Tek485 - hybrid HV resistor cracked - can anyone help with a replacement? - Page 1
Forum section: electronics / repair

author: Lorenzo_1
author_rank: Regular Contributor
timestamp: May 18, 2017, 10:49:57 am
content: My Tek485 started going in and out of focus - unlike it's usually crisp trace. Thought it might an HV supply problem as supply seemed to be wobbling quite a bit at the -2.95kV test point. All the other low power rails are spot on. HV supply hard to get at so started checking the Focus and DC restorer circuits on the more accessible parts at the rear of the horizontal amplifier board. Signals at all the other relevant test points check out fine (Anode of CR1660, TP1665, Anode of CR1656 and TP1653 (schematic & traces attached). However the hybrid resistor pair R1642 is cracked (photo) and likely explains the problem. It's a voltage divider between the -2.95kV and 50V rails (500k and 29.5Mohm).Can anyone help with a replacement, a source for a replacement, or suggestions as to other ways of getting around the problem?. Perhaps someone has a dead Tek485 or horizontal board lying on a shelf somewhere that could be plundered? The scope is working fine and dandy otherwise and I'd like to restore it once more.

author: Andy Watson
author_rank: Super Contributor
timestamp: May 18, 2017, 11:25:37 am
content: I had a similar problem with my 475. Fortunately the problem was with the connection at the low voltage side of the resistor chain - so I bridged it (the 500k ?) with a standard resistor. You could do the same with the high voltage side but remember that you will require several series resistors to attain the voltage rating. Also, be careful probing that area of the circuit - reducing the feedback can lead to excessive output voltages.

author: Lorenzo_1
author_rank: Regular Contributor
timestamp: May 18, 2017, 11:36:54 am
content: Whoops! I missed the other two components of R1642 - the 21M and 7.5Mohm on the focus pot side.Thks for that advice Andy - I'll get in a little closer tomorrow and see if I can work out what's actually dead. If it's the 21M resistor on the ground side of the focus pot, perhaps it's safe to bridge it. I'd much prefer to get a replacement for the original, but perhaps that's a forlorn hope.

author: tggzzz
author_rank: Super Contributor
timestamp: May 18, 2017, 12:16:17 pm
content: It would indeed be a shame to "lose" a 485, doubly so after you've recapped the HV!I presume you've looked on the sphere and qservice websites. Don't forget the Yahoo TekScopes group.For example...http://www.ebay.com/itm/TEKTRONIX-307-0386-03-Precision-Resistor-Network-400-Series-Oscilloscopes-NOS-/140744958989

author: Lorenzo_1
author_rank: Regular Contributor
timestamp: May 18, 2017, 12:43:54 pm
content: Yes I tried Sphere and qservice but they're showing out of stock on the 307-0386-00 I need. Not sure whether the 307-0386-03 shares some or all of the same spec. Will try and track it down as they seem more readily available. Not listed in my 485 manual so I'll have to try and find out what scope they're used in and get the specs from the service manual.

author: David Hess
author_rank: Super Contributor
timestamp: May 18, 2017, 03:39:40 pm
content: Quote from: Lorenzo_1 on May 18, 2017, 12:43:54 pmI think the -xx variations have different resistance divider ratios so they are not direct substitutes without other modifications.Precision high voltage thick film resistors are available which will work but the problem with using them is fitting them into the available volume. There are also precision high voltage thin film resistors which could be used. The ratio of R1642B and R1642C does not need to be exact as long as it is within the adjustment range of the regulator; if necessary, series or parallel trim R1642B for the desired ratio. Use plenty of voltage derating for R1642C and the focus divider chain.Depending on how discrete high voltage resistors are wired into place, some corona dope over the exposed wires and solder connections would be a good idea. Make sure joints are mechanically secured; do not rely on the mechanical strength of the solder joint. (1)The real trick is making sure R1642B and R1642C track over temperature which is where the original thick film network was a good solution. The precision of modern thick or thin film high voltage resistors helps with this but make sure to use the same type of resistor for both legs of the divider.(1) I cut the leads short and bend them into little hooks so they grab each other and then I lightly crimp the hooks. The solder goes over the complete joint.

author: james_s
author_rank: Super Contributor
timestamp: May 18, 2017, 10:46:13 pm
content: I would just make one. Simply chain a bunch of high ohm resistors together to get the appropriate resistance and voltage rating, then slip a piece of heatshrink tubing over them or arrange them in a zigzag and pot them in wax or epoxy. I've built lots of HV dividers and voltage multipliers and it isn't that hard. Another thing you could do is superglue the cracked hybrid and paint the crack with a bit of conductive paint like the stuff used to repair automotive rear window defroster grids.

author: Lorenzo_1
author_rank: Regular Contributor
timestamp: May 19, 2017, 12:40:02 am
content: Thks for the great advice. Good to know there's a few ways around this.Did some measurements today and it's clear what's happening. Attached photo is my markup of a hybrid advertised as 307-0386-00 - has same pin setup as mine but may not be an -00. Resistance measurements done in circuit but with focus pot leads disconnected. The hybrid is definitely cracked right through - separating the +50V and ground rails from the -3kV supply. With scope on, the Cathode Regulator test point is reading 118V (should be 95+/-5) and the input to pin3 on U1624 is sitting at -5.4V. Cathode Reg pot has no effect on the 118V reading. So I surmise what's happening is that, absent the ground and 50V rails the -3kV supply feed into U1624 is being clamped at -5V by CR1626 (just as well!), and there's no regulation of the 3kV happening via U1624. That would explain the fuzzy and variable focus.I was pondering last night whether, absent a replacement, it might be possible to glue and bridge broken trace on the cracked hybrid as suggested by James. However, the good news just in this minute is that Sphere has in fact got one in stock! Just have to pay the delivery cost from USA, which is always high to Australia. I prefer replacing like for like when I can. Nice to know my re-cap work won't be wasted - I was rather pleased with that. I do like the build quality on this old machine and it's nice to keep it looking good as long as it doesn't break the bank.I'd like to have a go at a glue and bridge repair of the old one, by way of a trial and - if it did work - to keep for a rainy day. Would appreciate a few more details on how precisely to best try that and any pitfalls/problems I might encounter. Are you sure a fix like that would survive the HV load? I don't have the expertise to judge that.

author: james_s
author_rank: Super Contributor
timestamp: May 19, 2017, 01:06:42 am
content: Nothing is ever sure, but I would say that with a bit of care, success is very likely. Those dividers use very low current and the original element is just printed with a special conductive ink. Cyanoacrylate ought to bond well to the ceramic.

author: Lorenzo_1
author_rank: Regular Contributor
timestamp: May 31, 2017, 03:09:21 pm
content: Replacement hybrid resistor arrived today - scope is now working fine and dandy. Cathode regulator voltage back on spec and a nice crisp and stable trace. Thumbs up to Sphere who help us keep these old machines alive.Interestingly, one symptom of impending failure was low intensity on the B-sweep at high sweep rates <10ns, even with B-intensity wound up fully CW. This happened quite some time before the A-sweep went bad. Anyway now have good intensity and focus across the entire range.Turns out the failure was of my own making. When I recapped the scope, one cap close to the hybrid was larger than the original and when I installed the safety cover it was pressing slightly on the unattached end of the ceramic plate. Hence the crack. My plan to try and repair the old one failed too - I discovered that ceramic hybrids and ceramic floor tiles don't get on. The former are remarkably brittle and the latter unforgiving. Two pieces might have been repairable but five pieces was asking a bit much! So much for that. Happy to have the scope back nonetheless. As always - thanks for the advice en-route.