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B&W TV Rediffusion "Guildford" RT 194/19 Dual Standard

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PYE625
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Posted by: @pye625

If you study the circuit, you will see that a diode 3D9 feeds the heater circuit. If you make arrangements for a switch to be temporarily fitted immediately before or after this diode, you could briefly interrupt the heater supply only.

I have made a rather glaring error in the above. The diode is NOT 3D9, it is 3SR2. Furthermore, if you remove the heater volts, you also remove the 75vdc screen feed to 2V1 (PFL200). Not as simple as I first thought then.

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Posted : 03/06/2022 11:21 pm
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Nuvistor
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While on the subject of the sync separator, has the electrolytic cap on its screen feed been replaced.

I remember if it was o/c is affected the frame sync more than line but if it’s low in value it could cause problems.

 

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Posted : 04/06/2022 9:00 am
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WayneD
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@pye625 I like this idea of a switch in the heater circuit, it sounds very me! 

I shall investigate and report back.

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Topic starter Posted : 04/06/2022 8:33 pm
PYE625
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Posted by: @wayned

@pye625 I like this idea of a switch in the heater circuit, it sounds very me! 

I shall investigate and report back.

I was looking at the wrong circuit when I dreamed up the idea.... it was the A774, not the A640 chassis.

However, the idea could be made to work with this set, even just for a moment that maybe enough to eliminate heater chain voltage induced hum. The other option is to substitute one by one, each valve. The PCL82 is probably not to blame or you would certainly be hearing it.

 

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Posted : 04/06/2022 8:50 pm
PYE625
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Posted by: @nuvistor

While on the subject of the sync separator, has the electrolytic cap on its screen feed been replaced.

I remember if it was o/c is affected the frame sync more than line but if it’s low in value it could cause problems.

 

I agree completely and it is 2C48, if not already replaced.

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Posted : 04/06/2022 10:42 pm
PYE625
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Speaking of 2C48, you need to be careful if just the heater circuit is made open because then there will be nothing to stop 2V1 screen voltage moving up towards 320vdc. The PFL200 may not approve of this.

Ideally, perhaps a temporary pair of high-ish value resistors could be used in a potential divider to drop the HT to supply this 75v for the screen if you decide to briefly remove the heater supply to all valves.

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Posted : 05/06/2022 12:43 pm
WayneD
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2C48 has definitely been replaced!

Screenshot 20220605 162615 Gallery

I put it on the other side of the PCB to minimise any heat going to it. 

I've just replaced the mains smoothing capacitor for a larger value one and it's made a huge improvement: 

20220605 161048

I'm aware that I've probably solved the symptom rather than the problem though.

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Topic starter Posted : 05/06/2022 4:44 pm
Nuvistor
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@wayned 

Check the HT current, if it’s higher than normal the extra smoothing may mask the fault.

 

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Posted : 05/06/2022 10:04 pm
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WayneD
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I'm an idiot! 🙄 

I mentioned earlier that I replaced all the dodgy capacitors... well it turns out that I missed C13: 

20220614 180428

For some reason my brain didn't even register this as a capacitor. What is going on with the leads?

On further inspection it's a 2000pF:

20220614 180700

 But according to the schematic, it's supposed to be 0.01uF or 10000pF! 

Screenshot 20220614 180850 Gallery

It tests bad anyway, but no matter. 0.01uF capacitor goes in and....

Screenshot 20220614 192640 Gallery
Screenshot 20220614 192617 Gallery

PERFECT! 😎 

I think we can wrap this up now. This had been quite a journey! I've learned so much from this TV and although it's been a little frustrating in places it has rewarded me every time I've learned something new. I started this project not even knowing what a tripler was as I'm used to everything being enclosed and replacing it all as one unit.

I'd like to say thanks to everyone who followed me on this thread as well as everyone who has given me advice through this!

Now, if you'll excuse me I'm going to watch Doctor Who, Tomb of the Cybermen on this...

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Topic starter Posted : 14/06/2022 7:42 pm
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PYE625
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Posted by: @wayned

It tests bad anyway, but no matter.

Was it open circuit?

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Posted : 14/06/2022 8:36 pm
WayneD
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@pye625 leaky according to the tester. However it only seemed to affect the picture after about 20 minutes (it was right above the PCF80 valve) so who knows what it was like in-circuit.

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Topic starter Posted : 14/06/2022 8:50 pm
PYE625
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Interesting and well worth knowing.

One for the memory banks.

 

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Posted : 14/06/2022 9:00 pm
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PYE625
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I wonder if it had ever been replaced in the past, especially as it was the wrong value. (Probably not too critical though).

My set has a standard Mullard mustard type, 0.01uf.

PS, You are NOT an idiot lol.... Who on earth would have suspected this particular capacitor?

Credit to you for locating this awkward fault.

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Posted : 14/06/2022 9:44 pm
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WayneD
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@pye625 it certainly seems to have been replaced. I don't understand why the leads were all over the place. 

I think I'm just annoyed with myself that I was so thorough with the resistors and capacitors but I missed this one. I think my brain seemed to dismiss it as a crystal oscillator.

 

 

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Topic starter Posted : 14/06/2022 10:04 pm
Nuvistor
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@wayned 

All sorted out, well done.

That capacitor being leaky would increase current through the line output valve which is probably why extra smoothing helped.

The line output, boost diode and LOPTX will not be worked as hard now.

 

 

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Posted : 14/06/2022 10:55 pm
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Jayceebee
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I’ve just checked my Z146 manual which was the last version of the A640 with some changes to the smoother and a slightly smaller chassis frame. 3C13 is unchanged from the A640 at 0.01uF. Would you really use a polystyrene capacitor for the line drive coupling, more suited to the oscillator surely?

We had an engineer who wound loops on replacement component leads, he thought it would help with heat dissipation. Well done finding it but makes me think if the drive was compromised could that have helped with demise of the original LOPT knowing their fragility.

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Posted : 14/06/2022 11:15 pm
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