Bush CT187CS - Back to the fray!

That's a lovely scene.....good mates helping out and some real progress with the set too

... and me keeping out of the way on coffee duties!

Actually we realised that all four of us were in at the start of '405 Alive' magazine way back in 1989, which seems a scarily long time ago all of a sudden.

Steve

colourstar said
... and me keeping out of the way on coffee duties!

Actually we realised that all four of us were in at the start of '405 Alive' magazine way back in 1989, which seems a scarily long time ago all of a sudden.

Steve  

The pictures remind me of a golden memory from my childhood where the visit from a Television Engineer was a big occasion. Someone who could understand such a complicated thing as a television set was a true genius in my eyes. (They still are !!  )

The first two photos took me back many years, I did enjoy the time fixing telly's.

Frank

Hi all,

          It was indeed a very enjoyable day, accompanied with Gary and Brian this was a memorable visit to Steve's house in Derbyshire. Excellent hospitality, a rather splendid pub lunch and then it was time to get to grips on a type of colour TV set I used to know well in the 70s, the Bush A823.   This particular set is an impressive looking 25" screen model housed in an immaculate  cabinet with fold away doors.  

Yes, at the beginning of the servicing session there was a few scary bangs and flashes including that plug on the  thyristor power supply board. That sorted we can now without any more drama switch on the set and it was not before long a bright raster appeared on the screen, however, the brightness control has little effect and neither do the A1 screen controls.  The three R G B screen controls consist of three parallel connected 2Mohm potentiometers supplied 700 volts by the line output transformer. The other side of the pots is returned to chassis though two series 220Kohm resistors and a forward biased 1N4148 diode.  Sure enough the resistors have gone high, in fact o ne was OC.  Resistors were duly found and fitted, but there was no improvement, the raster was as bright as it was when we started on this job. What's gone wrong?  OK, hear my confession, mea culpa!  It was one of the 220K resistors which is fitted is on the convergence board, checked it again and it was OK.    I instinctively replaced the resistor in the position anyone would believe should be as indicated by the straight line between the two lead through holes, in fact this is a manufactures mistake and the resistor is inserted into the board in what appears to be used for a wire link, i.e. a dashed line.   

So at last a controllable brightness control and the A1 pots now work so the R G B black levels can be set.  It wasn't long be a good monochrome test card "C" was displayed on the screen, now it's time to do some colour. Feed the set with colour test card "F". Results no colours, It all came back to me when the Ident control 3RV7 is turned to one end of it's travel the colour killer will be overridden.   3RV7 is situated at the bottom left side of the A807 decoder panel.

The set does display colours but that's all you can call them, these are bad colours and have the Hanover blinds effect.  The A823 does not employ the usual free running sub-carrier oscillator, instead the swinging bursts are converted to a constant phase and these are used to "ring" the 4.433 crystal. It follows in the absence of a colour signal no sub carrier is generated, this therefore can provide the colour killer function. To convert those bursts into a constant phase you need a bistable switch, transistors 3VT3 and 3VT4 perform this function. The bistable is driven by a line TB pulse to initiate the 7.8Kc/s square wave.  If running correctly there should be 6 volts at the collectors of 3VT3 and 3VT4. It was found that 3VT3 is 12V and 3VT4 is 0.3V, it's obvious the bistable has stopped flip-flopping.  Bistable operation can be monitored at 3TP2, this has the 0.3v reading. Short circuiting the collector of 3VT3 to ground results in the collector of 3VT4 going high @ 12V. So it follows I'm pretty stimmpt that the line drive pulse is absent and that only an oscilloscope can determine if that is so. Or, perhaps those BC148 transistors are faulty, they test OK but we know about those Lokfits.

Beside switching the burst phase the bistable also switches the R-Y signal in the normal manner.  Worth noting the blue colour bars is displays  just about normal, not so the other red and green bars of course.

So to sum up when the PAL switch is sorted out Steve will have a perfectly useable large screen colour TV set. The Thorn "Newlife" CRT displays really bright pictures.

Till Eulenspiegel. 

I was overjoyed to see the set working after the previous encounter. David and Gary did a marvelous job getting the set basically working and overcoming the excessive brightness problem. Funny how the grey scale was correct under the fault condition. None of the A1 controls had any effect. I guess all the A1's were hard on. Goes to show how good the CRT is with equally good emission from the three guns.

In the old days one would've tried a known good decoder panel to establish where the fault lay. A major design flaw is the inability to check volts in situ. Rank did of course supply specially extended course supply special extension leads to enable checks to be carried out with the panel out of the cabinet

I wonder how many survive today?

As for the luminance problem it was fiddly trying to solder a wire onto the broken coil. Really needs a magnifying glass and copper wire for the repair.

A brilliant day out. Many thanks again to Steve for a brilliant day out and David and Gary for making the drive there and back most enjoyable.

Cheers

Brian

I never did get the extention cables, luckily I don't remember needing them. Very occasional decoder chip failures, but mainly tants and the video output transistors on that PCB. I could have had other faults on that PCB but not ones I remember. The PSU seemed to have its fair share but that seemed to be the case for many sets, the tripler was another source but not that bad.

The rest of the set didn't give me a lot of problems, unless I am in denial.

A set of this age is a different matter completely, anything is possible. 

Frank

Focusdiode Said:

I wonder how many survive today?

A couple of years ago I had the opportunity to buy a number of items from an ex tv engineer who worked in the early 70's. Much of this included Thorn service aids, many of which have made a appearances in my threads such as the PSU/LTB test rig. One such box contains extension leads for 3K and the RBM A823 decoder kit, these will no doubt come in handy when I get around to working on my A823. So at least one set survive, I suspect there will be others out there.

Some second thoughts about the non functioning bistable.  It seems difficult to believe that the line TB pulse is absent on the decoder board, the reasoning for this is the fact the burst amplifier transistor 3VT7 is switched on by the positive line pulse. No pulse would mean no bursts to ring the sub-carrier crystal.   I'm sure the bistable steering diodes were not tested, these diodes are marked as 3D7 and 3D8 on the CTV182 circuit diagram.  Also check 3D22 and 3D6, all the diodes are the common type 1N4148.  

Information for the early production versions of the A823 chassis can be found in the 1971/72 Radio and Television servicing book.  The all transistor Rank-Bush-Murphy A823 was introduced late 1969. 

Till Eulenspiegel.

Till Eulenspiegel said

Information for the early production versions of the A823 chassis can be found in the 1971/72 Radio and Television servicing book.  The all transistor Rank-Bush-Murphy A823 was introduced late 1969. 

Till Eulenspiegel.  

And as I've mentioned on numerous occasions, the full Bush manufacturers service manual for the A823 range, is available in the library. This is far more comprehensive than the Radio and Television servicing book.

Thanks so much everyone for the help and encouragement so far. I will heed the good advice and changes those diodes and transistors and report back...

Steve

Chris said

Till Eulenspiegel said

Information for the early production versions of the A823 chassis can be found in the 1971/72 Radio and Television servicing book.  The all transistor Rank-Bush-Murphy A823 was introduced late 1969. 

Till Eulenspiegel.  

And as I've mentioned on numerous occasions, the full Bush manufacturers service manual for the A823 range, is available in the library. This is far more comprehensive than the Radio and Television servicing book.  

Just seconding Chris's comment - mentioning that it's in RATS is all well and good, if you happen to have a full set of them which I suspect most people won't have. Far better to use the company manual in the library, which means we all can see what is being referred to.

The other problem with RATS - and just one of the factors that led to its downfall - was the necessity of breaking circuit diagrams up into small chunks which then made them very difficult to follow. No such issue with the company manuals 

What I should have mentioned was because the A823 was introduced late 1969 one would have expected the servicing data to be found in the 1970/71 R & T book. 1971/72 is a bit late especially when you consider the data for 1967 models such as the Baird 700 series and the GEC 2028 was already in the 1967/68 book.  

It's possible that even as late as the end of the 1960s certain manufactures placed restrictions on the distribution of their servicing data, only to accredited dealers was the policy of many set makers.

Till Eulenspiegel.

I served my appreticeship with an RBM dealer and have a soft spot for the original A823 chassis. I was just having a search around to see whether the Plessey SL901 series of ICs were still available in any numbers. I'm sure there were three versions, the original SL901 with the metal tabs and 20 pins but extremely unreliable, SL901A with 20 pins(?) but different pitch which replaced it and the SL901B which had 24 pins used in the later and horrible Z180 two IC decoder version. I can find sources for the SL901B only, one in the UK and one in China which claims to have over 85,000 in stock.

Now this is where my memory gets cloudy, I think all but the very earliest PCBs could take the A version having two sets of pads/holes to cope with the different pitch of the pins. Was it possible to use the SL901B in the earlier decoder I wonder? Certainly, in the Z180 version the two extra pins at each end have no connections made. Looking through the manual failed to shed any light on this but the OH says I often look with my eyes shut. Anyone got any recollections on this?

John.

John, You have a better memory of it all than I do, I remember the various types but not whether they were interchangeable with PCB lands for both types.

Only suggestion if it helps you to remember is perhaps the very first PCB's could only accommodate the earlier type with metal tabs, a later PCB revision allowing either type depending what was available to manufacturing.

Frank

I should have plenty of nos decoder chips for the later type decoder in stock. I also have lots of nos various type coils which were fitted to these decoders. I aquired these when Rediffusion w/shop closed down and the Bush test rig was thrown out.

Just had a little time to spend on the Bush today, so here's an update: Transistors 3VT3 and 3VT4 have now been changed, along with diodes 3D7 and 3D8, thanks to David's kind donation of parts!

Results are still horrible but the colours appear to be basically correct (compared to the earlier photos- on the test card the clown's costume is now green rather than red). The Hanover bars remain, the test card gratings have patterns running through them and that coil mentioned earlier (L15 or L16) still has a wire off (from where I'm unable to see) making the picture smeary. There's not an awful lot of colour saturation; the control was more or less on max for the photos.

Meanwhile my Alba T1520 was also on hand to keep me entertained in good old black & white whilst all this was going on!

Steve

That's excellent progress indeed Steve, well done!

The smearing is no doubt due to that coil being damaged. It's repairable hopefully but time and patience will be needed. This should sort out the smearing and allow the setting up of the decoder.

Ignoring the smearing does show a dramatic improvement in the colour! The Blue B-Y component looks to be lacking which might respond to adjustment. The manual should of course be consulted beforehand.

Cheers

Brian