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Another Biffa TV22

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Terrykc
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Member Rest in Peace
 

... My "linearity coil" description was perhaps a bit misleading. It's actually a more of a transformer, one to one with one winding on top of the other ...

Peter, as the former can clearly be seen between the turns, it looks more likely to have been bifilar wound than in two separate layers.

The truth will become self evident when you unwind it!

When all else fails, read the instructions

 
Posted : 05/01/2015 4:13 pm
peterscott
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Hi Terry,

I don't think it is bifilar at present but that would certainly reduce the counting job although with no proper winder I'm not sure whether I wouldn't be better sticking to single file . It looks as if each winding has about 120 turns.

Thanks,

Peter

www.nostalgiatech.co.uk

 
Posted : 05/01/2015 4:32 pm
freya
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There is a nice article in the Feb 1958 practical television in the library that deals with the line scanning circuits, goes into details on the linearity coil. Might be an interesting read.

 
Posted : 05/01/2015 5:44 pm
peterscott
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Hi Stephen,

Thanks for the reference. I think there is a collection of PT in the Technical Library here but can you remind me how to access it?

Peter

www.nostalgiatech.co.uk

 
Posted : 05/01/2015 6:22 pm
Cathovisor
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I think there is a collection of PT in the Technical Library here but can you remind me how to access it?

See section 5 of the rules here

 
Posted : 05/01/2015 6:35 pm
peterscott
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Thanks Cath,

I just had senior moment but Chris sorted me out.

Found the article thanks Stephen.

Peter

www.nostalgiatech.co.uk

 
Posted : 05/01/2015 6:51 pm
peterscott
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Well, I took Terry's advice and rewound the linearity transformer T5. Each winding had 260 turns on two layers with paper between them. I didn't have the exact wire gauge and used 36 SWG whereas the original was maybe a couple of gauges heavier. I just used the same number of turns but not neatly laid, just piled over the area with primary and secondary separated with pvc insulating tape.

tv7

This did improve the linearity a bit but made no improvement in boost voltage or EHT.

tv8

From here I decided to disconnect the LOPT and test the Q as suggested by Cath. Using Chris' method monitoring on the EHT rectifier anode connection and connecting my scope calibration output to the Line O/P anode connection I get a rather inconclusive ring (displayed at 50uS/div). The EHT rectifier heater and all other loads were disconnected.

tv9

Peter

www.nostalgiatech.co.uk

 
Posted : 07/01/2015 3:13 pm
freya
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Have you tried moving the ion trap, just wondering if its positioned incorrectly throwing the raster off. Just an idea. Wouldn't help the low EHT though.

 
Posted : 07/01/2015 7:14 pm
peterscott
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Hi Stephen,

Thanks for the thought. I have tried many combinations of picture centring and ion trap magnet and can get the brightest and least vignetted image. Having said that my scan does tend to favour the left side of the screen and this is not because of leakage in the scan coil coupling capacitor. Perhaps my scan coil core has some residual magnetism?

Peter

www.nostalgiatech.co.uk

 
Posted : 07/01/2015 7:28 pm
Doz
 Doz
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I had similar issues although the lopt did turn out to have a problem, the bad lin was indeed the line form control.

 
Posted : 07/01/2015 7:30 pm
peterscott
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Hi Doz,

Did you try Trevor's cookery tip?

Peter

www.nostalgiatech.co.uk

 
Posted : 07/01/2015 7:47 pm
Doz
 Doz
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Hi Doz,

Did you try Trevor's cookery tip?

Peter 

Not yet but she's out tonight ....

 
Posted : 07/01/2015 7:56 pm
peterscott
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My overwind has had quite a few hours of operation as I continue failing to discover the fault but it never gets very warm. I suspect that it would be warmer if it contained a shorted turn. I think my next attempt will be to rewind the EHT rectifier heater winding.

Peter

www.nostalgiatech.co.uk

 
Posted : 07/01/2015 10:06 pm
peterscott
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Well, I unwound the heater winding and it made no difference to the ringing test picture so it looks like this one is truly destined for the Biffa bin! (Unless there's a convenient overwind I can slip over the existing former.)

Peter

www.nostalgiatech.co.uk

 
Posted : 08/01/2015 11:59 am
peterscott
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I see that the last time I posted about this set was in January 2015. I was really fed up with it but recently reading that running 30mA through the overwind for a few days can dry it out and restore normality. Well, I did try this but to no avail. 

When I last ran the set I could get a picture with distorted line scan but on revisiting it I could only get about 2kV EHT that disappeared if I turned the brightness above more than faint shadow. Discussing it with Jac Janssen he suggested that I try generating the EHT from a voltage doubler off the LOP anode. I tried this and got 3.7kV and a bit more brightness. Jac then suggested that I try removing part of the overwind just in case the shorted turn(s) were near the surface. After a few days soaking in a bath of paraffin I got the pitch off the winding and started carefully unwinding it hoping to find a shorted turn.

tv10

Unfortunately this was not the case but I continued to use Chris's test of feeding the scope calibration output to the LOP anode end of the overwind whilst viewing the trace on the other end. I did this continuously whilst removing more and more of the winding in the hope that I would see a sudden jump in Q factor if I got to a shorted turn.

No luck! So I now had a LOPT with no overwind and no EHT rectifier heater winding so I connected it back into the set with a silicon voltage doubler and now have a set with a future again.  See the current screen shot.

tv11
IMG 20190912 143945 (Medium)

Lots of my nasty temporary fixes (that didn't work) still to sort out and what with a CRT heater isolation transformer to overcome the HKS I guess it might not have quite the video bandwidth that it had when new but we'll see.

Very many thanks to Jac for his suggestions and encouragement.

Peter

www.nostalgiatech.co.uk

 
Posted : 12/09/2019 3:14 pm
peterscott
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I forgot to mention that with the LOPT overwind removed the EHT from the doubler has risen to 6.5kV.

Peter

www.nostalgiatech.co.uk

 
Posted : 12/09/2019 4:44 pm
peterscott
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Got the alignment a bit better but the LO should be at 63MHz but in practice I'm struggling to tune the sound IFs there so had to go a little lower.

The vision response is fine even with the HKS transformer. I see that when I fitted the transformer I added a large ferrite bead around the heater wires at the CRT base.

Picture centring suffers from vignetting if I try to push the picture over to the right and moving the ion trap magnet doesn't help much.

Peter

P1040521 (Medium)
P1040522 (Medium)

 

www.nostalgiatech.co.uk

 
Posted : 13/09/2019 3:00 pm
PYE625
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Probably has been tried, but does it help by taking off the ion trap magnet and flipping it over ?

It may seem a silly suggestion, but could it be worth degaussing the CRT gun assembly ?

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

 
Posted : 13/09/2019 4:42 pm
peterscott
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@pye625

Hi Andrew,

Thanks for the suggestions. I just removed the focus magnet and degaussed the gun then put  things back with the ion trap facing the other way (so ends up on the bottom of the neck). Unfortunately the scan is right back where it was. The focus / centring assembly is on slotted holes and I find it is best fairly well back and twisting it on its slots helps but I'm at the limit of twist just to get the left skewed raster as in my previous photos.

Thanks,

Peter

www.nostalgiatech.co.uk

 
Posted : 13/09/2019 7:07 pm
peterscott
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As of last night I had two faults that I was trying to fix.

  1. Inability to centre the picture left right.
  2. A slow rolling distortion of the frame scan.

 

Number 1. still eludes me but having replaced most of the decouplers in the set I was trying to think where the mains could be corrupting the scan and wondered if the heater voltage in the frame oscillator and output could be somehow coupling. So this morning I swapped the two ECL80s over and to my delight the fault disappeared.

It's a bit strange though because the cathodes in the frame timebase are both at ground potential so you might expect any leakage from the heater to be invisible.

Peter

www.nostalgiatech.co.uk

 
Posted : 14/09/2019 7:40 am
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