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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

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Old 14th Jul 2004, 12:33 pm   #1
Ed_Dinning
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Default Bush TV22 fade out

Hi Gents, I recently re-discovered a TV22 that had been put away some time ago with the fault of the picture brilliance fading out over 1/2 hour. After a couple of hours off the pic would be OK again, only to repeat the proceedure over the next 1/2 hour. Most caps and high value resistors had been changed, nothing appeared to get hot. Ht was OK but boost dissapeared. New valves for LOPT ,Boost Diode and line osc/sync had no effect. Anyone had this fault before, prior to me doing battle with it again?
Many thanks, Ed Dinning
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Old 14th Jul 2004, 4:18 pm   #2
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Default Re: Bush TV22 fade out

Did you check the metrosil? My was defect.

Darius
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Old 14th Jul 2004, 9:56 pm   #3
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Default Re: Bush TV22 fade out

Dont forget the dreaded lopt rust on the core.
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Old 19th Jul 2004, 12:05 pm   #4
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Default Re: Bush TV22 fade out

I'm not sure about the TV22 but the TV12 has a " visconol " capacitor accross the EHT that when faulty (e.g always these days) gives these symptoms. It can be replaced with few HV disc ceramics which are tiny and easily hidden. A clue is that the focus drifts before the picture disappears.
 
Old 20th Jul 2004, 12:17 pm   #5
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Default Re: Bush TV22 fade out

OK, here's something else you won't want to hear: My TUG12A (same thing but 12 " ) had an intemmitent short in the tube heater ( " M " shaped) that would occaisionally dim the picture (clouting the tube neck would bring it back). When faulty, there was only 2V across the heater and the glow was much diminished, though of course the valves were unaffected.

Let's hope not...
 
Old 20th Jul 2004, 11:08 pm   #6
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Default Re: Bush TV22 fade out

Run the set till it goes faulty then unplug it and feel the lopt winding to see if it is getting hot?
if not i would again whilst it is in fault condition check the line drive at the grid of the line output valve.
does the line output valve glow more than normal. also it is possible that the scan coils are faulty.
Have you replaced the boost cap ?
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Old 21st Jul 2004, 1:35 pm   #7
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Default Re: Bush TV22 fade out

Ed,
Does the picture shrink or expand as the brightness changes? If the brightness changes independently of the geometry I would think that the fault lies in the video or tube biassing departments rather than the timebase.
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Old 21st Jul 2004, 2:08 pm   #8
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Default Re: Bush TV22 fade out

If the picture expands as the brightness drops then it's possible that the EHT rectifier is losing its heater supply. THis could just about be the problem even if the picture does not change size. Try a BY140 or similar EHT diode across the EHT rect as a test. If this cures the problem then either leave it there (sacrilege!) or find the fault with the heater winding. It's just possible that the insulation of the heater winding is breaking down when hot (this would also drag down the whole line output stage) in which case you'll need to remove the old EHT rectifier before trying a diode.
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Old 31st Jul 2004, 12:53 pm   #9
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Default Re: Bush TV22 fade out

Doesn't take much leakage from the heater winding (at EHT potential) to the core to bring down the EHT. The insulation really is critical here, as in any part of the EHT circuitry.
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Old 1st Aug 2004, 4:37 pm   #10
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Default Re: Bush TV22 fade out

Hi Ed,
here you can see the wafeform at anode PL38
2KVppH (I made a special 1:100 probe for this) ; 8)

www.pixum.de/viewalbum/?id=1383170

The plate voltage is 8KV

g2 (from Metrosil) 350V DC (DMM)

at the other end of the metrosil you must
have a H-pulse of 400VppH sitting on 200VDC

I used the RS-Components 289-6926 metrosil.

@Jeffrey, I added a picture of g1 and anode pentode
ECL80 with the reduced level V-sync in this album for
you.
I can sent you more via email if you want.

Kind regards

Darius
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Old 2nd Aug 2004, 12:36 pm   #11
Ed_Dinning
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Default Re: Bush TV22 fade out

Thanks All, a new metrosil has produced a cure with no fade out after 3 hours.

Ed
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Old 2nd Aug 2004, 9:04 pm   #12
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Default Re: Bush TV22 fade out

Quote:
I used the RS-Components 289-6926 metrosil.
Hi Darius,

RS no longer seem to stock your Metrosil. Do you have any characteristics
for it?

Thanks and kind regards,

Peter.

Last edited by Paul Stenning; 29th Dec 2004 at 1:49 pm.
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Old 3rd Aug 2004, 8:28 am   #13
oldeurope
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Default Re: Bush TV22 fade out

Good morning Peter,

It is the S05K150 VDR it conducts at ca.250VDC .

I used the S05 because it has a lower capacity than
the stronger VDR's.

I think the S07K150 RS ordercode 204-2040
will work as well, but I did not try this one.

I have seen several S05K150 at digikey.

The VDR DC value must have the half of the ACpp level.

In my Bush with the PL38 I have 400VppAC sitting
on 200VDC.
So the peak voltage is 600V and with the voltage
drop of the VDR you get 600V-250V= 350V (Ug2 CRT)

When you switch off the TV, the VDR discharges the
g2 cap a little bit down to 250V, so there is enough
voltage to discharge the EHT . This circuit is a burning
point suspression (is this the right word for Brennfleck-
unterdrückung?) [=switch off spot suppression].
And the VDR makes an additional positive voltage
in this case 150VDC to the 200VDC.

This kind of generating DC from the H-flyback pulses
is used in later applicationes with the for ex.PL500
to generate a negativ g1 voltage to regulate
H-with and EHT .

Kind regards

Darius
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Last edited by Duke_Nukem; 15th Feb 2005 at 2:03 pm.
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