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B&W TV HMV Dual Standard - BRC 1400 Chassis

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Jamie
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Dare I venture into televisions? Yet alone a dual standard... May aswell! I actually "acquired" this from the local tip, its pretty much immaculate externally. Internally it shows signs of heavy use. The dropper has had many patchwork repairs, which I understand is the norm for these... But other than that, nothing seems amiss as such. I made a quick repair to one of the wires going to the CRT neck which had come loose.

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I applied power, and nothing happened. This was tracked down to a blown 1.5A fuse, I wasn't sure if it had "blown" or broken during it's trip to and from the tip, so I replaced it. Of course the fuse lit up and poof. So now the fun begins.

I'm pretty useless at reading diagrams but i've tried my best with this one. The first and most obvious place to begin is the dropper resistor, I checked all of the "repaired" sections, which were within tolerance, and also checked the few remaining original sections of the dropper.... Which were also OK. So where to now! 

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I'm thinking there's a dead short somewhere, and I remember reading about BRC Capacitors being prone to this? Not sure if it applies to this model... The can is an original Dubiler. At the moment progress has stalled as I need to order more fuses. 

One thing that did catch my eye (literally) , is R133. It should be 220K but was measuring half that.. Could this be part of the problem? (As you will see aswell my R&TS have many hand written faults from its previous owners. Which may well come in handy as the journey commences. Can anyone help shed some light as to where to look?

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Topic starter Posted : 03/04/2022 9:32 pm
Jamie
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This is what was wrriten on UKVRR.. By Heatercathodeshort:

"The unreliability problem with this particular type of capacitor employed in the UK in mostly Thorn television receivers in the 1970's and 70's, was confined to the black plastic cased single ended types with a dark red sealer.
They really were very bad going short circuit in even very low voltage circuits. The Thorn 2000 suffered with them as did the 3000 colour chassis. Both 1400 and 1500 monochrome chassis suffered breakdowns caused by these capacitors as any TV engineer of the period will confirm.
When overhauling these receivers it was normal practise to replace all the black cased series callins capacitors on sight to ensure reliable operation.........."

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Topic starter Posted : 03/04/2022 9:49 pm
Jamie
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Hmmm, two more points. The system switch has been "tied" up at the end with some old wire, fixing it into its position. 

I also read one of the notes in my R&TS saying that C101 often causes constant blowing of the fuse... 

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Topic starter Posted : 03/04/2022 10:11 pm
Nuvistor
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@jskinner97 
Callins capacitors were notorious for failure in other makes of set.

Did you check the value of the resistor in or out of circuit, in circuit the reading could be misleading.

If there are no obvious burn ups or smoke I would check the HT rectifier diode for s/c and the capacitor C101 0.1uf, that reference number is from the LLJ article from this sites data library. It’s from one side of the fuse to chassis.

 

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Posted : 03/04/2022 10:12 pm
Jamie
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Posted by: @nuvistor

@jskinner97 
Callins capacitors were notorious for failure in other makes of set.

Did you check the value of the resistor in or out of circuit, in circuit the reading could be misleading.

If there are no obvious burn ups or smoke I would check the HT rectifier diode for s/c and the capacitor C101 0.1uf, that reference number is from the LLJ article from this sites data library. It’s from one side of the fuse to chassis.

 

I think our posts overlapped! Indeed that C101 is my next course of action, with a bucket load of fuses arriving tomorrow... 
Resistor was checked out of circuit. 

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Topic starter Posted : 03/04/2022 10:16 pm
slidertogrid
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@jskinner97 Hi Jamie. A nice set! I have just restored exactly the same model, all of the brown and blue/grey capacitors were leaky it is surprising though how well the set will perform in spite of that! If the system switch has been fixed into the 625 position make sure the VHF tuner knob is switched to "U" because if the microswitch on the rear of the tuner hasn't been disconnected the system switch will be trying to move to the 405 position and being solenoid operated will draw a large current, the solenoid is only rated for momentary use and will quickly burn out the coil if it is powered for more than a second or so.  Check the HT rectifier diode if the fuse blowing persists.

Rich

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Posted : 03/04/2022 11:20 pm
slidertogrid
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This is my example with the parts that needed to be replaced to get it working.

HMV Dud bits
HMV ITV

 

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Posted : 03/04/2022 11:25 pm
Jamie
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Ta for your help, I'll get to work... What's the best way to clean those "open" potentiometers by the way? Isoprop on a cotton bud?

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Topic starter Posted : 04/04/2022 9:39 am
slidertogrid
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@jskinner97 That will clean them but I would suggest a little shot of switch cleaner ( I use Servisol) would be better as it leaves a trace of lubricant. check there is no corrosion on the inner wiper 'ring' a fibre pen will clean that off if necessary. The preset pots on these sets especially if they are the ones with the little plastic disc on them can be troublesome the width/set boost pot on my set had dropped drastically in value and was gently bubbling away on it's track.

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Posted : 04/04/2022 12:14 pm
Jamie
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I’ve just set about replacing C101 while I’m waiting for the fuses to arrive from Amazon.

 

I thought I’d also replace the Callins capacitors lurking.

One being 12uF. Is 10uF an acceptable substitute? C81

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Topic starter Posted : 04/04/2022 12:22 pm
Jayceebee
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10uf is acceptable as electrolytics can have a wide tolerance spread. One thing I wouldn’t take any risks with is the massive multi cap can. It would be prudent to reform each of the five sections before applying power.

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Posted : 04/04/2022 1:44 pm
Jamie
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I’ve replaced the above caps, aswell as both diodes W11 and W10…. Fuses continue to blow unless W11 is disconnected?

 

I’ve also removed the solenoid just incase but this doesn’t seem to make a difference. With W11 disconnected I get heaters no smoke and the can stays nice and cool 

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Topic starter Posted : 04/04/2022 1:51 pm
Jamie
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Well what do you know it.

a deeper look and I noticed a capacitor sitting at an odd angle within its plastic “seat” (C104) 

 

I then thought I’d connect the leg of W11 back up again, and see if the fuse blows on the lamp limiter, then I’ll know if the large capacitor can is faulty or not

low and behold the fuse didn’t blow!

I left it half an hour and no magic smoke appeared, after which I applied full mains. No fireworks!

 

One of the few remaining sections of dropped resistor smoked a bit but I wasn’t bothered by that

Withon 30 seconds we had a picture! Snow, sound, and some strange goings on.

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I let the set warm up for a few minutes and was presented with a better picture… but it’s not taking up full screen, and there’s a large white line down the centre, so this is where I’ll need a hand from people who know about TVs! From someone who can only just read a circuit diagram yet alone understand the ins and outs!

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Topic starter Posted : 04/04/2022 3:13 pm
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Jamie
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Another point i've just noticed, C104 I replaced with a 0.1uF as per data, but have just read here ( https://www.radios-tv.co.uk/vintage-electronics-blog-forum/thorn-1400-chassis-part-1/) this was later changed to a 1uF so may be best I alter this.

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Topic starter Posted : 04/04/2022 3:46 pm
crustytv
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Posted by: @jskinner97

I’ll need a hand from people who know about TVs! From someone who can only just read a circuit diagram yet alone understand the ins and outs!

On the contrary, Jamie, Give yourself credit, you're doing very well, and congrats on getting a raster. The younger Jamie would have just blanket recapped or even given up when it was dead. Armed with the LLJ article you linked to, you'll have this sorted in not time. The field timebase is where you need to go looking for your lack of height. Keep up the great work. 👍 

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Posted : 04/04/2022 4:33 pm
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Jayceebee
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Great start, well done. C104 wouldn’t normally cause the mains fuse to blow but if you’re changing it definitely put in a 1uF, it’s value was changed to prevent spot burn of the CRT under certain fault conditions. 

For the frame fault check the various plastic moulded caps in the field stage. It would also be a good idea to change the two line S correction caps C107/8.

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Posted : 04/04/2022 4:49 pm
PYE625
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That is a really nice set there Jamie, and it is looking like you will soon be showing cracking pictures ! 👍 

I am sure a NOS 1400 mains dropper is available for replacement at a later date, the existing one has seen better days perhaps.

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Posted : 04/04/2022 8:27 pm
Jamie
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Posted by: @pye625

That is a really nice set there Jamie, and it is looking like you will soon be showing cracking pictures ! 👍 

I am sure a NOS 1400 mains dropper is available for replacement at a later date, the existing one has seen better days perhaps.

Thanks, That is the plan once it's working. I don't want to fry a new one haha.

I thought i'd sling a signal into it and see what I got... A mess, basically!

OK, As recommended I changed the two S correction caps 107/108. no difference. I also changed C104 so it now meets the mod (ie 1uF instead of 0.1uF) 

C100 stand out, being A big red blob, It had appeared to melt slightly... And a hand written note in my R&TS says "Bottom Fold C121 O/C" 

So I changed these, Which gave me a slightly bigger (wobbly up and down) picture, alnd also tidied it up an awful lot but it's still cutting off a substantial amount of the lower half... 

It's also sounding like a dalek, But im not bothered by that yet...

So, where too? I can't identify the field timebase (no idea what part of the circuit it is) so may need help there..

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Topic starter Posted : 04/04/2022 8:54 pm
Jayceebee
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That's looking pretty good but what format is the signal you are feeding in? it looks to me that it's 4:3 letter box rather than a standard 4:3 and that is probably why you have the black bands top and bottom. 

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Posted : 04/04/2022 9:49 pm
Jamie
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It’s being fed from a Freeview box so I’d imagine it’s 16:9 squashed into nearly 4:3

 

There’s definitely something amiss with the set, as the picture balloons up and down all the time it’s nowhere near as good or stable as it looks on this photo

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Topic starter Posted : 04/04/2022 10:05 pm
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