Should I clean scope CRT anode cabling and connections or leave alone? - Page 1
Forum section: electronics / repair

author: mmagin
author_rank: Frequent Contributor
timestamp: April 10, 2016, 05:56:38 am
content: I've got a Tek 7904 which isn't working (ticking sound). So far I've measured the resistances at the test points and I'm pretty sure that there's a shorted tantalum on one or two power rails.Anyway, I was listening around to be sure it's the power supply hiccuping and not the 21 kV anode supply flashing over. I'm pretty certain the HV is fine, but naturally it's got a coating of super-fine dust around the connections as you'd expect to see. I was wondering if it's best to try to clean this up (and with what), or if I would be better off just leaving it the hell alone.

author: SL4P
author_rank: Super Contributor
timestamp: April 10, 2016, 06:18:16 am
content: Discharge the EHTSoft paintbrush - don't flex or bend the wireIf it's easy to get to, you may like to gently work with some non-residualsolvent based cleaner that won't attack anything else (like the insulation!)The inner conductor is often compressed-carbon impregnated, and any flexing will/may break the core.

author: mmagin
author_rank: Frequent Contributor
timestamp: April 10, 2016, 03:45:45 pm
content: Thanks for the advice about the fragility of the wire.I think I'll stick to my usual low-pressure, clean compressed air dusting plus a paintbrush maybe.In this model it snakes around a bit and I can't even get to all of it without some additional disassembly, which I'd rather avoid

author: TerraHertz
author_rank: Super Contributor
timestamp: April 12, 2016, 10:01:50 am
content: Quote from: SL4P on April 10, 2016, 06:18:16 amWhat?You're surely thinking of car spark plug leads, which use resistive carbon cores to reduce EMI emissions. But I've *never* seen CRT EHT leads with anything other than copper wire cores. Never, not once in 50 years of disassembling/repairing CRT systems. Including plenty of Tek 7000 scopes. Why on earth would anyone use resistive leads for DC voltages? Especially when regulating the voltage at the CRT is important to system calibration.Typically EHT cables are either PVC or silicone insulated. With sometimes a rare teflon cable. The only thing to be careful of, when cleaning, is that very old PVC insulation may crack if flexed much. Also scope anode supplies often don't have bleed resistors, and can hold charge for many hours. ALWAYS short the anode connection to chassis ground before working around the EHT. (I have a very dead Tek 7104 due to forgetting this, and assuming the supply would have bled away after several hours. It hadn't.)For cleaning the ionization gummy dirt that accumulates on cables and glass with a high electric fields, I don't find it necessary to use anything fancy. Plain old turps, on a wad of tissue held in your hand or forceps, works fine. On some types of PVC, kerosene works better. Wipe it dry with fresh tissue when finished.

author: SL4P
author_rank: Super Contributor
timestamp: April 12, 2016, 10:28:35 am
content: Agree with the automotive & insulation points, but I was specifically referring to TEK, SONY, and other analog broadcast video monitoring gear from years gone by - perhaps for the same reason - to reduce RF emissions in high-frequency low-level signal environments. Or just to be proprietary!The leads even had sticky tags attached that said 'do not bend', and upon further investigation (e.g. dismantling old gear) - you could actually feel the core 'crack' if it had been in a fixed orientation for a long time - then bent out of shape (no not the insulation sleeve!) ... not sure how I would have approached any need for 'softening' the old, 'rigid' HT leads... !

author: mmagin
author_rank: Frequent Contributor
timestamp: April 12, 2016, 09:37:11 pm
content: Quote from: TerraHertz on April 12, 2016, 10:01:50 amI wasn't going to use anything more aggressive than pure isopropanol.I think it's probably silicone in this scope, it has the appearance of it anyway. But parts of it are hard to reach without disassembly, so I might not clean it too obsessively for now.

author: mmagin
author_rank: Frequent Contributor
timestamp: April 12, 2016, 11:55:57 pm
content: Quote from: SL4P on April 12, 2016, 10:28:35 amI'm guessing that regular raster-scan monitors (and regular old CRT televisions) have a lot nastier EMC problems on the HT lead due to the sawtooth they're driving it with. On the other hand, with this, the multiplier on this is driven by a winding off the main power transformer -- a switching supply, but I think it's some kind of resonant converter.

author: timb
author_rank: Super Contributor
timestamp: April 13, 2016, 01:08:13 am
content: Quote from: mmagin on April 12, 2016, 11:55:57 pmThe older Tek stuff (at least to the early '80s) used Resonant Royer converters for the HV supplies.As far as I'm aware, most of the 2400 series (designed in the 80's) took a ~100V feed from the main supply and converted it to high-voltage pulsed DC through a small transformer before running it through a Cockcroft-Walton voltage multiplier. (The multiplier would often be in a sealed, potted module that had a connector for the anode lead.)The 2200 series was the same, but used lower voltage CRTs and would often tap off the ~100V inverter output before rectification and feed that right into a voltage multiplier.I loved the old Resonant Royer supplies. Elegant, self-oscillating and their outputs had exceptionally low amounts of noise and ripple (often in the hundreds of uV range).

author: mmagin
author_rank: Frequent Contributor
timestamp: April 13, 2016, 11:14:09 pm
content: Quote from: timb on April 13, 2016, 01:08:13 amLooking around the schematic of the 7904 is quite an education. I didn't realize they actually regulate the anode current in scopes until now. And the vertical deflection is pretty interesting. It's almost like a balanced transmission line inside the scope of multiple plates, driven at one end and terminated at the other.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 14, 2016, 01:15:53 am
content: Quote from: SL4P on April 12, 2016, 10:28:35 amI've worked on a lot of Sony BVM 1301 & BVM 1410 Picture Monitors as well as the truly horrible Tek 650 series,& I've never seen any such tags.The EHT leads were always fully flexible.The old 529 waveform monitors & 520 Vectorscopes had leads which were,as far as I could see,wire,& fully flexible.Maybe the ones you saw were the next generation.The automotive carbon ignition wires were a massive source of annoyance,as they would go high resistance "if you breathed on them".It was not uncommon to find leads which were nominally around 15k,measuring values ranging from 50k to 0.5 meg!