Bush CT187CS - Back to the fray!

Thanks Till for the diode coding. Very useful indeed and I shall act accordingly

And thanks for the extra capacitor tip, Chris. I will be a very happy chap indeed if I can achieve anything like the results on your CT187CS. Out of interest, what work (if any) did yours require? Or was it all singing and dancing as donated from Stan? I'd be intrigued to know it's history. I suspect my set has suffered from being out of use for many years. Perhaps yours was kept in service for longer than mine.

Steve

A high voltage pulse derived from a winding on the line output transformer is clipped and clamped by the diodes 8D4 and 8D5 and fed via the power board (for some reason) to the RGB amplifiers. The pulse serves to set the black level reference level during the back porch period. Thus, no pulse = uncontrollable brightness.

Till Eulenspiegel.

Chris said

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.

Chris, I absolutely agree with you on the picture quality, the A823 did indeed give a good account of itself. The sound quality was also reasonable unless you had the earlier A809 IF board which had the TAA350 intercarrier IC, they were prone to caption buzz.

It was the A823A and subsequent versions fitted with the Z180 two IC decoder (SL901B/SL917A) which drew criticism and rightly so in my opinion. Out of the box they were just lacking, even the amount of colour saturation was only just about adequate. A year of use and they just didn't look right at all, Hanover bars and generally poor chroma rendition.

The best picture I ever saw on an RBM CTV was the Murphy 22" hybrid dual standard CTV2210D (Bush eqiv CTV174D) this was the chassis where the SL901 IC first saw service, they were very thin on the ground and I seem to remember the IC was mounted on the print side of the PCB (?), a worthy addition to your collection if one ever became available. 

Steve I'm puzzled about  your stated lack of A1 voltage, the manual states you should be aiming at around 500v on pins 4,5,and 13 and with that quality of picture I would say you easily have that.

John.

The A823 earlier single chip gave a much better colour picture, the later sets suffered from poor colour Hanover blinds, also no or colour dropout was a common problem. Some of the no  colour problems were due to the if panel, apart form that they were quite reliable sets even though some of the components were difficult to get access to. The chassis CRT lasted quite a long time compared to thorn sets . I recall the first time I was given the job of replacing the two line output transistors it took me more that half a day  the first time . 

Mod Note: the following posts were moved from Crusty's A823 thread.

Posted by: @colourstar

there was a bright white flash from the rear and the set was dead. I think an ancient thermistor on the power panel had given up. 

Ooops! Hope that's not the notorious dry jointed thermistor, that might mean carnage on the decoder. 😲 

https://www.radios-tv.co.uk/rbm-sstd-stock-faults/#comment-5841

Posted by: @crustytv

Posted by: @colourstar

there was a bright white flash from the rear and the set was dead. I think an ancient thermistor on the power panel had given up. 

Ooops! Hope that's not the notorious dry jointed thermistor, that might mean carnage on the decoder. 😲 

https://www.radios-tv.co.uk/rbm-sstd-stock-faults/#comment-5841

Oh my gawd! Just read that post.  I hope not. I do dearly hope not.....

Steve

Just thought I'd better take a look at my power panel (sorry Chris- I don't want top hi-jack your thread!)

Looks like TH2 is where the action is. The thermistor is physically intact and the connections appear to be sound. I'm not sure whether the blackening of the board is the cause (carbon tracking) or the result of the flashover. Maybe the plan would be to remove TH2, thoroughly clean the board, re-fit TH2, stick my fingers in my ears and power up. All three fuses are intact, incidentally.

Steve

Has anyone had the "flash-up" experience with KC series Blaupunkts?

I used to cringe every time I pushed the power button! HORRIBLE things they were! Maybe it was just  the ones made for the Oz market where they had to run full-wave.

Please don’t reuse that TH2, it looks in a poor state. The lead connections to it look bad and ready to break off the component. Arcing often occurs here and across the whole body.

They were fitted to the 8500/8800 but mounted on the mains filter choke and often put on a spectacular firework display, thankfully that chassis was a bit more resilient and no collateral damage caused.

Posted by: @jayceebee

Please don’t reuse that TH2, it looks in a poor state. The lead connections to it look bad and ready to break off the component. Arcing often occurs here and across the whole body.

Yep, good advice John. In fact the moth-eaten appearance may indicate where it's arced across and started taking chunks out of itself. I guess this wasn't a problem with later encapsulated types. I did find a second power board in my spares stash which looks OK on top (on the right of this photo with my original on the left):

But the underneath shows what can only be described as a hard life! Amazing how these things were kept going.

Luckily my original board is more or less as RBM intended, with only the bridge rectifier BR1 having been replaced.

The original thermistor is a VA1104 type and a quick google shows that NOS ones are available, but is there a more recent type that would be better? I think these old un-encapsulated carbon ones are asking for trouble unless the room is kept warm and there's any damp in the air.

Steve

The GEC 2110 chassis did without the thermistor by employing a slow start circuit, I'm pretty sure this could be incorporated into the A823 quite easily as the basic design is very similar. I have two A823 sets in storage and will attempt this as soon as my new workshop is finished, watch this space.

I'm not sure about a modern encapsulated type, I wonder if you mean the degauss thermistor which became two PTC thermistors in a three legged device? The most common types were white and made by Philips although one chassis did use one as a combined surge limiter and degauss. They were prone to fuse blowing, if you removed one you gave it a shake and if it rattled it was faulty.

Latterly I did incorporate the slow start modification in many of the remaining A823 sets still on rental. This allowed the removal of the troublesome thermistor. The LEDCO company made a replacement power supply PCB for the A823 which had the slow start circuit.

Another well known modification was the addition of a diode in series with the anode of the thyristor. The diode ensured that only DC going half cycles are supplied to the thyristor.

Nevertheless, the thyristor power supply is a horrible method to supply HT. Takes a high current bite out of the latter part of the positive half cycles of the mans supply. 

Till Eulenspiegel.

I’ve not seen a LEDCO replacement in the flesh but am led to believe it suffered the effects of heat worse than the original. Was the circuit ever published? I’d be interested in their take on it.

Thorn used a diode W704 a BY127 in series with the original thyristor W703, a 2N4443 or BRC4443 in the 8000/8500 as you will be aware. Failure of W704 was a common fault of a dead set but with CRT heaters. It used to fail o/c and you often find just two stumps of the diode with the middle of the body missing. Mid production deleted W704 adding a link and W703 became a 2N4444, this was a disaster for reliability. Failure of W703 also resulted in a burn up of R713,5 and C710. Repair consisted of replacing said components and refitting W704, later production went back to the original circuit.

Posted by: @till

Another well known modification was the addition of a diode in series with the anode of the thyristor. The diode ensured that only DC going half cycles are supplied to the thyristor.

That'll be the one published in RBM Service Skill of 74' 

Is this still a worthwhile modification 50 years later? After all I shouldn't think heavy spikes are present nowadays.

1974 was the year I jumped ship from RBM to Thorn 😀. Wouldn't hurt to fit but as you say mains is probably much cleaner than it was. I believe I read somewhere that being near a newspaper printing works was bad for mains spikes.

Posted by: @jayceebee

I wonder if you mean the degauss thermistor which became two PTC thermistors in a three legged device? The most common types were white and made by Philips although one chassis did use one as a combined surge limiter and degauss.

To help, this is what John is describing

p.s. Yes I know the tray has a spello

The diode was added to protect the thyristor from negative going spikes from the mains. What would the amplitude of spikes be?

I do remember the MR508 diode was the type with thick wires and was chosen because current during the time the thyristor is conducting is very high compared with the steady power consumption of the receiver.  When one considers the short time the thyristor is conducting, less than 5mS in a 20mS duty cycle the reservoir capacitor has to hold a lot of energy. 

Do a simple experiment, insert a 1ohm resistor in series with the mains neutral of the A823 receiver. Connect the oscilloscope across the test resistor and observe the waveform. Measure the voltage and then calculate the current through the resistor.

Till Eulespiegel.

For spikes probably better to fit a VDR across the mains input, we fitted one with the 3000/3500 to prevent damage. I can't remember the type number but it looked like a disc capacitor colour coded brown, white, orange I think.

That'll be the VDR to protect W602, your banding value recall is as ever, spot on.

That's the fella, shame I can't remember what I had for breakfast though. 🤣