Odd CRT failure mode

[T3sl4co1l]

What? Deflection is not affected by spot (beam) intensity. Thankfully. Imagine how messed up trace intensify highlighting modes would be if that were the case.
Edit to add: Oh wait, by 'spot intensity' you meant EHT voltage (higher EHT allows maintaining spot intensity despite faster sweep speeds.) Not variations in beam intensity during operation.

Yes, not in an operating sense, but as a design tradeoff.  You can make wide deflection, dim CRTs, or narrow deflection, bright CRTs, but doing both is harder.  (Doing both, and with high efficiency in terms of light per beam power, even more so!)

Also "anode is at the same potential as the deflection plates"... What? Is that a typo, or did you mean it?

"Simple CRT case", like a 5AQP1 or whatever -- a CRT simpler than the ones we're talking about here.

Sorry for the lacking clarity

But this is all kind of irrelevant. Point is, does grumpydoc's CRT have that kind of internal resistive EHT field shaping? (Which is primarily to achieve higher deflection gain, regardless of fine details of which way the beam curves.)
And if his CRT does have that, is the resistor open circuit?

Yeah, definitely, there'll be something funny with the deflection sensitivity, and probably linearity too, if some of that trace burns off.

Got a megger handy?  Or.. maybe a picoammeter?   Not sure offhand what resistance those things are, but I'm guessing it's more than most DMMs read.

Also, if there were a broken spot, would that break down, even in vacuum?  Would it have burned away enough that there's a gap or burn, visible by inspection perhaps?  (I don't know exactly how much clearance is needed in vacuum, but I would think it must be at least sparking if it's dropping a large fraction of the HT across a << 1mm gap in the stuff.  ...Would the sparking be detectable, say by a sensitive amplifier or radio?)

Tim

[tautech]

Would the sparking be detectable, say by a sensitive amplifier or radio?

Hmm
SMPS HV supply and trebler noise is probably all you'd hear. 

[grumpydoc]

But this is all kind of irrelevant. Point is, does grumpydoc's CRT have that kind of internal resistive EHT field shaping? (Which is primarily to achieve higher deflection gain, regardless of fine details of which way the beam curves.)

Not that I am aware. As luck has it the CRTs have available data sheets. The 200Mhz 'scope has a D14-383GH/123 and the 100MHz 'scopes have D14-373GH/123 although I found a D14-374GH/123 in the PM3382

Note 5 in the data sheet says that the CRT uses permanent magnets for fine tuning geometry and astigmatism

 D14-383GH: http://frank.pocnet.net/sheets/186/d/D14383-123.pdf
 D14-373GH: http://frank.pocnet.net/sheets/186/d/D14373-123.pdf

I assume that the D14-374 is very similar but I cannot find a data sheet - there are, however, photos including the neck assembly

http://koalakits.net/Tube_Images/D14-374GH-123_FP.jpg
http://koalakits.net/Tube_Images/D14-374GH-123_2_FP.jpg

And if his CRT does have that, is the resistor open circuit?

Don't forget I have tried two CRTs so a fault in both is unlikely.

The RCs are for filtering and compensation.  Without (or with incorrect values), it will most likely oscillate rather than deliver stable regulated DC.  Or be overdamped and too slow, which is probably the side it's erring on.

OK, good - that makes sense.

I still don't quite understand the divider there though - the total resistance is 31.7666Mohm and the voltage should be 2210V - 2210*(31.6/31.7666)  is 2198.41 so the top of R1103 should be at -1.59V. However it is connected to the non-inverting input so, given that the inverting input is at 0V the top of R1103 should be a virtual ground.

I wouldn't touch R1103 (other than cold testing).  Adding 15K in series with R1123 and R1101  would probably be a better bet to drop the 2200V supply a bit.  Use a 22K trimmer with the wiper wired to one end of the track so it *CANNOT* go open. and you should be able to 'dial an EHT'.

I might try that. R1103 seemed an easier target as it is through hole and on the "large component" side of the board whereas R1123/R1101 are SMD on the other side of the board crammed in with all of the other stuff around the op amp. However there aren't too many values to choose from in VR37 (which is the Vishay high voltage range used) - 30, 31.6 and 33Mhom and only DigiKey stock all three.

I agree about making sure the wiper can't go open but there is also protection from V1100 and V1105 which clamp the non-inverting input to +/-11.6V or so.

[Ian.M]

In that case *IF* you decide you need to tweak the EHT for testing, its probably easiest to desolder the SMD R1123 and patch in the preset and an 18K resistor, soldered to a scrap of perfboard.  I'd keep the wiring short and twisted together to minimise the risk of instability, and due to the small pads you'd be soldering to, I'd probably choose Kynar wirewrap wire.

[grumpydoc]

In that case *IF* you decide you need to tweak the EHT for testing, its probably easiest to desolder the SMD R1123 and patch in the preset and an 18K resistor, soldered to a scrap of perfboard.  I'd keep the wiring short and twisted together to minimise the risk of instability, and due to the small pads you'd be soldering to, I'd probably choose Kynar wirewrap wire.

Interesting - just about to look at getting a trimpot in there and I notice that they have been playing with the values of R1123 and 1101. The original 338x and 339x had 19.6K and 121k, the "A" had the same and then the "B" has 19.6k and 147k.

Of the two PSUs that I have out the original 3394 one has the values as per the schematic whereas the 3394B one has 11k and 147k

Curious.

HighVoltage is sending me another PSU to play with - it will be interesting to see what values it has.

[grumpydoc]

I replaced R1123 with a 50k multi-turn pot mounted on the top edge of the PSU board with a bit of hot snot.

Tweaking the cathode supply down to -2150V allowed the calibration marks to be aligned with the graticule and the autocal runs past the point that it gave up before (it still runs aground later on but HighVoltage thinks that is a problem on the digitiser board). I need most of the gain from the final X/Y amps - perhaps coming down another 50V would help.

The CRT isn't especially bright and I can't quite adjust the grid bias/dark level but that might just be the CRT that is in the 'scope which is the one from the 3394B. The one originally in this chassis was better.

I suppose that I could have a look where the trimpot is now set and replace the 19.6K resistor with that value. I don't much like that as a solution - too much like a hack but it is a solution.

I suppose the fact the Phillips themselves put so many different values here makes it more acceptable.

[grumpydoc]

The PSU from HighVoltage turned up yesterday

Its cathode supply is around -1950V which is interesting. The CRT is bright with this PSU so there is probably another problem with the other "B" series PSU that I have. R1101 and R1123 are 147k and 11k which is different to the B series service manual that I have but the same as the other B series PSU.

I'm now pretty certain that Philips lowered the cathode supply in the "B" series 'scopes (and maybe, in practice, some A series instruments) but did not fully/correctly update the service manual to reflect the change. We go from -2200V/140k (121k+19.6k) in the original series to -1950V/158k (147k + 11k) in the "B" - the PSU I modified is sitting somewhat in the middle with a total resistance of 150k (121k + 30k on the trimpot) and cathode supply of -2100V.

I'm not sure why they might have done this - perhaps they had problems with deflection sensitivity. I am also not at all clear why the combination of the CRT in the original 3394 and the PSU from the same chassis would not work.

However it does make me comfortable in increasing the resistance at the top of the R1123/R1101/R1103 divider to something in the region of 150-160k

Just need to fix the A8 boards now (hopefully)

[justanothercanuck]

Over time remaining things break down inside the tube and adhere to the internal structures causing electrical leakages internally. Additionally barium can be evolving from the electron gun and causing problems

can't you tilt the tube vertically and blast the guns with a crt tester?  should fry that stuff off without damaging the tube phosphor.