Tektronix 7704a Won't turn on and flashes the neon starters - Page 1
Forum section: electronics / repair

author: tedybear315
author_rank: Contributor
timestamp: October 10, 2015, 01:51:42 am
content: This is a bit low on the repair list as I was given a pair of Oscilloscopes for no charge.One is an older military type: os-245(p)u This one I was told had no vertical deflection---Brought it home and fired it up! Works fine. (so far)The other one is a Tektronix 7704a. This thing has all the options and the gent also gave me several other of the plug in modules. (Along with a box of NOS (New Old Stock) CRT's for smaller Oscilloscopes. (Even have one CRT that is only about 1 1/4 dia. Really tiny screen) Not sure what I"ll do with them, but it is an oddity. Might toss those up on eBay or donate them to a tech school-under the "Remember what these are?" heading.Okay, back to the problem. He let me know that the unit failed when he turned it on during a humid day. He felt it might have a carbon track that is preventing it from starting.What it's doing: Turn it on and the neon starter flashes in the power supply and the neon starters on the high voltage board also flash rapidly and it sounds like it's arcing in several spots. I say "Sounds" like it, because I can not find any signs of arcing or carbon burnt marks. (Sorry if I kinda use older TV CRT terms--I'm decent with older CRT monitor repair. Scopes are new to me for repairing)http://elektrotanya.com/tektronix_7704a.pdf/download.htmlWhat I could find for a service manual. Probably more to be found--but it's been a long day.I did a full inspection didn't find any signs of brunt traces or carbon paths where high voltage could 'jump' causing issues. I did note when I pulled the high voltage lead and turned the unit on? Of course we had no picture-- But the unit seemed to fully power up and it was nice and quiet. Plug in the high voltage lead--and all the neon starters flickering rapidly--the arc sounds return (sounds like it's coming from 3 different places--power supply, high voltage board, and the CRT itself. I suspect the sound is traveling around the case a little bit and is being deceptive.Sense the one that was supposed to not work--appears to be fully functional--I'm not in a rush to get this one working. In fact I might just put the modules up for sale on eBay or such. But as the old saying goes: In for a penny--in for a pound. I'm only out fuel for the pickup, so it's no big deal if I can't get this one up.Any ideas would be great. I'd hate to see this one get sent to the trash heap if it's something simple.S-

author: DC912
author_rank: Contributor
timestamp: October 10, 2015, 02:06:30 pm
content: I've been trying to repair a 7904a supply recently. I recall reading a post or two describing your symptom, and the fix was to blow out all dust that might be around that connector going to the crt. You might give that a try. Dave

author: c4757p
author_rank: Super Contributor
timestamp: October 10, 2015, 02:34:03 pm
content: Got a high voltage probe? I'd start by checking to see whether the CRT anode bias comes up properly when not plugged into the CRT.

author: tautech
author_rank: Super Contributor
timestamp: October 10, 2015, 07:46:07 pm
content: Quote from: tedybear315 on October 10, 2015, 01:51:42 amI've had humidity problems with CRT's before too. Dust provides the film for it to flash over at the 10kV PDA connection. Clean and dry, check grommet for carbon tracks and ensure the PDA cable itself is not breaking down to the chassis. Many CRO's have a designated path that the cable must take to reduce the chance of breakdown.Then you should consider if the HV multiplier has failed as a result of a breakdown.If you disconnect just the multiplier does the neon stop flashing?

author: tedybear315
author_rank: Contributor
timestamp: October 10, 2015, 09:41:38 pm
content: I've got some reading to do this afternoon. Just got back from work.I'll have to see if I can get a high voltage probe to check.When I fully remove the lead from the 'flyback' (high voltage wire. Not sure if the terms would be the same as I used to do a lot of CRT repairs many years ago-we moved to flat panels now) Anyrate, when I removed the high voltage wire and powered the scope up, everything (except the screen) came on and worked fine. At least the buttons where fully lit up and not pulsing in time with the neon starters. (forgot to mention that)I did a quick and dirty arc test. I had the HV lead about an inch pulled out of being fully seated. I could hear the arc jump and hiss. So I'm fairly confident we have the high voltage.What is odd, I'm used to discharging CRT's. I can't even get a minor spark to ground from the HV lead to the tube. I did remove the tube and cleaned things up. I did see some very odd discoloring in the same corner the HV lead is attached to the tube. My first thought was that it had arced there--but I've cleaned that up and it's still doing it. Maybe it might be worth while to cover the area with some electric tape and ensure it's fully encapsulated--so the electric has only 1 path to go.Just messing around with ideas. I'll check into a HV probe. I'm slowly getting more test eq. as I can afford it and as donated.My thanks for the replies! I'll do more reading and checking later this evening after the shopping trip with the wife.S-

author: tautech
author_rank: Super Contributor
timestamp: October 10, 2015, 10:00:26 pm
content: Let's get on the same page with terminology.CRT HV for grid etc @ ~ 2kVPDA (post deflection acceleration) @ ~10+ kVQuoteSome CRO's will display a faint trace without PDA voltages, check for this in a darkened room.QuoteI've used Silicon sealers and Silicon grease in this instance.You could also check the PDA boot is properly connected with the cable.Have you another multiplier to substitute as a check?For CRO HV checks and adjustments I keep old AVO meters with an 3kV range just for this purpose.There's a thread here with a homebuilt HV PDA multiplier for Teks, I'll see if I can find it and link it here.

author: Deathwish
author_rank: Unknown
timestamp: October 10, 2015, 10:05:37 pm
content: One of the oldest diagnostic tricks I ever learnt from old TV sets was to remove the covers and turn the thing on in a dark room looking for arcs or such. Who knows might help you

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: October 10, 2015, 10:32:32 pm
content: +1 on dark roomHelped me find arcing x-ray cables a few times. Lit up the room like a flash bulb.

author: tautech
author_rank: Super Contributor
timestamp: October 10, 2015, 10:44:36 pm
content: Most of what you need to know about a CRO multiplier repair/replacement is in this thread:https://www.eevblog.com/forum/repair/tektronix-465/

author: tedybear315
author_rank: Contributor
timestamp: October 10, 2015, 11:04:52 pm
content: Quote from: Deathwish on October 10, 2015, 10:05:37 pmYou are not kidding on that trick! I used to use that all the time when working on pinball machines for the company I work for. Great way to find arcing coils for the flippers and such.I'll report back once I know more. I love an adventure, and this one so far is pretty fun! (Okay, HEAVY to lift onto my work area...but still fun)(side bar note: I have a couple of tubes of Silcone Gel used for car/auto ignition systems and the spark plug wires and boots. Great suggestion on that as well.)S-

author: tedybear315
author_rank: Contributor
timestamp: October 11, 2015, 01:12:24 am
content: Well, looks like a combined effort. So my thanks to everyone.Looks like the main issue finally surfaced. As suggested I removed the front bezel and kicked it 'on'.I left it running a bit longer then I did before and I noted the tiny tiny flash. That and some smoke and smell.Pulled out the tube and took the attached photos. The one is the tiny hole where it burnt through. The second shows more of the damage once I pealed back the insulation tape and the 'tin foil'.I'm not used to repairing this kind of high voltage damage. So let me run this past the group:Clean the area up a bit more and use Silicone Glue to pack and build up the damaged area so it will hopefully allow the voltage to stay where it's supposed to be.2nd part would be to peal the tin foil back a bit more on all the sides where the damage is so all that is showing is the plastic insulator.3rd part would be to put a thin layer of plastic over the damaged area with the silicone glue hardened.4th part would be to cover the area again with some sticky back tin foil. Looks to be the same as used on HVAC systems. Thin strip. I'm guessing it's used to prevent interference. If not and it can be left off without causing problems let me know.Looks like it went past the rubber seal....ate past the thin plastic cover...and was arcing under the tin foil. Looks like it wouldn't be seen until it broke through.Suggestions or improvements to my ideas on repair are welcome. Much as I love to tinker with these things I'm not looking forward to ripping it apart again.Ah yes. The scope did have a jumpy bit of the line showing. Pin point dot and then it skewed down and out. Looks like this ol' Scope might have some life to it.Thanks again!S-

author: tautech
author_rank: Super Contributor
timestamp: October 11, 2015, 01:46:06 am
content: Who knows why Tek put that foil tape there.Probably a damn good reason but I doubt it will worry you for hobbyist use.I'd remove just enough of it to ensure you don't have the same problem again.

author: tedybear315
author_rank: Contributor
timestamp: October 11, 2015, 01:53:44 am
content: Think you're right. It would only be a small section carefully removed to allow better access and cleaning of the damage.Should be okay with the Silicone glue to insulate and the plastic covering?(Quite the adventure....I love the challenge)The only sad part? Lost a couple of the wires for the deflection. Old and a bit on the brittle side. Looks like a bit of modify work coming my way.

author: Deathwish
author_rank: Unknown
timestamp: October 11, 2015, 02:08:19 am
content: I have never had the misfortune of having to dismantle an oscilloscope to repair one, so I cant give any experienced reply to how to repair it, but I am rather surprised that Tek would do that , it seems to look like the tape goes under the rubber connector, that would bring about a short sooner or later to my mind. is it possible that someone else has tried a repair with some rubber or such then taped it in place with the aluminum tape Tautech ?. If it was an old TV I would just replace the whole LOPT or the anode cap from a similar unit.weird.

author: tautech
author_rank: Super Contributor
timestamp: October 11, 2015, 02:50:07 am
content: Quote from: tedybear315 on October 11, 2015, 01:53:44 amWhen I've had a damaged grommet I've used Silicon glue/sealant. For arcing tracking prevention use Silicon grease. Scrape/cut away any carbonising first.Where did the deflection plate wires break?@tedybear315So often these are quite straightforward repairs often overlooked by those scared by the high voltages involved and few these days have worked on CRT's. With your background and hopefully a successful repair you may get bitten by the bug too.The challengegot me into fixing CRO's too. Be warned, success leads to addiction.Quote from: Deathwish on October 11, 2015, 02:08:19 amYep weird Gary.One must remember many modern CRT's are covered in a metalised coating, maybe Tek didn't have that technology back when these were made. Those I have seen had this coating pulled back a few cm from the anode terminal, presumably to prevent arcing.Yes a new anode cap might be a good idea.

author: tedybear315
author_rank: Contributor
timestamp: October 11, 2015, 01:09:42 pm
content: Most of my time working on things has been spent over the last 20 years as an arcade manager and tech. I've recapped monitor boards, replaced flybacks, anode leads, etc.. Always have spare parts floating around for those repairs.I'd have no clue where or how to obtain a new anode cap. More research on the way! But in the mean while I believe if I clean up the damaged areas, used an exact-o blade and trimmed the damaged area back. Silicone glue it back with probably 1/4" bead around the existing cap. And polish it off with a few wraps of electrical tape for good measure. Should keep that voltage where it belongs.As for the two wires to the tube itself? Both broke off the board. About 1" long. I salvaged a couple off the old Phillips scope that is being scrapped. I found a lottttttttttt of heat damage and distortion on the rear section of the main board, and a simple repair of that one was starting to get expensive and ugly. (The IC that is likely damaged---can't find it anywhere. Even a buddy overseas with a scope 'graveyard' didn't have one.)The wires off the Phillips scope are made for a thicker lead from the tube--so while it was a bit sloppy of a tight fit, it at least helped move the project along. I'll continue with trying to find a connector in my parts boxes that should be secure.I'm not scared of high voltages--25 years ago I used to be an automotive tech. I've been nailed by HEI ignition systems. One gets a very healthy respect for electricity after the first zap. I've got a cardiac deflib implant that can be triggered if I get nailed hard enough- Even more reason to be careful around it.Once I get this ol' beast running correctly? I'll have to take a course called "Oscilloscopes for Dummies" Last time I used one was back in 1985. I've become dependent on my trusty multimeterAnd of course that lets all of our bench techs in the company I work for have all the fun.Thanks again for the help!! I'll take some photos and update as things progress.S-

author: tautech
author_rank: Super Contributor
timestamp: October 11, 2015, 06:47:06 pm
content: Sounds like fine progress and you have it all under control.Here's some resources:https://www.eevblog.com/forum/beginners/oscilloscope-training-class-(long)/http://www.tek.com/downloads

author: tedybear315
author_rank: Contributor
timestamp: October 11, 2015, 09:24:12 pm
content: Well, had progress. LOL.My gut feeling is that something is amiss with the anode lead itself as well. I gave it a good coating of silicone glue and let it harden up. I then used something recommended by the guys at the home center that is rated to insulate 6,000 volts per layer. It's a rubberize type of tape that is used for high voltage applications.Kicked 'er on and she arc'd worse then ever.Much as I love a good spark show... Pulled everything off and figured out the arc is jumping a good inch--all the way over to the tube metallic coating. I cleaned the carbon up using 90+ proof rubbing alcohol, got 'er nice and cleaned up and let it dry before applying the silicone.Think it's time to bring out the dielectric "grease" and give it a good coating and try to wedge some under the corner edge of the anode lead. I cut away the burnt up section and it's a mess.While I'm not beaten yet? My pride took a kick in the tuckas. Had any chance of an external arc covered--so it was arcing to the tube metallic coating.I'll give 'er round 3 in a day or two. Worse case I can pick up the same model off eBay as a 'parts' machine. If I go that route I'm likely to ask the seller to plug the thing in and see if the tube kicks up a trace line. (Cheap enough for the machine as a parts one--but the shipping cost seems like I'm sending the shipping carriers children to collage with the high rates)And the saga continues....S-

author: tautech
author_rank: Super Contributor
timestamp: October 12, 2015, 02:18:03 am
content: LOL great fun isn't it.More links for you;https://www.eevblog.com/forum/repair/repair-documents-and-links-sticky-me-please-mods/msg477909/#msg477909In particular Sphere.

author: tedybear315
author_rank: Contributor
timestamp: October 12, 2015, 10:08:00 pm
content: Well this just got more annoying. (said in jest.)Repair on the bottom part of the anode seems to have held this time!Sadly I'm still hearing a buzz/snap and I can see a lot of arcing under the top section of the anode cap.I believe that connection is pretty well going to be full of carbon crud and will have to be removed to fully clean it up.Not quite what I planned upon, as I'm used to the anode that looks like a suction cup. This looks molded into the tube itself. Any suggestions or just start working it off with an exacto blade?S-

author: tautech
author_rank: Super Contributor
timestamp: October 12, 2015, 10:21:30 pm
content: Quote from: tedybear315 on October 12, 2015, 10:08:00 pmI guess you're talking about the Tek anode caps that look like auto spark plug caps.http://www.sphere.bc.ca/test/tek-parts/tekparts5.html(scroll down)Could you cut the old one off and mod a spark plug cap to fit?

author: tedybear315
author_rank: Contributor
timestamp: October 12, 2015, 11:52:18 pm
content: I wish it was that simple. I found the tube itself and 'borrowed' the photo. It's the white anode cap that is glued to the tube itself.I gave it another go with more silicone glue after I cleaned up about 1/4" of the conductive coating from next to where the cap connects.Just for 'fun' with everything unhooked except the anode lead. I powered it up (of course it's clicking away like mad and not to happy) I placed a screw driver between the white anode cap and it threw an arc about 1/4" to the closest part of the conductive coating.I didn't see where it was coming out of from the cap itself. Turning the light off and letting it have it's fit for a moment showed several blueish arc type lines off the cap in a couple of spots. Not a pin-point crisp arc..more like a wide fuzzy arc.If I didn't know better, I would think the white cap is compromised/porous and not sealing the voltage to the tube like it's supposed to.Said the heck with it and gave it a rather thick coating of the silicone on the entire cap this time, and the surrounding areas. Oh yes, one thing that the tube DID do that it failed to do before. I drew an arc when I grounded the anode lead. So I know it's starting to act correctly. Just need to plug up the leaks.

author: tautech
author_rank: Super Contributor
timestamp: October 12, 2015, 11:57:33 pm
content: Radiator stop leak?

author: tedybear315
author_rank: Contributor
timestamp: October 13, 2015, 01:43:24 am
content: Quote from: tautech on October 12, 2015, 11:57:33 pmIf I figured it would work I'd give it a try!I'll let this silicone harden overnight (it's about 9:40PM here where I'm at, and I've been up sense 4am for work. Kinda going brain dead at this point.)And I'll give those eBay soldering stations some kudo's. The one I picked up a few months back has been flawless. The soldering iron hits temp wicked quick!. (Yeah, had another of the CRT tube wires break. That makes 3 so far. Old and brittle.)S-

author: tedybear315
author_rank: Contributor
timestamp: October 14, 2015, 12:04:30 am
content: All right.It's put back together and working somewhat now. After another coating of silicone glue, it's no longer arcing to ground. When it is turned on there is still a static hissy sound from the anode cap. (of course....there' a small handful of screws and what nots hanging around.....always have left over parts)But it turned on and has a full picture and writing on the screen.The only lingering issue is one of screen stable and focus. I can get the focus on the trace lines decent...or the writing on the screen. Both in focus seems to be a stretch.The picture on the other hand? Tends to have a bit of a jumpy hop to it. It almost seems to be a lingering bit with the static sound... Or it could be that this 1972 scope is in bad need of recapping. My money is on the capacitors being fairly well roached out.At least progress is being made.Thanks for the help in getting this as far as we've taken it!S-

author: tautech
author_rank: Super Contributor
timestamp: October 14, 2015, 12:20:11 am
content: More than likely PSU, it should always be checked first to see if it is to spec.Study the service manual PSU section carefully.Another handy link:http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

author: tedybear315
author_rank: Contributor
timestamp: October 14, 2015, 12:41:25 am
content: I'll meander down to the PSU in a bit. I've got the other one that seems to be functional and I'm giving it a bit of work. (larger screen) The only downfall of the older military one would be one of the knobs on the control board got smacked clean off. Not sure if I should attempt to repair it or just leave it alone. The main knob for the voltage is fine, it's the center built in knob that got snapped clean off.I'm back to work tomorrow, so I'll be calling it a night pretty soon anyway. Thanks again for the help!! I'll start to watch the video on scope basics once I get the bugs worked out.S-

author: tautech
author_rank: Super Contributor
timestamp: October 14, 2015, 12:46:56 am
content: Quote from: tedybear315 on October 14, 2015, 12:41:25 amSphere have many of the knobs.