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Bush CT187CS - Back to the fray!

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colourstar
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Thanks Brian. It's only when you compare the photos to those at the top of the thread that the difference is clear.

That coil assembly is very prone to being knocked when the decoder panel is being removed or replaced. It must have caught out countless engineers down the years. The damage happens when you squeeze your hand in to access one of the panel's plug-in connectors. Board replacement is quite a fiddle.

I'll take the panel out again and use a magnifier to see if I can see where the wire has been damaged.

Once the smearing is sorted, it will be easier to see what the remaining colour issues are.

 

Steve

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Topic starter Posted : 10/12/2016 8:17 pm
colourstar
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OK well I managed to repair the broken wire on L15/L16 after checking it against the circuit diagram. That at least has cleared up the smeary picture. The coil can be seen at the centre of the first picture.

Now I am left with the odd colour fault(s), which can probably be best seen on the colour bars and the odd fringe effect. Generally colour is very lacking. Also the brightness is far too high. The picture is overbright even with the user control on minimum. I'm not sure what to adjust for that.

Steve

Bush5a.jpgBush6.jpgBush7.jpgBush8.jpgBush5.jpg

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Topic starter Posted : 10/12/2016 10:37 pm
PYE625
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Well done Steve, it is really coming along well. thumb_gif

It would be tempting to give likely looking presets a little twiddle and observe the effect, but I would not advise it to be honest. It can cause more problems and is something I now avoid until I know exactly what they do in the circuit. When these set's were ten-a-penny back in the day, it didn't matter too much.

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Posted : 10/12/2016 10:50 pm
Jayceebee
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Goodness me an SL901B with tabs, is it original Steve? I thought that version of the IC had no suffix. Its's not the original version of the decoder as it has the holes to accept the larger IC.

Those small skeleton presets can be troublesome so first make a note of the position of the slider, give them a good squirt of cleaner and rotate them back and forth a few times putting the slider back to it's original position and see if that improves matters.

As to the overbright picture what voltages do you have on the CRT A1s?

John.

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Posted : 10/12/2016 11:07 pm
Till Eulenspiegel
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Hi Steve,

             Encouraging results.  With regard to the damaged coil, it is in the luminance channel and functions as the 4.43Mc chroma trap. Reference to the circuit diagram: locate R41 which is soldered on the coil former, connect a wire link across it and you'll find the smearing will have vanished.  The reason for the smearing is simply without the coil connected across R41 it resistor functions along with a 200pF capacitor as a RC HF filter.

Looks like the bi-stable switch is working, you can confirm this by measuring voltage at the collectors of the bistable transistors, a reading of about 6 volts on both transistors will confirm that the flip-flop circuit is working.

That blue bar does look strange and as we know the B-Y signal is not on the switched axis, some call it the NTSC phase.

Switch off the red and green guns and examine the blue bars. With the colour saturation control turned up the gaps between the blues bars should appear as black.

Till Eulenspiegel.

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Posted : 10/12/2016 11:12 pm
Focus Diode
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Excellent progress indeed. Well done in repairing the coil.

As for the excessive brightness didn't the set suffer from that when David and Gary first got it going? A couple of resistors were replaced curing the problem so why the fault has returned is puzzling.

Did this re-occur following replacement of the diodes and transistors out of interest? Is it possible to "extinguish" the raster using the A1 controls? How are the voltages?

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Posted : 11/12/2016 10:55 am
colourstar
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Hi everyone and thanks for your comments.

John, I think the SL901B may not be original- there's evidence on the solder side of the board of it having replaced a larger IC.

Andrew, presets will be duly waggled although I'm loath to upset any settings that may cause further grief!

Brian, no it's not possible to extinguish the picture with the A1 controls. The picture is viewable with them turned all the way down. I'm not sure where to measure the voltages on these.

As per David's advice I tried the colour bars with only the blue gun connected. Results are as per the photo

I note that on one of the Television articles that 3VT2 can be suspect in the case of Hanover bars. As it's another BC148 I changed it, although it's made no difference. There's also mentioned of tweaking preset 3VR3 but I'm darned if I can see it. VR1, 2, 4, 5, 6 etc are all there but try as I might, 3 remains elusive...

The picture with the colour turned down to b&w is really first class, by the way.

The images below are all affected by the fact that the brightness is quite high. The alleged 'blue' raster - a sickly yellow-  looks like it probably would be black if the brightness was normal.

 

Steve

 

Blue-Gun-Only-1.jpgRed-Raster-1.jpgBlue-Raster-1.jpgGreen-Raster-1.jpg

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Topic starter Posted : 11/12/2016 3:18 pm
PYE625
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colourstar said
Hi everyone and thanks for your comments.

Andrew, presets will be duly waggled although I'm loath to upset any settings that may cause further grief!

 

Steve

 

Hi Steve, yes but please be careful....a fraction of a turn and then back to the same position as near as you can get if no difference is made.

As to the high brightness, could that be to do with the fact the IC has been changed ? Could be totally barking up the wrong tree, so again treat with caution. I presume that there is no pre-set brightness control. There are people here with way better knowledge than me with these sets   grin_gif

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Posted : 11/12/2016 4:34 pm
Cathovisor
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Monochrome picture in, voltages on cathodes and A1s please. Let's see some science here wink

If the set is capable of giving a good monochrome picture...

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Posted : 11/12/2016 5:00 pm
PYE625
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Not wishing to teach my grandmother to suck eggs, but I would be inclined to attempt a set-up as per the full service manual. A check on voltages too as Catho suggested above would be in order....get it right in black and white first.

Sorry to state the obvious, especially if such data is not available for the setting up of the chrominance decoder.

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Posted : 11/12/2016 5:15 pm
Focus Diode
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If the picture is viewable yet not possible to completely extinguish the individual guns may suggest a different fault to the earlier problem. I recall the A1 control settings made no difference to the excessive brightness yet the grey scale was correct!

 

Perhaps someone could advise where the problem may lie?

Cheers

 

Brian

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Posted : 11/12/2016 5:25 pm
Cathovisor
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Focusdiode said
If the picture is viewable yet not possible to completely extinguish the individual guns may suggest a different fault to the earlier problem. I recall the A1 control settings made no difference to the excessive brightness yet the grey scale was correct!

 

Perhaps someone could advise where the problem may lie?

Cheers

 

Brian  

The only time I've come across this fault where it wasn't down to A1 settings was where the G1s were wrong. The set concerned was a Telefunken and somehow, the G1s were no longer tied to the set's 12V rail.

I note the control grids are used to apply blanking; there could be some fun and games going on here but as is often said, look at the voltages and make sure all PSU values are as they should be.

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Posted : 11/12/2016 5:41 pm
colourstar
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Just thought I'd show off the very good b&w picture the set currently displays....

It's been a few weeks since David, Brian and Gary's visit but I remember trying the set after they'd departed and noting that the brightness was high then. I know it had been tamed earlier in the day so I'm not sure when or how it came to be up again.

Anyway here's that picture... 🙂

 

BW.jpg

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Topic starter Posted : 11/12/2016 8:04 pm
Jayceebee
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That looks pretty good other than being a tad bright and the line and frame blanking look OK, is that with the brightness control at minimum? What sort of voltage swing do have you at the red A1 pin (4) of the CRT with the red pot on the convergence board at minimum and maximum? Have a look at section F of the manual for the circuit and description, the bottom end of the A1 circuit is a little unusual. In your first post you mention the two 220K resistors causing the excess brightness, I would suspect they were 4R3 on the CRT base and 7R8 on the convergence board.

Now it's well over 45 years ago since I last looked at an A823 but I have a sneaking suspicion that the brightness control range wasn't that great even when new, is there anyone out there with a working set who can tell me if my mind is playing tricks?

 

John.

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Posted : 11/12/2016 9:50 pm
Till Eulenspiegel
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The range of the brightness control was rather limited, especially when the beam limiter cut in.

The rules of setting up the basic black and white picture are to set the black levels with the A1 controls.

According to the information in the 1971/72 R&T book the first thing to do is remove the tuner socket 2Z1from the IF panel to produce a noise free raster. Set the R.G.B. controls to maximum, the A1 R.G.B. controls to minimum and the Brightness control to maximum. Adjust the A1 controls in this order: (leave all gun switches ON).  1. The Green A1 control to a just visible raster. 2. The Red A1 until red is just introduced into the raster. 3. the Blue A1 until blue is just ntroduced into the raster.   Restore the tuner socket and the signal, adjust the Contrast to a normal picture and set the Brightness control for the correct black level.   Adjust the appropriate A1 control to remove any colouration if any, in the low lights close to black level.   Adjust the appropriate Drive control to remove colouration if any, in the peak white areas (Illuminant D) Finally check that the overall grey scale is satisfactory.

This information most probably was gleaned from the Rank-Bush-Murphy service manual.                                              So it follows the basic monochrome picture must be correct before any colour content is added.   

Till Eulenspiegel.

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Posted : 11/12/2016 11:47 pm
Jayceebee
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Thanks for that Till. Actually I got the years wrong, it was only 43 years ago when I defected to Thorn.grin_gif

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Posted : 12/12/2016 12:07 am
colourstar
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Hi all

Yes John, you are quite correct - those two resistors you mention have indeed been replaced. The excellent service guide in Television mag (in the library) also mentions D4 and D5 (both BA148 types) on the power supply going leaky and causing brightness faults. It may be worth replacing them as a matter of course. I wonder what a suitable modern equivalent might be?

With some further tweaking of the A1 controls, minimum brightness on the user control does now provide a picture that is just a bit too dark, so advancing it slightly brightens it to a normal level. I think you are right about the overall range of brightness on colour TVs not being great; perhap's I'm just used to b&w sets. Incidentally I found I could get no voltage reading at all on crt pin 4, let alone a swing. Possible dodgy diodes notwithstanding, I will heed Till's advice and do a set up as per his notes and see how that goes.

One interesting characteristic of the colour present on screen is that when the user control is slowly advanced from zero, the colour comes in by bleeding down from the top of the picture. Very strange indeed.

 

Bars.jpg

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Topic starter Posted : 12/12/2016 9:44 am
Focus Diode
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I remember similar strange effects when I converted the G6 to tripler operation. The whole picture was the correct colour yet was missing in small areas on some scenes on a moving picture. In other words black and white areas on a colour picture!

 

cheers

 

Brian

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Posted : 12/12/2016 11:50 am
Till Eulenspiegel
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The BA148 was recoded as BY206, PIV = max 350V, If = max 0.4amp.  BY207 has a higher PIV = max 600V. The BA159 is a near equivalent. These diodes are used in the luminance black level clamp and can be found on a tag strip close to the main HT smoothing capacitors. Designated as 8D4 and 8D5 in the RBM service manual.

BA148 info:  http://www.radiomuseum.org/tubes/tube_ba148.html     Developer was Valvo.

I've seen that colour shading before, could be 50Hz component entering the decoder, smoothing or earth returns.

Till Eulenspiegel.

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Posted : 12/12/2016 11:51 am
crustytv
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Another key component to check in addition to diodes 8D4 & 8D5, is the 0.1uF 1kV capacitor 8C11.  This is the clamp/brightness pulse feed cap and can also be responsible for uncontrollable brightness.

The brightness control although not as great when compared to other sets of the period, does give adequate swing from dark to bright. And despite often being slated as having a poor picture compared to its peers, I think the A823 gives a very good account of itself and is very easy on the eyes if you know what I mean.

Here's what you can and should be aiming to achieve, my Bush CT187CS.

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Posted : 12/12/2016 12:06 pm
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