CTV 1975 22" Granada C22NV2; Tandberg CTV2-2

It was worth it just to see the grin on your face when you opened your front door! 😊 

That cabinet has come up lovely! 

I note the presence of the Tandberg Viking in the service manuals - rather like the Tektronix 'freak' in their late 60s/early 70s manuals! I first met the Viking in a cassette deck manual.

The area surrounding those RGB output transistors looks like it's had quite the roasting! 😮 

Found a more appropriate stand for the Granada, than the Thorn one I had initially used. This one has wheels, and more importantly, enough room to place a video underneath. I had a Granada Stereo VCR, model VHSAH3 in the collection, so now I have a full Granada set up.

The set my parents used to rent from Granada had a trolley stand like that.

That's a nice looking set Chris, displays well with the VCR.  I didn't see many Tandberg sets in this neck of the woods, half a dozen at the most. Never saw one badged Viking I didn't realise Granada rented them, we had a local Granada ex-rental shop (remember the "clever Leopard" adverts?) so I am surprised there wasn't a few about. 

Posted by: @crustytv

I believe, using the Granada chassis decode guide, the model might well have been C22NV2. Here's why:

• C = Colour
• 22 = CRT size
• N = 1975
• V = Tandberg
• 2 = UHF/VHF tuner

Looks like I was not wrong on the model number. Out of curiosity, I searched the television mags for C22NV2 and came up with and article from Apr 1988.

This is a VERY high hour set. The A56 140X red gun was very low, and I mean very low. I left it cooking at about 8V to see if it would cook the red cathode, it did not. I then ran a clean and balance, again no improvement. Nothing to lose with a rejuvenate, that did it.

As mentioned above, the set is a high hour ex rental, it's caked in fifth and the tuner has all but fallen apart, hence the wanted ad here.

I decided to power it up and for once threw caution to the wind, giving it full mains. I noticed the tripler energised, as a very large cobweb stood to attention. However, no sound, no raster and no signs of life except an ominous ticking/clicking sound from I believe the PSU area. The set employs a SMPSU, my first instinct the set is tripping, and a safety circuit is operating.

After removing the PSU, and getting absolutely covered in grime, all was revealed.... Yikes.

I had the back off this set with the full intention of making a start, however that has all changed. I'm not sure I want to get bogged down with this, especially as the tuner is going to be a real problem. I don't think there is one area that does not need some work. I'll approach this as a long term phased project, a little work here and there, whilst I work on other sets. For now the back is going, back on this one.

Oof! 😱 Better you than me. It looked like it had a hard life. Kinda feel bad for having so much success with the two TVs I got from the same vendor!

Are any of the boards/parts interchangeable with your other non-hybrid Tandberg?

Ok, so looks a bit of a challenge but that’s never stopped you before. Replacing that can may be all that’s wrong with the PSU after all, your observations of the spider web as you say suggests the LTB and tripler look to be doing something. Before going too much further I personally would like to see a ring test of the LOPT especially if it has an AT number.

For the selector unit if you can’t obtain a replacement there are still options. If one button can be latched then that’s all you need really. If no button latches but you can access the button contacts then you could employ a touch tune IC such as an ML237 with one of those banks of tuning pots from a VCR and some LEDs for indicators.

What I have found curious is on another well known forum, there was a discussion about what solid state TV's can and cannot be woken up on a variac. A member categorically states the CTV2 cannot, and I quote, “do so at your peril".

Yet, the manufacturer's manual I have clearly states the opposite. So who is right?

I'd go with what the Viking says, Chris 😉 

Seconded, and it even tells that you can bring it up from 0V AC input which is something I wouldn’t normally do. Maybe Trevor can add something as he seems to have some experience of this chassis?

Hi.

A word of caution on the CTV2. We rented Tandbergs both the CTV2 & 3, The  CTV3 is a terrific set, extremely reliable and long lived, the older CTV2 has a very rough and ready PSU, which often would either just be dead or blow many components. One of the big issues was it didn't cope particularly well with mains fluctuations and they cannot be wound up on a variac, the manufacturer repair kit clearly stated that in the instructions.

The PSU is best repaired with the kit as individual component replacement never really was successful or you continued to blow various items, Tandberg never recommended taking that route.

I personally had a CTV3 for at least 10 years and was given to a friend who used it a good further few years, I think it blew a fuse once! 

The CTV2 sounds like a nightmare set. Very bad design!

Peter

Background on Tandberg CTV Range

CTV1
https://www.radios-tv.co.uk/1970-tandberg-ctv-1/

The Tandberg CTV1 chassis, is the predecessor to the CTV2 & CTV3 range of 110-degree solid-state colour Televisions. In contrast, the CTV1 employs an A56 120X 90-degree deflection CRT and hybrid chassis. The line oscillator uses a BC157 and an ECH84. The BC157 is the oscillator stage, with the heptode section of the ECH84 as a reactance stage and the triode section as a voltage amplifier to give sufficient voltage swing to drive the PL509. Some earlier receivers did not have the transistor oscillator stage, beware the service manual only shows it with.

The mains transformer has two l.t. windings, apart from the 6.3V winding for the CRT heaters, and these feed two bridge rectifiers followed by regulator circuits. One gives a regulated +22V rail and an unregulated +29V rail, the other regulated 18V and +6V rails. The h.t. supply is straightforward, consisting of a half-wave rectifier with a choke filter.

The three PL802 RGB output valves drive the CRT grids, NOT the cathodes. A.C. coupling is used, with single diode clamps. The clamp voltage is set by the brightness control. The CRT cathodes are used for beam limiting and field flyback blanking. Line flyback blanking is carried out in the CRT first anode circuit.

A56 120X, 3 x PL802 RGB, ECH86 line osc, PL509 Line output PL509 and PY500 efficiency diode.

CTV2-2
https://www.radios-tv.co.uk/1975-granada-c22nv2-tandberg-ctv2-2/

This early CTV2 (CTV2-2) uses a 110 degree delta gun CRT A56 140X. It has the control panel at the bottom right. The CTV2 has a large metal chassis frame, with a "signals" board on the left, with several plug in boards (tuner/IF, decoder, line oscillator and sound IF/output). On the right is the line output and EHT section and in the middle, at the bottom there is the screened switch mode power supply module. There is also a small board at the top, between the two large boards, which is mainly concerned with pincushion correction.

CTV2-4
https://www.radios-tv.co.uk/197x-tandberg-22-ctv2/

Later CTV2 sets (CTV2-4) which I also have in my collection looks very similar at the front to the later CTV3 series. The CTV2-4 employs a multiband touch tuner, there are eight square pushbuttons in a horizontal row. Beneath a flap, four horizontal rotary controls, and preset tuning for the tuner and multiband selectors, a headphone socket and a push-push on-off switch. The non-remote versions have eight touch-tune pads, three slider controls and a pus-push on-off switch. That also had a pull down flap concealing the preset tuning pots. Finally, there was a 1/4" jack socket for headphones and a seven segment red LED number displaying the touch pad programme selected.

CTV3

The CTV3 which I do not have but would like to find, used a 30AX CRT. It had two separate vertical PCBs, one either side of the CRT. There was no common metal chassis. The left PCB was RF/IF/decoder/RGB drive/sound and the right PCB was power supply, timebases, EHT and convergence.

There are superb and useful articles By Eugene Trundle and W.S.J. Brice, on the function, and servicing of the Tandberg CTV2-2. These can be found in 'Television', Oct 1975 & May 1976.  It's worth repeating them in this thread, as there is very little out there on the web covering real-life repairs of the Tandbergs. As ever, VRAT will aim to fill that gap via threads such as this.

The Tandberg On My Bench

The following will help to provide a sense of the design, and function, of the Tandberg CTV2-2 PSU.

The CTV2-2 Switch-mode Power Supply

Oct 1975 E.Trundle

The Blocking Oscillator Circuit In the Tandberg CTV2-2 110° colour chassis uses a novel form of switch-mode supply, which could become more widespread in future. The circuit is shown below. The incoming mains is rectified by the full-wave bridge rectifier D976-9. The resulting -280V output being smoothed by C977 and used to power the blocking oscillator Q977, which supplies power to the whole of the receiver except for the c.r.t. heaters. The primary winding of the blocking oscillator transformer is winding a -d which forms Q977's collector load.

The feedback winding is i-h, which is connected to the base of Q977 via the limiter resistor R984, capacitor C979and the speed-up diodes D980/D981. Q977 is either fully saturated or cut off, so its dissipation is low. The output is a square wave of varying mark -space ratio depending on the frequency, which is varied to provide the regulation. The square wave output is rectified by diodes D984/D987 and filtered to provide the various supply lines required by the receiver. The oscillator's high basic frequency ( > 25k Hz) greatly eases the filtering and accounts for the low values of the smoothing capacitors. The transformer T975 itself is a surprisingly small ferrite-cored device.

Stabilisation

The voltage appearing across winding g -h is rectified by D982, with C975 as its reservoir. This voltage (approximately 24V) is proportional to the unit's output voltage .and sets the base current of Q975, whose emitter voltage is stabilised by Zener diode D975. The conduction of Q975 is thus governed by the unit's output voltage. As this increases, the collector of Q975 moves in a positive direction. Taking the gate of thyristor Q976 with it. The regulator's “earth” line is point P8, at Q977's emitter, but as a result of the presence of R982 there is a sawtooth ripple voltage present here.

The amplitude of the sawtooth is proportional to Q977's emitter current. This sawtooth appears at the gate of thyristor Q976, which fires on the peaks of the sawtooth voltage. As its gate moves positively, due to the action of Q975, so a lower amplitude sawtooth voltage corresponding to less current in Q977 is required to fire the thyristor. Each time Q976 fires, it passes a negative pulse from the -280V line via C982 to the base of Q977, cutting it off early. The output voltage is thus stabilised against mains voltage and load current variations. Capacitor C982 is charged to about 4V by D989 and D990 from winding k -f on the transformer: this holds the anode of the thyristor at the correct voltage.

Crusty Note: In light of what Trevor has stated, can someone brighter than myself explain why the CTV2-2 PSU does not supposedly cope with voltage fluctuations, when it appears to be designed to do just that.

Kick -Start Circuit

At switch on, the reservoir/smoothing capacitors C983-C990 present a heavy load to the transformer on the positive half -cycles of its output, due to the charging current. Thus, C975 will not charge fully via D982, and the regulator transistor Q975 will not conduct. The negative half -cycles are not loaded, however, and as a result C976 is charged to about -30V by diode D983. This voltage is stabilised at -7.5V by Zener diode D988. The potential divider R980, R991 supplies -3V to the gate of thyristor Q976. This means that the current in Q977 must be very high (3.7A) before the voltage developed across R982 becomes high enough to fire the thyristor and cut Q977 off.

The feedback voltage from point i on the transformer is still too small at switch on (due to the loading effect of the charging filter capacitors) to maintain oscillation. Thus, we need a starting mechanism to get the circuit going. At switch on, the charge on C982 increases because winding k -f is so phased that diodes D989 and D990 conduct on the “unloaded” half-cycles of the output. C997 therefore charges through the base -emitter junction of Q980. As a result, Q980 conducts, tending to discharge C982, which is already being drained by the conducting thyristor Q976. The net result is that the charge on C982 becomes too low to maintain the thyristor in conduction. The thyristor is thus held “off” until C997 has charged and Q980 cuts off. This takes about 100msec, by which time the output filter capacitors have charged, and the excessive load has thus been removed. The circuit then operates normally.

Overload Protection

If a heavy load is presented to any of the outputs from the unit (a short-circuit line output transistor for example) the regulator will function as described above but without Q980 conducting. C982 will eventually discharge through the thyristor, cutting it off and allowing Q977 to conduct briefly. C982 will rapidly charge again, switching the thyristor on for a long period until the charge on C982 has once more leaked away. The mark -space ratio of the blocking oscillator's output waveform thus becomes high, reducing the average current to a safe level. The repetition frequency in fact falls to about IkHz, and a characteristic buzz is heard from the oscillator transformer. These conditions are maintained (even if the load is removed) until the set is switched off for a few seconds and then switched on again, allowing the quick -start circuit Q980 to operate once more.

May 1976 W.S.J Brice

The regulated power supply uses a chopper arrangement, the chopper itself consisting of a blocking oscillator whose on/off time is varied by feedback to give the required regulation. 160V, 250V and +24V and -24V regulated supplies are given from a -280V rail provided by the bridge rectifier across the mains input. The operation of the circuit was described in detail in the October 1975 issue of Television. The article also provided useful fault tracing hints.

The circuit is shown below. Blowing of the 2A mains fuse is usually caused by failure of the BU126 chopper transistor Q977. This usually damages the BRY55 thyristor Q976 and the BC157 transistor Q975. Feedback from the blocking oscillator transformer T975 controls the conduction of Q975 which sets the firing point of Q976, thus varying the blocking oscillator's triggering. The BU126 itself can be damaged by a faulty C981 in parallel with it. And since the value of R984 depends on the gain of the BU126, it is a good idea to change all these components whenever the BU126 has to be replaced.

When presented with an overload, the oscillator frequency falls and the power supply produces a distinctive buzz. Typical faults which can cause this condition in the power supply are failure of any of the four supply rail rectifiers D984/D985/D986/D987 or their associated reservoir capacitors. A method of checking which supply rail is causing the overload is to disconnect each diode in turn and see whether the power supply operation returns to normal. Disconnect only one or two diodes at a time, since if the transformer is left without a load, it is possible for the BU126 to be destroyed.

Outside the power supply, the line output transistor going short-circuit is the most likely cause of the overload condition arising. A faulty BRY55 thyristor can cause a hum bar with associated line sync pulling. This is similar to the symptom when the bridge rectifier reservoir capacitor C977 loses capacitance: this shows up as hum modulation of the picture width.

Finally, me again

Looking at the description of the 'Kick-Start' & Overload Protection circuits above, I think I can see the failure of C977 is why the TV failed to power up and the ticking I could hear.

Although I did have 400uF cans in stock, they were multisection cans, physically too large for the available space, and a waste of the other sections which would not get used. Therefore, I've ordered a new single 400uF, and will tackle the PSU when it arrives.

Posted by: @murphyv310

One of the big issues was it didn't cope particularly well with mains fluctuations

That seem strange as the very nature of a SMPS was that it should be capable to cope with a much wider variation than say a linear type, how did it cope with variations of load also? The variable frequency mode of operation which seems to be pioneered here was certainly used in other PSUs, one of the best known to me is the one utilising a TDA4600 and used by many manufacturers. 

Another question for anyone that knows the CTV2, when and why did Tandberg about turn on the recommendation of using a variac?

What I'm having trouble with is why is C978 drawn the way it is?

I thought that was how a delta filter cap should be drawn