Britannia B1014R - Fidelity chassis

[Techman]

I’ve got this rare relic to look at. It’s a Britannia B1014R, which I think uses the Fidelity ZX4000 chassis. I think this set is an early version and it’s possible that the circuit diagram I’m looking at is a slightly later revised version, as it mentions the Britannia B1014’S’ as opposed to ‘R’ suffix. It's pre-scart, but later than my time working on TVs, which were over by the early 80s. The set has led a hard life, living high up in two different kitchens for most of its life, latterly above and just to one side of a deep fat fryer, so it needed a thorough external clean before it could be handled to any degree.

The fault is lack of audio. The volume can be advanced for the first third of a turn of the volume control, but any further adjustment of the control causes the audio to decrease to silence. Originally this set started to develop this fault near the top end of its range, but this has gradually moved down to the position that it now is, with only a low level of audio near to the lower end of the control range. I was tending to think that this was perhaps some kind of biasing fault due to leaky coupling. I’ve not looked at the innards yet but having examined the circuit diagram (if it’s the correct version), I note that there’s basically not a lot to go wrong, other than basically two ICs. The set has been subject to EHT crackling and flashing over when the set has been turned on during cooking (the set would need to be turned on to warm up slightly before commencement of any cooking), but this didn’t always happen, so I’m wondering if the first ‘multi’ IC has been damaged by this. The video/picture side of the set is absolutely perfect, although when in a damp steamy atmosphere the tuner would drift completely off frequency accompanied by the sizzling EHT, so this set has done well to survive above a deep fat fryer and a gas cooker for many years of its working life.

Below are pictures, first of the set itself to jog memories (taken today after it had dried off from having its external cabinet clean up) and the next two showing clips from the relevant parts of the circuit diagram. In the first of the two circuit pictures it can be noted that within IC2, a TDA4503 (assuming this is what’s actually in the set – we don’t know yet) there’s an audio amp preceded by a volume control, with the actual control pot shown down at the bottom left corner of the picture and connecting in via 1V6 to pin 11 of the chip. The audio output from this chip goes out at pin 12 via 2V5 and on and into pin 3 of the audio output chip shown in the second circuit picture via R27 and C16. There’s not a lot component wise to go wrong, other than possibly that first chip.

I’ll no doubt have to give the innards a good blow out and clean when I take the back off to have a look, so what do we reckon before I look at the innards - a dud section in that TDA chip, or something simple (and cheap) in the few other associated components around it...any thoughts?

[Techman]

Couldn't resist having a quick late night look at this set.

It's not too bad inside, just gave it a bit of a clean. Those are of course expected voltages marked around the chip. There doesn't seem to be any R56 on this version, otherwise things seem to agree with the diagram. Taking a few readings round the chip didn't seem to throw anything up, so I'm starting to suspect the chip itself.

[jayceebee]

Don’t have any experience with this model but have repaired sets using the TDA450x series ICs. The unusual audio symptoms you describe are most often caused by the 6MHz detector coil drifting off tune. A very small adjustment should return the audio to normal.

John.

[Welsh Anorak]

Not seen one of these for years! It's an odd fault, certainly. A scope will be invaluable here to have a look at the signal exiting the TDA4503 to see if it increases in amplitude as the volume control is adjusted. I have a vague recollection of the 10 ohm feed resistor to the output stage going open circuit but that's all I can remember - these were pretty good basic TVs.

[Techman]

Thanks chaps.

R56 was actually there when I looked closer. There's voltage both sides of R28, 10 ohm. A scope test showed there was audio at the output pin 12 of the TDA which rose and then disappeared with adjustment of the volume control. Looking at the data sheet for the chip shows there's a lot more going on inside it than is indicated in the simplified block shown on the diagram. I'll look at the possibility of the detector coil adjustment, but other than that it's looking like the chip itself. I see they're available quite cheap, although I may actually have one - I'll have to have a look in the loft if I can face looking through boxes of old chips, all out of order, that I was given a few years ago by a chap that must have had a misspent youth stripping components from hundreds of old boards. He was having a clear out due to getting married and moving into a new home with his wife, so gave me 'half' (so he said) of his stock. There's not enough time left in life to sort all these chips and other components into some sort of order and list them all so that I know what's what, although I've sometimes found what I've been looking for with just a casual rummage. At least the chips are separated and all together in several containers and I know the physical size of what I'm looking for, so I may get lucky, but a couple more tests to be made first.

[Techman]

Originally Posted by jayceebee

Don’t have any experience with this model but have repaired sets using the TDA450x series ICs. The unusual audio symptoms you describe are most often caused by the 6MHz detector coil drifting off tune. A very small adjustment should return the audio to normal.

John.

You were absolutely right!

I would never have thought that would have caused a fault where there was some low audio that disappeared as the volume control was increased past a certain point. It must be to do with the circuitry that we can't see within the chip, but identified as the sound synchronous demodulator, shown as a block on the chip data sheet and connected to L5 via pin 13. I would have expected just a general lack of, or low, thin audio, increasing throughout the volume control range. It's probably down to slight component value variation around the coil, there's an internal 'un-valued' capacitor shown, which may not even be a physical capacitor, but perhaps just the stray capacitance of the coil itself. Anyway, as you described, adjustment of the coil slug cured the fault, so many thanks for the pointer, I think I would have gone round in a few circles fault finding that one.

As shown in the first pictures in this thread there's no mains on/off switch, just a hole where it once was and the set was always switched on and off at the mains wall socket. I assumed that the switch had been previously broken and bypassed, but on looking at it the switch was all intact and the wiring looked original. Shining a light in there I could see what looked like the broken off push button wedged to one side and there also seemed to be a metal washer on the spindle of the switch itself. I removed the switch and retrieved the plastic button which had one side of its fixing broken off and missing altogether. Someone had obviously had it apart before and added that washer for some reason. The remaining side of the switch fixing lug looked brittle and there was a fine crack in it, naturally, a bodge was called for. So out came the reel of RS tinned copper wire of an appropriate gauge and a needle file to produce a couple of notches round the plastic lug. The two bits of wire were wound tight and then the soldering iron used to melt around the area of the crack and the wire - as shown in the pictures below. It's a bodge and it may not last, but so far it's been switched on and off a few times and seems to be holding up well.

While working on the switch I found that the degaussing coils had been disconnected. I thought that perhaps I'd accidently pulled the plug off the board, but the plug had been pulled right up out of the way through its tie. I don't know why this had been done and reconnecting the plug doesn't seem to be causing any problems, so that's a bit of a mystery.

So that's it, the set will live to fight another day, pics of the switch etc. below:-

[McMurdo]

Our trade-only cash and carry sold Britannia badged fidelitys. Is that a remote power-off facility? Only seen those on the Rediffusion sets before! I have the non remote version in grey.
Perhaps the set had developed the old habit of firing the knob across the room. If it's a kitchen set it might've ended up in the tossed salad...

[Richard_FM]

The Goodmans set my family had as a second set used to spontaneously change channel or turn off. This used more or less the same chassis.

Quite annoying when used as a monitor for our Acorn Electron in the middle of a game!