1967 Baird M702W

700 Series Chassis

Model: M702W

Year: 20th December 1967

System:405 Line Black & White – 625 Colour

Original List Price : £304.10

Valves: 14

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
V22 PD500 EHT shunt stabiliser

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    “”

CRT Fitted: RCA 25″ shadowmask

Power Consumption: 320W

Notes of Interest:

One of the first generation colour televisions you could have bought or rented for the launch of colour television in 1967. This is fitted with the RCA cathode Ray. This particular set is date stamped 20th December 1967.

The 700 series sets are very 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.

A number of alternative versions of the 700 series receivers were available for use on other systems other than the British 405-line and 625-line standards. The receivers involved were used on the following:-

  • Top Rank System
  • Hong Kong System
  • European C.C.I.R System
  • Rediffusion System

Introduction to this set

This TV was a set high on my list to obtain but never thought possible to find, this has now come my way. and I was offered the chance to purchase it from a very well respected engineer in the vintage TV arena, thanks Steve. This pretty much now completes the early colour ( doubt I will find the others on my want list) TV collection and will complement the other Baird models I have which can be found elsewhere in the collection pages.

All the system switches and associated linkages are in place, all being well it should operate on 405 & 625. The set had been stored in a shed for the past 18 months so particular attention will be required to see what or if any moisture damage has occurred.

This is another first generation set like the G6 and CTV25 ( these can also be found in the collection pages) that were produced for the brand new British colour television service that launched in 1967

m702-1 m702-2 m702-3 m702-4 m702-5

The chassis consists of two units – main chassis and separate tuner section with user controls. Four vertically mounted printed panels within the upright section of the main chassis, with a further printed panel on top of the main chassis tray. Solenoid operated system switching linked to three printed panel switches by a horizontal shaft.

Fitted with a 25″ shadow mask CRT, the user controls are On/Off, Colour, Volume, Contrast, Brightness, UHF tuning and Channel select for VHF.

m702-7 m702-8 m702-9 m702-10 m702-11 m702-12 m702-13 m702-14 m702-15bd700-1bd700-2bd700-3In the past I had the pleasure of being in contact with a Baird dealer, thanks for a very interesting chat Byron. I was especially interesting to hear about the early days of being a TV service engineer and owing your own shop. Byron passed among other useful parts for all sorts of TV’s, a full service manual for the Baird series. Along with the service data there was the installation guide and many technical bulletins Baird released for the series in 1967/68.


servicebd1Update: The Power Supply

As can be seen by the illustration above, the chassis withdraws on runners then tilts forward to a locked position. To achieve this two retaining screws left and right need to be removed. Once pulled and tilted it gives great access, I’m struck by the similarity to the G6 with regards to all the hard-wires……. I drop to my knees at this point and utter the following incantation, “Oh TV spirit of the great ozone, protect me from the pinged wire gremlins”.

Immediately my beady eye is off to a start, I’m sure there will be more but for now :-

On the decoder coil L230 which feeds into the luminance delay line, was hanging off and will need fixing and there’s a cap rolling about on the cabinet floor, I need to see where on earth that’s come from. Most worrying of all though are the main smoothers, C603+C604, C607+C608 both pair are 2x400uF. One looks totally shot, having vented on one side, see below. I’m not even going to bother to see if this can be reformed. I will be replacing both dual section cans with new STC stock cans from the 80’s. These were kindly donated by Malc.

These New old stock STC 400+400uF are 30 years old and required reforming, these took two days and happily reformed perfectly.

I also tested the RCA CRT tube, it tested fine and should give a good picture.

Right-hand cap badly vented
NOS cap reforming
Chassis in service position

bd702-2 bd702-3Update:

Task for today is to replace the main smoothing cans now that the NOS STC 400+400uF have been reformed. Then to replace the boost cap C454 .47uF 1kV on the LOPT.

Decided to removed the entire chassis to make it easier to work at the bench with good lighting. The more I get familiar and work on this set, the more I think what a thoroughly well thought out chassis and consideration for servicing. Especially when compared to the G6 and the Decca Battersea which were competitors of the time, the later two are sheer difficult and daunting with regards to service access.

Clearly this electrolytic has seen better days!
Bleed resistor doubled in value
Nice new electrolytic pair installed, should last another 50+ years probably more with the light use the set will get now.
Chassis ready for removal
Chassis removed leaving just the CRT, tuner and user controls.
M702 chassis on the bench component side view
M702 chassis on the bench component print side view
Further evidence of the great though given to access for servicing. The decoder panel swings out.
Need to replace C454 .47uF 1kV
LOPT released for easier access to C454
This is going to be a tricky removal
Replacement cap
New cap installed

more-2 more-7more-8bdlop-1 bdrear-1

625-Line Colour Standard

The First Power Up & Tripler Failure

The chassis was reinstalled into the cabinet and everything hooked back up. Final checks that everything was in place and ready to go…. power was applied, the valves began to glow and eventually sound could be hear from the speaker. This was followed by some loud snaps and cracks which seemed to be coming from the cage containing the LOPT/Tripler and EHT Shunt stabiliser.

Then a raster was presented on screen nice and bright, I tried tuning into the Test card but unfortunately it would appear the drive cord to the UHF tuning mechanism was not working, yep I didn’t check that before all the preparatory work. At this point I adjusted the contrast and brightness which was greeted with a further snap but this time incredible hissing.

This was still all coming from inside the cage, there was no way I was going to remove the cover and have a look due to the X-ray’s and the inherent danger. I got out my EHT wand and probed the CRT final anode and this showed I was only getting about 4-5kV, this should be around 25kV and must have been what I had when the raster first showed. My gut was saying tripler at this point.

Nothing for it but to remove the chassis once more and relocate to the bench, Once on the bench the cage was removed and the smell of component grief was evident. Upon dismantling the tripler the fault was clearly identified, the HV caps 1000pF had failed. One in particular has destructed and all the others were cracked.

Looks like another tripler rebuild is on the cards, at least this one is going to be easier than the BRC 2000. The Baird tripler is not potted but constructed from Paxolin panels that slot together the HV diodes (BY140) and HV caps ( 1000pF 8kV) are strung inside.

bdlight-1 bdlight-2 bdlight-3 bdlight-4 bdlight-5Update: UHF Drive Cord Fix and Tripler Rebuild

The UHF tuner drive wheels were seized, when the dial cord was removed and the various wheel rubbers I at least found the tuning gang was free to operate so its just the drive transfer. I cleaned all the wheels, rubbers and spindles, reattached the drive cord and now it operated OK, a little bumpy due to the rubber wheels having dents in them. I guess this happened due to tuner being static in one place all these years and the set having been stored in an outside shed with all that thermal cycling causing the rubbers a bit of grief.

bdtuner1 bdtuner2Next the tripler, the parts were all removed from the casing, now it was much clearer to see how the HV caps had failed. I cleaned up what was going to be reused, namely the diodes and wire coil wraps then set about the task of rebuilding the diode chain. The rubber insulated wire nuts were potted with “Hot-Melt” as an added precaution. Its not pretty but will be sealed in its Paxolin box so out of sight, all being well it should function.

bdtripgutsbdtrippartsbdtripbuildtripdone1 tripdone2bdchas-123Second Power On After Tripler Rebuild:

Rebuild of the tripler and repair of the UHF tuner drive was a success on both counts. Now the work begins on setting up converging and fixing the faults. The CRT does indeed look like its going to be good.


Width, height and focus from the initial first test card are a little better. Vertical linearity needs attention also need to address the shading to one side which I think is a decoder fault, this might take some tracking down. I had a similar fault on a BRC 3000. Then there’s all the usual installation set-up (degauss, Purity, grey-scale, static and dynamic convergence) to perform.

bdtcf2Update: Failing Width Boost Resistors

All sorts of fur-balls recently, the width had been steadily reducing to the point where I started to panic thinking it was the LOPT. Thanks to Tas who suggested the high value boost line resistors R486A, R486B and R486 might be the culprits.

widthIndeed they were, it looks like someone has been there before me and made boo boos on R486A/B. Instead of fitting a 2.7M, a 270K has been installed for R486B quite a significant discrepancy. Adding to that problem, R486A has also been replaced with a 2.2M not a great difference from 2.7M but add to that its gone sky high, so high my meter reads infinite resistance. Finally R480 which should be 1.8M has gone high to 2.2M but that is within tolerance. This must explain the problem with the width and why L405 was wound right out.

resistorsTypically I don’t have any 2.7M in stock I do have some 2.2M and 470K  so putting these in series gets me all the way to 2.67M which is close enough until I can order some 2.7M. Installed the three new boost resistors, not pretty but it will do. This has now restored the width. I now had a new problem, now I have the boost sorted the XRay chamber is hissing and a smell of ozone present, then the line sync was lost.

newres aggghhFor some reason R477 625 line hold, was out of whack, adjusted as per manual and its all back. I set EHT for 23kV and its settled down, the width thus far is stable. I need to adjust R420 Frame shift and then monitor to see if it sits OK on soak. Not sure why but the card is looking much better, the shading less prevalent.

aaaaNext job is to tackle the wobbly verticals

Update: Line Lock Lost

I’ve now lost line lock

The test card above is shifted to the left, this must have been an existing fault on this set as when I tried to adjust R495 which is the line shift, I discovered someone has already adjusted it to its fullest extent. Adjusting it to centre sends the test card way further to the left.I read in the manual that reversal of the direction of line shift can be obtained by reversing two flying leads. I thought this might be worth a check to see if I can get some control back on R495, It might just allow me to shift it to centre.

So after reversing the leads I needed to run through the line set up procedure. This involves shorting out TP31/32 and TP30/TP36 . You then adjust R477 (625 line hold) to get a floating lock running horizontal. Then remove TP31/32 short, adjust L432 (625 line stabilising coil) to get the floating lock once again. Finally removing short TP30/36 and you should have a nice line locked picture.

Well It would not resolve at all, It was also at this point I noticed a small but bright light in the system switch on the timebase board. The switch had moved ever so slightly and one spade was sparking. Quickly powered down, set the system switch, powered on and all was OK.

I decided as I was unable to get a lock with the fly-leads swapped, I would put things back as they were. I reversed the two flying leads so they were back in the original config. I would however need to run through the line set up procedure as they had been adjusted in the precious procedure and off from where they were when I started. To my dismay I could not get a lock whatsoever back in the original config.

I’m now worried that the sparking has shorted something, hopefully nothing irreplaceable. I’m also wondering if it might be the diodes in the sync circuit, the phase comparator diodes D31/32? These are sort of in the vicinity of the sparking. The symptoms do suggest these might be a possible cause as the output of these are fed to the line oscillator and subsequently the flywheel. Other than that I haven’t a clue what else it might be.

syncUpdate Aug 22nd 2015: Lock fault Found

I was joined by a good friend who was an ex TV engineer, with a grey-scale signal fed in, we went back over the various scope points that had been covered. John could see and indeed confirm the waveforms/voltages were just way too low. That is until we got back to TR7, the Second I.F. amp, now the waveform and voltage looked spot on . TR8 BC107, Video emitter follower came under direct suspicion now. A few more checks and then a decamp with chassis to the workshop bench.

Upon removing TR8 it tested OK, however the HFE was very wrong “4”, it should read around “350” This was checked against a new BC107 and that confirmed it was not right. A new TR8 was installed and whilst we were at it removed TR9, 10 and 11, all tested fine and had the correct characteristics so were reinstalled.


Back to the set with the chassis it was installed powered on and it could be seen straight away the video was being presened correctly. We then ran through the line set-up procedure and gained perfect lock. The picture was still shifted over to the left and we still had arcing from the spark gap.

shifttcfThe set was now back to where I was with regards to the picture shifted, so those fly-leads were once again transposed. With this done and the line/frame shift adjusted we had a centered picture. Now it was time to see why the arcing was occurring. R513 (1K 1W) was found to be open which means the PD500 was not conducting and therefore not keeping a lid on EHT. John reckons we probably had around 30kV.

With a new R513 installed the arcing was better but we still needed to run through the EHT set-up. This involved adjusting the set EHT current preset R496 for 1.2V. This could not be achieved and all manner of odd readings were obtained, it was time to replace this 50k preset. Once again we decamped back to the workshop with the chassis

With R496 replaced it was set to the centre point, whilst we were at it we also found R498 (820K) was open, this is also in the Set EHT current cct. With this duly replaced the chassis was reinstalled back to the set. Now we ran through the set EHT procedure. It was found to be almost spot on at 1.298V, just a minor twiddle to achieve 1.2V and it was set. With this correct things picture-wise were a little worse. Geometry, width and vertical needed sorting also colour was virtually gone. John explained we would need to go through all the line set-up again as now EHT was correct, its all interdependent.

After going through the set-up indeed things are starting to look a whole lot better the test card now fills the screen with the correct amount of over-scan, geometry is good and colour is back.

So what next? The red & Blue A1 pots need cleaning or replacing as they are either all or nothing. The focus is not as good as it should be. In the focus chain a bunch of 5.6M R501 – R508 resistors are suspect and probably high so I will need to check and likely replace. When these jobs are done I maybe able to get the test card looking even better. John reckons there might still be some work to do on the IF panel with regards to alignment. First I need to go though the board and check its electrolytics and of any resistors are out of tolerance. Then there’s the 405 side to look at which I’ve not even looked at yet.

Tonight I will sleep properly, with that I mean not just going to bed to only end up tossing and turning due to Baird circuits continuously passing before my closed eyes like some demented Disney cartoon.

TCFBackUpdate: The A1 pot challenge

Next job was to tackle the faulty A1 pots, R433 (Blue) & R436 (Red), they were either on or off with nothing in between.

I found a couple of smaller 2M lin pots identical in size and construction to the A1 pots used on 3500 and those are presented with 400V-900V. So I would have thought 670V the Baird presents, was within their capability.

The problem to be solved here is the original pot is physically larger then the new and the new has pin-out in a different configuration than the old. The new pot will not solder in to the existing PCB holes. the solution is to dismember the old pot and use the original pot base as an adaptor plate. It worked a treat.

pota potb potc potd pote done fin aftera1changeAfter the two A1 pot changes the gun balancing it was was much easier to perform grey-scale. It shows I need to work a bit more on the dynamic convergence, just need to sort out the final geometry before attempting that. Focus will hopefully get sorted in the week when the 5.6M resistors arrive. I also want to run through the decoder set-up procedure to make sure the luminance, reference oscillator. burst, Ident and chroma are all as per the manual.

Update: Focus Chain Resistors.

The majority of the focus chain resistors arrived today 5.6M 2W, I set about testing and removing R500, 501, 502, 503, 504, 505, 506, 507 & 508.

R501(3.9M) was spot on, as was R503 (5.6M) the rest had virtually doubled in value. The final one in the chain R508, someone had fitted a wrong value (4.7M) and besides that it read out of limit for my Fluke.

They were all replaced, the convergence was again slightly out of whack which I adjusted. I now have good range on the focus pot and a nice sharp picture.

focuschain1 focuschain2Think I’ve finally got on top of the geometry and convergence. I could sit and twiddle for hours but as someone said you just have to walk away. I’m sure others will see glaring issues but my eyes are out on stalks at the moment and I can’t see me bettering it, so for me, it will do. Just need to get to the bottom of the shading on the right.

The colour killer is working as can be seen when switching the test card generator from 625 B&W to colour test card. By the way this is the first early colour set I have where all the frequency gratings are resolved, pretty damn impressive Radio Rentals


geom1 geom2Update: Shading Fault Found

For some reason C273 was missing from my board. I assumed it must have been a mod but after scouring the manuals modification section, nothing could be found. I could not see any reason why it would be removed. More so when I read the stock faults which listed the very condition I’m having. I therefore fitted a 47uF 450V, powered on and the shading had gone.

That’s concludes 625 Colour, now to see what’s up with 405 Black & White.

Next Instalment: 405 Line Standard

Not much to say, switching to 405 brought up a full raster but the line speed was way off. Adjusted R478, L431 & R402 405-line is working fine, just a little fold-over on the right, other than that, this set is done. The RCA tube seems to be getting better with prolonged use.

The black and white is superb on 405, I think I now need to adjust the controls to balance B&W on both 405 & 625, There is a procedure for this.

One thing I have not done yet is check the EHT since first obtaining a raster.

During the troubles and the unregulated EHT with the arcing a spitting going on, as advised I fixed the “Set EHT” slider to minimum. I have not since measured EHT with my wand, frankly on three colour sets now I had flash-overs using them so its proved too risky. What I needed was a simple and reliable way to measure EHT without using the wand type meters.

Thankfully, in steps a good friend who had a professional Brandenburg EHT monitor going spare. After an exchange of beer tokens, said EHT meter arrived this morning. This meter is ideal when it comes to testing EHT on a colour TV as its in-line. You plug the input lead into the final anode of the CRT, the sets final anode lead then plugs into the top of the meters input lead. Safe testing with no risk of flash-over and the ability to constantly monitor EHT.

I set EHT to a little over 20kV and checked the EHT current was still set to 1.2V. This 20kV will do nicely and gives the old RCA tube lopt and tripler a gentle life. I’ve read its good practice to run with reduced EHT so all my TV’s are set around 22kV but this Baird gives superb results at 20kV.


The End:



4 thoughts on “1967 Baird M702W

  1. Great work I wish I had your perseverance lots of memories of the days and nights working on these sets and yes went to bed and like you the faults go round in your mind kind.

    Regards from one tv engineer to another EX tv eng brill work and thank you again for the memories

  2. Only just spotted this comment, sorry Frank, thanks for the kind words,

    Somehow working on these sets with the wistful tunes of the test card music wafting through the workshop along with the unique aroma’s these sets give off, one could forgiven for thinking no time had passed and it was their hey-day again.

    Glad you and many other engineers are finding the site good for a trip down memory lane and a peek inside the CTV’s of yesteryear.

  3. An RCA CRT and from circuits of the set I have seen probably quite a bit of RCA know how in those circuits as well. This was the first CTV I saw, it was in late 1966 at the local tech.
    Well done sorting it out.


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