Join Date: Jan 2007 Location: Beckenham, London, UK. Posts: 373	Re: Bush Dual Standard Colour Restoration Yep that about sums it up, though some sets did have circuitry which claimed to compensate automatically for greyscale drift with ageing

15th Dec 2021, 4:19 pm	#29
Niechcial,Steve Hexode Join Date: Jan 2007 Location: Beckenham, London, UK. Posts: 373	Re: Bush Dual Standard Colour Restoration Having aligned the decoder as per the book from beginning to end, I now have a good colour picture with all voltages as per the manual. I have one fault left which is causing me to pull my hair out. The fault is a horizontal band of slight tinting of magenta or green which moves up or down the picture with picture content. It looks very much like a decoupling or earthing fault but I'm running out of suspects. Anyone have any ideas?

16th Dec 2021, 12:56 pm	#30
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Bush Dual Standard Colour Restoration Or possibly the reference level clamps in the three colour difference amplifiers? The triode sections of the PCL84s. DFWB.

16th Dec 2021, 1:55 pm	#31
Welsh Anorak Dekatron Join Date: Apr 2005 Location: North Wales, UK. Posts: 6,921	Re: Bush Dual Standard Colour Restoration Could it be H/K leakage in a PCL84? __________________ Glyn www.gdelectronics.wales

18th Dec 2021, 1:32 pm	#32
ctc15 Pentode Join Date: Oct 2005 Location: Barnet, London, UK. Posts: 167	Re: Bush Dual Standard Colour Restoration Hi Check the 10M resistors and 4700pf capacitors in the triode anode of the PCL84s' Keith

19th Dec 2021, 4:38 pm	#33
retroteck Tetrode Join Date: Jul 2006 Location: Harrogate, North Yorkshire, UK. Posts: 68	Re: Bush Dual Standard Colour Restoration Just a thought, I had a similar fault once on a Decca CTV25, turned out to be the tube. Rod

20th Dec 2021, 9:37 am	#34
Niechcial,Steve Hexode Join Date: Jan 2007 Location: Beckenham, London, UK. Posts: 373	Re: Bush Dual Standard Colour Restoration Thanks all. I had already had the three PCL84s as my main area of suspicion and changed everything around there including the 10M resistors and coupling Cs. I also changed the three A1 de-couplers. In the end it turned out to be something very stupid. This set has only internal contrast controls. I have been operating it at what seemed to me to be a good contrast setting. However, looking at grey scale bars showed me no distinction between the darkest two bars. In other words, slightly over- driven. Correcting that with the internal contrast control seems to have cured the fault. We will see. I'm sorry I have no photos to offer at present. I am viewing in a mirror where I can't get a good angle. Before I turn it round for final convergence adjustments I want to see if the panels from the other CTV25 I have in the garage are redeemable. That set had been stored by its previous owner in very bad conditions, and I would not be at all surprised to find o/c coils etc. At the moment I've got the IF panel from that set in circuit. That works with an apparent agc fault (severe overloading). Putting a 12 db aerial attenuator in produces a reasonable UHF picture. I will go hunting for that one.

20th Dec 2021, 11:10 am	#35
Welsh Anorak Dekatron Join Date: Apr 2005 Location: North Wales, UK. Posts: 6,921	Re: Bush Dual Standard Colour Restoration Having a good set to use as a 'mule' for the panels and units will be very helpful. Looking forward to seing the pictures! __________________ Glyn www.gdelectronics.wales

20th Dec 2021, 11:21 am	#36
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Bush Dual Standard Colour Restoration Well worth making sure the DC restoration of the luminance circuits is in order. If the black level is wandering about this will have an effect on the colour reproduction. In the Bush CTV25, like other sets such as the Pye CT70 the final summation of the Y signal and the colour difference signals from the R G B amplifiers takes place in the CRT. DFWB.

21st Dec 2021, 2:40 pm	#37
Niechcial,Steve Hexode Join Date: Jan 2007 Location: Beckenham, London, UK. Posts: 373	Re: Bush Dual Standard Colour Restoration Thanks David. I did check out the heater/cathode idea, by changing all PCL 84s, the PL802 again. I also powered the CRT heater up from an isolated source. No change. I have now sorted out the second IF panel (agc fault was a poor contact on tuner plug pin 1 which returns agc bias to the first IF transistor), and the fault remains with that panel in circuit. I am now going to overhaul my second decoder/CDA panel so that should be a further test. What I haven't done is scoped the beam limiter voltage from the LOPT to the brightness control network. I will give that a go.

30th Dec 2021, 7:49 pm	#38
Niechcial,Steve Hexode Join Date: Jan 2007 Location: Beckenham, London, UK. Posts: 373	Re: Bush Dual Standard Colour Restoration I got so far with working on the second receiver/decoder chassis I have. Having done a blanket replacement of the usual suspects, plus several transistors with a rotted through leg, I finished up with a good monochrome picture and sound, with just a trace of the red colour bar. At this point I ran into a totally exasperating problem. The first chroma amp stage was showing very little gain, and the gain didn't change with the saturation or preset saturation controls. It's a very simple one transistor circuit which sits on a board of its own under the IF strip. Several of the resistors in the base bias circuit were well out and so replaced. Transistor (BC108) checked OK but replaced .I checked for signal at the base, DC voltages (ok) and for cracks in the board etc. The Collector load is just an air cored coil shunted by a 1K damping resistor. Presumably the coil is designed to resonate around 4.43 MHz. I'm wondering if the prolonged exposure to damp has damaged the response curve. Feels unlikely as it's wound with DCC rather than enamel wire. I can't think what else it could possibly be though. Anyway, I've had enough for the moment. All suggestions welcome. I have re-assembled the set with the original chassis and the picture is excellent. I attach photos. The horizontal lines are entirely camera artefacts. That funny colour smearing on both b/w and colour pictures was entirely due to the preset contrast being set too high. I was able to replicate it on chassis number 2. Once set up as per the manual the streaking vanished. Attached Thumbnails

13th Feb 2022, 10:04 pm	#39
Niechcial,Steve Hexode Join Date: Jan 2007 Location: Beckenham, London, UK. Posts: 373	Re: Bush Dual Standard Colour Restoration Number 1 TV now has legs on it. I was very lucky to find a set which are a pretty close match for the originals (pic attached). The set has run faultlessly for several hours now. I would rank this design in 2nd place for picture quality among its contemporaries- excelled in first place by the G6, 2000 and hybrid Bairds, and about the same as the GEC hybrid. As Mikey points out in his gallery, the set up of the decoder is critical. This is because the burst take off is after the user and pre-set saturation controls. When you throttle back the saturation to a good level for watching, the burst is only just large enough to operate the colour killer. A very silly design feature really. In terms of reliability, looking at what had been replaced previously, I would guess it was pretty average. The carbon resistors are of a type where many more than usual were over 20% out of tolerance, and the LOPT is a very weak link as it was on most of these dual standard hybrids. I have now finished rebuilding the timebase unit on number 2 TV. I am less optimistic about the LOPT rewind I did on this one because I had to go one winding more than on number 1 TV, and the winding concerned was a bifilar one. I shall plug it in, switch it on and beat a hasty retreat later this week Attached Thumbnails

4th Apr 2022, 7:01 pm	#40
Niechcial,Steve Hexode Join Date: Jan 2007 Location: Beckenham, London, UK. Posts: 373	Re: Bush Dual Standard Colour Restoration Just to finish the story, set number 2 lopt rewind did fail quite quickly by shorting between lead out wires where I had been rather careless and allowed at least one of the windings to overlap the lead out wires of another winding. So I rewound it again. This time it is fine. I now have everything I need to restore set number 2, but I'm a bit Bushed out so I will leave it a while. Set number one is going strong with lovely pictures.