Bush TV22 flyback pulse
 

I seem to be stuggling to get good EHT from my TV22. When the brightness is turned up the picture expands and defocuses ending up with a blank screen until the brightness setting is reduced again. I am wondering if this could be caused by incorrect flyback resonance. The flyback pulse at the LO anode is in excess of 800V but has a kink in it (see attached). Any advice appreciated.

Re: Bush TV22 flyback pulse
 

Keith,

Some more info could help:

Does the peak amplitude of the flyback pulse drop on the LO anode as the CRT brightness is turned up, or not ?

Also what type of high frequency capable high voltage probe are you using to connect to the line output anode ? (I had to manufacture a special probe for this as in some sets its over 5kV peak).

Do you have any kind of DC EHT probe/meter ?

If the peak plate voltage of the PL38 is fairly stable as the CRT brightness is turned up and the picture expands (suggesting EHT drop) it means that the internal resistance of the EHT as a supply is high, so I would check the EHT rectifier, also is its heater glow visible in the dark ? Tack a modern silicon EHT rectifier across it, if there is any doubt as an experiment.

If your CRT has external aquadag, make sure it is properly earthed.

With regard to the resonance during flyback is it important that half its period is no longer than the H blanking time. If it is lowered though, to a shorter time than this, by lowering the tuning capacitance value on the transformer, the peak anode voltage will increase, as will the EHT. So if the flyback appears too long, that would/could be associated with a lower EHT, so perhaps measure that on the scope.

PS: make sure the tuning cap on the Lopty is a new 470pF high voltage silver mica type works well.

Hugo.

Re: Bush TV22 flyback pulse
 

Hi Keith,
Check the EY51 EHT rectifier as Hugo suggested.

Also check the brightness control, if it has a break in its wirewound track it can cause this symptom.

Cheers
Andy

Re: Bush TV22 flyback pulse
 

This was a common problem with valve EHT rectifiers when they were past their best. Check/change the EY51 first.

Re: Bush TV22 flyback pulse
 

Many thanks for your replies. The probe was just a bog-standard X10 so the waveform was probably not an accurate representation. My concern was the fact that it was not a smooth half sinusoid. However, the concensus seems to point to a low emission EHT rectifier so a new EY51 has been ordered. I'll report back with the result of the replacement.

Re: Bush TV22 flyback pulse
 

Keith,

I can't recall exactly but I think the primary of the TV22 lopt has around 2000 turns and the secondary close to 3000. So for arguments sake if it is all working ok and say there was some EHT around 7kV to 9kv (But I don't know off hand what the value is for a TV22) then one could expect the peak anode voltage on the H output valve to be about 2/5 x 7kV or well over 2kV peak, which would normally destroy a x 10 scope probe (usually only good for 700V) and damage a scope's input circuits too. Even a x 100 probe might not be rated for such a high peak voltage, but some are rated to 2.5kV, but it still can be pushing it for the anode of a line output valve:

https://docs-apac.rs-online.com/webd...6b80c645f1.pdf

The best probe is one of these but they are darn expensive:

https://au.rs-online.com/web/p/oscil...robes/7966702/

Probably lucky when you did the test the EHT and other voltages were low.


Hugo.

Re: Bush TV22 flyback pulse
 

.....just getting the data sheet out of my files on the transformer, the peak voltage on the plate will be roughly about 1350/4310 of the EHT + the boost voltage, So if the EHT is about 8kV, the plate voltage peak will be 2.5kV plus some. So probably its in the region of 2 to 3kV at a guess, when all is normal. As I say, I'm not sure what the exact EHT is supposed to be on a TV22, so it could be higher.

So it pays to keep test instruments & probes away from that anode cap. I have a 1000:1 50kV high frequency capable probe. Next time I have the TV22 on the bench I will document the anode waveform and EHT value.

One way to measure it though is with a capacitive divider, a 10pF and a 100pF high voltage capacitors works to divide it down by about 10, but the DC component of the boost is lost and if doing that make sure the earthy end of the 100pF doesn't fall off!

Re: Bush TV22 flyback pulse
 

The EHT should be 8kV when synchronised with a signal and zero beam current (brightness turned down).
I suspect the CRT may be gassy causing excessive beam current which is dragging down the EHT to around 5kV.
Not as technical as Hugo, but seen in a few sets now.

Re: Bush TV22 flyback pulse
 

That makes sense, and the beam current could be checked fairly easily. Also one would expect the gas would cause a fairly significant beam defocus and possibly a violet glow in the gun area near the base.

Re: Bush TV22 flyback pulse
 

That's why I suggested the brightness control pot as well. It is a 50K wirewound with its cold end at 0V and the hot end connected via a 56K resistor to HT, so if the winding breaks then the pot will jump from 0V to full HT causing excessive beam current. I mention it because I have seen it on a couple of these sets.

Another thing to check is the A1 voltage from the metrosil, it should be a stable 300V. Measure this with a digital meter or valve volt meter as it is high impedance. The metrosil is also known to fail, but can be replaced with a diode and potential divider.

Cheers
Andy

Re: Bush TV22 flyback pulse
 

Thanks all,

Sounds like I was lucky not to damage probe or scope input. Either that or the pulse voltage is not as high as it should be. There's an old AVO high voltage probe in an auction that I'm attending next weekend - at least it might be a way of determining my EHT voltage. I'll also check the brightness pot, although the effect seems quite gradual as I turn it up. The metrosil has been replaced with a 1N4007 at some point. I added a potential divider as the diode was giving about 700V on A1. What values are recommended for this, I wonder? Also the beam current would be easy to test as suggested by Hugo. Looking at the gun end in the dark I can't see any ionisation glow so hopefully the vacuum is still reasonable.

Re: Bush TV22 flyback pulse
 

Hi Keith,
I have attached the circuit that I have used in the past to replace the metrosil in TV22 sets. The potential divider has a high over all value in order not to dampen the line output stage too much.

Cheers
Andy

Re: Bush TV22 flyback pulse
 

Thanks Andy,
Very similar values to what I used, in fact. I gather that the maximum A1 should be about 500V.

Re: Bush TV22 flyback pulse
 

Well, I have replaced the EY51 with a NOS Mullard part. The initial result was not promising. The picture was still dim with the focus lever needing to be fully clockwise. Increasing the brightness control still caused ballooning of the picture and eventual extinction. The picture also seemed to be shifted down somewhat and the line linearity was very poor (expanded on the right) - adjusting the linearity core would improve but not correct this, and reduced the brightness further.

Suddenly, after about five minutes, everything changed. Brightness was restored and the focus now optimised at about half travel of the lever. The vertical shift was gone and the good horizontal linearity could be obtained. Switching off and on a couple of times did not induce the fault condition to return.
I still don't have any means of measuring the EHT but the HT and boost HT were measured at 192V and 264V respectively in the non-fault condition.

I am concerned that this fault will come back to haunt me more permanently during an exhibition being staged for our open village event in early June. Any suggestions would be appreciated.

Re: Bush TV22 flyback pulse
 

It is always possible that there is an intermittent problem in the LOPTY, but I think this is far less likely than a problem with the capacitors associated with the circuit:

On my trader sheet, I would suspect initially C32, the electroltyic 2uF boost capacitor connected between the HT and the linearity control. Also replace C37 the 2uF yoke coupling cap , another electrolytic. I would replace both of those with non electrolytic poly caps, the Solen Fast 400 or 630V ones work well.

Then, also in this circuit just the smallest amount of leakage in C29, the G1 grid coupling cap to the PL38, grossly upsets the circuit so make sure that is a new part, and after that, make sure C33 a 0.2uF the capacitor across the lin coil is renewed and also the tuning capacitor C34 on the lopty secondary is new.

Check for other things like a bad screen or cathode connection on the PL38 or a problem with the damper rectifier. All that doesn't exclude everything but its a start. At least if those parts are all new you can look elsewhere for the cause of the fault if it recurs. These parts may already be new ?

Re: Bush TV22 flyback pulse
 

Hi Keith,
adding to what Argus25 has already suggested, I would also suggest fitting a nice new ECL80 which forms part of the line oscillator. The triode section of the ECL80 can have a short life.

Cheers
Andy

Re: Bush TV22 flyback pulse
 

The 820k ECL80 Triode line oscillator anode load resistor has a habit of rising in value way above tolerance as well.

Re: Bush TV22 flyback pulse
 

Given that your NOS ey51 had not been used for tens of years, I suppose its always possible that its cathode needed some time to re-activate and become emissive again. If that's the case, the "fault" won't recur. Anyone seen a NOS valve need a few minutes to become fully emissive again on initial use?

Re: Bush TV22 flyback pulse
 

Definitely happens with CRTs - haven't knowingly experienced it with valves but I don't see why the same effect should not apply

J

Re: Bush TV22 flyback pulse
 

Yes, I've heard that said about CRTs. I suppose its more obvious in a CRT. Can't say I've noticed it with NOS valves but I'll look out for any such effect when next I try a NOS valve in a set.