Telequipment D67 resistor query

[martiR2R]

Recently aquired this - it's c1971 and in very reasonable nick. On unwrapping a lump of insulating tape I found six 1M resistors daisy chained together between the brilliance pot and the focus pot, the cct diag for the 67A ( which I'm assuming is very similar) shows R304 as 750K in this position - am I ok to proceed with replacement.
I don't do scopes but it would be nice to see if there's any life in it before passing it on. All, visible, components have been checked for burning, bulging and leakage.

Thanks for looking
Martin

[Diabolical Artificer]

AFAIK the focus pot is basically like a volume control on an amplifier valve which controls the grid, turning it up lets more electrons flow thus brightening the trace so replacing the bodge with a single resistor should improve matters.

Someone's put them there for a reason though, I wonder why? Are the pots ok, can't think why six 1 meg R's have been used? Someone with more knowledge might know.

Andy.

[Radio Wrangler]

The focus control is a pot which adjusts the voltage of a hole-in-a-disc-shaped electrode in the gun part of the CRT. It is part of an electron lens system and is used to make the beam converge in a small spot at the phosphor screen. It should have little effect on beam current or total brightness. You twiddle this pot for the smallest size of spot for the sharpest picture.

The 'grid' is another hole shaped electrode very close to the cathode and like ordinary valve theory, it enhances the space-charge effect to control emission, and hence controls the beam current and overall brightness.

There's another electrode voltage twiddler, usually a screwdriver slot type on the front panel, labelled 'astig' This one adjusts the astigmatism of the spot. Set one way it defocuses the spot into a horizontal smear, set the other you get a vertical smear. Adjust this one to get a round spot. It interacts a bit with the focus knob, so when you set the astig twiddler, alternate between turning it and the focus control a few times.

In a very basic scope you'll find a long chain of resistors and pot tracks all in series running from about -2kV to ground, with various CRT electrodes tapped off from various nodes and the pot sliders. Changing one resistor from 750k to 6meg is going to change the current in the whole string and upset all sorts of voltages. It looks like a bodge to cover up for something else which has gone wrong.

Better scopes have DC coupled blanking so the beam can be cut off during flyback of very slow sweep speeds, and it allows all sorts of tricks with dual timebases. Some instruments used AC coupling with a DC restorer, Tek and HP liked twin windings for the -2kV EHT and had separate supplies for grid and cathode. A 'Z' amplifier running around ground potential was used to offset one of the twin supplies and so control the grid-cathode difference.

David

[MotorBikeLes]

First thing before you do anything is to put some sort of insulation around the mains fuse which it just sitting there ready to bite you hard, so you knock the scope off the bench, thereby shattering the CRT as well as killing you.
I have never had trouble in this part of a D67 or D67A. My advice would be to check ALL the resistors from R301 down to the Brill pot. Then put everything to standard and see what happens. If there is an underlying fault, it will show you what it is, so you tackle that logically.
David mentions the unblanking circuit. In the D67, ALL the unblank section had CRT grid voltage sitting there also ready to bite, so work with caution.
One other common problem with the D67, often intermittent in nature, is to do with the double sided circuit board. There are a number of hollow rivets carrying earth continuity thro the board which become O/C with age and use (heating effect). Well worth dealing with, and the best way is by sucking out any solder, making a little "walking stick" from suitable tinned copper (I use silver plated copper wire), and solder the "handle" on one side. Then shorten the stick on the other side, leaving just enough to bend over into another handle. Solder that too. If you can keep some "air" all thro the hole, your joint will last for ever!
Les.

[ronbryan]

Martin

Don't forget to check the voltage rating of any replacement resistors used in the Brightness/Focus circuitry. Metal glaze VR37 resistors are the components of choice.

Ron

[martiR2R]

Andy, David, Les, Ron, Thankyou gentlemen for taking the time to reply and my apologies as I now feel a bit of a fraud. As I said I don't do scopes , and all this is way over my head. I have replaced the chain of resistors and there are signs of life, but no amount of twiddling will produce a spot or line. I saved this from the scrapman because it looked in such good nick, I shall now offer it to forum members in the appropriate section, hopefully it will be of use to someone more skilled than I.
Thankyou
Martin

[Radio Wrangler]

Hi Martin,

All of us 'didn't do scopes' right up until we did our first one!

I built my first scope (complete with mains-driven EHT) as a 13 year old schoolboy and wound up on the design team of one at HP many years later. As long as you take precautions to prevent them being life-ending, messing with scopes can be life-changing.

The D67 is a useful beast and there are plenty of folk to talk you through it if you fancy a go. If you got it for nowt (I'm from the other side of the Pennines) and parts are cheap, then it's a good one to learn on.

David

[avocollector]

Check the eht resistor chain to earth - have had several of these scopes with way out values up and down in that chain (on the back board). Once replaced things have been fine. Seems they get a bit easily fried in that area.

[martiR2R]

Thankyou David for your encouragement but I think this time around I'll pass. I have two rather challenging valve testers that'll keep me busy for quite some time and don't have space (long term) to store the scope, so I will pass it on.
Regards
Martin