Bush TV 12B To restore or not to restore advice please

[beery]

Hi Andy,
A good thing to do with sets which use EF50s or earlier valves in the RF section is not to mix them up.
If you do have to take them all out at once, label them up so that they can go back in the same sockets that they came from.
No two EF50s are exactly the same, despite the fact that the all glass base does enable internal capacitances to be fairly tightly controlled, and mixing them up can mess up the alignment. With EF91s, EF80s and later RF/IF valves this is not really a problem.

I brought my own TV11A (a wooden TV12) to Ware from my parents place to look into rewinding the LOPT. I have ruled everything else out I think. When the set has been on for an hour or so the width starts to flick inwards by about an inch every so often, I've seen this before in sets that have quickly failed, but my TV11A seems to be able to keep this up without the transformer finally dying.
Maybe more investigation is required, but it is a long way back in the queue.

When working poperly these are great little sets. As Fernseh said, the line output stage is an improvent on the TV1/TV2 circuit. That's putting it mildly. I've restored a couple of TV1s and I can say that without the efficiency diode, the poor old EL38 (6.3V version of the PL38) runs out of steam, resulting in poor EHT regulation. The Bush TV91 which had mains EHT must have performed much better in that respect, but I have never seen one running.

Cheers
Andy Beer

[bionicmerlin]

Thanks Andy for the information. I did read about the valves before.. I don’t intend taking more out than I need to . The set is slightly on hold for now . Andy

[Radio Wrangler]

If I'm replacing multiple components on a tagstrip, I may do desoldering as a job lot, but I then unpeel the legs of one component and replace it before I move on to unpeeling the next. Soldering is a job lot again at the end.

This way I never have more than one component out at any time.

It also works with components hanging from valve sockets, where you have access.

David

[bionicmerlin]

Well I’m almost ready to power up.
I have been slowly putting things back together after getting the LOPT rewound by Ed ( thank you )
I have replaced the visconol and the bleed resistors as 2 of the old ones had gone high. I found a new old stock PL38 in my stash to replace the original that the cap fell off. I didn’t want to risk pulling the cap off the CRT so soldered the HT wire on the old cable . I have checked my work hundreds of times not to make mistakes.
Only thing being as the rewound LOPT is in a metal can it’s not easy to tell if over heating.
As I’m connecting it up as the chassis being neutral . The internal fuse is in the neutral line. What’s the view on that I was thinking of changing it the the live line ?
Any tips before powering up On a variac Andy

[mark pirate]

Just connect to a variac & a lamp limiter and go for it!
I would recommend fitting a new EY51, the one on my set was possibly the original, and fitting a NOS valve increased the EHT by 1.5kv.
Make sure you have a good PZ30 fitted, these can make a world of difference.


Mark

[bionicmerlin]

Mark.
I have gone for it. So far to start had partial raster ,that’s the max with the hight on full but to start had a thin white line. I do have a week Sound tone from the Aurora.
I will change the EY51 . I have a new one but I don’t think I have a new PZ30 .
I think nearly all the pots want cleaning as well as the valve holders. Andy

[mark pirate]

It looks like frame collapse possibly due to dirty or open pots, at least you have first light, so a good start.
I had problems with duff EF50's in my set, well worth checking.
I take it you have replaced the wax caps in the receiver, unlike the TV22, there are only a handful to do.

I also had problems getting a signal through, again due to a dead EF50.
Make sure the Aurora is switched to channel 4.

Don't worry about the PZ30, as you obviously have both HT & EHT present.


Mark

[bionicmerlin]

Mark
I have changed the EY51.
On powering up and a bit of tweaking I got a full raster. The frame collapses unless vertical hold at full position.At that position the frame is running far to fast. Raster is somewhat affected by the brightness.
I’m going to check ,clean and possibly replace the vertical hold pot and others as soon as I got time.
I have changed all the caps. I got a picture but it’s not as good as it looks .
Thanks for your help Andy

[mark pirate]

Hi Andy, I am sure repair or replacement of the vertical hold pot will help, also check the associated resistors.

If you have the means, check the EHT, it makes a huge difference to picture brightness, these sets struggle to produce 6kv, my set does produce a clear bright picture, as the replacement tube has very good emission, although cranking up the brightness makes the picture balloon slightly.

I would also check the heater voltage once the set has fully warmed up.
Once the set has been run for a while, the tube should improve, you will see that the ion burn becomes less noticeable too.


Mark

[bionicmerlin]

Mark I don’t have the means to check the EHT easily.
I will make a point of checking the ht in due corse.
The problem I got at the moment is the vertical hold pot is open circuit near one end. I did find a replacement but that’s a carbon track and that’s iffy. It’s not easy to get at so ideally iwould like to find a good replacement. Maybe I will put a post on the wanted section Andy

[Heatercathodeshort]

The main problem here appears to be picture positioning. The whole raster is deflected downwards causing extreme corner cutting. It needs to be set with the cenering screws either side of the focus unit.

Once this has been set up you should be able to reduce the height and set up the frame. It will then be a lot easier to check if you actually have a frame fault.

Data from Bush service manual FEB 1949. John.

[bionicmerlin]

Thanks john . That makes sense.the picture I took is not a true image of what I’m seeing. I guess my camera is taking a picture of a locked screen where Infact the image is rolling way to fast due to the faulty pot . I will try and get a replacement then follow your advice. Thanks Andy

[Heatercathodeshort]

Should read centering of course! watch the mains fuse when adjusting. Use rubber gloves or better still cover the fuses with some insulating material as recommended in the service data.

[bionicmerlin]

Thanks john ,I see what you mean the fuses are very close.
And thanks to Andy for supplying the replacement pot.
I have made some progress. The volume was low but that’s Improved on its own to be more than acceptable.
I’m struggling on the picture side. i now have the test card on the screen but lacking in hight at the top.As can be seen adjusting the hight gives me the Upside down egg shape. The Vertical Form pot only helps a little and didn’t appear to do much . Brightness is not fantastic and adjusting the brightness increases the picture a fair amount as mentioned before .
I will keep going but hopefully I can get some more advice. Andy

[Heatercathodeshort]

Nice bright tube! Try adjusting the frame linearity control to correct the bottom extension together with the height control and entering. they will be interactive.

If this fails to produce a circle on the test card check the EL42 frame output valve particularly if the bottom extends as the height is advanced. You could try swapping it with the sound output EL42 but the sound one may not be as good as the frame! Check also the cathode bias resistor for a rise in value. Check also that you have the correct value capacitors in the frame output circuit.
John.

[mark pirate]

Brightness is not fantastic and adjusting the brightness increases the picture a fair amount as mentioned before .

As you have changed the EY51, I would check the PZ30, this is not only the HT rectifier, but the boost diode as well.
They do have a hard life, well worth trying a new one, I have had a few that have got weak, usually on one side.


Mark

[bionicmerlin]

First of all.
The linearity doesn’t seem to do much. It does do something but not much . I have changed the valves over and Infact it makes it worse.i can’t get a circle at all. I tested that resistor you mentioned and it is spot on.
The output from the PZ30 measures 210volts , the service info says it should be 214 volt. So not to far out.
I don’t have any new valves to check . I might have some more old ones to try if I go through all my boxes of pulls. But again they might not be any good
I will also treble check the right value caps I have fitted later.
If I can’t find any valves maybe the way forward is to try and get some NOS
Thank you both Andy

[mark pirate]

The linearity doesn’t seem to do much. It does do something but not much . I have changed the valves over and Infact it makes it worse.

I would try a NOS valve, the same goes for the PZ30, although the HT is not far out, the boost side may well be low.

A quick & dirty trick is to lower the mains tapping as a temporary test, if an improvement is noted then it would point to the valves.


Mark

[beery]

Hi Andy,
I'm glad to have been able to help with the frame hold pot.

The boost diode on this set is a PY31, which does indeed seem to have a hard life, see if you can get hold of a new one.
The TV22 uses one half of the PZ30 HT rectifier as the boost diode, which was asking a lot of it, but it seemed to work, athough I don't know of any other sets that did this. The Invicta T102 went one further and used the PZ30 as a voltage doubler, very nasty and a heater cathode short would make the HT cap go off like a rocket.

Anyway, back to the TV12. Check the value of R6 (Bush manual)/ R32 (Trader sheet 971), this should be 470K and unusual for resistors of this era has a 5% tolerance. Check of course the frame form pot and its 1.5Meg standoff resistor. The frame transformer can give problems on this set. My TV11A had fluctuating height which turned out to be a frame transformer. It is now fitted with a transfomer from a scrap GEC BT2147 which works fine after replacing R6 with a different value. I've since rewound the frame transformer, but have yet to refit it.

I'll let you know if I think of something else.

Cheers
Andy

[bionicmerlin]

Mark and Andy thanks for the suggestions.
When I got the set the voltage was set to 230 volts. I put it up to 240 for our area.
I will try putting it back to 230.
I checked R6 and it tested 260k so I suppose it’s acceptable.one thing I did find is in parallel there is a .001mf mica cap under the board , it appears to have a small hole in the middle exposing the inside. I did test it . It tests ok but I will change it tomorrow.
I got loads of valves at my mums so will check before trying to track down replacements Andy