开云体育

Hi all,

I recently picked up a couple of 7904s with a stack of plugins. While most of the plugins are OK, the mainframes have some problems. One of them looks very good but is completely dead despite having intact fuses. It doesn't even tick. The other looks like it's had a hard life but at least powers up. The vertical and horizontal systems work OK but the calibrator and the readout do not. Also the displays look Ok when the beam is at the lower part of the screen but develop a kink when it is shifted upwards. The geometry and astigmatism controls do not correct it. This happens regardless of the vertical or horizontal settings and al all timebase speeds. Is this a sign of a damaged CRT?

I'd like to end up with two working 7904s but If the CRT is the problem than I might as well try to make one good one by swapping modules.

Any advice gratefully received!

Morris

Morris,

by shear coincidence I just got a 465B what sounds like the same problem with the CRT: a kink in the traces in the upper half of the screen. I had thought (and still do, until proven otherwise) that this must indicate some damage to the tube itself, probably to the scan expansion mesh. Hearing that another scope has a similar failure, however, makes me skeptical. Specifically, if two scopes had damaged expansion meshes I would expect the damage to be in different parts of the screen, rather than both affecting the upper half.

The distortion I am seeing affects both the vertical and horizontal axes. A level trace deforms in a wave-lake way as it moves higher up the screen, and a repetitive signal shows horizontal compression in regions of wave-like vertical displacement (I home that’s comprehensible).

I don’t have any advice, as I have barely begun to investigate this issue myself, but I wonder if the location in the upper half of the display tells us something important. Maybe it’s a failure of one half of the vertical amplifier? I’m having a hard time imagining what kind of damage or failure would cause the observed symptoms.

— Jeff Dutky

Thanks Jeff,

That is indeed very interesting, your description sounds exactly like what I am seeing in the 7904. I certainly hope it's suggestive of something other that a CRT fault. I'll wait and see what other replies say.

Morris

Morris,

I understand the desire for this to be something other than a CRT fault, and that’s mainly why I don’t trust my (motivated) reasoning, but what are the chances that some chaotic destructive process would produce similar damage in two different CRTs in two different instruments?

I’m very interested to see other replies too.

— Jeff Dutky

Here's what I'd do to make a definite diagnosis: Look at the vertical deflection plate voltages. If they look fine, then that pretty much leaves the crt as the only culprit. In that case, there's still a (slim) chance that the problem is external to the crt (e.g., a magnetized bit that is kinking the beam, although I don't recall any magnetizable bits being near the crt).

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070

Bonjour Monsieur
Bravo to have acquired several of the great 7904, classic 400 MHz

A photo of the screen displaying all aspects will be helpful

1/ shipping damage can ruin delicate CRT deflection plates, electron lens or gun

2 / deflection plate termination is on CRT neck, the pins can get disconnected

3/ HV power in CRT circuit affects beam focus
Bon chance



Jon

Morris,

I have created a photo album with an image of the distortion I am seeing /g/TekScopes/album?id=272587

Does this look similar to what you are seeing?

— Jeff Dutky

Tekscopes troubleshooting rule number 1: Check all the power supply voltages.

I recently had a similar problem on a 7904 except my distortion was at at the bottom of the waveforms. Turned out to be low -15V . If I were a betting man I would guess your +15V is low.. . The power supply stack is a PIA to troubleshoot and the +15V? problem ( if that is what it is ) can be in the supply or one of the boards that +15V feeds ....Have fun

Dave

NR1DX

manuals@...

--
This email has been checked for viruses by Avast antivirus software.

The power supplies are all perfect. I'll post a photo tomorrow (it's quite late here in Australia) but it's looking a lot like a CRT problem. This scope looks pretty grubby and has been knocked around a bit so something may have come loose inside the CRT.

Hi, one shall never place a magnetic circulator on top of an instrument having a CRT. You would get a permanent damage.
G?ran

Don't know how to label what that photo looks like. With a square wave, the lit part of the trace on one line should be exactly above the off part of the other line. this looks particularly strange in the middle of the screen one division below the top of the screen where there are more cycles displayed than on the bottom trace.

Set the time/div to 1 mS and push in the VER red knob to better examine this. Adjusting the scope probe would give a better picture also (the adjustment slot in the hole in the BNC connector housing.

John

John,

I know, it’s very strange looking.

I’ll set the scope up as you advise (except for “pushing the VER knob” as that’s not a thing on the 465B: I’ll just rotate all the VAR knobs into the calibrated position) and get some more pictures with the trace at different vertical positions. The square wave looks the way you would expect in the lower half of the screen, it’s only when you cross the centerline that the distortion starts.

Tom,

I’ll try to get voltage readings, but my bench is currently occupied by another, unexpected project. Checking the signals going into the plates is excellent advice (as always), but I will need to clear the bench before I can do that.

— Jeff Dutky

Morris,

If something had come loose inside the CRT wouldn’t we expect that the symptoms might change with orientation? I haven’t tried rotating the instrument (which is more difficult for you than for me), but I have tried tapping the case to see if the distortion moves (it doesn’t, but I wasn’t tapping very aggressively). I will try operating the scope horizontally as well as vertically and see if that makes a difference (which would, I think, confirm the hypothesis of CRT damage).

I’d really like to see some pictures of what your 7904 is doing, just to verify that we are seeing similar faults.

— Jeff Dutky

I have seem some TEK CRTs that used small magnets taped to the CRT used to fine tune the CRT. I bought a 475 that had these magnets on the CRT and they were obviously original to the CRT. The tape that secured them had become brittle over the years so I had to add some new tape to keep the magnets in their original position. Could it be possible that the problem is related to such magnets? Perhaps they are present and have shifted out of place? Or perhaps they might have attracted a piece of foreign mater that is distorting the beam?

--
Michael Lynch
Dardanelle, AR

Odd and seemingly "impossible" trace distortion (especially when power and signals internally are good) usually has one of two causes:

1. Internal CRT damage caused by physical shock (glass supports broken or parts of the gun/deflection system bent due to inertia). Sometimes visual inspection can reveal this, but not always. The actual damage can be quite small to the eye.

2. Something is magnetized, which can be internal to the CRT, or external in the chassis area. If it is possible to lift the CRT from the chassis and still be powered, it can help to localize the source. if the effect goes away or dramatically changes when the tube is lifted, the problem is an external field in the chassis. Some earlier Tek CRTs had gun alignment MAGNETS glued to the glass around the gun area, it is possible the magnet may have fallen off, and is now in the chassis area, a double magnetic problem.

If the issue is mechanical damage, only CRT replacement can fix it. If the issue is magnetic interference, many remedies are possible. Long ago I built up a very large de-gaussing coil that could pass over an instrument or tube, it was powered from a beefy 24VAC transformer and could demagnetize most objects. This is a very handy tool (a left over from the color TV era), and can resolve many odd issues that seem to be "impossible". They were automatically powered on the clean the color CRT mask every time the unit was switched on.

How do things GET magnetized in a scope? easy, something with a DC magnetic field was too close, a speaker, circulator, battery operated drill, magnetic tool, electromagnet or other item, and stayed there too long. An alternating field randomizes this effect, and helps to eliminate the temporary polarized field in ferromagnetic materials like hardware and tube elements. If the damage is too strong, the coil cannot fully eliminate it, and part removal or a much more powerful field can be the only recourse. The smaller the coil diameter, the more powerful the field flux will be, so one that just passes the CRT diameter can be more effective at clearing a bad tube than one that passes over the whole instrument..

Don't forget that the CRT itself is VERY easily influenced by external magnetic fields of all kinds when it is operating (that's why the SHIELD is there), and the shield can be damaged by physical shock, machining and heat so that is loses its properties. It is a lot more fragile than it looks.

all the best,
walter (walter2 -at- sphere.bc.ca)
sphere research corp.

Just thinking - one way to eliminate large parts of the scope’s electronics is to test the X-Y capability. Driving the scope this way should help point to crt or deflection amplifiers. Another way is to directly drive the deflection plates,

Cheers Roy

If the issue is mechanical damage, only CRT replacement can fix it. If the issue is magnetic interference, many remedies are possible. Long ago I built up a very large de-gaussing coil that could pass over an instrument or tube, it was powered from a beefy 24VAC transformer and could demagnetize most objects. This is a very handy tool (a left over from the color TV era), and can resolve many odd issues that seem to be "impossible".

Oh yes! Been there, done that many times. And it usually works great.

You can make a very effective degaussing coil by purchasing a 100 foot roll of #14 NM with ground electrical cable at your local building supply store. The cable will usually be in a coil about 12 to 14 inches in diameter and wrapped in plastic.

Leave the plastic wrap on the coil. Carefully pull out both ends of the coil far enough to be able to strip all three wires at each end of the coil. Using the START and END of the coil, connect all three windings in series with each other like this:

START BLACK wire connects to AC power

Connect END BLACK wire to START BARE WIRE

Connect END BARE wire to START WHITE wire

END WHITE wire connects to AC power

This will give you a 300 foot long coil of wire with one free black and one free white wire to connect to the mains power.

When connected to the 120 VAC 60 Hz mains voltage, the combined DC resistance and AC impedance of the coil results in a current of about 12 amps through the coil, which #14 can handle satisfactorily. Because the wire is in a coil, it will eventually become too warm for continued operation, but that usually takes about 5 to 10 minutes depending on the ambient temperature, but certainly long enough to degauss an instrument or two.

If you have a Variac which can handle the current, you can do even better. Connect the coil to the output of the Variac. When degausing, turn the Variac up to the highst level you are using. Then, over a period of several seconds, gradually and smoothly reduce the voltage to zero. This will produce the gratest amount of randomness in the final domain orientation. Suddenly removing power would tend to leave the domains oriented in the direction they had at the instant the magnetic field collapsed.

Stephen

Hi all,

I have uploaded some pics of the 7904 problem to Jeff's photo folder of distortion on his 465B . They show two 7904 traces, one with the calibrator square wave and one without. They are OK on the lower half of the screen and distorted on the upper half. I tried putting the scope on it's side in case of something loose inside but it made no difference.

Morris

I have added 16 images to the album showing the cal signal via a properly adjusted probe at 1 ms per division (horiz) and 1 volt per division (horiz). Each image has the trace position half a division up, starting at the bottom of the screen, to show the distortion over the entire face of the CRT.

I also updated the title and description of the album to reflect Morris Odell’s contribution.

My apologies for the dimness of the trace, but it was in the middle of failing altogether.

Morris,

that certainly looks like very similar distortion. I’m a little bit impressed that you put the 7904 on its side.

Tom,

I measured the regulated supply voltages and ripple. All the voltages (-8, +5, +15, +55, and +110) were within 0.02 V of expected value, but at four of the rails (all but the -8 V rail) had 0.5 V of 60 Hz ripple (the +15 V rail’s ripple read as “0 Hz” but I suspect that was not correct). I already know that 60 Hz was getting through to the Z-axis amp (ah, my old nemesis, we meet again), so finding 60 Hz ripple on the supply rails is only expected.

I have not looked at a service manual yet, but I’m guessing that this implicates the bulk filter caps.

— Jeff Dutky