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Tektronix scope issues


Thomas Dodge
 

Hi,
This is Tom Dodge, and I am posting this topic again because I didn't post it right before. I have three Tektronix scopes. I have a Tektronix 533 that doesn't work. When I turn it on, it does not have a trace at all, and just a beam. I also have a Tektronix 7704 which I just powered on. I got it from a friend, and when it is on, it just shows a beam that is very bright, so I turn it off so it doesn't burn the CRT. The switches for the Time Base and the Volts per Division have no effect. I also have a Tektronix 684A which shows the most promise. I bought it from a surplus place, and is in very good condition. When I took it home and powered it on, it didn't come on at first, but after a few more tries, it did power on and went through the Self Test process, and it seemed to be ok. When I turned it off and on again, it worked ok. When I powered it on the other day, panel lights came on, but the CRT did not. I even tried to connect an external monitor using the VGA output ii the back, and I didn't get any signal. I remember reading somewhere about this issue, and the solution was to replace a chip on the board, but I can't verify that. I did look at the manual to see what it said if the self test doesn't work, but I didn't see anything. Does anyone have any ideas about this? Thanks.

Thomas Dodge


 

On Sat, Nov 28, 2020 at 07:07 AM, Thomas Dodge wrote:


I also have a Tektronix 7704 which I just powered on. I got it from a friend,
and when it is on, it just shows a beam that is very bright, so I turn it off
so it doesn't burn the CRT. The switches for the Time Base and the Volts per
Division have no effect.
Often, an uncontrollable, very bright CRT image indicates a problem in the DC restorer circuits.
If combined with absence of vertical and horizontal deflection, it most often indicates a problem in the (low-voltage) power supply (LVPS), which may also affect Z-axis behavior, causing a high intensity beam.
Fault finding may not be too difficult but you must protect the CRT - and yourself. Since I have no idea what level of experience yo have, I have to warn you WRT the dangers involved in working on these instruments. No intention to insult you...
With deflection absent, you can't move the beam off-screen to protect the CRT, so at least turn focus to either completely CW or completely CCW. That's probably not enough to protect the CRT (burning), neither will be removing the post-deflection acceleration (PDA) voltage (red wire to front of CRT).
Be very careful with the high voltages present and don't forget that the PDA voltage may take hours (or more) to bleed away - and that the 6.3 Vac filament supply is floating at about - 3 kVdc...
Once you've excluded the LVPS as the cause of your problem (a good chance the problem lies there), it would be very informative to see if there's a negative voltage between cathode and grid of the CRT: The CRT's grid should be at least 50 volts or so negative (averaged DC) with regard to the cathode with intensity turned down all the way. If the voltage is very low, intensity will be max. Measure using a battery-supplied high-impedance (>= 1 MOhm) voltmeter, well-insulated, and placed at least 5 cm away from any conductive areas. Connect first, then power on!

But first you need to protect the CRT's screen from being burnt. To achieve that, you have several options but you need to know what you're doing. Examples in decreasing order of my preference are:
- Temporarily disconnecting the filament supply (6.3 Vac).
- Temporarily removing the CRT. A bit dangerous, not as difficult as it may seem but not attractive until experienced...
- Temporarily stopping the inverter producing filament- and high voltage. This would take away the possibility to (among other things) measure grid-cathode voltage though and therefore in my opinion be the least attractive option.

Raymond