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B&W TV RE: KB Royal Star; PVP20

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sideband
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I've just bought one of these from a fellow collector who is thinning out his collection. It's mostly been restored, but hasn't seen mains for some time. I have a special reason for wanting one of these, as it brings back memories of my first experience of Silly Things That Customers' Do! Notably, using the set to dry damp washing (see Trade Tales 'It gets nice and warm behind there... link above top of page).

This one has not been subject to such abuse, so I'm not expecting to many problems. Considering it's a portable, I wouldn't like to carry it far! I suppose it was OK moving from room to room, and the fact it has a handle just about fits the qualification of 'portable'. I have yet to get it into my loft workshop and will probably require the assistance of my son.... I get into the loft, and he hands it up to me (he does weights, so it will be a trifling to him)!

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Topic starter Posted : 10/07/2021 4:12 pm
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Nuvistor
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I have always liked the look of these sets, very futuristic for 1958.

Frank

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Posted : 10/07/2021 5:28 pm
Till Eulenspiegel
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I guess the shape of the cabinet was designed to resemble the Sputnik. The PVP20 has a 90 degree CRT, either Brimar C17SM or Mullard AW43-80 and the 1959 QVP20 has the 110 degree CRT.  The QVP20 runs much cooler because a mains transformer is employed instead of the dropper resistor.

Till Eulenspiegel.

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Posted : 10/07/2021 6:37 pm
sideband
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I've just printed out the circuit. All looks fairly conventional and straightforward. A couple of valves I'm not familiar with, the 9D7 and (although I've heard of it) the 6BW7. I suspect they are similar to EF85 and EF80 respectively.

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Topic starter Posted : 10/07/2021 7:15 pm
Till Eulenspiegel
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6BW7 and 9D7 have a slightly better gm compared with the EF80 and EF85.  Ferguson employed the 6BW7 as IF amplifiers in the model 505T, a special set made for use in fringe areas. The developer of the 6BW7 and the vari-mu 9D7 was STC.

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Posted : 10/07/2021 8:44 pm
sideband
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Well it's on the bench with the cabinet off. I noticed that most of the capacitors had been replaced including the smoothing. Apparently it hadn't seen mains for around 7 years so I decided a gentle wake-up was in order. I connected the Aurora on Ch1 and I started the variac at 200V and after about 3 minutes I was rewarded with a dim testcard C. It looked a bit tubey so increased the mains slowly up to 240V. It improved the picture and I suspect it will get better with use. Line and frame were spot on with plenty of lock. The HT was almost spot-on at just over 205V. The only obvious problem was no sound. There was just a small amount of vision buzz in the speaker but nothing when touching the volume control tags (I should mention at this point that I was also running on isolated mains).

Switching off, I checked the triode anode load resistor which was fine and then I noticed two of those dreadful 'torpedo' capacitors that look like fat resistors. Checking the circuit, both of them are audio coupling, one from the noise limiter, the other from the volume control to the triode control grid. I'm betting that one or both are faulty and will investigate next time.

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Topic starter Posted : 11/07/2021 10:15 pm
crustytv
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If I'm not mistaken, this KB PVP20 was restored by ex Vrat member "Crackle" 8-years ago, back in January 2013. Rich, If you've not already found the original thread, you may find it of use to evaluate what work was carried out.

https://www.radios-tv.co.uk/community/black-white-tvs/kb-pvp20/

Posted by: @sideband

after about 3 minutes I was rewarded with a dim testcard C. It looked a bit tubey so increased the mains slowly up to 240V. It improved the picture and I suspect it will get better with use

I think the tube might be past its prime, it looks like the tube was treated to the non-destructive rejuvenation method, for similar symptoms you're experiencing.

https://www.radios-tv.co.uk/community/black-white-tvs/kb-pvp20/paged/3/#post-72622

About 18-months later, Oct 2014

Posted by: @anonymous

Ho Hum, I thought I had finished the set, and put the case back on, but if I can get more improvement then why not try. The picture does take an age to come on about 3 times the time for the sound

https://www.radios-tv.co.uk/community/black-white-tvs/kb-pvp20/paged/4/#post-72636

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Posted : 12/07/2021 9:14 am
sideband
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Posted by: @crustytv

If I'm not mistaken, this KB PVP20 was restored by ex Vrat member "Crackle" 8-years ago, back in January 2013.

Absolutely correct! How did you work that out? Thanks for the link to the old thread. I'd completely forgotten about it...and I even mentioned the set I remembered back in the early 70's having the washing draped over the back!

I remember back then it wasn't the easiest set to work on. I think I will 'cheat' with this and cut any suspect components off close to the lead-out and solder the replacement component to the stubs. Not the most professional way of doing things but it solves the problem of broken leads and weak print lifting of the board. If done neatly it looks OK. I'm hoping a couple of replacement .01 caps will restore the sound.

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Topic starter Posted : 12/07/2021 1:56 pm
crustytv
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Posted by: @sideband

Absolutely correct! How did you work that out?

Three clues from your opening post, 1) a collector thinning out their collection, 2) previously restored, 3) The photo showing the TV with a display label, suggesting an organised and displayed item.

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Posted : 12/07/2021 3:28 pm
Nuvistor
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Wasn’t cutting the leads the preferred way on early PCB’s, the print on later PCB’s was much stronger and replacing the component completely became normal.

I worked on very few early PCB’s so it’s only from reading about it.

Gadget sold in the USA to help with this problem. Fine tinned copper wire will do the same job.

https://worldradiohistory.com/hd2/IDX-Service-&-Sales-IDX/Archive-Radio-Retailing-IDX/IDX/60s/Electronics-Technician-1965-04-OCR-Page-0009.pdf#search=%22solder%22

Frank

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Posted : 12/07/2021 3:28 pm
Cathovisor
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@nuvistor

I do recall it being suggested in the pages of "Radio & Television Servicing": when printed circuits started to become more common they did an article on it in the pages at the front but I couldn't tell you what year. I do recall suede brushes being mentioned for clearing solder from things like valve holders.

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Posted : 12/07/2021 6:18 pm
Nuvistor
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@cathovisor

Radio Servicing edited by Pat Hawker.

8B1E7795 FB2C 461D 8E25 0BF885D9BC6D

In the Radio developments section of R&TV 1957-58 there is a reference to PCB servicing that’s is contained in the Television developments section, I unfortunately don’t have that section.

Frank

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Posted : 12/07/2021 7:03 pm
sideband
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It is essential to make the connection as quickly as possible otherwise you run the risk of melting the solder on the print side of the board.

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Topic starter Posted : 12/07/2021 7:59 pm
PYE625
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Posted by: @sideband

It is essential to make the connection as quickly as possible otherwise you run the risk of melting the solder on the print side of the board.

Or causing a dry joint and the prolonged heat required to tin the old lead can lift the track too.

I find that with care, a component can be replaced ok providing a good hot iron together with swift removal of the old device with no wiggling. Straight up and out. Excess solder can be quickly removed with solder wick, never a plunger type of sucker though. This will lift the track by the jerking action. I cut the lead of the new component to length first and then make a quick joint. With practise, it can be done without damage. The only problem is if a PCB has been reworked before, but if the pad has lifted, an absolute minimum of superglue applied underneath can solve that problem and it be stuck down again.

But the fact is, it is best to do what you feel comfortable with.... 😉

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Posted : 12/07/2021 9:17 pm
Cathovisor
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Posted by: @pye625

The only problem is if a PCB has been reworked before, but if the pad has lifted, an absolute minimum of superglue applied underneath can solve that problem and it be stuck down again.

NEVER use superglue as it has toxic decomposition products when heated.

Use epoxy if you must.

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Posted : 12/07/2021 10:19 pm
sideband
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OK back to the sound fault. It didn't involve replacing any components. A quick check on the suspect capacitors mentioned (those nasty little torpedo shaped things) showed they weren't shorted. Touching the grid of the triode produced nothing either. It's very difficult to measure any voltages since the print side of the PCB has very little access but I found that waggling the PCL82 produced crackles from the speaker and then if pushed in one direction, sound would be heard. I cleaned the valve pins and sprayed them with contact cleaner which made a slight difference but still very touchy. Carefully checking the contacts in the valveholder showed everything was OK. I then turned the whole chassis upside down to get a better look at the print side. Immediately I saw what looked like a poor solder joint on pin 9 (anode of the triode section) of the PCL82...the telltale black ring around the solder joint. Getting the iron in there was another thing.....! It was made much easier by removing the smoothing cap from its clamp on the chassis....these sets were not made with quick engineer repairs in mind....! As soon as I touched the joint, the solder fell off the pin completely....Fortunately it took a fresh lot of solder without any problem and soon pin 9 was made good. Refitting the valve and putting the set right way up I switched on and after 30 seconds or so, up came the sound and no amount of waggling the valve made it go off.

So a nice easy fault in the end even though actually getting to the dry joint was not so straightforward.

It looks as if the tube is way past it's best although I might try a gentle boost.

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Topic starter Posted : 12/07/2021 10:42 pm
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turretslug
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Posted by: @cathovisor
Posted by: @pye625

The only problem is if a PCB has been reworked before, but if the pad has lifted, an absolute minimum of superglue applied underneath can solve that problem and it be stuck down again.

NEVER use superglue as it has toxic decomposition products when heated.

Use epoxy if you must.

Some years ago, I was asked to repair (by soldering) some items of jewellery- what I hadn't been told (until after the event....) was that a previous attempt had been made to repair with superglue. After thorough cleaning, there must still have been a trace present, and the resultant fumes were astonishingly painful in my eyes and I thought for a few moments that I'd been blinded- fortunately, the pain soon cleared and my vision seems no worse for it. It certainly highlighted the hazards of superglue and heat, and a worthwhile warning. Not a lesson I'll forget in a hurry!

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Posted : 13/07/2021 1:29 pm
Katie Bush
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Posted by: @cathovisor

NEVER use superglue as it has toxic decomposition products when heated.

Abso-flippin'-lutely.

Cyanoacrylate is a tad too close to cyanide for my liking. If you've ever inadvertently heated superglue with a hot iron, you won't make that mistake again. Apart from choking on the fumes, the acrid smoke burns your eyes and inside your nasal passages - It's evil stuff!

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Posted : 13/07/2021 1:32 pm
Cathovisor
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As I understand it, hydrogen cyanide is emitted.

I encountered a pocket transceiver at work where a previous SMD push-to-talk switch failure had been addressed with copious quantities of superglue on the PCB; I just binned it and refused to rework it. When I'd been the first to deal with such failures, I just mixed a small amount of Araldite to hold the component in place.

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Posted : 13/07/2021 2:43 pm
PYE625
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I have done it once or twice on PCB's before, but with only a tiny drop on the end of a needle applied. Shall not use superglue again though. Had no idea of the consequences. Oops.

As for the CRT emission, I would give it a bit more time before thinking of boosting it. But the gentle way might be best if really needed...

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Posted : 13/07/2021 5:17 pm
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