Mystery Decca (CZ8150)

[PaulM]

Can anybody identify this 26" Decca, please?

It appears to be some sort of prototype and it came from Bradford.

The set has no model number and the CRT label just says: 'Toshiba Engineering Sample 660AB22'. There's also a date of 1978. There is nothing on the fibre back whatsoever and nothing inside.

It's a single board chassis with a teletext module added.

After a duff field scan chip and red video output transistor it's now a 'go' other than it shuts-down after about an hour or two. It can then be re-started with no problem. I feel that I need the schematic to go further and it needs to be identified to be able to find that.

The CRT is like new and the set is really quite good.

Many thanks for any help - I was never really up to speed with Decca models.

Best regards,

Paul M

[FERNSEH]

I can't give you a specific model number but the chassis is the 90 series with teletext.
The text board is the Mullard VM6101 and the smaller PCB below it is the special inteface to convert the Decca remote control codes to those required for the Mullard VM6101.
Decca made another chassis which is similar to the 90 series, the 110 series. This chassis was designed to drive 110 degree Mullard 30AX CRTs.
The Toshiba 660AB22 is likely to be a 110 degree deflection CRT so I'd reckon that this development chassis will have beefed up timebases to drive the wide angle tube. The 90 series was designed for 90 degree in-line CRTs.


DFWB.

[Nickthedentist]

Wasn't the 70-series chassis similar too?

[FERNSEH]

Hi Nick,
The 70 series was very similar to the 90 series except it was designed to drive the smaller CRTs used in portable sets. The EHT might have been lower.
I became a Decca dealer in 1979 and all the first sets I received from the company were 70 and 90 series models. The 110 series came out in 1980.

DFWB.

[PaulM]

Thanks for the replies, guys. It is indeed a 110 degree CRT.

On the back of your information, I found a useful article in the September 1983 'Television', page 596/7 by Neil Dobson on the Decca 70-90 chassis power supply. It includes the full circuit and the helpful tip that intermittent shut-down can be caused by dry joints on the TDA2581 SMPSU PWM controller chip and its associated O/P transformer. Having re-soldered same, the set now seems stable!

Best regards,

Paul M

[ben]

Interesting beast and from my favourite era of sets - wonder what its history was?

[Maarten]

It would be very interesting to see some higher res pictures of this set (in which details of the electronics, labels, etc are visible). Also of the picture tube, neck, yoke and any printing that may be on the cone. I'd like to find out if it was an attempt by Toshiba to make a 30AX compatible tube.

[FERNSEH]

It would also be interesting to determine the neck diameter of the Toshiba 660AB22 CRT. The Philips/Mullard 30AX has a 38mm diameter neck. The S4 PIL type of tube has a neck diameter of 28.5mm.

DFWB.

[PaulM]

Some time ago, I flagged a rather unusual 26" Decca set with Teletext which appears to have been a prototype as it has various 'issues'. There's what seems to be design department 'handiwork' in various places, the CRT is labelled only as a Toshiba 'engineering sample' (1978 date) and the set came from the family of a late ex-Decca engineer in Bradford.

I had put this on one side for a rainy day (!) and finally its turn came round again. It has always suffered from random shut-down and re-starts. Re-soldering the SMPSU as per the recommendation in the September 1983 article in 'Television' (mentioned in the original thread) made a vast improvement. However, it still has a 'mess about' now and again, just when it fancies! Thus, I set to once again, and after a swapping out lots of 'low' SMPSU electrolytics, the last one to be changed (connected to IC601, pin4) seems to have fixed the problem. The capacitor goes to ground and is the decoupling for the 'counter' shut-down system which the TDA2581 IC features.

All well and good, but during my deliberations, I applied the DVM probe to the raw HT (C624 positive, rectified mains) in with no DVM ground connected and the whole SMPSU cycled through its shut-down and recovery mode. This could be repeated and was not a fluke. It is categorically not mechanical - this is either residual radiated rubbish and/or capacitive loading (the meter is on the bench). Since changing every electrolytic (apart from the main smoother which was checked and good), it's still incredibly 'twitchy' if you stab it with just a floating DVM probe. The bottom end of the smoothing capacitor is fine - you can connect at will - but the 'hot' end is super sensitive. Adding extra low ESR decoupling makes no difference.

My question is, is this normal behaviour for this series of Decca? It's now stable and providing I don't annoy it in this way, it's good. I've never known a SMPS do this (and I've also designed them, so I know the wrinkles . . .), but this one is a 'floater' tied to chassis ground only by a small cap and a 3M3 resistor. Apart form the venerable CTV19, Deccas have not been something that I've tussled with and I just wondered whether this was known 'normal' behaviour? Extra decoupling/bypassing does not affect things. Any Decca experts out there who might have an insight on this, please?

Many thanks for reading this rather long post!

Best regards,

Paul M

The circuit (or an approximate version) is attached form the 'Television' article.

[Welsh Anorak]

I remember the unusual BFR91 (I think that was the one) transistor in the base drive could cause odd symptoms - Decca used to supply an equivalent. This must make a change from the CTV19!
I have a prototype Decca 100 with Nixie tube display and remote control. Seems Decca gave sets away instead of scrapping them.

[neil29]

I also had a 100 chassis set which had an early texas chip based teletext system in it from someone who I think worked for decca. and I passed it onto someone who broke it up unfortunately.

Cheers
Neil.

[PaulM]

You're sort of close with the driver transistor number, it's a BSR59. That's a Darlington device and so far as I can see, it's all good and the pulse drive looks OK, plus the SMPSU output transistor is relatively cool, so running efficiently. The associated transformer runs a little too hot for my liking - is that normal? Bear in mind that this set will be greedier for power than earlier ones. As I don't have the manual (if there ever was one for this variant), I've set the HT to 165V as it seems happiest there and that's in the middle of the adjustment's range. Everything else on the secondary side also then comes out about right too with the width correct and EHT plausible.

I've never seen a SMPSU behave like this - why is it so sensitive to connecting almost anything to the incoming HT line? 3ft of wire and it's fine, connect the meter (no ground wire) and the trip cycles. I can see no clues on the waveforms - they all look about right for this class of supply, and apart from a few 'wobbles' in the width for a few seconds after switch on, it's fine. Very odd indeed.

Cheers,

Paul M

[michamoo]

The Decca 100 really did give superb pictures.

[PaulM]

I've finally found the model number!

It's a CZ8150

See this rather good show of Decca TV sales brochures: https://www.radios-tv.co.uk/decca/

It's towards the bottom. I wonder how many they made of this 30AX based 26" teletext beast?

Can't find a manual for the 110 chassis for sale or on line, so still guessing about the recommended HT voltage but 165V seems 'comfortable'. Anybody know what it should really be?

Best regards,

Paul M

[Welsh Anorak]

Never heard of this chassis - I thought the 120 series (flat chassis) was the next when Tatung took over. I suppose as they'd always had a large-screen set it made sense.
In fact the front panel and cabinet look very like the 26" 100 series prototype I mentioned earlier.
I quite liked the 70 series portables - except when I accidentally snapped the obsolete thin-shaft customer controls. That happened more often than I care to admit.
Glyn

[Maarten]

Toshiba made a 30AX tube?

[PaulM]

Well, it looks like a 30AX. On the bowl, it says: 'Toshiba Engineering Sample 660AB22', but I can't find any definitive references.

How many of these sets Decca sold is absolutely unknown, but my guess is that my example is a prototype and that very few (if any?) were actually sold - especially ones with the Teletext option.

I'm open to opinion and knowledge, but it is what it is - and it works pretty well too!

Best regards,

Paul M

[Maarten]

Quite interesting indeed!

[Robint 47]

Paul. From the Decca service manual for the 110 series CTV, setting HT is as follows. With the receiver operating at zero beam current set the ht control VR618 to give a meter reading of 148V at the junctionof R404 and C402.

Sorry I can not answer any of the other points it's been so long since I worked on colour TV, I swapped over to audio just after the 60 series.

Regards, Robin.

[FERNSEH]

With regard to the Toshiba 660AB22 CRT. I'm sure this tube was fitted in the UK made Toshiba model C2695B. CRT neck diameter was 28.5mm.
Production of Ernesettle made Toshiba TVs started 1979.

DFWB.