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Alba T1520 (BRC 1500 chassis) revival

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Cathovisor
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Tackling VT6 first, with the BF184 fitted, the base is now at 0.6v and the emitter at 0v, which seems more normal. The base was at 25v and the emitter at 0v with the old lockfit in place.

The collector voltage should be 11.5v. With the old lockfit it was 25.7v. With the new transistor fitted it's 7.27v, which is a bit low. I wondered if R20 (3.9K) may have drifted but it reads 4.2k so it's not far off.

So if I assume that the HT is about 26V, we can safely say the previous transistor was o/c b-e.

I'm a little unsure about that floating shield lead: I'm wondering if there's enough of a stray capacity issue to detune the circuit. The trouble is, the circuit is designed in such a way that the DC potentials around VT6 are somewhat dependent upon the hFE of the transistor.

A BF254 or BF594 may have been a better replacement, to be honest.

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Posted : 26/06/2016 10:41 pm
colourstar
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I'm a little unsure about that floating shield lead: I'm wondering if there's enough of a stray capacity issue to detune the circuit. The trouble is, the circuit is designed in such a way that the DC potentials around VT6 are somewhat dependent upon the hFE of the transistor.

Well, funny you should say that, as since fitting replacing that transistor I've lost the picture! Just a blank grey raster remains. Perhaps one of the other transistors you suggest would be a better way to go...

Steve

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Topic starter Posted : 28/06/2016 10:05 pm
Nuvistor
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Have you checked that the transistor is faulty or perhaps you have you another fault?

Frank

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Posted : 28/06/2016 11:15 pm
colourstar
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Nurse! More transistors please!

Back to this sickly 1500 to pick up where I left off. It looks as if the sudden loss of signals is due to the 1st and 2nd IF amps (VT4 and VT5 respectively). They were reading pretty much spot-on when the set was first woken but appear to have now thrown in the towel. Both are BF196 types. The equivalents book suggests a BF167 which I don't have. I wonder if there are any others I could try?

Steve

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Topic starter Posted : 27/08/2016 4:38 pm
crustytv
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Nurse! More transistors please!

Both are BF196 types. The equivalents book suggests a BF167 which I don't have. I wonder if there are any others I could try?

Steve

All these BF198, BF225, BF310, BF367, BF596 will all work as substitutes.

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Posted : 27/08/2016 4:51 pm
colourstar
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Great stuf, thanks Chris. I'll see what I've got kicking around...

Steve :aad

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Topic starter Posted : 27/08/2016 6:36 pm
colourstar
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Hi again from the land of dead transistors.

It seems the battle is finally won. After replacing every single transistor in this set, it's now a runner at last. I'm well chuffed! :bba The picture has that odd trait that I always associate with 1500s of having decent amounts of brightness in reserve, but appalling corner focusing. I don't remember other similar sets having that issue for some reason. The crt is a Mazda Goldstar and I have another I can try to see if that improves matters. Not that the present incumbent isn't perfectly watchable.

The broken tuner buttons need attending to and there are some cosmetics to address, but it's great to have an example of the set I grew up watching back in action.

Steve

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Topic starter Posted : 08/09/2016 12:42 am
Katie Bush
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I say, Steve, that's pretty ruddy good! :aad

I know what you mean about the corners being defocussed, and to be honest, that's one thing that always put me off owning one of these - so went on the get through four of them, although they were never what I'd call "keepers".

I recently took delivery of a 1500, though I was fresh out of hospital that day so never really looked at it... Perhaps I should give it due consideration?

Marion

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Posted : 08/09/2016 12:52 am
Marc
 Marc
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Hi Steve,

So it lives ! :aad
Nice one that man :bba

Marc.

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Posted : 08/09/2016 12:54 am
colourstar
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Thanks folks! And I forgot to say a very big THANK YOU for all the advice received from members along the way.

This 1500 was a bit of a basket case and has had far more effort expended on it than would normally be justified. A complete re-cap, a new dropper and all new tr*ns*stors. Blimey. I'm old enough to remember these sets being ten-a-penny and generally always working, so it was quite a strange experience carrying out something more akin to a full restoration than a mere repair. It was just that this was the particular model I was after and they are thin on the ground now.

And very well done Marc on your new G11- it's a cracker! :aad

Steve

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Topic starter Posted : 08/09/2016 1:06 am
neil1974
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Hi Steve. I've got several more 1500's here that need fixing :qq1 :aak .

Cheers
Neil.

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Posted : 08/09/2016 1:18 am
Lloyd
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Hi Steve,

It look pretty darn good to me! Mine has fuzzy corners too, only notice it on the test card. It also has a tube labelled 'Supervision' I guess they were a re-gun company.

Regards,
Lloyd

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Posted : 08/09/2016 1:35 am
Cathovisor
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Can't help but feel defocused corners are inevitable with a 110 degree tube and a 'cost-optimised' chassis....!

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Posted : 08/09/2016 1:46 am
ntscuser
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"Transistors last forever" don't you know? "They don't wear out like valves do". :aak

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Posted : 08/09/2016 7:38 am
Focus Diode
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Brilliant! I've also read, "Valves break easily, transistors are more robust". That of course may be the case if you were dropping them onto the floor from a height but as far as electronics are concerned......

Great the Alba is now working as it should do. Well done!

Cheers
Brian

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Posted : 08/09/2016 8:01 am
colourstar
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Hi Steve. I've got several more 1500's here that need fixing :qq1 :aak .

Cheers
Neil.

Bring 'em on Neil! I'd love to have a go. And thanks very much indeed for kindly donating the Alba to me in the first place. It's proved a very useful introduction into the world of the transistor, whilst still having plenty of friendly familiar valves around.

The focus control is at one end of it's travel, so I may just check a resistor or two in the locality. I always think it's a shame the 1500s were never fitted with convergence controls!! :qq1

Steve

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Topic starter Posted : 08/09/2016 8:30 pm
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