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B&W TV RE: KB Royal Star; PVP20

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Nuvistor
(@nuvistor)
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@sideband 

According to Pat Hawkers Television Engineers Pocket Book, CP ERP was 200Kw, your memory is near enough not to make any difference. Piece of wet string would overload the front end.

The book is well worth a look at for historical information, it’s in the forums Data library.

 

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Posted : 28/09/2021 9:05 am
PYE625 liked
sideband
(@sideband)
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Posted by: @nuvistor

@sideband 

 Piece of wet string would overload the front end.

 

...That's what my brother used to say!

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Topic starter Posted : 28/09/2021 9:32 pm
sideband
(@sideband)
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Had a rest for a few days since boosting the tube so tried it again tonight. I had severe vision buzz last time and after having my memory jogged, I tried an attenuator in the aerial lead as suggested. I could only find a 6db and 18db. 18 was too high so tried the 6. Very much improved and although there is still some sound buzz it's not intrusive. I thought the contrast lacked range, plenty of good brightness though with no silvering effect. A new PCF80 video amplifier worked wonders and now there is plenty of range and the blacks are really black. It even looks good with the bright workshop lights. I need to make some adjustments to the line linearity and possibly the picture centering but basically I think it's done. I have to say that it has the best interlace I've seen for a long time.

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The middle picture shows the exceptionally good interlace....it really looks spot-on. The testcard looks pretty good but shows up the line linearity problem. The overall focus looks reasonable even in the corners. Then just a quick off-air screenshot.....with a ghostly apparition apparently taking a photo.....

Had the set running for about two hours with no problems.

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Topic starter Posted : 30/09/2021 10:51 pm
sideband
(@sideband)
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I've spent some more time on this set tonight. After the shock of Mike's (Crackle) death I decided that it was time to finish the odd bits that needed doing. One of those was noisy tuner contacts. This set uses a fireball tuner which I find are actually quite easy to service. They are very compact tuners and the components are really packed in there. Complete removal of the tuner makes the job much easier and there are only four connections plus the aerial so a quick diagram of where each wire goes is easy to draw out.  Once the tuner is removed you just need to release the metal cover over the coils. The coils are fitted to a carousel which has to be removed before you can get to the contacts. One thing I did check was the 6.8K feed to the oscillator anode which used to go high. It had in this case and measured 11.2K....some way off the 10% tolerance allowed. Fortunately this resistor is easy to change but you still have to be careful not to disturb other components nearby. It's a two watt type so I unearthed a similar 6.8K 10% replacement. A slight complication was that one capacitor was soldered halfway up one of the resistor leads so I unsoldered the resistor first and then carefully unsoldered the capacitor. I spent a few minutes cutting the new resistor leads to length and bending them to shape and then trying it in position. Once I was happy I soldered it into place and then carefully resoldered the capacitor halfway along the resistor lead as original. 

Next to remove the coil carousel. There is a large nut holding it to the drive spindle which has to be removed. Once this is done, the carousel just slides off the spindle allowing access to the studs and contacts. These contacts are silver plated and you shouldn't really use anything to clean them with other than a dry cloth. A couple of minutes rubbing and they should come clean. Same with the spring contacts although these need to be treated carefully since the profile is important. Some engineers applied a thin smear of petroleum jelly or silicon (MS grease I think) to prevent tarnishing but I assembled them dry since with the heat generated within, dust and dirt attracted by convection is likely to stick to the contacts. Reassembling after cleaning is easy since you just locate the carousel with a keyway that prevents incorrect positioning, tighten the nut and job done.

The tuner is now ready for refitting but I'll leave that for next session.

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First picture shows the old 6.8K, second shows replacement fitted (compare the two), third shows the coil carousel with dirty studs, fourth shows dirty contacts, fifth shows cleaned contacts and sixth shows cleaned studs. 

Some may wince when diving into tuners. Admittedly some repairs can be nigh on impossible but generally they are very reliable and a resistor change such as this if carried out carefully wont upset anything.

 

More pictures with next post. 

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Topic starter Posted : 31/10/2021 12:26 am
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PYE625
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Using your first photo, I have indicated where a fine wire runs along the bottom edge of the tuner chassis. The sleeving can cause the wire to corrode and go open resulting in a deaf set. It leads from L8 to the anode of V2B.

I had this failure on my Marconiphone VT161 tuner. I wince thinking of it now as it was perhaps one of the most hard to get to repairs I have done for a very long time !

Inked10062 js10241000435 LI 50

 

 

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Posted : 31/10/2021 9:43 am
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sideband
(@sideband)
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@pye625 Thanks for that. I'll give it a check over before I refit it.

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Topic starter Posted : 31/10/2021 10:29 am
sideband
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Went back to this for an hour or so tonight. Checked the tuner over. I checked the fine wire mentioned by pye625 above and was pleased to note it was all in good order. Time to refit the cover and put it back in the set. Note: Reconnect the leads unsoldered before refitting. Not possible to do it afterwards. Once this was done, I was ready to check that everything still worked. 

Pleased to say it all worked, just a small amount of fine tuning. The 'scratchy' channel change was now clear so the contact cleaning was a success. 

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 First shows coil carousel refitted, second shows tuner reassembled ready for fitting, third shows tuner fitted, fourth, it's all working again. Some picture adjustments required but basically OK.

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Topic starter Posted : 31/10/2021 10:01 pm
sideband
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Well the magic smoke escaped....from the tuner! Just shows how careful you have to be with these fireball tuners (maybe there was another reason for calling them fireballs)! Anyway what happened?...Well I switched on Monday evening and the set had been on about 5 minutes when I noticed an interesting column of smoke arising from somewhere over near the mains switch end. I looked over the top and the Magic Smoke was emerging from a slot in the tuner casing....the set was still working but I switched off PDQ! Somewhat puzzled by the turn of events I decided to leave it for later. The later was tonight. 

I removed the tuner again and wondered if for any reason it was the new 6.8k that I'd fitted since that is all I had replaced. I removed the cover but the 6.8k looked fine. Looking at where the slot in the cover was I soon noticed a rather frazzled resistor. The circuit (Trader) showed this as R12 a 10k feeding the screen grid of the PCF80. I couldn't initially work out why the resistor had decided to burn up unless the PCF80 had developed a fault but the set had continued to work....all very odd. The screen grid is decoupled by a coil and capacitor. The coil is L8 (Trader). L8 is actually a couple of turns of tinned copper wire and I noticed it was very close to the chassis....in fact it was almost touching. On a hunch I carefully refitted the cover and as I clipped it down I was able to observe through the slot that the cover was just touching the new 6.8k resistor which in turn pressed against L8 shorting the coil to chassis. You can see the tiny coil in the second photo in the fourth post above. It is very close to the chassis. 

I had a very delicate job replacing the 10k which is somewhat buried. You can just see it in the same photo where the lead from the 6.8k crosses over a wire link soldered to a feedthrough. Where the two wires appear to cross, there is a 10k buried under there. I had to unsolder the wire link from the feedthrough and then unsolder one end of the 10k. The other end was soldered to the valveholder. I cheated and cut the lead at the valveholder. I carefully prepared the new resistor by cutting one lead short to a few cm, soldering that to the valveholder pin and the other end I formed a loop and soldered it to the feedthrough. Finally resoldered the wire link. About 20 minutes work altogether. I slightly repositioned the 6.8k so that it wouldn't touch the cover and very slightly moved L8 away from the chassis. A few resistance checks showed no shorts and when I replaced the cover, it cleared the 6.8k sufficiently and the coil remained clear of the chassis. 

The good news is that it all works after this and no more Magic Smoke was lost. I had the set running for about half an hour and all was well.

Just shows how careful you have to be with this particularly compact tuner. Admittedly the new 6.8k was slightly larger than the original but I hadn't appreciated how close it was to the screening can....or the coil. Obviously the short wasn't a complete short otherwise the screen grid volts would have been removed completely and there would have been no signals. 

Fortunately no real harm done other than the demise of a resistor. I don't have a problem working on tuners but it's the first time I've had to 'dig' further into one of these.

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Topic starter Posted : 05/11/2021 12:08 am
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PYE625
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I must confess to thinking that the new 6k8 was a bit large physically, but if you can get it to fit ok, then all well and good in the end. 👍 

Quite a bit of heat is generated in the tuner (mainly the valves) so it is not surprising it can look a bit toasted inside over the years. Blackened valve sheilds may have helped radiate the heat slightly more than shiny ones, but a bit late to worry about that now. It will hardly get the use it would have had when in it's heyday.

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Posted : 05/11/2021 11:08 pm
irob2345
(@irob2345)
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Hotter than normal tuners could often happen because to the reception environment.

I had occasion recently to look at a mid 60s parts chassis that spent its life in Wollongong (regional city south of Sydney). Although local channels were available, there was more choice of programs from the Sydney channels - in the years before aggregation. This was deep fringe reception and the tuner front end obviously ran most of the time with zero AGC. The condition of the getter flash in the 6ES8 (ECC88) showed it had had a hard life.

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Posted : 06/11/2021 8:30 am
sideband
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Posted by: @pye625

I must confess to thinking that the new 6k8 was a bit large physically, but if you can get it to fit ok, then all well and good in the end. 👍 

 

Yes agreed it is larger than the original but not by very much. It was the only 6.8k I had that was suitable. I'm sure it doesn't need to be that big even originally but probably reduced tuner drift.

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Topic starter Posted : 06/11/2021 9:27 am
sideband
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DISASTER!! I switched the Royal Star on last night after a couple of weeks rest and after a few minutes, it died with a fizzle! The fizzle was accompanied by the picture dimming and expanding very briefly before disappearing. A hasty switch-off and general sniffing over the line stage revealed a plasticky smell. Removal of the cage and a cautious switch-on showed that the LOPT was arcing between the primary winding and core. 

For the moment that curtails any further work and I wondered if it was worth carrying on. However I think I'll give it a second chance and see about getting the transformer rewound.

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 First shows where the arcing occurs, Second is the transformer itself, third, the dampened ring showing shorted turns. Oh dear!

 

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Topic starter Posted : 31/12/2021 1:39 pm
Nuvistor
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@sideband 
That’s a beggar, be nice to have it rewound, depends how much rewinding will cost, I have no idea.

 

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Posted : 31/12/2021 2:23 pm
PYE625
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I am so sorry to hear this... It is a real pain. Let's hope something can be done in way of a re-wind.

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Posted : 31/12/2021 3:36 pm
Cathovisor
(@cathovisor)
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@sideband 

One to wave under Mike Barker's nose, I reckon.

If nothing else it'd be nice to keep something that once belonged to Mike Edwards going.

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Posted : 31/12/2021 4:08 pm
sideband
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Posted by: @cathovisor

@sideband 

One to wave under Mike Barker's nose, I reckon.

If nothing else it'd be nice to keep something that once belonged to Mike Edwards going.

That is exactly why I've just contacted Mike Barker via PM. See if he can help. I'm pretty sure the overwind is OK since that is completely sealed.

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Topic starter Posted : 31/12/2021 4:15 pm
PYE625
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I am not saying that the following is the cause in this case, but I always like to at least partly dismantle a LOPT (where possible) and give it a thorough clean. Carefully using a vacuum cleaner and fine brush to get rid of all dust and surface contamination, and checking insulation, I believe this helps avoid surface tracking and possible breakdown. Obviously, this does nothing to prevent internal faults and breakdowns deep within the windings, but it may at least help give the LOPT a fighting chance.

I believe in damp and humid weather (as we seem to get quite often), any muck or contamination on the surface will aid tracking and eventual breakdown of insulation. This is one reason that I use a dehumidifier in the workshop. It certainly helps to keep set's in storage reasonably moisture free.

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Posted : 02/01/2022 6:47 pm
sideband
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Mike Barker has been in contact with me via PM and email. He can certainly help but can't give definite details of what is required until he has seen the transformer which is understandable. I've sent him a couple of photo's to start with and will now pack the transformer ready for sending. 

 

I think the Royal Star will live again. I wonder if I can find a really good CRT from somewhere....I don't think the boosted one will last.

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Topic starter Posted : 03/01/2022 2:32 pm
sideband
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Well, the line transformer was posted off to Mike at lunchtime so I'll now leave it in his capable hands. All I have to do now is wait!

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Topic starter Posted : 08/01/2022 9:59 pm
sideband
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Mike just PM'd me to say the transformer has arrived safely. There's no rush for the rewind so I'll just wait until Mike is ready to do it. The KB will live again!

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Topic starter Posted : 11/01/2022 7:39 pm
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