Isolated Variable Transformer? - Page 1
Forum section: electronics / beginners

author: FourthDr
author_rank: Newbie
timestamp: February 28, 2023, 03:40:54 pm
content: Hi:First post on this forum. Recently acquired a non-working Tektronix 465 scope. Found thishttps://www.eevblog.com/forum/repair/170-tektronix-465-review-and-troubleshoot-part-1/useful info with two videos. In the first video, an isolated variable transformer is mentioned, but no details. I don't have one of these and was wondering if anyone can recommend one? So I can work on the scope without damaging it or myself.Regards

author: Terry Bites
author_rank: Super Contributor
timestamp: February 28, 2023, 05:12:12 pm
content: Ordinarily you'd feed a 1:1 isolation transformer from a variac. Lots of options.Be aware isolation transformers only protect you against shocks from the hot side of the line to mains ground.The output of an isolation transformer will happily kill you- though not through a path to ground.Read up on the risks an benefits of isolation transformers. Make sure you understand before switch-on.HV dangers from internal circuits in your scope (or whatnot) are not defeated by isolation transformers.

author: coppercone2
author_rank: Super Contributor
timestamp: February 28, 2023, 05:14:45 pm
content: fooling around with isolation transformers is like banging a wrench against the wall in alien isolation

author: bdunham7
author_rank: Super Contributor
timestamp: February 28, 2023, 06:02:49 pm
content: For reasons I won't go into anymore, you will get all sorts of weird comments and warnings you didn't ask for. I don't know that you actually need a isolation transformer to work on a 465, but if you decide to get one, I use a BK Precision 1655. The 1655A is a current model and is about $600, the 1655 was the previous USA-made model that you can get for about $200 if you are patient. There is also a much smaller 1653 model.Sencore and Tenma also made similar products, but I'm not familiar with them.

author: james_s
author_rank: Super Contributor
timestamp: February 28, 2023, 06:19:06 pm
content: I can't imagine what you'd actually need one for if the goal is fixing the scope. The 465 uses a linear power supply with a transformer that inherently provides isolation. There is nothing on the primary side that is likely to require anything more than a multimeter to diagnose.

author: Gyro
author_rank: Super Contributor
timestamp: February 28, 2023, 07:27:43 pm
content: Quote from: james_s on February 28, 2023, 06:19:06 pm... or indeed connected to the mains. Checking the primary side can be done just with resistance measurements.Be very careful bringing the 465 up with a variac, it is very easy to cook its transformer without blowing the rear panel fuse. I nearly cooked the mains transformer on a 475A (virtually identical PSU) while bringing it up slowly that way a few weeks ago. The transformer was giving off a very unpleasant smell before I managed to get around all of the supply rail measurements! Luckily for me, it survived, but I didn't deserve to be that lucky!https://www.eevblog.com/forum/repair/tektronix-475a-issue/msg4666933/#msg4666933Your first checks should be for shorts on the DC supply rails and rectifiers (unpowered).

author: bdunham7
author_rank: Super Contributor
timestamp: February 28, 2023, 07:32:19 pm
content: Quote from: Gyro on February 28, 2023, 07:27:43 pmOne of the very handy features of the BK 1655 is the switchable meter that selectably reads voltage, current and leakage current. I'll know right away if I have a shorted rectifer or something.

author: FourthDr
author_rank: Newbie
timestamp: March 02, 2023, 04:47:35 am
content: I think it's strange that so many people recommend such an out dated power supply. Even the manufacturer hasn't noticeably changed the design in the most current version of the B&K Precision 1655A. Why anyone would still need a variable output for a soldering iron boggles the mind. And the gall to charge $700! That's more than the scope is worth. Are there any cheaper options? Since I don't see myself getting that much use out of a BK, I can't really justify paying that price. I mean how much does a few 1:1 copper winding cost? And no digital readout? Come on BK! This isn't the 1960's!

author: bdunham7
author_rank: Super Contributor
timestamp: March 02, 2023, 04:56:10 am
content: Quote from: FourthDr on March 02, 2023, 04:47:35 amIt works well and there aren't many good alternatives that I know of. Yes the new price is a bit steep, but as I said, for under $200 used it rates as a pretty useful tool, retro or not.QuoteThat's a strange comment from someone restoring a Tek 465!

author: FourthDr
author_rank: Newbie
timestamp: March 02, 2023, 10:14:05 am
content: Not really. With the Tektronix scope I know it is 40+ years old. But paying full price for any new piece of test equipment, I expect something more than a stale decades old design. I'm surprised there aren't more choices out out there. I mean how much does two transformers and a few diodes cost? Certainly not nearly $700 after tax +shipping.

author: FourthDr
author_rank: Newbie
timestamp: March 02, 2023, 10:27:14 am
content: Had a quick look on ebay, and some one is trying to sell the original version of the B&K used. Except it's all rusty and of unknown condition for over $400 and they want an extra $23 for shipping + tax.Who know how long that thing was sitting in dank damp basement. Seem like it will be some time before I could manage to find a decent one for anything near $200.

author: jonpaul
author_rank: Super Contributor
timestamp: March 02, 2023, 12:37:23 pm
content: You,are getting ahead of the game.Before any isolation , Variac,or debugging with a,scopebeware the 1970s Tektronix scopes 465/475 typically fail in 3 ways1. High Voltage transformers, multiplier or resistors2. LV psu3. scratches switches., controls.suggesta. get,full service manual free pdf on w140, tekwiki or printed version off,epay/ Artechb. send us photos of,sx0cope front and rear and traces showing up the issueBon couragej

author: FourthDr
author_rank: Newbie
timestamp: March 02, 2023, 02:08:54 pm
content: You're Probably right. But that's just how I am. I've already done some research on the BK unit and found there is a Global Specialties 1420 which is a re-branded BK and identical. And can be had cheaper. I also found the manual and full schematic. In case I get a used one and need to repair it. So I'm all set once I see an opportunity to get one on sale or a cheap used one.I haven't cracked open the scope yet. So far I have only researched what's involved with a repair and common problems. My 465 is missing the feet used to wrap the power cord onto. So I found a 3D model of the feet to print replacements in either PLA or ABS.I have powered it on once just to see what it does. Or in my case, what is doesn't do. I get power lights, and intermittent trace on the screen. However, switching settings the display will go out and it's not displaying the calibration signal correctly or at all. So definitely contacts and power supply I would say without opening it. Seems to have many of the same symptoms as the one in the linked video from this forum.I'll post some pics once I have a chance to get inside the unit.

author: jonpaul
author_rank: Super Contributor
timestamp: March 02, 2023, 04:58:47 pm
content: Simple testopen coverget VOM on DCVtest every DC PS voltagetest AC rippleFinally suggest that you hold off on Variac, isolation, BK, these scopes are only worth $20,,50 topsJon

author: james_s
author_rank: Super Contributor
timestamp: March 02, 2023, 07:02:40 pm
content: A good working Tek 465 is worth a lot more than $20. My friend sold his about a year ago for $120 and it sold within a day of posting it. I don't think I've ever seen one go as cheap as $50, typically average condition they've been around $100 for quite a few years.

author: tooki
author_rank: Super Contributor
timestamp: March 02, 2023, 07:32:27 pm
content: Quote from: FourthDr on March 02, 2023, 10:14:05 amThe main use for these things is for repairing hot-chassis devices. We stopped making those half a century ago. So there is very, very little demand for such things. Why go to the effort of designing an updated, “modern” one for which there is no market? You’d never recoup the development cost…As for the soldering iron control: yes, totally irrelevant now, but the reason for keeping it could be something as simple as that removing it might be a design change that invalidates the UL certification, and they sell so few that the cost of recertification would exceed the parts savings.Quote from: FourthDr on March 02, 2023, 10:14:05 amHave you seen the cost of copper lately? Transformers are only going to get more expensive.Plus, it’s also a piece of test gear, not just a transformer.

author: FourthDr
author_rank: Newbie
timestamp: March 02, 2023, 09:43:03 pm
content: JonPaul:Thanks for the w140 tekwiki link. That's a great resource I didn't know about. Very comprehensive. Although, I have already obtained all of the relevant user manuals and service manuals with full schematics from alternate sources.

author: FourthDr
author_rank: Newbie
timestamp: March 02, 2023, 10:29:57 pm
content: Tookie:True that you don't see many hot-chassis devices. But it also has a handy leakage tester. And even if the chassis isn't hot it could become hot under some conditions. If nothing else, peace of mind.

author: Electro Detective
author_rank: Super Contributor
timestamp: March 03, 2023, 12:44:21 am
content: Make yourself a dim bulb tester to save the oscilloscope in case of a sudden 'BANG'rather than dimming it up with a variac and risk frying somethingor put a cheap glass fuse rated for the max current draw of the scope, in the line somewhere safeand let that blow if there's a dramacheap fuses are easy to troubleshoot btwand cheaper to replace than hitting on Ebay for scope partsLike the other commenters stated, check if the scopes transformer Live (1) and Neutral (aka Live 2) is isolated from the mains with a multimeter,but don't count on the Ground/Earth and chassis connectors to be isolated from the mains Neutral (aka Live 2)compliments of the Neutral and Ground/Earth Link on most mains switchboardsNot forgetting gloves, eye shields, rubber sole shoesor boutique flip flops as seen on Youtube fixit channels etc = cheap isolation

author: james_s
author_rank: Super Contributor
timestamp: March 03, 2023, 01:52:20 am
content: Quote from: tooki on March 02, 2023, 07:32:27 pmThere's a fair bit of mechanical construction too, and it's probably a quite low volume specialist item. The lower the volume, the less you can take advantage of economies of scale and the higher the price. If everyone and their mom needed one of these things they would probably be $100 or less.

author: jonpaul
author_rank: Super Contributor
timestamp: March 03, 2023, 11:16:40 am
content: Check schematics and sev manual SN vs your unit, many major revs orcer the long life of these!Jon

author: tooki
author_rank: Super Contributor
timestamp: March 03, 2023, 11:36:37 am
content: Quote from: FourthDr on March 02, 2023, 10:29:57 pmWell yeah, but from your feigned outrage at the price, it wasn’t clear thatyourealized it was more than a simple variac.

author: jonpaul
author_rank: Super Contributor
timestamp: March 03, 2023, 12:22:34 pm
content: ISOLATION: Most TE needs are modest perhaps 30..150 VA ~ 1A on 120v.Cheap solutions: Find y PAIR same rating of mains>< LV fil/DCPS trsfe.g. 120V:12V <> same in reverse 12V><120V = zero cost isolationVariac: Cheap Chinese avail ~ $25..50 eaCkt: Mains><TR1><TR2><VariacORMains><Variac><TR1><TR2.Jon

author: FourthDr
author_rank: Newbie
timestamp: March 08, 2023, 02:03:12 pm
content: [ Specified attachment is not available ]I'm wondering if at this point I should start another thread?I finally had a chance to open the case. after looking around I didn't see any of the usual suspects like separated resistor leads, cold or missing solder joints or leaking caps. The the semi's all look in good shape.The two power transistors that mounted on to the chassis look like a newer type than the ones used in Radio Tech's scope that had separated leads. So maybe those are ok.I did notice that the design of the sockets that the removable transistors are in is different. Instead of having a socket body above the board, it uses tiny individual sockets that fit in the circuit board's component holes. At first I thought this scope didn't have removable transistors. But after looking more closely they are in fact socketed.After re-seating a few transistors used in the low voltage section I'm getting a thick beam across the display. Stays that way on all ranges, with or without the calibration signal connected. I checked the 110v and seems a little low by a few volts. Any guesses? See pic.

author: jonpaul
author_rank: Super Contributor
timestamp: March 08, 2023, 02:18:19 pm
content: Suggest you start a new thread in REPAIR, title like Tektronix 465 repair and also post at Tek scopes and tekscopes2 @ groups.io, many very experienced Tek vintage mavens and even retired Tektronix folks.Your scope photos show good HV, CRT, but scratchy CRT controls or adjustment.Suggest that you open up HV cage (scope OFF, wait a few hrs)Find focus, inten, astig HV pots and trimmers.Spray into the control pots and trimmers a good quality control cleaner/ lubeExercise controls, note trimmer position, exercise and return to the original positionAllow cleaner to dryreplace HV cagetry againjon