Dead Tek 2235

Service manual and Operators manual here:

Also you might join the tek yahoo group and describe your findings there.

Good luck.

Thanks.

Isaac

One known weak point in this family is a chain of high-value carbon-composition resistors which divide down the high voltage to provide the focus voltage. They run fairly hot, and tend to drift over time, resulting in a fuzzy image which can't be focused properly.

If one of these were to fail "open" I suspect it might disrupt the operation of the CRT badly enough that you wouldn't see a trace.

Repair costs a few dollars in parts (either some old-stock carbon comp resistors selected for the right value, or more modern leaded resistors of a type rated for high voltage) and an hour or two of work (a few of the resistors in the chain are tucked into slightly tricky places on the board). It's not a bad job, though: I picked up a 2235 at a flea market for $60, found it had this problem, and had it repaired by the end of the afternoon.

Failure-to-light-up might mean a more serious problem in the high voltage supply - if I recall correctly it's a switcher. Dried-out (high-ESR) or leaky caps would be another common suspect.

The simplest and easiest thing to do is check the CRT heater lights - a dry joint somewhere would probably be more likely than an OC heater.

This is where an old VTVM and 1090 meg HV probe come in handy. The self con tained ones most people used for TV work could not measure negative voltage s and it is a hell of alot easier to get to the cathode than the anode on a scope.

Those voltages are usually on the print, so you can tell if it is in regula tion or not. If I am not mistaken, the dividing resistors for the focus are part of the HV regulation feedback loop, so a fairly accurate measurement of the cathode voltage can be quite useful. Also, turn it off and let it al l discharge, then turn it on with the probe connected and see if you get H V for a split second and then goes away. If so it is going into shutdown, d ead giveaway.

Most of the scopes we bought/buy we get a chance to plug them in and tend t o get the ones that at least produce a trace. We are sorta in the market fo r one of those 1090 ohm jobs but the ones I see are a little to high for so mething that might get used once a year. Also that the market for them is s o limited. So that means I get to wrestle with the fun ones, like B sweep n ot starting on a 465B. You REALLY want B sweep to work on a 465B because it has the split function where you can see the whole trace and the expanded trace at the same time. Even though it is not something you would use every day, it is still a feature and it should work.

And don't overlook the possibility of a failure in the main power supply. C heck everywhere it says 15V, 5V, 8V -5V, -15V, 130V, 50V, -50V and so forth . Those seem to be the supply voltages Tek used to like to use.

snipped-for-privacy@gmail.com wrote: >

The self contain HV probes are easy to change polarity. Either reverse the meter leads, or connect a small bridge rectifier between the meter and the probe and ground lead.

Not so fast. These things had a current detect function for the horizontal output TUBE (vale for those overseas). Now if you put a DPDT switch there a nd reverse the meter coil polarity that would work.

However, a bridge rectifier would introduce error, and that error would be greater at lower voltages. Many meter movement barely need a volt but a sta ndard diode drops .6 of that. At scope cathode voltages which might only be like 2 KV or less, this is significant.

We now have a Keithley HV probe but acn't really use it. Dude found a good deal on it, but it has a 350 meg output resistance which means you need an electrometer to use it. A regular VTVM will not work without alot of math, and even then... We do have one meter is would work on and it is seriously aged. (two syllables there, really) Actually it agrees with the best of the Flukes we got, but nobody is going to do the NIST routine on it and certif y it.

I have seriously considered building one, but not with 1090 megs. The meter is 10 megohm, so 90 megs gives it enough range to measure scope CRT voltag es. We are sort of committeds to repairing and restoring CROs. I do not lik e DSOs, let alone LCD based scopes. I believe that every electronics class in highschool should have a CRO for the kids to play with, once they know e nough to not f*ck it up of course. I also think they should not graduate wi thout certain other knowledge, like the different types of firearms ad how to at least unload them and make sure there is not one in the chamber. If w e are going to have guns here, dammit, make sure people know how to be safe with them.

They should know how to change a tire on the care, some at least simple mec hanics. How many times has someone called on a neighbor or whatever to do o ne of the simplest of tasks claiming that are "not mechanically inclined" ? We put a Man on the moon and now we are down to this ? No wonder conspirac y theorists think it was fake.

But I can tell you the moonshot was real. Not that I was there but it is no t hard to figure out. This was a milestone in human history, a worldwide ev ent. There were Russian kids with telescopes all over the place and it was during the cold war. If ANY ONE of them caught even one little piece of evi dence that it was faked they would be rich and the USSR would have never le t us live it down. And if you really think about it, and I mean really, it would have cost more to fake it than to actually do it. Plus they really di d want to do it.

But I have digressed quite a bit. As I look at my old 561A Tek that was bui lt before I was born with dual trace and dual time base, sometimes I wonder just how far we have come. They have minuturized things and packed more tr ansistor and other devices into one case, but what have they really done ?

When I was young my Uncle gave me the basic books from the air force. Whate ver branch of the US military you serve(d) in, if it was the air force you usually had the least risk and got the best jobs when you got out. At least back then.

But those books said there are only three electronic circuits, rectifier, a mplifier and oscillator. And every circuit to this day you see can be descr ibed as one of those. It might be gated or controlled, fancied up in all ki nds of ways but it is still the same thing. Consider the function, a comput er gate can be considered rectifiers feeding an amplifier.

But anyway, I am probably boring the OP (et al) so it is time to stop.

To the OP, do you got 15 volts ? Do you got 130 volts ? Before tearing into the HV shit that doesn't like to be touched, make sure you got everything else.

Not all of them have a current function. The bridge rectifier would introduce a little over 1 volt error. There is a shunt in the ones that monitor cathode current for the horizontal output. You have no need for hat in a scope repair. The reason I wouldn't use the switch is that there isn't enough wetting current, so after a few years it can start arcing. Even at low current, you don't want to create plasma. Calculate the current, through the resistor for the full scale range. It is quite low to keep from loading the HV supply and affecting the reading.

I remember meter shunts being available in 2 different types - one was 100mV and the other might have been something like 70mV.

It was usually printed in a corner of the dial plate so you could order the right shunt.