1968 Baird M718

710 Series Chassis

Model: M718

Year: 22 October 1968

System:405 Line Black & White – 625 Colour

Original List Price : £356 July 1968 (RER- Trade magazine)

Valves: 13

V1 PC97 R.F VHF amp
V2 PCF802 VHF Mixer Oscillator
V5 PFL200  ‘F’ section sub carrier drive, ‘L’ section luminance output
V6 ECC81 Chrominance demodulators
V7 PCC88 R-Y and B-Y video amps
V8 PCC88 G-Y video amp and line pulse shaper
V9 EB91 R-Y and B-Y DC restorers
V10 EB91 G-Y DC restorer ( other side of diode not used)
V15 PCF80 Sync separator and frame oscillator
V16 PL508 Frame output
V17 ECC82 Line oscillator
V18 PL505 or PL509 Line output
V19 PY500 Efficiency diode

Transistors: 27

TR1 BF161UA or BF181  UHF RF amp
TR2 BF161UB or BF181 UHF mixer oscillator
TR3 BF167 First vision i.f. amp
TR4 BF173 Second vision i.f. amp
TR5 BF173 Final vision i.f. amp
TR6 BF184 or BF194 First sound i.f. amp
TR7 BF185 or BF195 Second i.f. amp
TR8 BC107 Video emitter follower
TR9 BC107 Video inverter
TR10 agc peak-level detector
TR11 BC107 agc amp
TR20 BF184 or BF194 First burst amp and gate
TR21 BC107 or BC147 Second burst amp
TR22 BC108 or BC148 DC amp
TR23 BC107 or BC147 Crystal oscillator (sub-carrier)
TR24 BC108 or BC147  7.8kc/s switch generator
TR25  ”               ”                     ”
TR26 BC108 or BC148 Switch generator pulse-splitter
TR27 BF184 or BF194 First chrominance amp
TR28 BF184 or BF194 Delay-line amp
TR29 BF173 B-Y chrominance r.f. amp
TR30 BF173 R-Y chrominance r.f. amp
TR31 OC75 Colour killer
TR32 BC108 or BC148 First luminance amp
TR33 BC108 Sub-carrier emitter follower
TR34 AC128 Red/Green line convergence DC
TR35 AD162 Blue line convergence DC restorer

Diodes: 27

D1 AA119 a.m. sound detector
D2 OA90 f.m intercarrier sound detector
D3 OA90            “”
D4 OA81 Sound interference limiter
D5 OA90 Video detector
D20 OA90 Burst gate pulse limiter
D21 BA115 Burst phase detector
D22 BA115    “”
D23 B102 Sub-carrier oscillator control
D24 Ba115 Luminance DC restorer
D25 OA90 Saturation law control
D26 OA90 Chrominance switching diodes (matched pair)
D27 0A90  “”
D28 OA90 Colour killer rectifier
D29 OA90 Protection diode of TR25
D30 M3 Frame sync gate
D31 BA144 Line flywheel phase detector
D32 BA144    “”
D36 BY140 Part of EHT tripler
D37 BY140   “”
D37 BY140   “”
D38 BY140   “”
D39 BY140   “”
D42 BY140   “”
SR40 LT156 ( westinghouse) selenium l.t. rectifier
D40 BY100 h.t. rectifiers
D41 BY100    “”

Integrated Circuits: 1
RCA CA3034V1

CRT Fitted: Mazda 19″ A49 11X

Power Consumption: 320W

Notes of Interest:

One of the first generation colour televisions you could have bought or rented following the launch of colour television in 1967. This one was manufactured 22nd October 1968 as evidenced by markings on the cabinet interior. This particular model has the Mazda cathode Ray tube installed.

The 700 series sets quite rare, this is partly due to the Radio Rentals Recover and destroy policy. I have this information on good authority from an Independent TV engineer, who had his own business when colour TV was launched and was a Baird dealer. This particular 19″ model is less frequently seen than the larger 25″ variants. In fact I’ve only seen one other and that is owned by a chap called Tas. If you have one or know of other 19″  that have survived, please leave comments below.

Introduction to this set

This set was bought from new and owned its entire life by the original owner, he also happened to be a TV engineer so he maintained it himself. He eventually passed it on to the next owner 20+ years ago, this new owner did nothing other than store it away until I purchased it in Feb 2017.

Physically the cabinet is much smaller than the large 702 (see below), much easier to move. From initial observations of the 19″ version of the 700 series chassis has some slight differences to the 25″ 700 series chassis. The 718 employs a push button tuner for UHF whereas the 702 uses a rotary one. It also has a rather nice control fascia which shimmers and has a mother-of-pearl appearance, though its just a machined finish.

Overall the cabinet was om great condition just suffering from being stored in a cold garage loft for 20+ years. I spent the morning with beeswax and ‘00000’ grade wire wool, finally much buffing revealed a splendid finish. Thankfully the mas is al there with some minor cracking to the bottom right corner. Baird used two types of mask one which was brittle the other slightly more flexible. The former is often found in a very sorry state even more so when a replacement push through tube is installed that put pressure on the mask. This is not the case with this set as it does not have a push through but has its original steel shrouded rim-band. The cracking I have can easily be repairs with some fiber matting and epoxy to strengthen the whole assembly.

At the rear the chassis has been squeezed and where the two bottles were on the M702, on the M718 is the 25kV V.D.R. focus control. A further observation peering into the rear shows the unique Baird tripler has also been moved. Further reading reveals although housed in a similar  Paxolin box, the 710 tripler is a later type. This results in the PD500 shunt stabiliser being done away with and we get the addition of the focus VDR as shown below.

Another interesting feature is the use of an integrated circuit device, the RCA CA3034V1. It was used for signal processing components with the exception of the phase-detector transformer  and contained within a  10 pin TO-5 can. Would 1968 see this as the earliest use of an i.c. in a British set? If you know please leave a comment below.

listed as follows:-

• RCA Solid State
• General Purpose Video Amplifier
• Min 3dB Bandwidth (Hz)=45M
• Minimum Input Impedance (Ohms)=2.0k
• P(D) Max.(W) Power Dissipation=26m
• Nom. Supp (V)=10
• Status=Discontinued
• Package=Can
• Pins=10
• Military=N
• Equiv= CA3030 equivalent CA3005 or CA3006

Additional Photos

Update

Time to check the status of the Mazda A49 11X. Initially all three guns read low. The Red and Green guns after a while stated to recover with the emissions climbing up to be quite respectful. Blue on the other hand rather than climbing up, fell away quite dramatically.

I decided to try a clean & balance but there was not even a slight kick on the meter. I decided to do a rejuvenate, purple sparks from the tube, this time the needle kicked and when it dropped to .4 released. A check now showed blue with good emission.

Once the set is repaired and has been given a long run I will make the call as to whether I should install a new 19″, for now its good to go on this CRT.

Now I have the chassis on the bench I will spend the evening giving it a baseline evaluation, looking for anything untoward and making notes and priorities to address.The obvious ones to start with being the smoothing caps C603+C604, C607+C608. The one which looks better is starting to show signs of fatigue. I have two NOS 400+400uF 350VDC caps in stock so will be fitting those.

Also there’s one of those nasty .47uF plessey caps, C454 @ 1kV on the LOPT, that’s getting changed.

Now I have the chassis on the bench I will spend the evening giving it a baseline evaluation, looking for anything untoward and making notes and priorities to address. I might also take a peak inside the tripler to see if there’s anything untoward happened in there. If you remember the M702 the caps in that had destructed.

Update

This evening I decided to replace the main smoothers. Whilst the new cans reformed, I took the opportunity to hoover up all the black soot as I was fed up getting covered in grime every time I touched the chassis.

The main smoothers have been reformed, I replaced the bleed resistor (220K) the old was OK but whilst I’m in there might as well.The worst can was snipped out not before taking notes on the wiring. Then the new ones were then installed.

LOPT housing fixings removed, LOPT rotated 90 degrees, this now gives me access to the boost cap C454 .47uF @ 1kV.  There is visible signs of stress developing along a seam, the old RS one was splitting in two places so was a wise move to tackle that now. It was replaced with a new Axial in stock .47uF 500VAC which should be good for 1500VDC.

I decided not to go for application of power, my reasoning being and assuming I did get a raster, I would want to quickly move to the next stage which would be applying a signal. That would then mean hauling the chassis out and addressing the seized tuner.

Therefore I’ve decided that tackling the seized tuner now is the way I want to proceed, also I needed to get at that filter cap to give it the snip. All this means the control assembly would need to come out. The procedure for this is quite simple, remove all the user control knobs, undo two long hexagonal  nuts, remove the panel which holds the power cord and UHF/VHF aerial connection and out it all comes.

The UHF tuner is now operational, as simple clean and oil has restored normal operation. Replaced the old filter ‘Bomb’ with a new safety cap.

More work

Today I’m tackling treating the cabinet, this involved removing the CRT Sheild and the CRT.

The reason for all this was the discovery of a number of worm flight holes in the rear of the cabinet and there were little piles of dust on the ledges beneath. This would indicate the little blighters have already flown but who knows what remains dormant.

In some respects the worm hole observation is fortuitous as it sent me down a more thorough approach to this set. With everything removed I can clean up the CRT, Shield, check the degauss coil, scan coils and static convergence clover and fix the cracked Mask. Had there not been worm holes I might have put all that off.

Speaking of which the scan coils and static clover are Plessey Corporation.

Mask

The mask is cracked in two places, Baird seemed to use two types the brown very brittle and the grey which was a little more flexible. I have tow with the brown and both have suffered with cracks and breaks. This one will get resin matting applied to the rear in an effort to provide additional strength and repair the cracks.

the mask strengthening (micro-fibre matting and epoxy resin) has set, this should help to stop the existing cracks spreading whilst also giving the mask some extra rigidity.

I also decided to disguise the very obvious reinforcement I had applied to the rear of the CRT mask. I found a near colour match and its certainly made the whole repair blend in much better. I know you’re never going to see it but I knew it was there and it bugged me.

The mask from the font is looking much better and the whole thing doesn’t flex like it used to, a much stronger mask altogether.

Now to await the cabinet to dry then reassemble and power up, more to follow……….

The cabinet is fry so it was all put back together ready for power application.

Update

I got fed up working in the TV display room so moved the whole set to the bench, thankfully I built it to take large console TV’s.

Powered up after a while EHT came up at around 17kV, after a further short while began to fall, the was a flash followed by smoke and the set shut down. The 3A mains input anti-surge fuse had blown. Also the thermistor that thermocouples to the degauss had gone open circuit. Replaced the 3A fuse powered up with the chassis fully extended so as to see where the flash and smoke were coming from. Again the EHT rustled up but this time only around 10kV, the the flashing started. It was arcing on the top panel of the LOPT where the PL509 anode lead connects. Powered down and investigated. Turns out the connection tag had been tracking much carbonisation  had occurred. No amount of cleaning would fix this, it would need replacing.

I had some thicker paxolin and some eyelets so set about replicating the to panel

Powered up again and this time 22kV and stable. ( meter shows H.T. B+ for our American friends)

Now to feed in a test card and see what we get. Well not a lot just a field collapse and a wiggle one at that.

This is where I now descended down a rabbit hole for a couple of hours. First I feared for the the Field output transformer but after copious tests with the meter it ohm’d good, Then I started to fear the worst and suspected the scan coils, again after much checking they were found to be innocent. The suspicion started to fall upon R437 on the convergence panel, this is the field deflection balance pot and is 5R. I removed it and when tested was erratic, most erratic and very very high. Once replaced and powered up field scan was restored. So after many years of sitting in a barn the set now lives. There’s still plenty to do, the static convergence for a start and there’s a decoder fault as we don;t have colour. More to follow soon.

Next Update

This is by no means definitive, just a quick ‘Rough N Dirty’ static, height, linearity and focus set-up to get a feel for whats what and how things might shape up. Loved fumbling on that scary looking focus arrangement. A full purity static and dynamic will follow but first I think I might disable the CK, see if that reveals anything about what colour is doing.

Update

The use of the eyelets proved to be a folly the fault of tracking between the two connections happened again. I decided to build another LOPT top plate but this time following the same tags that Baird used with one additional mod, an air gap.

Below

#1. Original; #2. 1st attempt; #3. 2nd attempt

So far this revised 3rd attempt seems to be holding up.

Next was to get the B&W picture to a reasonable state, “get it right in black and white”

Now Where’s the colour

So now to see why the colour is missing. I did a check to see if the reference oscillator was running, it was.

As was the burst

After following a few rabbit holes as I often do on decoder diags, I found the fault was in the 7.8kHz switch generator. TR24’s emitter decoupling capacitor C235 12.5uF 25V, was acting as a resistor thus resulting in a low ident signal. Replacing with a 10uF ( nearest) restored colour.

What Next?

Purity, grey-scale, Static and Dynamic convergence then a full  decoder setup.

Update

The degauss circuit was disconnected ( by me) since the fuse blew a few days ago and spotting N600 was o/c I had assumed this was the cause, I was wrong. Today looking to reestablish the degauss circuit,closer inspection revealed the situation with regards to degaussing.

N600 being open had nothing to do with the 3A anti-surge fuse blowing and everything to do with the arcing from the LOPT. N600 would appear to be an old historic failure that was left in place ( its now snipped out to save future confusion). VA1103 has been replaced by the fitting of a E299DH up on a tag strip on the CRT shield. This type of thermistor was used for degaussing on the Thorn 3000 and other such chassis’ of the time.

I checked all this against the Baird M702 which still has the original N600 strapped to R601 and just the P600(VA8650) up on the CRT shield tag strip.

The M718 modded degauss circuit was reconnected, tested and is working fine as it always was.

Now on to sorting out the purity I think I’ll have another go at this as I still need to lose a little blue tinge from the bottom.

I was also reminded by Frank I need to do a manual degauss to remove any magnetism that cannot be sorted by the sets circuit. With this sone and another purity, static and grey-scale, things are steadily getting better.

AFC

Tried as per book, defeat the AFC short tp9 to adjacent pin. Connect AVO 8 on 10V range to output test pin on AFC module, tune channel. Now read the voltage ( in this case 4.5V). Remove the short, meter will drop, then adjust L50 to raise meter back to where it was before.

Tried adjusting the discriminator coil L50 as described, this appears to make no difference, the meter stays put. Defeating the AFC and tuning correctly has now given better definition to the verticals, especially noticeable on the left but its still not right.

Not forgetting this colour TV is also a dual standard, today I set up the 405 side. A little tweaking of the 405 set EHT pot, and some tweaks to the height and lin TCC is looking fine. I also did a complete check and clean of the system switch, it switches smoothly between standards.