CRT anode discharge procedure for Tektronix video waveform/vectorscope - Page 1
Forum section: electronics / repair

author: kinap35
author_rank: Contributor
timestamp: February 16, 2025, 10:16:00 am
content: Hi,I have to recap a dead power supply board into a Tektronix 1761 video waveform/vectorscope. The power supply board is connected to the CRT's anode so i need to disconnect it to be able to fully remove the boardI've made some research (including here) about to safely disconnect and discharge this anode, and most of the time instruction is to use some kind of long isolated and ground connected screwdriver and slip it under the grommet on the CRT itself before disconnecting it.Except that the kind of CRT used in these scopes has no grommet, but some kind of potting (see attached picture).Other instructions say to simply disconnect the big anode connector (presumably by hand) and to short it to the chassis, like shown in these videos:-at 4:10-But i wonder:- Should the scope still be connected to the main plug or the chassis somehow connected to earth so that released energy is diverted to earth? Or should the scope be completely unplugged/unearthed, but then where does the energy go?- Is it really safe to handle this connector as shown on theses videos? Is it long enough to ensure no arcing can happen to the hand? Of course one should not touch the chassis while discharging.- Some people says it is better to use a resistor to discharge more progressively. Is it advisable in this case, or is it completely safe for the CRT and electronics to directly discharge to chassis?Thanks for your advices!

author: George Edmonds
author_rank: Frequent Contributor
timestamp: February 16, 2025, 10:26:28 am
content: HiUse a current limiting resistor to prevent any possible damage and keep the CRT connection grounded at all times as CRT’s can and do acquire a charge over time. Whist this would not be lethal it may cause a reflex action when touching it which could result in damage to the CRT.G Edmonds

author: kinap35
author_rank: Contributor
timestamp: February 21, 2025, 09:27:49 am
content: Thanks for these advices. But I still be worried by those other questions (located after the videos, so may be some of you missed them):- Should the scope still be connected to the main plug or the chassis somehow connected to earth so that released energy is diverted to earth? Or should the scope be completely unplugged/unearthed, but then where does the energy go?- Is it really safe to handle this connector as shown on theses videos? Is it long enough to ensure no arcing can happen to the hand? Of course one should not touch the chassis while discharging.

author: ondraN
author_rank: Regular Contributor
timestamp: February 21, 2025, 10:36:36 am
content: Yes, it can be pluged into socket, but not powered ON.Yes, this handle is safety, no arcing cant happen.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: February 21, 2025, 02:14:44 pm
content: HiNEVER ever work on anything that is still connected to the mains supply. If the mains on/off switching is in the neutral not the phase/live wiring (you would be surprised as to how often this is the case) the entire internal circuitry can be at full mains voltage although appearing to be turned off.It is also not a good idea to have a large metal grounded object to provide a direct path to earth near you should you accidentally come into contact with a high voltage.G Edmonds

author: kinap35
author_rank: Contributor
timestamp: February 22, 2025, 10:33:11 am
content: So you mean that the CRT anode can safely (for the electronics) be discharged to the chassis even if the chassis is not linked to earth at all? In this case, where the large amount of energy is being dissipated?

author: Gyro
author_rank: Super Contributor
timestamp: February 22, 2025, 11:04:16 am
content: The energy is stored between the CRT final anode inner conductive coating and the outer coating (chassis). The outside world is not involved.

author: Tantratron
author_rank: Frequent Contributor
timestamp: February 22, 2025, 11:44:44 am
content: Quote from: kinap35 on February 21, 2025, 09:27:49 amBonjour, first normally most CRT system do have a bleeding resistor in parallel of the anode with the ground, such resistor can be very high for logical reasons. If the schematics does not show such resistor, you have no choice to access the anode (either by puncturing a bit the potting or disconnecting the high voltage transformer plug linked to the CRT's anode).To my knowledge, there is no reason to earth connect the chassis if you use a screwdriver or metal short between anode and chassis. Just use good isolated gloves to protect your hand, your body then discharge.The energy will be dissipated through the chassis as a short value close to zero resistive load or you can add say big resistor in between active as resitive I2R load. Just wait enough time for the energy to be dissipated and do not be afraid of the initial high voltage plasma arcing.If your oscilloscope has not been running for a long time, at some point the CRT's virtual HV capacitor will self-discharge, again there might be a bleeding high imepdance resistance added by tektronix to automatize this discharge as a safety.Gloves in your hand, do not think too much and go for it.Amicalement from France, Albert

author: Manolo Mos
author_rank: Contributor
timestamp: February 22, 2025, 12:15:15 pm
content: Hi. I have a Tek 1741(now is 1745 by software) and 1765. I've replaced all the caps to the 1741 and some caps in the 1765, and there is not need to remove CRT anode connector.The most important is to backup calibration configuration and replace Dallas NVram, if battery have not discharged yet.

author: kinap35
author_rank: Contributor
timestamp: February 22, 2025, 04:12:37 pm
content: Quote from: Tantratron on February 22, 2025, 11:44:44 amThanks Albert. The manual indeed says to short anode to chassis, but what i am wondering is:- if chassis was connected to earth, the released energy would be "evacuated" to the earth, not to the internals of the scope- but if chassis is NOT connected to earth, since chassis = common ground to all electronics boards, my thinking is that since anode is at positive high voltage, it would mean connecting a high voltage positive to GND input of all circuits. So i wonder how/where the energy is dissipated without possibly frying som components?This is may be a stupid question, but i would like to understand what is happening when shorting anode to the floating/not earthed chassis.Quote from: Manolo Mos on February 22, 2025, 12:15:15 pmUpgrading the internal software to upgrade functionnalities is interesting. It would indeed make my 1761 a dual standard PAL/NTSC like the 1765. I also have a 1755A, but i don't know if it could be software upgraded to 1765 for example.Is there somewhere a source of softwares/ROM files needed to do such upgrades?About caps replacements, the power supply PCB is connected to CRT's anode. It is possible to disassamble it while keeping anode connected, and i just did it to replace the RIFA caps. But it is not really convenient to have the PCB still attached to the device, and i'm unsure if it is safe either, since anode voltage is probably exposed at some points on the PCB. So disconnecting the anode to fully free the power board seems to be a better idea to me.About Dallas NVRam: i should indeed take care of that quickly! Is there a warning message displayed on a (working) scope if battery is discharged?

author: Manolo Mos
author_rank: Contributor
timestamp: February 22, 2025, 04:56:10 pm
content: Hi. Here are the calibration files.https://groups.io/g/TekScopes/message/202894?p=%2C%2C%2C20%2C0%2C0%2C0%3A%3ACreated%2C%2CNVSAVE.EXE%2C20%2C1%2C0%2C103797339Yes, you can convert a 1761 in a 1765, but not a 1755 in a 1765. 176x has a different hardware.But, for to do it, you have to calibrate completely.It is not difficult, but it long and its needed some instrumental, like video test card generator, etc...So, the must important, it to backup your calibration data. NVSAVE.exe and a null modem cable.There is another thing, IEC Schaffner inlet filter conector should be replaced, but I haven't found any similare replacement, for PCB direct soldering. I haven't replaced it, so, I'm cautelous because it can give any fright.

author: kinap35
author_rank: Contributor
timestamp: February 22, 2025, 05:24:19 pm
content: Quote from: Manolo Mos on February 22, 2025, 04:56:10 pmThanks!But do you mean that upgrading from a 1761 to 1765 is just a matter of calibration? No ROM file needed?Also, for the moment the power supply board of my 1761 is dead, so i should probably first dump the Dallas NVRAM using an EPROM reader, just in case. It seems it is also possible to replace the original Dallas NVRAM by an FRAM without internal battery, will have a look at that...Quote from: Manolo Mos on February 22, 2025, 04:56:10 pmOh yes, you're right! I've already replaced it with a FN9226-1-02: it is almost identical to the original FN326-1/02, except that the black plastic part does not extend to the screw holes.And by the way, i also had to replace not only the infamousYELLOWRIFA caps, but also theWHITEones, which is new to me! They were all damaged in several devices (from different sources), including one in which i had previously replaced only the yellow ones.

author: kinap35
author_rank: Contributor
timestamp: February 22, 2025, 05:25:55 pm
content: My third picture of white RIFAs didn't went through, this is it again...

author: Manolo Mos
author_rank: Contributor
timestamp: February 22, 2025, 06:09:32 pm
content: Yes, no ROM file is needed. When you start to calibrate, the software ask you which model is the 17xx, and then you select 1765.Don't remove the NVram. Repair the power supply first and then backup the calibration data. I say it because I removed a NVram for to place socket, and when I placed the Dallas NVram, it lost the calibration. I've read here and other forums, that when the battery is low, it is not safe to resolder or solder the NVram, because data can be lost, and I can assure it.So, don't remove the NVram. Repair the power suppy, check Nichicon caps and all suspicious stuff. And when the power supply works, backup it. I used for it a HP dc5800 with Wxp and worked perfectly. I used a portable Dell with native serial port, and gave me problems.

author: Tantratron
author_rank: Frequent Contributor
timestamp: February 22, 2025, 07:30:32 pm
content: Quote from: kinap35 on February 22, 2025, 04:12:37 pmWhen a chassis has enough metal massic volume, it becomes like a small earth.Think of an airplane flying, its all metal mainframe becomes a local earthing.Same with a car running on tires or a boat floating in the ocean, where is the local earth or relative ground for all components.There is no absolute earth, just relative so what matters is enough metal solid presence which will be quickest path to discharge the CRT HV capacitor. The electrical energy is lazy, it goes where it has less resistance plus when the plasma arcing manifests, lot of energy is dissipated localy like a lightning rod.Albert