BUSH TV22 lopt failiure?

[Tim]

Hi all.
Currently fiddling with one of above. Have replaced all wax caps and was able to get a raster( but poor shape and width) when suddenly there was a crack sound and raster vanished. EHT very low( can't draw spark to chassis) and tube A1 volts about 72v(180 coming from lopte.) There may have been a flash from inside the tube too, but not sure about this.
Lopt failed?
Is the tube likely to be OK?
Is the lopt easy to re-wind or does someone have a spare( don't laugh ) or is some clever soul able to rewind the lopt for me, or give me some advice on how to fix this old beast, as I don't know much about tellies.
Thanks
Tim

[oldeurope]

Hi Tim
maybe C21 and C24 (tangens cap)
I used 2u2F 250V axial MKT
same size as the electrolytics

Darius

[Tim]

Thanks Darius
I have already replaced both the 2u2F electrolytics, one on the tube neck and one on the line timbase section.
Which service data are you refering to. I have the " trader " sheet number 1091/T38.
Regards
Tim

[jim_beacon]

Tim,

LOPT failure is common with the TV22 / TV24 Chassis - but it is usually the primary which goes.

Check this page for details:

http://freespace.virgin.net/andy.valve/techa.htm

Good luck!

Jim.

[Tim]

Thanks Jim.
Sounds like your right. The EHT winding appears to be continous.
Which winding is the primary?
Could I wind another on top of the existing windings?
When the set was working, I had a miss-shapen raster and CRT A1 voltage and feed to brightness control were well down(metrosil replaced) from quoted values, even with a DVM. I could not adjust width at all, so pretty sure problem lies in the line/timebase area somewhere.
Anyone wound one of these before? How easy(or difficult !) is it?
Yet to remove lopt and test.
I have already seen Andy's pages,(and printed them out) thanks Jim.
Thanks
Tim

[jim_beacon]

Tim,

I think the primary side is fairly easy to re-wind, though I think (from Andy's page), that the failure my have a lot to do with the laminations, rather than anything else (shades of Tek 545B scopes?). Even so, cleaning and re-varnishing the laminations is not beyond reason. The primaries are straightforward windings, so they can be hand wound, with a little care. I have an AVO coil winder here, but unfortunatly, it needs restoring before I can use it (though it is getting close to the top of the list - I've got a couple of jobs that I need it for !).

Have you tried changing the small (mica?) cap that tunes the transformer? If it is badly out of tolerance, or leaky, it may reduce the Q enough to stop oscillation.

I'll be interested to see what conclusions you reach, as I have a TV 24 here with much the same symptoms - very low EHT as well (about 40% of what it should be).

I have an electrostatic votmeter and EHT probe for a DVM if you need them - I can drop them in next time I'm down in Bath.

Jim.

[oldeurope]

I measured the primary resistance of the lopt
between plate of the PL38 and Metrosil.
It is 28 Ohms.

For metrosil use the RS-components 289-6926

I think your deflection coil is anyway disconected?!

Hope that helps you
Darius

[oldeurope]

PL81
Do you think this is the right replacement?
This is a self oscillating line output stage.
The transconductans (hope it is the right word for Steilheit) of the PL 81 is much lower and the
current at 0V on g1 is dangeraus high

I think this is your problem. You are in GB so it
is no problem to get the PL38 ( I think...)
What does it cost in GB?

Darius

[Tim]

Hi Darius.
The PL81 is correct for later models of this set. (Trader 1091 shows circuit for both PL38 and PL81. )Judging by dust etc the PL81 is original.
Where is the best place to check for oscillation with a 'scope?
Cheers
Tim

[Hybrid tellies]

Hi Tim
Not to familiar with these really old sets but sounds like it uses flyback derived EHT much like the hybrid mono tv's of the 1970's did.
So one sure way to check the EHT winding for shorted turns is to let the set cool down, then disconnect the top cap to the EHT rectifier and run the set for say 10-20 mins. Switch off and if it makes you feel safer short to chassis the top cap of the line output valve.
Next with your fingure feel the temp of the EHT winding. It should be cool. If it feels warm or hot then your problem is shorted turns. I hope this is of some help. best of luck.

[Danny]

Hi it could be the boost voltage is being draged down?
Probrably by the frame circuit?
If the frame osc has stalled then it draws current from the boost supply and sometimes burns the blocking
tranceformer out .
but also A heater to cathode short in the boost rectifier
valve can cause these simptoms.

[Tim]

Thanks Danny.
I have changed the PZ30. I think the fault is around the line/horizontal stage somewhere.
I'm gonna keep this one for a rainy day!
Tim

[oldeurope]

Do you have a sine wave generator?

I had a look at trader sheet 1091
Bridge C37.
Bridge the lin transformer T1 ( all three connections).

Darius

[Tim]

Thanks Darius
Yes I do have a sine wave generator. AF or RF?
C37 (500pf?) is this a large mica capacitor connected to the width transformer?

Tim

[oldeurope]

oh no !!! it is the 2uF on the deflection coils

[oldeurope]

Hi Tim,
I measured something out for you.
(TV switched off of course)

Take a sin wave generator . Connect ground to
chassis. Put the output over 100K Ohms to the
plate of the PL. Adjust the outputlevel to 6Vpp
and watch with the oscilloscope at the plate.
At ca. 38KHz must be a resonanz with 1Vpp.
Please check it out and tell me...

Darius