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Old 11th Jan 2017, 2:17 pm   #41
Argus25
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Default Re: Bush TV22 Mk 1

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Once you have EHT if there is nothing visible on the screen briefly connect the CRT cathode to ground via a resistor of around 10k to 47k. If all is well the screen should light up.
Liam,

The reason for above is that the polarity of the video detector diode drives the video output tube grid positive, that is when the TV station RF carrier is received. This increases the video output tube's plate current and so the plate voltage drops, which takes the CRT's cathode more negative with respect to its grid (as does adding the temporary shunt resistor) This increases the CRT's beam current. So with no RF carrier the CRT beam can be cut off, out of range of the brightness control which takes the grid in the positive direction to get beam current and light up the CRT.

The raster looks compressed at the bottom, usually a sign of poor emission in the vertical output tube, or out of spec R's and C's in that stage. Or it might have a combination of off center beam alignment and low amplitude (height). Check the vertical running frequency.

(I think I mentioned that if the set is powered before the full restoration of componentry R's and C's etc and a set of trial nos tubes, there can be a few faults to chase down).

It is good to see the CRT light up, well done.

Many vintage CRT's have ion spots sometimes called ion burns. This is really ion contamination & de-activation of the phosphor. Non-aluminized phosphor is vulnerable to this effect. The reason the ion spot is in the center is that the negative ions have a higher mass and are deflected less than the electrons in magnetic deflection systems. Ion trap magnets help to minimize ions in the electron stream but they are not 100% effective. Aluminized screens solved this problem.

(In electrostatically deflected tubes ion damage to the phosphor wasn't as noticeable because ions are deflected in that case on the basis of their charge, not mass, so they get deflected the same as electrons).
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Old 11th Jan 2017, 2:55 pm   #42
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Default Re: Bush TV22 Mk 1

wd40addict - Thanks I'll give that a try. I'm not sure what the set is tuned to. The tuning knob at the rear is missing and has been broken off at some stage, although the stump is still there and the core seems to screw in and out. The gauge alongside is numbered 1-5.

Argus25 - I'm fairly sure the compression of the raster is due to my camera. It was mounted on a tripod and I only lit up the screen long enough to take the photo. The exposure was about 1/60 I think. I took a few others and I think they showed the bottom as ok. I'll check through them and make sure.

Regarding the EHT: does the presence of a raster confirm that it is present? So far I can't get a spark from the CRT or PL38 after switch off. I also cant be sure the EY51 is lit as it's fossilised under pitch. I do see a faint glow, but it could be a reflection of the PL38 heater.

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Old 11th Jan 2017, 4:29 pm   #43
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Default Re: Bush TV22 Mk 1

You certainly have EHT with a raster.EY51 do not glow that bright or should not!
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Old 11th Jan 2017, 9:30 pm   #44
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Default Re: Bush TV22 Mk 1

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Argus25 - I'm fairly sure the compression of the raster is due to my camera.
That sounds good, sometimes camera shots will give a false impression of the raster or TV image with a grey or dark wide band as part of the raster gets under exposed.

All the light you see from the phosphor comes from the scanning electron beam which gets its energy from the EHT supply. The fact the raster is good in width also indicates the line output stage is fine and normal height means the vertical output is fine and the HT must be good too. So seeing a good raster checks multiple parts of the TV's system (except the radio frequency stages and sync separators).
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Old 13th Jan 2017, 9:49 am   #45
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Default Re: Bush TV22 Mk 1

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Most old school service generators had a fixed AM setting of 400Hz at 30% modulation depth. Turn this on and sweep freq around 45MHz and you should see something on the screen.

Sweep around 41.5MHz and you should hear the tone.

Note the above frequencies assume your TV22 is set to London Ally Pally frequencies.
I tried the above with the sig gen connected to the aerial terminals on the r.f. deck via a dummy aerial pad I made for radio work. So far I can't get any response from video or audio. There are still about 4 wax capacitors in the r.f. deck so I'm focusing on replacing those and checking the remaining resistors for now.

Once all components have been checked I'll revisit it and do some voltage checks signal tracing. Hopefully the tuner is ok despite the missing knob.
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Old 13th Jan 2017, 10:56 am   #46
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Default Re: Bush TV22 Mk 1

Do you know which region this TV was set up for? If it's from the North West it wouldn't respond to a 41.5MHz signal. Try sweeping the sig gen from 41MHz to around 70MHz. If the RF side is working you should get some response at some frequency in that range.

The sound channels are 48.25MHz (ch2) 53.25MHz (ch3) 58.25MHz (ch4) and 63.25MHz (ch5).

When I did my TV22 I replaced all the wax paper caps plus the 25uF electrolytic on the RF chassis and the two 2uF electrolytics on the main chassis before switching on. I had no raster on the screen until I connected the Aurora. Then it was a case of adjusting the controls to get the correct height, width and linearity and focus. I didn't replace any of the resistors as they appeared to be OK.

The fact that you have a raster, albeit distorted, on the screen is very encouraging as it shows the timebases and tube are OK.

The 16uF electrolytic C25 can cause problems with apparent ghosting and displacement at the top of the screen if it fails. It didn't reform well on mine. I fitted a 10uF cap in its place.

I don't think it will be too long before you have it up and running.

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Old 13th Jan 2017, 11:17 am   #47
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Default Re: Bush TV22 Mk 1

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The 16uF electrolytic C25 can cause problems with apparent ghosting and displacement at the top of the screen if it fails. It didn't reform well on mine. I fitted a 10uF cap in its place.
Is that the 16uF smoothing capacitor under the main chassis? My Trader calls this C63 but it's the Mk1 version so the numbering may be different. I haven't checked it yet, though there seems to be almost no hum on the speaker so smoothing should be working fairly well.
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Old 13th Jan 2017, 3:15 pm   #48
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Default Re: Bush TV22 Mk 1

Yes it's C63 on the trader sheet but C25 in the Bush manual. It decouples the supply to the Line oscillator and the RF chassis. If it fails there is interaction between the two causing the problems I mentioned. I'd wait until you get a picture before you check it.

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Old 16th Jan 2017, 9:31 am   #49
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Default Re: Bush TV22 Mk 1

I did more testing on the TV22 over the weekend and managed to get a signal through using the method suggested.

The problem which prevented it working initially seems to have been the contrast control on the r.f. deck. When taking voltage measurements I noticed that the cathodes of the first and third EF91s were measuring more than double the Trader voltage. Since the contrast control is in the cathode circuit I tried turning it, and found that at one extremity the correct voltages were restored and interference could be heard from the speaker. I’ll have to examine the contrast pot again to see what the problem may be.

With the above problem discovered, I was able to get tones from the speaker and lines from the CRT using the sig-gen, and to confirm the operation of the tuner despite the missing knob. The various picture controls seem to work mostly ok, however I noticed that as the height control is moved towards its upper extremity a low frequency mechanical sound can be heard. At one point I also heard a spark but did not see where it came from. Can anyone shed any light on what might be causing this?

All wax capacitors have now been replaced on both decks. Aside from figuring out the contrast problem and replacing the brightness and volume pots, it seems there is little more I can do now until I can get hold of an Aurora.
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