LOW EHT Tele D43 scope.

[ronbryan]

The scale illumination pot is shown as RV401 in the power supply drawing in post #1. The scale lamps are dim at switch-on and increase in brightness as RV401 is rotated clockwise.

Ron

[Diabolical Artificer]

Thanks Ron, I wondered if it was wired in some odd way, over thinking things and missed RV401 on the PSU drawing, it's wired as I described.

Andy.

[Diabolical Artificer]

Am pretty sure the dim trace is down to a faulty CRT. I swopped over the grids, cathode and a2's with no improvement. I've removed and cleaned up the socket pins and cleaned the pins on the CRT, can't think what else to do.

I've taken voltage readings again, there is a bit of disparity between a3's, but nothing out of the ordinary for valve circuits, IE 20v.

I noticed a few things, 1) moving the brilliance control on the iffy trace does not dim the trace 2) I removed the sockets on the a3 pins to take a reading right at the CRT pins - the good trace goes out of focus and there is a buzzing/arcing sound, not so on the faulty trace.

So that's it, unless I can find a replacement CRT, it's not really useable and not worth going further and calibrating or improving noise etc on the plugin's.

Has anyone had this fault before on a scope?

Andy.

[Ed_Dinning]

Hi Andy, when varying brill control on iffy trace what happens to the measured voltages on the pin?

Ed

[Diabolical Artificer]

After looking at your question ed, I went to measure the voltages at the pins and after some confusion the good gun measures - 0.5 (full brightness) to - 75v whereas the duff gun measures 0v (full brightness) to + 95v, this is with the black lead of the DMM on cathode, red lead to grid.

I was getting muddled up before taking readings in relation to ground/0v. So I'm thinking why have I got a positive voltage? If you look at the schematic, one gun (the good one) has the top of the brightness pot joined to the focus pot via a 820k R, whereas our duff gun's brightness pot is joined to it's focus pot through the X sens pot. So I twiddled with this preset - nada. Head scratch. I'd replaced the neon and thought I'd checked
MR301 but have just measured 0v across it. So I tried shorting out the X sens circuit with a jumper lead which I would have thought would establish our potential divider and cut the X reference part out, it did nowt.

I'll change MR301, but am not optomistic, there's something else cracking off. Will check the rest of the potential divider circuit out too. I'm getting confused here, swopping over g1/k/a2 should have improved the trace, unless the problem is a3? Which I measured yesterday and thought ok.

Andy.

[Diabolical Artificer]

Well I've gone trough the divider from top to bottom and checked just about all components. MR301 had indeed gone, so I replaced it. All pots are ok and there is continuity from top to bottom. I thought the cause of the grid being positive in relation to the cathode might be a OC between the focus and brightness pot.

I rechecked all connections and made sure I hadn't got the probes of the DMM the wrong way round, black to cathode, red to grid right?

Stumped. It has to be the CRT. Any input welcome.

Andy.

[Ed_Dinning]

Hi Andy, the higher voltage on the duff gun sounds as if that gun has low emission as it is not drawing mush current.

Ed

[Diabolical Artificer]

Right, so less voltage drop across the resistors/pots and hence zero to positive voltage rather than zero to negative? That would mean one gun has a dicky cathode then, maybe? Do both guns share the same envelope or are they physically separated?

Best look for a CRT or scrap D/S43

Andy.

[ronbryan]

With the 'X sens' control at the top of its travel, both cathodes should be at the same potential, as they are connected to the same point.

I am not sure which grid (pin 1 or pin 12) controls the dim trace gun, so can you check that the bottom connection of both brightness pots are definitely connected to the 10k resistor R320. It is possible that one of these is floating, allowing the grid to go positive. A wire may have dropped off on its way from the front panel.

The only other thing that I can think of is that there is a leak from the +250V on A1 (pin 6) to the appropriate grid with the +ve voltage on it. Why the trace is dim when the grid is held positive of cathode is a mystery, as is how the grid can be +50V positive without being clamped to a much lower level by the grid-cathode diode.

I have attached a sketch of the small board that shows location of parts on the PC4B board on my scope - hopefully the same as yours, in case it is some help.

Ron