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Old 8th Nov 2021, 9:48 am   #21
Welsh Anorak
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Default 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!
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Old 17th Nov 2021, 6:25 am   #22
Niechcial,Steve
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Default 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
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Old 1st Dec 2021, 2:04 pm   #23
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Default 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 donít 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.
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Old 10th Dec 2021, 8:20 pm   #24
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Default 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
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Old 10th Dec 2021, 8:30 pm   #25
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Default 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.
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Old 10th Dec 2021, 8:44 pm   #26
Niechcial,Steve
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Default 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.
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Old 10th Dec 2021, 9:44 pm   #27
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Default Re: Bush Dual Standard Colour Restoration

Quote:
Originally Posted by toshiba tony View Post
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.
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Old 10th Dec 2021, 10:41 pm   #28
Niechcial,Steve
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Default 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
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Old 15th Dec 2021, 3:19 pm   #29
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Default 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?
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Old 16th Dec 2021, 11:56 am   #30
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Default Re: Bush Dual Standard Colour Restoration

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

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Old 16th Dec 2021, 12:55 pm   #31
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Default Re: Bush Dual Standard Colour Restoration

Could it be H/K leakage in a PCL84?
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Old 18th Dec 2021, 12:32 pm   #32
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Default Re: Bush Dual Standard Colour Restoration

Hi Check the 10M resistors and 4700pf capacitors in the triode anode of the PCL84s'
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Old 19th Dec 2021, 3:38 pm   #33
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Default 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.

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Old 20th Dec 2021, 8:37 am   #34
Niechcial,Steve
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Default 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.
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Old 20th Dec 2021, 10:10 am   #35
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Default 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!
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Old 20th Dec 2021, 10:21 am   #36
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Default 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.

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Old 21st Dec 2021, 1:40 pm   #37
Niechcial,Steve
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Default 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.
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Old 30th Dec 2021, 6:49 pm   #38
Niechcial,Steve
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Default 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.
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