What you have is the original Radio Rentals replacement mask, the one they adopted due to the frailty of the bronze rigid type. My 702W has the same mask fitted as the one on your 708, personally I wouldn't paint it. The mask you have fitted is all part of the sets life-cycle/history
And I agree. It's an original replacement and will stay as it is. I was a bit undecided, but now I'm convinced. Thanks guy's for all the input ?
To understand the black art of electronics is to understand witchcraft. Andrew.
I think the problem is that the eye is being misled by the absence of the grey of the tube faceplate to think that the mask looks much brighter; when the tube is in place it will appear to tone down.
I'm ready to re-fit the CRT, but my back isn't, so it will have to wait a while. Certainly a few days as I have somehow managed to strain it again, heaven knows how though.
In the meantime, I have a little puzzle for you....this is something I noticed a while back but forgot to do anything about it. There should be a 1k damping resistor in parallel with the width adjustment coil L405. This had been snipped away and is missing on my set. Now, the question is, what effect would this resistor (absent or present) have upon the circuit and the visual effect on screen?
In this earlier picture, you can see a burn mark upon the coil former where the resistor would have been....
To understand the black art of electronics is to understand witchcraft. Andrew.
Not sure about the width coil but the damping resistor across Line Linearity coils very often causes ringing of the scan, not video, on the left had side.
Be interesting to see what changes on the display with the resistor back in place.
I am pleased to say that the CRT has now been re-installed along with the control panel and speaker. As I was missing a corner spring to secure the CRT degaussing shield, I used a suitable spring and some drive-cord to secure the bottom right corner.....
Here is another corner with the correct fixing.....
And the assembled ensemble....
To understand the black art of electronics is to understand witchcraft. Andrew.
Fitting the CRT was not without some difficulty as the required distance with which the CRT protruded into the mask surround had to be selected exactly. This I found just right with a thick washer (2mm) between the cabinet wood corner stops and each CRT mounting lug. Too little thickness of washer, and the CRT would put too great a pressure on the mask. Too thick, and of course a gap would be visible between the screen and mask. Fitting extra nuts proved too much, for example.
To understand the black art of electronics is to understand witchcraft. Andrew.
Let's hope it will still work after the chassis and deflection assemblies are re-installed. Naturally, purity and static convergence will need setting up for a start. I intend to do this tomorrow and weekend as my back is not happy after humping the CRT in and out several times already this evening !
To understand the black art of electronics is to understand witchcraft. Andrew.
Yesterday afternoon and evening was spent re-installing the scan coils and chassis with some quick setting up of purity and static convergence (it is far from perfect as yet). We are back to a working set now. The fault of right hand foldover of the raster has been cured with replacement of both D31 and D32 in the line oscillator. Whilst I was checking the diodes, one of them was split and just separated in two with light pressure from the test probes. It has made no improvement to the left side of the raster and the inclusion of the damping resistor across the width coil appears to make no difference in any respect.
To understand the black art of electronics is to understand witchcraft. Andrew.
So, as it stands at the moment, we are left with the odd chrominance and to some extent luminance timing fault at the start of line scan. It appears almost to be "over-blanked" for want of a better expression, or excessively delayed.....
To understand the black art of electronics is to understand witchcraft. Andrew.
So, as it stands at the moment, we are left with the odd chrominance and to some extent luminance timing fault at the start of line scan. It appears almost to be "over-blanked" for want of a better expression, or excessively delayed.....
It is a very long time ago but when I worked on these receivers I remember that the adjustment of L216/L216B on the decoder panel adjusted the chrominance/ident timing. I'm not sure about the line flyback timing. The two issues may be connected, but as I say it has been a very long time since I had sight of the 700/710 circuit
I'm not entirely convinced there is a fault on the decoder as deliberate manipulation of the line hold can be made to correct the symptom, but then on the other hand, this could just be shifting timing and masking it. This particular issue will wait until the full service data is available as I am uncomfortable trying to fault-find or making adjustments without it. There are of course many similarities with the 700 chassis data, but if it was exactly the same, then Baird would not have published any 710 data.
Another issue I'm having is setting up purity. This is interesting because purity errors on an individual coloured raster may be seen to vary very slightly in accordance with a hum-bar effect slowly moving up from the bottom of the screen. It makes correct adjustment almost impossible. I'm wondering if there maybe a slight residual current in the degaussing coils.
To understand the black art of electronics is to understand witchcraft. Andrew.
Have you a manual degaussing coil? If so disconnect auto and manually degauss the set, then try setting up purity.
I always manually degaussed a set before setting one up, it did occasionally help when the auto circuit had failed and left a residual field that the repaired auto circuit could not remove.
I'm not entirely convinced there is a fault on the decoder as deliberate manipulation of the line hold can be made to correct the symptom, but then on the other hand, this could just be shifting timing and masking it. This particular issue will wait until the full service data is available as I am uncomfortable trying to fault-find or making adjustments without it. There are of course many similarities with the 700 chassis data, but if it was exactly the same, then Baird would not have published any 710 data.
Another issue I'm having is setting up purity. This is interesting because purity errors on an individual coloured raster may be seen to vary very slightly in accordance with a hum-bar effect slowly moving up from the bottom of the screen. It makes correct adjustment almost impossible. I'm wondering if there maybe a slight residual current in the degaussing coils.
Yes you are probably right, L216 that I remembered adjusted the ident timing and you adjusted the coil so that the ident switching occurred outside the active line period, mis adjustment gave rise to no colour at the extreme edge of the picture depending how far out of adjustment the coil was. From what you say about adjusting the line hold, the effect you have in associated with the timing of the line flyback pulse from the line timebase to the flywheel sync circuit. Without the circuit to hand I'm not sure which components would affect the line sync phasing.
On the degausing issue I remember the degausing thermister having to be very close to the dropper in the bottom of the chassis. If I remember correctly, this thermistor used to fall apart too and it may well be missing causing the symptoms you describe which is indeed residual current in the degausing coils. Im not sure which way round the NTC and PTC components are physically but I think problems with these are a key to the degausing issue.
I'm not entirely convinced there is a fault on the decoder as deliberate manipulation of the line hold can be made to correct the symptom, but then on the other hand, this could just be shifting timing and masking it.
It isn't clear from that if you are pulling the free running speed of the line oscillator off frequency to eliminate the effect or not. If you are, I'd carry out further checks of all the other components around D31/32 that could also add imbalance and thus affect the phasing
Posted by: PYE625
I'm wondering if there maybe a slight residual current in the degaussing coils.
I can't think of any other fault which could cause this effect.
Moderator Note: I had to fix this post.
For some reason although displaying OK in the thread the "source code" for quoting and bold appeared in the recent posts side-bar as codes. My assumption is you were using source code to do the quoting and bold as when I came to edit your post the quote was only source code and not as expected. Please use the "quote" feature on the previous post or the " on the post editor menu tab above, to construct the quote.
After my correction using the correct quote features as detailed above the "recent" side-bar displayed correctly, please refrain from using source code commands as there really isn't any need. If you didn't use source code the please ignore me, it must have been a glitch.
Andrew, I thought you modified the degauss circuit to the more common type, it’s possible the degauss coils are drawing too little current to warm up the thermistor enough to reduce the current to an acceptable level.
Andrew, I thought you modified the degauss circuit to the more common type, it’s possible the degauss coils are drawing too little current to warm up the thermistor enough to reduce the current to an acceptable level.
That is why in the heyday of those TVs we were told to put the thermistor as close as possible to the dropper. Some engineers didn't and customers complained of moving colour patches. As I said the thermistor regularly fell apart.
