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Old 10th Aug 2004, 10:07 pm   #1
fableglade
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Default Bush TV12AM 405-line TV help please?

I'm currently restoring a Bush TV12AM set, it looks to dfate from 1950, I've prinyed off the service data but the set is a little different from the Bush TV12A (presumably the TV12AM is a modified version of the TV12A ?).

I have replaced all of the capacitors in the set and it powers up, the audio side of things is fine. There is no EHT and I noticed that the PL38 has a very slight purple glow, so I replacved both the PL38 and the PZ30 with brand new (old stock of course) replacements. When the set was then powered up the new PL38 also had the same slight purple glow and there is still no EHT.

All of the capacitors have been replaced so I'm not quite sure what to try next, also as I don;t have the precise service data this makes it a litle harder. Can anyone help please and suggest what the problem may be? There is also no line whistle.

Any help or advice greaty appreciated.

Thanks for any help.
 
Old 10th Aug 2004, 11:54 pm   #2
Duke_Nukem
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Default Re: Bush TV12AM 405-line TV help please?

It's getting late and brain is fading, but given lack of whistle I suggest you check the volts on the PL38's grid which, if the line osc is running, should be nice and -ve.

Wouldn't do any harm to check the PL38's screen and cathode resistors just in case.

Check the volts on the cathode resistor ; if not excessive yet PL38 is glowing it just might be the NOS PL38 is duff.

TTFN,
Jon
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Old 11th Aug 2004, 12:06 am   #3
jim_beacon
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Default Re: Bush TV12AM 405-line TV help please?

Blue glow usually indicates ionisation in the valve, could either be gas in the valve (faulty valve), or extreme overvoltage (check the anode DC volts, also check the decoupling is working, RF parasitic oscillation can sometimes cause this kind of thing)

Jim.
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Old 11th Aug 2004, 8:27 am   #4
Studio263
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Default Re: Bush TV12AM 405-line TV help please?

There does tend to be a little blue glow in the centre of some PL38's, it's OK if it's in the middle of the electrode structure, but the valve should be replaced if it occurs at the glass. The EL84 displays similar characteristics.
If the line O/P stage is running but there is no EHT, check the visconol capacitor across the EHT. A meter check is not enough, they tend to break down only when over 1.5kV is applied. There are also some high-value resistors mounted behind the EY51, if they are bad use proper HV rated replacements.
 
Old 11th Aug 2004, 10:01 am   #5
fableglade
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Default Re: Bush TV12AM 405-line TV help please?

Well I have replaced the PL38 and that has (as far as I can see in daylight) eliminated the blue glow problem. I did also try another replacement PZ30 just in case, but that made no difference. This set is a little difficult in that the TV12A service data on the CD ROM is not accurate for the TV12AM, so parts can only be used as a guide.

I have taken the following (DC) voltage measurements:

PL38:

Pin 2 -6V [er, heater!]
Pin 4 +176V [[Grid 2 (screen)]
Pin 7 -4V [[er, heater!]
Top Cap +174V [[Anode]

PZ30:

Pin 1 +1V
Pin 2 -6V
Pin 3 -10V
Pin 4 +192V
Pin 5 +188V
Pin 6 -7V
Pin 7 -8V
Pin 8 +174V

EY51:

Anode +172V

Also, I measured the voltages present at the two main electrolytic capacitors, the 250uf capacitor has 189V and the 60uf capacitor has 191V

When the set arrived the mains tapping was set with the red lead at Terminal D and the green lead at Terminal E, the voltage rating plate is no longer with the set but I used the voltage settings in the TV12A Service Sheet and set the leads as follows: Red to Terminal G and Green to Terminal J. This is for the 250V setting and our local mains averages 247V.

I've checked the 4 resistors around the base of the PL38 and the PZ30 and all are very near to their original values.

As it now stands the set has audio but still no EHT and no line whistle. I'm a bit stuck now, any other ideas would be greatly appreciated.

Thanks for any help.

[PL38 pin connections added by Jon, you're friendly neighbourhood moderator ]
 
Old 11th Aug 2004, 12:38 pm   #6
Duke_Nukem
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Default Re: Bush TV12AM 405-line TV help please?

Whoa!, avoid measuring PL38's anode and the EY51 dirrectly ! You don't mention that your meter has blown up so I think it is now safe to assume that the frame timebase isn't running and this is cook the PL38, LOP, etc since it needs the timebase running in order to generate a negative bias on it's grid 1.

PL38 connections can be found here :-

http://www.r-type.org/exhib/aaa0497.htm

If it were me, I'd disconnect the PL38's top cap AND disconnect it's screen (important) whislt investigating the problem to avoid damage. Even with these pins disconnected, you should still get a negative voltage developed on it's grid if the timebase is running.

So, on the assumption it 'aint running, and that you have already mentioned you've done a cap replacement then :-

1) Check the primary and secondary of the blocking transformer for O/C.

2) Check DC resistance between V14 osc valve's grid and 0v, and check the grid resistors.

3) Double check you have correctly connected the replacement caps and ensure connections are good. Please don't take offence with this suggestion, we've probably all been there and got the T-shirt

4) Check picture width control - if the slider contact is poor / open then again no frame osc !

5) Well, caps checked, transformer checked, resistors checked so that can only leave checking the oscillator valve by replacement.

If it turns out to have been the oscillator at fault then I'd recommend changing the PL38's cathode resistor anyway as it will have taken a pounding.

TTFN,
Jon
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Old 11th Aug 2004, 1:15 pm   #7
fableglade
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Default Re: Bush TV12AM 405-line TV help please?

Thanks for the very helpful advice Jon. I wouldn't usually have measured the voltages on the PL38 and the EY51, but as there was no EHT present I decided that it may help to identify the fault if the voltage present at these points was known.

You make a very good point about checking replacement caps etc, I shall certainly do so, it is all too easy to make a bad soldered connection and not realise.

I shall work thorough the points you have given me and report back. PLease excuse my ignorance but which actually is the oscilator valve? Is this the PL38?

Thanks again for the help, now back to the set.........
 
Old 11th Aug 2004, 1:16 pm   #8
Studio263
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Default Re: Bush TV12AM 405-line TV help please?

Frame timebase (see above)? Line surely?

I agree though, if it was working then (assuming that the meter is a digital one) everything would be at -1888V!




 
Old 11th Aug 2004, 1:20 pm   #9
fableglade
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Default Re: Bush TV12AM 405-line TV help please?

It is a digital meter, yes, the voltages above are those which it displayed, as I mentioned before though I would not have taken these readings if the EHT had of been present.

I'll see what else I can find out by following Jon's guidelines.
 
Old 12th Aug 2004, 11:56 am   #10
peter_scott
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Default Re: Bush TV12AM 405-line TV help please?

Quote:
Thanks for the very helpful advice Jon. I wouldn't PLease excuse my ignorance but which actually is the oscilator valve? Is this the PL38?

Thanks again for the help, now back to the set.........
I don't have any ccts to check but I'm still assuming TV12 is the same as the TV22 that uses the PL38 LOp stage.

If I remember correctly the line oscillator is formed as a multivibrator from cross coupling the PL38 with the triode of the adjacent ECL80.

Good luck,

Peter.

Last edited by Duke_Nukem; 1st Mar 2005 at 7:11 pm. Reason: Changed V12 to TV12
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Old 15th Aug 2004, 6:10 pm   #11
oldeurope
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Default Re: Bush TV12AM 405-line TV help please?

Good evening,
sorry for my late answer. I am just back from Berlin
watching a Baird Televisor and things like this...

I had a look at trader sheet 971.
The TV12 has a blocking oscillator ( " Sperrschwinger " )
with the EF50 V14.
The H-osc. works with T3 R59 R60 and C51 between
katode g1 and g2. The H- puls is formed at the plate.

I don't have this set, the TV22 is very different.

Kind regards
Darius

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Old 20th Aug 2004, 7:36 am   #12
mjizycky
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Default Re: Bush TV12AM 405-line TV help please?

Quote:
I'm currently restoring a Bush TV12AM set, it looks to dfate from 1950, I've prinyed off the service data but the set is a little different from the Bush TV12A (presumably the TV12AM is a modified version of the TV12A ?)
Certainly is - TV12 RF unit, TV22 [top] deck.

<blatant plug for own site>
See www.bushradio.co.uk, navigate to the 'design classics' link and click on " TV12 and variants "
</blatant plug for own site>

Last edited by Duke_Nukem; 1st Mar 2005 at 7:13 pm.
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