Bush TV22 totally stuck see pics

[Argus25]

lloyd,

If all the other suggestions above have been tested/eliminated, I'll make some suggestions and the reasons for them, they might not be right but its worth a look.

1) In a sense you can regard the picture tube as a kind of oscilloscope, the CRT's beam won't lie about the value of the yoke current, so a lot of the time, even without an oscilloscope a lot is gained by looking at the scan(raster) , or the picture information in scan time. So in this case you can be sure that the output tube's plate current has the same oscillations/fault in it that you see on the scan on the picture tube face.

2)Since the total amount of line deflection is about right, its probably unlikely there is a yoke problem, or that the line output tube is faulty or that the total amplitude of the peak output tube plate current is low. The EHT is obviously also ok, supporting these notions.

3)Since the left 1/3 of the raster is basically ok and the scan oscillations are largely on the right side, there is probably nothing wrong with the booster diode & circuit either.But note the word probably.

4) So the above implies the oscillations in the output tubes plate current are probably also in the grid drive waveform to this output tube also. It looks like the grid drive voltage is the cause. However, since this circuit is a self oscillating type (using the output signal from the line output transformer for feedback), the output tube's grid waveform depends on the feedback, which is positive. So in this particular circuit, a small error or tendency to oscillation gets amplified in the feedback pathway. Its not as if the line output tube is being driven by an independent oscillator which is the custom for most TV's.

5) Referring to the component numbers on the pdf attached by UKCOL in this thread, one check you could do is disconnect the sync coupling cap C28 to isolate that from the equation. Check the values of C29 (replace it) and check replace R35 and R34 in case the fault is in the output tube's grid drive circuit. If R33 went high (as suggested by Dangerman) or C27 was the wrong value it could upset the grid drive. If you had a scope you could look at the output tube's grid drive voltage to confirm oscillations on the scan time.Since the feedback is derived from the same connection on the transformer as the Metrosil, that part and its associated capacitor have to be checked too.

6) So in summary what I'm suggesting, for one reason or another, is that it looks like there is excessive and oscillatory grid drive voltage to the horizontal output tube. But the whole problem is complicated by the feedback for this circuit. One way ultimately around this problem for a circuit with feedback and a fault, is to break the feedback loop, or disconnect the feedback pathway by removing C29 and substituting in a new correct drive signal to the grid of the line output tube and looking in the circuit after that for the problem, but this sort of thing could be tricky unless you have a lot of experience.

[Argus25]

Actually, I'm inclined to agree with DangerMan, if the set is upside down in the photo, then most of the disturbance is in the timing area on the left side(start of scan) where the damper diode would by conducting, so its likely that the fault is in that area.

On the other hand, the oscillations in the scan current are still centered on the time where the H output tube begins to conduct about 1/3 the way into the scan from the left side and the very left side of the raster is still ok (so possibly not the damper diode area), so I'm still suspicious of something wrong in the grid drive circuit to the output tube. Have you had any luck with it yet ?

[GeoffK]

Originally Posted by Argus25

Actually, I'm inclined to agree with DangerMan, if the set is upside down in the photo, then most of the disturbance is in the timing area on the left side(start of scan) where the damper diode would by conducting, so its likely that the fault is in that area.

I put the picture the right way round, see post 18. Could be just a lose valve holder or bad solder joint.

[ukcol]

Originally Posted by ukcol

Check out the line timebase, the first suspect being C32.

Originally Posted by Argus25

Actually, I'm inclined to agree with DangerMan, if the set is upside down in the photo, then most of the disturbance is in the timing area on the left side(start of scan) where the damper diode would by conducting, so its likely that the fault is in that area.

I agree with Augus25, referring to my circuit snip in post 3; it looks like V10a is not contributing to the scan. If as you say you have replaced C32 then do a cold check of the circuit to eliminate a wiring error. First check you have continuity from the cathode of V10a via winding b of the line linearity coil to the positive plate of C32 before you look elsewhere.

[Argus25]

Yes I agree with ukcol, not only must C32 be in good condition (have a low ESR), the wiring must be right, but there is one more thing.. it is very important that the negative terminal on C32, which connects to the power supply, supplied by the junction of two inductors (L29 and L30 on my version of the circuit) not shown in the circuit snip, that this junction, between the power supply filter inductors, has a good electrolytic capacitor C66(on my diagram) to ground. If this capacitor C66 went high ESR or low capacity, it would have the same/similar effect as a faulty C32 because significant AC voltages would develop on the negative leg of C32 and also the yoke coupling capacitor connected there too. This would also cause ripple on the output tube's screen grid supply (positive leg of C32) and therefore its plate current. If you have access to a scope there should be little ripple at line rate on either side of C32 if C32 is good and the power supply filter electrolytic C66 is also good.

[Argus25]

PS: Lloyd, Don't forget if you end up doing any scope checks on these sets (which could possibly go hot chassis with mains power wires reversed) it's better to run them from a mains isolating transformer and make sure the chassis is properly grounded before attempting to attach the scope.

[lloydwells]

CURED!! While it was annoying me and misbehaving I replaced anything I didn't like the look of. Made it up a new lower tag board and spotted the problem broken wire on coil! Some very delicate soldering and it's back up and running. The coil had, by the look of it, (see pic) been bodged before.

[Hunts smoothing bomb]

Well done that man! I must say that I find it very satisfying when you eventually get to the bottom of something that's been niggling you for a long time, sit back and enjoy!!


Cheers

[ukcol]

Well done Lloyd, it looks like the set has a reasonably good CRT as well which is always a bonus.

That's the line linearity coil in your first picture with C33 at the top. With the exception of C33 & C32 all the wires to that coil carry HT to the line output valve so I imagine the wire that was disconnected was the one to the boost capacitor C32.

[Argus25]

For the circuit to have been working, even with the fault, there must have been continuity of both windings a & b of the lin coil and windings d & e on the lopty. So like ukcol says the break must have somehow disconnected C32, but at the same time not made windings a & b of the lin coil go open.

[lloydwells]

The wire in question was completely broken nothing connected to it at all ergo one winding was doing absolutely diddly squat no one is more amazed anything was visible than me.

[Argus25]

Yes it sure is interesting, because on the lin coil, if coil "b" was open there would be no plate or screen grid supply to V8, so there could be no line deflection and no EHT.. and if it was coil "a" that was open there would be no plate supply and again no line deflection, so all a bit mysterious. It can be frustrating when a fault doesn't seem to match the observed symptoms. Perhaps your unit has a variation of the lin coil circuit.

[lloydwells]

Funny you should say that I had two and both were totally different which made.working on it slightly tricky.

[Heatercathodeshort]

I thought exactly the same thing Argus. It can't work with the coil O/C. John.