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Alba T1520 (BRC 1500 chassis) revival

 
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Alba T1520 (BRC 1500 chassis) revival

Post by Colourstar » Wed Jun 08, 2016 6:15 pm

To add some background to this thread, the set in question is a 20" Alba T1520, dating from 1974 and one of the few 1500s to be blessed with a wooden cabinet, as opposed to plastic. This was our family set for a few years in the mid 70s, before being replaced with a Ferguson 9000 series colour mode in 1977/8l. The Alba wobbled on it's silver legs for many years as a second set and latterly as a VDU for my Sinclair ZX81 computer. I'm sure the set went to the tip still working sometime in the 1990s. How did I ever let that happen? :ccg

Fast forward to a year or so ago and a visit to Neil in Newport who had rescued some sets from an old cellar of a shop in Bristol - correct me if I'm wrong Neil! :) One of these refugees was a T1520, which Neil very kindly gave me when I told him it was a model I knew well.

The cellar storage had done the set no favours at all and any thoughts of bravely switching on were switfly curtailed after a cursory glance at the state of the capacitors. Several of the electrolytics were oozing and bulging. All were replaced, along with a couple of wirewound resistors whose legs had rusted through. I also treated the set to a NOS dropper. During this time Marc and Glenys of this parish dropped by for a visit. Marc had his tube tester with him and the previously poor reading given by the A50-120WR crt in the Alba improved markedly after a gentle clean and balance, enough for Marc to give it the thumbs up.

The first power-up of the set proved surprising, as when the line scan started the frequency sounded about spot on for 405 lines! A dim raster lacking height was obtained. Clearly the line frequency was way off. It was also noted that the initial snow visible on screen had vanished leaving a blank raster. IF trouble and a brace of dying transistors no doubt!

What EHT there was evaporated and the line whistle dwindled to something sounding not unlike the dying moments of a bluebottle trapped in a jar. My good friend Brian has VT10 the flywheel DC amp in his sights as being highly suspect. I wouldn't be surprised if all the transistors in the set turned out to be duff. I can give you a couple of voltages:

VT10
Emitter 8v (should be 1v)
Collector 10.1v (should be 44v)

So that's where things currently stand, folks. There's also the question of whether damp ingress has affected the line blocking oscillator transformer (T1)

Steve
(With grateful thanks to Brian for his invaluable help)
Attachments
Alba2.jpg
Alba1.jpg
Very manky caps!
Last edited by Cathovisor on Fri Jun 24, 2016 9:56 pm, edited 1 time in total.
Reason: Thread split from the "Thorn 1500 VT10 type TVT 7" thread.

 
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Re: Thorn 1500 VT10 type TVT 7

Post by Focus 2 » Thu Jun 09, 2016 8:09 pm

John Law's excellent article in the June 1976 issue of "Television" gives the resistance windings on the blocking oscillator transformer. Fortunately it's identical to that fitted in the 1400 series. I can send one from a scrap chassis if it turns out to be defective.
The set has clearly suffered from the effects of dampness. As Steve says all the transistors will probably be in need of replacement.
I understand the line oscillator has now failed completely with -20V from cold to 0V at the PL504 control grid on warming up.
Any suggestions, comments and opinions from others is most welcome.
Cheers
Brian

 
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Re: Thorn 1500 VT10 type TVT 7

Post by Colourstar » Thu Jun 09, 2016 8:22 pm

I think had it been any other 1500 I wouldn't have bothered so much, but as this specific one has 'family history' I'm prepared for the long haul. There are probably loads more faults in this set than the average 1500 would have, but at least this will prove to be a good educational exercise.

I'll check the resistance of the transformer windings over the weekend, but will meanwhile order a BC117 to replace VT10. In fact I'll order a handful of them.

Steve

 
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Re: Thorn 1500 VT10 type TVT 7

Post by neil29 » Sat Jun 11, 2016 8:50 pm

Hi Steve. Actually it was taunton. and I've already had a go at two of the other 1500's replacing several caps and a dropper or two and have managed to get something out of them but they like the others will need a fair bit of fettling to get them up to scratch. and I actually had the 24" alba version back in the 80's and as luck would have it there was one amongst the pile which is now in my shed :bba .

Cheers
Neil.

 
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Re: Thorn 1500 VT10 type TVT 7

Post by Till Eulenspiegel » Sat Jun 11, 2016 9:15 pm

Alba TS1520? I had loads of these on rental during the seventies. The tuner panel has the appearance of one designed for a six button tuner. One of the nicest looking single standard receivers. Well liked by customers.

Till Eulenspiegel.

 
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Re: Thorn 1500 VT10 type TVT 7

Post by Colourstar » Thu Jun 16, 2016 8:57 pm

Some positive progress to report. I changed VT10, using a BF257 in place of the original BC117. Result? No change. However, John Law's excellent servicing article on the 1500 in the July 1976 issue of Television mag (thanks Brian!), provided an explanation of the operation of the stages and gave me a few other items to check.

The upshot was that R61 which should be 270k had risen to nearly 3 meg. It's part of the 'line blocking oscillator time constant network'. Oddly this resistor wasn't an original, clearly having been replaced at some time. Changing it brought things back in line with a healthy negative voltage on the grid of the line output valve and, at last, a fairly normal looking raster of moderate brightness. Whilst I was at it, I thought it prudent to check R58 and R62, which, as it turned out, were within tolerance.

Unfortunately the signal stages aren't working at all at the moment, so I have only a blank raster to look at, but... it's one small step to victory!

Steve :cca

 
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Re: Thorn 1500 VT10 type TVT 7

Post by Focus 2 » Thu Jun 16, 2016 10:05 pm

Great news. Jon's article states VTV7 is the most vulnerable IF transistor. You never know replacing it could bring results.
Cheers
Brian

 
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Re: Thorn 1500 VT10 type TVT 7

Post by Focus 2 » Fri Jun 17, 2016 3:38 am

VT7 and VT8 are BF197s. The BF199 should work in this position.
Cheers
Brian

 
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Re: Thorn 1500 VT10 type TVT 7

Post by Colourstar » Fri Jun 17, 2016 8:42 am

Hi Brian, some voltages from VT7:

Collector 30v (Should be 23v)
Emitter 0v (Should be 2.9v)
Base 5.3v (Should be 3.5v)

Not sure how to interpret this. I need to brush up on some theory! :aal

Steve
Attachments
Alba R.jpg

 
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Re: Thorn 1500 VT10 type TVT 7

Post by nuvistor » Fri Jun 17, 2016 8:49 am

Appears to be O/C base emmiter on that transistor. Not had chance look at the circuit but that's my first thought.

Frank

 
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Re: Thorn 1500 VT10 type TVT 7

Post by Focus 2 » Fri Jun 17, 2016 9:08 am

Agree it sounds defective. I've had IF trouble on various sets soon after being woken following a long sleep. On each occasion the final IF transistor was the culprit.

Cheers
Brian

 
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Re: Thorn 1500 VT10 type TVT 7

Post by nuvistor » Fri Jun 17, 2016 9:18 am

I did not know it was the final IF transistor, this was a common fault on some Bush B/W TV's, the transistor was inside the final IFT. The sets were not 50 years old then though.
Frank

 
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Re: Thorn 1500 VT10 type TVT 7

Post by Cathovisor » Fri Jun 17, 2016 11:42 am

Colourstar wrote:Hi Brian, some voltages from VT7:

Collector 30v (Should be 23v)
Emitter 0v (Should be 2.9v)
Base 5.3v (Should be 3.5v)

Not sure how to interpret this. I need to brush up on some theory! :aal

Steve

Simple answer. For a silicon transistor operating in a linear mode - such as an amplifier - the Base should be about 0.6V higher than the emitter. Anything else spells trouble.

 
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Re: Thorn 1500 VT10 type TVT 7

Post by crustytv » Fri Jun 17, 2016 2:48 pm

Colourstar wrote:Not sure how to interpret this. I need to brush up on some theory! :aal

Steve

This may help, then again it may not :qq1
http://www.radios-tv.co.uk/test-a-transistor/

 
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Re: Thorn 1500 VT10 type TVT 7

Post by Colourstar » Fri Jun 17, 2016 7:13 pm

Thanks Frank and thanks Catho - pearls of wisdom duly noted!

And thank you Chris for the link to the beautifully annotated guide. That is precisely what I need. :aad

Steve

 
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Re: Thorn 1500 VT10 type TVT 7

Post by Colourstar » Fri Jun 17, 2016 10:05 pm

Another update: VT7 (4th IF amp) has now been replaced, with the effect that the voltages here are now within about 1v of those specified, so certainly the original BF197 was faulty.

Checking VT8 (Video Driver) showed no volts on the emitter, so this transistor, another BF197, was changed. Catho's advice that the base should be about 0.6v higher than the emitter holds good- the base is 10.7v and the emitter 10.0v but.... the specified voltage for the emitter is 5.2v, so it's currently about double what it should be.

The 1st and 2nd IF amps VT4 and VT5 are spot-on for voltages, but it all is not well with the 3rd IF amp VT6, which has no emitter voltage and a high reading on the collector (25.7v against the 11.5v specified). VT6 is a BF194. Could I use a BF199 as a stop gap?

Twiddling the contrast control caused some disturbance on screen, so I was encouraged to try giving the set some signal. The tuner has the usual BRC problem with the buttons, but to my surprise I was able to tune it in with some degree of success to reveal that even a sickly 1500 can still produce something worthwhile, despite a partially working IF section, scratchy controls and a duff tuner.

Have to say that although success is still some way off I'm really enjoying the detective work so far! :aal

Steve
Attachments
First pic!.jpg

 
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Re: Thorn 1500 VT10 type TVT 7

Post by Cathovisor » Sat Jun 18, 2016 1:05 am

Reading what you've written, VT6 looks a good candidate for being o/c.

VT8 though is not so good. If the emitter is that high, twice the current there should be is passing through the emitter which will pull the collector voltage down correspondingly, unless there's summat clever going on... I'm going to assume the map for this is in the library; I shall take a peek at it tomorrow as now, it's bedtime!

ETA: Okay, I couldn't resist. First things first, Steve: what's the voltage at the junction of R79 and R136, and the collector voltage of VT8? And now I *am* going to bed....!

 
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Re: Thorn 1500 VT10 type TVT 7

Post by Focus 2 » Sat Jun 18, 2016 9:14 am

That's a great result seeing how poorly the set was when we first powered it up! I was kindly given some transistors by another forum member which I'm sure includes a BF195 which should replace the BF194.
I'll have a look later.Unfortunately I can't seem to find the UHF tuner that would've been ideal for substituting the p/b switch bank. Sure I must have it somewhere.
Cheers
Brian

 
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Re: Thorn 1500 VT10 type TVT 7

Post by nuvistor » Sat Jun 18, 2016 9:58 am

That picture of the test card looks good, does it have AGC on VT6? If it does check the voltage with and without signal, these sets very often used forward AGC, I.e, the stronger the signal the harder the transistor is turned on, lowering the collector volts. See what signal conditions are quoted in the manual for various voltage readings.

It is just a thought, the transistor could still be faulty but worth a few minutes checking.

Frank

 
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Re: Thorn 1500 VT10 type TVT 7

Post by Colourstar » Sat Jun 18, 2016 10:03 am

The high voltages around VT8 were down to R136 having doubled in value. Reading the John Law article led me that way, but Catho has confirmed it. Sure enough, replacing R136 brought the voltages back to where they should be.

VT6 is certainly looking ill and I've also no sound (although the PCL82 stage is working), so the sound IF amp, another BF194, may be worth a visit.

The Mazda crt is improving all the while, no doubt thanks to Marc's clean and balance.

Steve

 
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Re: Thorn 1500 VT10 type TVT 7

Post by sideband » Sat Jun 18, 2016 10:21 am

I tend to treat all those 'lokfit' transistors with suspicion. Those and the BC14x all seem to be suspect. They didn't seem to have much of a production run and soon were replaced by alternatives in 'standard' packaging.

 
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Re: Thorn 1500 VT10 type TVT 7

Post by Cathovisor » Sat Jun 18, 2016 2:17 pm

I'd certainly have a look around VT11 (the Lokfits in this set do not seem to be covering themselves in glory ATM), but also the 82pF coupling cap (C36) from the video driver and the stabilising cap C65 (4µF) as well for a lack of sound.

 
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Re: Thorn 1500 VT10 type TVT 7

Post by Colourstar » Sat Jun 18, 2016 3:22 pm

Cathovisor wrote:The Lokfits in this set do not seem to be covering themselves in glory ATM)


I don't think this poor old thing has been in use for years. I must be still in my 1980s mindset, when all thse b&w hybrids were ten a penny in the 'reconditioned' telly shops. It seems quite odd to be having to do so much work to get one of them up and running again. I'm finding it very educational. Good fun too, as like almost everything else we like to tinker with, it's 'Beyond Economic Repair'. And then some.

Will check C36 and C65 by substitution, if I've got them.

Slightly better photo attached of the current state of play.

Steve
PS- Perhaps it's time to change the thread title to a more general purpose 'Alba T1520 BRC1500 revival'
Attachments
Alba testcard.jpg
The mists are clearing...

 
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Re: Thorn 1500 VT10 type TVT 7

Post by Lloyd » Tue Jun 21, 2016 9:56 am

Hi Steve,

Good to see you found one of these sets! Yours looks good so far :thumb

I think I must have been lucky with mine just needing a few capacitors and a couple of resistors to get it going.

Regards,
Lloyd.

 
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Re: Thorn 1500 VT10 type TVT 7

Post by slidertogrid » Tue Jun 21, 2016 4:03 pm

It's strange I have a couple of Thorn 1500's both had multiple faults, one set was particularly nasty with all the 1500 common faults and some! I wasn't expecting that as I have had three 1400's that had hardly anything wrong, the usual tuner clean and a cap or two was all they needed.
The 1500's were almost rebuilds in comparison, maybe they had seen a lot more use?
Well done getting this one going it looks well worth the trouble. It's funny you mentioning how cheap and plentiful these sets were in the early 1980's. I thought the same when I struggling with mine, I can remember buying a whole room full of ex rental monos for £2 each. A Most worked, I didn't try too hard with the ones that didn't if I couldn't fix it in about half an hour with bits from the scrap pile it went On the pile! :aah
My mum wasn't too happy when she came home to find a row of sets three high stacked on their ends all down the hall...
Rich.

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