Tektronix 561A Faulty Trace - Page 1
Forum section: electronics / repair

author: Cyberdragon
author_rank: Super Contributor
timestamp: May 15, 2017, 08:14:51 pm
content: I just turned it on to work on something else and this happened!This is with both focus and intensity fully clockwise. Input module is disconnected. Any ideas? I'll have a look inside to see what I can find. I do have the manual.EDIT: Both modules removed now. Brightness works, focus moves the spot for some reason.

author: tautech
author_rank: Super Contributor
timestamp: May 15, 2017, 08:29:39 pm
content: Check the sweep ramp linearity after the obligatory PSU rail check.

author: Cyberdragon
author_rank: Super Contributor
timestamp: May 15, 2017, 10:28:54 pm
content: The voltage regulators are working perfectly. The only one I haven't checked is the CRT supply, which unfortunately I can't check since the highest voltage on any meter I have is 1500v. I'll check everything else first, but the manual says that it could be the HV regulator.EDIT: I'll check the modules after I check the rest of the main unit.

author: tautech
author_rank: Super Contributor
timestamp: May 15, 2017, 10:43:10 pm
content: Quote from: Cyberdragon on May 15, 2017, 10:28:54 pmNothing wrong with the HV IMHO to be getting the crisp tace you already have. No need to check.EditThe start of the Sweep triangular waveform is what I'd want to confirm as correct, hence the hint in my first reply.

author: Cyberdragon
author_rank: Super Contributor
timestamp: May 16, 2017, 12:32:14 am
content: Quote from: tautech on May 15, 2017, 10:43:10 pmThe trace wasn't crisp at all, the tablet camera just didn't pick up the fuzz. It seems to have improved after adjusting the voltages a bit. The trace isn't fuzzy anymore, but it still seems a bit too fat (I remember it having a thin trace). The splotch has faded but is still there.I'll have to check both triangle waves at the plug of the sweep module then. It's weird though, because the splotch expands when I turn up the intensity (or is this just an arifact of the distortion in the sweep?).

author: tautech
author_rank: Super Contributor
timestamp: May 16, 2017, 01:41:15 am
content: Quote from: Cyberdragon on May 16, 2017, 12:32:14 amYes, that's what I would suspect. Hopefully the SM for these has a good # of waveforms so it should make troubleshooting simple. Be sure to set the scope to the correct default settings for waveforms and any DC voltages listed to be correct.

author: Cyberdragon
author_rank: Super Contributor
timestamp: May 16, 2017, 02:34:43 am
content: Well then, they aren't uneven, but they look nothing like the sawtooth shape they show in the manual. They look more like thin triangle spikes with lots of dead space in between. I did have to use an attenuator probe given that the signals are over 200V p-p, not sure if that is why. It also seems to be temperature related, I let it heat up for about an hour and the spot is gone.The trace is still oddly fat too, over half a small division wide. EDIT WHILE POSTING: dirty voltage range switch is causing noise, that's why it's fat.EDIT AGAIN: WOAH! It's now making a faint high pitch squealing when I input a signal! I thought it was the flyback making that noise but obviosly not. It's coming from the modules, it might be the suspect component.

author: Cyberdragon
author_rank: Super Contributor
timestamp: May 16, 2017, 07:38:49 pm
content: It has started exhibiting further symptoms leading to the trigger stability and sweep lockout circuits. It is adjustable with a trimmer on the front so hopefully tweaking it will fix it. If not, I'll have to start looking for bad components.

author: Cyberdragon
author_rank: Super Contributor
timestamp: May 19, 2017, 07:32:23 pm
content: Never assume an intermittent fault will be defeated so easily. It's doing it again, but this time it's only past a certain sweep speed. I did find one out-of-tolerance resistor (measured high) connected to the sweep switch, but it appears to be part of the position control. I have checked all the tubes (with transconductance tester) and found none bad. If it's more calibration issues, I'm screwed since I don't have a time marker generator.There is an electronics aution this Sunday, so if I get lucky I'll find one.EDIT: I FOUND THE SOURCE OF THE BLOB! It's a noisy power bus. It's affecting both modules (hence the 2D shape) and can be affected by both modules. What's odd is that it only appears during normal opperation when the sweep switch is on a high speed. There is also a large circle on the screen when the horizontal module is disconnected.

author: tautech
author_rank: Super Contributor
timestamp: May 19, 2017, 08:38:04 pm
content: PSU's are interesting in equipment, yes noise can originate from sick smoothing caps manifested as ripple easily seen with another scope or as AC on the rail with a DMM.However when components in the circuit it powers go out of spec, the design of the PSU can limit its specified performance to only normal operation so when loading exceeds that all sorts of strange things happen like the increased noise you have spotted.Fixes like this one teach us lots and wise us up to the unexpected.Time to get the schematic out again and have a good study to maybe spot the components that are in circuit when settings produce noise on the rail.

author: Cyberdragon
author_rank: Super Contributor
timestamp: May 19, 2017, 10:50:29 pm
content: There were weird spikes and stuff on the 300v rail. I found the faulty component, and I never would have guessed it, it's that weird. It does have to do with component specs though, but they were not exceeded, so I don't know why it failed.There is a heater-cathode short in V667B, and I don't think I have a spare. Well that's annoying... The heater bias resistor (to reduce heater-cathode voltage) is still good, so I have no idea why it shorted.

author: tautech
author_rank: Super Contributor
timestamp: May 20, 2017, 12:21:28 am
content: Quote from: Cyberdragon on May 19, 2017, 10:50:29 pmWithout digging through schematics and old valve books......are you sure ?Hot cathodes and heater to cathode links are not uncommon.

author: Cyberdragon
author_rank: Super Contributor
timestamp: May 20, 2017, 01:03:59 am
content: It didn't look like AC hum at all, so I thought I had ruled that out. It was just some weird random pulses which I thought was an intermittent connection. Plus, the cathode is rated at 300v, and it's sitting at less than that because of the resistor across the heater to the +125v (which probably caused some weird feedback through the cathode leak that resulted in random pulsing).Any chance that the shorted cathode caused any further damage? (I'll check the other tube in the 300v regulator just in case it was damaged by any leakage current)EDIT: I guess that would be a trap. Just because it doesn't look like ripple, doesn't mean it isn't caused by ripple.

author: Cyberdragon
author_rank: Super Contributor
timestamp: June 09, 2017, 11:49:19 pm
content: Wouldn't you know it? It's doing it again with the dimness and the spots!Now with more weird symptoms. If I turn the tim/dev switch to external (sweep off) and move the position to just off the screen, this happens (brightness turned to max for camera):I think the problem is that during blanking a part of the beam is being thrown to the side of the crt where it bounces around until it hits the screen forming strange marks. These marks may be normal for running the beam off the screen. Whatever failed must have been damaged by the unstable psu when that tube failed (it also caused the connected tubes to act funny and they had to be changed too).