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Height appears to increase at the Right Hand Side (HMV901)

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peterscott
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My HMV901 has always had a little of this problem but it appears to be getting worse. The edge focus has always been a little poor on the right but I suspect this is unrelated. My first thoughts were of some element of the line scan intruding in the frame and I wondered about the HT rail but it appears to be well decoupled. I also wondered if the common ground of the line and frame scan coils might be poor but shunting that makes no difference. 

Any pointers would be gratefully received.

You can see how is was previously here if you click on the test card image and the time base schematic is available if you click on the thumbnail version and scroll down a little.

Here's what it currently looks like.

P1070484 (Medium)

Thanks,

Peter

www.nostalgiatech.co.uk

 
Posted : 03/10/2018 4:10 pm
PYE625
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I take it the actual scan-coils are fully forward and sitting evenly against the crt neck and bowl ? Also, that the line and frame scan coils are properly seated and assembled together ok.

If they are ok, you could try reducing the raster size and seeing if the same shape is maintained regardless of picture size. It could simply be magnetism effecting the shape of scan, or lack of shape correcting magnets. I guess the EHT voltage and focus adjustment are normal ?

It don't look bad though   ? 

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

 
Posted : 03/10/2018 4:40 pm
peterscott
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Scan coils are tight to the cone and no freedom for tilting. EHT is normal and focus can be adjusted either side of good focus. 

Reducing the scan size doesn't appear to improve the distortion. There are no shape correcting magnets although I confess that I recently re-installed what EMI called the "push about coil" (a device for picture centering) after running the set for many years without this because it made the RHS focus worse. I have now removed it again but as the CRT is surrounded by a steel box then perhaps this has become slightly magnetised.

Many thanks for your pointers.

Peter

P1070485 (Medium)

www.nostalgiatech.co.uk

 
Posted : 03/10/2018 6:09 pm
peterscott
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I sensed around the steel cage that surrounds the area where the push about coil had been and it is quite strongly magnetised. I did try a few sharp blows with a screwdriver handle but it hasn't reduced it by much. I don't have a degausing coil so I guess I need to remove the cage for some more brutal treatment. That coil was nothing but trouble. It did shift the picture but not without distorting the edge focus. I don't understand why EMI thought it a good idea. You can achieve as much picture centering simply by moving the CRT around within the confines of the rubber screen mask. Having said that I think the earlier sets had rather less compliant masks so perhaps that is the excuse.

Peter

www.nostalgiatech.co.uk

 
Posted : 03/10/2018 7:53 pm
PYE625
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Posted by: peterscott

I don't have a degausing coil so I guess I need to remove the cage for some more brutal treatment.

Placing it in close proximity to a CRT colour set upon switch on from cold may degauss the cage, but I must admit to never have known a black and white set to need degaussing. Of course, I could be wrong.....

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

 
Posted : 03/10/2018 8:07 pm
peterscott
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That's a good suggestion. I only have one CRT colour set, a wide flat screen. I'll try it although it does involve removing the CRT in the HMV901 to get the cage out. Always a task approached with trepidation.

Thanks,

Peter  ? 

www.nostalgiatech.co.uk

 
Posted : 03/10/2018 8:14 pm
PYE625
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Hmm....it may be better to invest in a degaussing wand, or borrow one rather than go to all the trouble and strife of removing a fragile CRT.

(Ebay is a good place to look, I saw a degaussing coil just now on there.)

There is of course another option.... leave it be and accept it as it is.  It is rare to find any CRT set with perfect raster geometry.   ? 

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

 
Posted : 03/10/2018 8:19 pm
MurphyV310
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Hi Peter.

That looks like a bit of trapezium distortion and that could be due to an unbalance in the scan coil windings or any soldered tag that just might be going dry and high resistance.

Magnetism usually will bend the picture, you could try winding a number of turns of single core wire round the CRT framework, feed it from a meaty 6.3v transformer and wind up the primary on a variac then slowly reduce it to zero, this should degauss any residual magnetism, you may need to do each leg at a time.

Cheers,
Trevor.
MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

 
Posted : 03/10/2018 8:39 pm
peterscott
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Thanks Trevor. I'm certainly going to attempt some degausing as a first step.

Peter

www.nostalgiatech.co.uk

 
Posted : 03/10/2018 10:20 pm
Doz
 Doz
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Has someone got a fitted degaussing coil for a scrap colour set you can steal? Connect it to the the mains in series with a lamp, and don't leave it on for long. I thought I still had mine, and you could have had it for nothing, as I have a proper coil now, but I must have slung it....

 
Posted : 04/10/2018 11:58 am
peterscott
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Thanks Doz,

I have the cage out of the set now and tried placing it on top of a CRT colour set. I've tried powering up the colour set quite a number of times from cold and also tried placing the cage in front of the screen but I haven't seen any detectable reduction in the magnetism. It's a tall item and is clearly magnetised top to bottom. Unfortunately I don't know for sure what metal it is made of. If it were simply steel I'd start walloping it but perhaps it's Mu metal so that would be a bad course of action. That said I suspect it's not Mu metal because it appears to retain its magnetism rather well.

I have submitted a bid for an eBay coil but if I don't get that I may wrap wire around the cage and see if I can change the magnetism.

Peter

www.nostalgiatech.co.uk

 
Posted : 04/10/2018 12:40 pm
Till Eulenspiegel
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Hi Peter, I've a degaussing  coil in the shop, you are welcome to borrow it.

Took a look at my Marconi 702 today, it also has the "push-about" coil fitted to the CRT. However, it's never  been connected as long as I have owned the set.

This is the set that has never supplied sufficient video drive to the CRT grid, the best the video demodulator can deliver is 10 volts P - P.  CRT requires 15 volts of video for a fully contrasted picture.

Till Eulenspiegel.

 

 
Posted : 04/10/2018 7:18 pm
peterscott
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The offer is much appreciated Till and I might take you up on that if I fail to get the one I have a bid on on eBay. 

That TRF chassis is certainly an awkward thing to troubleshoot and especially the detector end that won't work if you remove the little cans from the back side.

Thanks,

Peter

www.nostalgiatech.co.uk

 
Posted : 04/10/2018 9:23 pm
peterscott
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Well I think my knowledge of magnetism is rather lacking!

Standing the CRT cage big end uppermost my compass told me that it had a North pole at the top and a South pole at the bottom. Well I then inverted the cage only to find that it still had North at the top and South at the bottom and if I lie it on its side then the compass shows little change from one end to the other.

In other words I was just seeing the earth's magnetic field passing through the cage.

Peter  ? 

www.nostalgiatech.co.uk

 
Posted : 07/10/2018 5:32 pm
Till Eulenspiegel
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Hi Peter,   during the course of production of the first generation EMI TVs different CRT cages were employed.    Tomorrow,  I'll take pictures of the CRT assemblies fitted in my Marconi 702 and HMV 901 TV receivers.   The push-about coil in the 702 is not energised, it doesn't need to be because the picture centering is perfect.   Can't display pictures on the 901 because the EHT transformer is burnt out.

Till Eulenspiegel. 

 
Posted : 07/10/2018 8:57 pm
peterscott
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Hi Till,

I think my 901 has the same cage as your 702. I know the early sets like your 901 extended the metalwork down over the scan coils. I do have photos of your 901 and 702 that I took about 10 years ago so please don't go to lot of trouble if my photos are adequate.

901 702 (Medium)

I'm still very sceptical about the push about coil. Like you I find it's not really needed to get picture centering and I'm convinced that it distorts the focus at the edges if you run any current through it. If your 702 has good edge focus I would certainly be interested to see it.

Thanks,

Peter 

www.nostalgiatech.co.uk

 
Posted : 07/10/2018 9:45 pm
Till Eulenspiegel
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The attachments show the different CRT cages employed in the Marconi 702 and the HMV 901.     Pictures go to show there has been no alterations to the receivers.   The HMV is out of service because the EHT transformer has burnt out.  

My early 900 has a CRT cage in which the tube is fully encased.  Seemingly an impossible task to remove the CRT.

Till Eulenspiegel.

Marconi702
HMV901

 

 
Posted : 08/10/2018 2:04 pm
peterscott
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Having decided that magnetism of the CRT cage wasn't the problem I checked the scan coil core but couldn't sense any problem there. I also checked the core of the push about coil (that I removed) and it did have some residual magnetism.

Anyway, I re-assembled the CRT in the set and after struggling to get a picture due to unreliable contacts between the CRT heater pins and the socket and failing to remove the screws on the socket (due to corrosion) I gave up but managed to get the heater lit with some switch cleaner.

The edge focus is better than it was with the push about coil fitted but I still have some height growth at the right hand side and am struggling to think what is causing it.

Peter

p.s. Till, I see you've been raiding your 901 for MSP4s. That's another area I'm in trouble with. My RF gain jumps suddenly between high and various lower levels. I can't see any supply voltage changes on the outside world and I've scratched all the valve pins and top caps all to no avail!  ☹️ 

www.nostalgiatech.co.uk

 
Posted : 08/10/2018 2:46 pm
Till Eulenspiegel
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Posted By: peterscott

p.s. Till, I see you've been raiding your 901 for MSP4s. That's another area I'm in trouble with. My RF gain jumps suddenly between high and various lower levels. I can't see any supply voltage changes on the outside world and I've scratched all the valve pins and top caps all to no avail! 

Hi Peter,  Like yourself I'm having difficulties finding decent quantities of the 5 pin MSP4 RF pentode.   A seven pin version of the valve was made and if this valve is found it might be possible to fit a five pin base. There's sure to be continental equivalents of the MSP4 although none appear in the Radiomuseum website.   The gm of the MSP4 about 3mA/V.  The Mullard TSP4 has the 7 pin base. Considering this valve is a Mullard there might possibly be a continental Philips electrical equivalent with a different base, B5 or ct8.   The ex-services CV1341 can be considered if one can be found.

Till Eulenspiegel.

 
Posted : 08/10/2018 7:28 pm
peterscott
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Hi Till,

I got one in the form of an MS-PEN. It works well.

http://www.r-type.org/exhib/aad0250.htm

Peter

www.nostalgiatech.co.uk

 
Posted : 08/10/2018 7:54 pm
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