Tektronix 485 scope repair help needed

This is the place for any magazine-related discussions that don't fit in any of the column discussion boards below.
Post Reply
[email protected]
Posts: 2
Joined: Tue Feb 03, 2004 1:01 am
Contact:

Tektronix 485 scope repair help needed

Post by [email protected] » Tue Feb 03, 2004 9:50 pm

Tektronix 485 scope repair help needed. My 485 wont come on. It get s power but nothing lights and no low voltage power on the 5v,15v,60v etc. The high voltage section just ticks. <p>It gets power to inverter board but transistor (diode) on inverter board only gets 13vdc rather than the 32vdc it should get. Rectified DC supply is ok 320vdc. Any help appreciated. Also willing to give a clean copy of my 485 service manual to anyone who can help.

russlk
Posts: 563
Joined: Thu Dec 06, 2001 1:01 am
Location: New Hampshire
Contact:

Re: Tektronix 485 scope repair help needed

Post by russlk » Wed Feb 04, 2004 7:33 pm

Sounds like a simple problem, but I need a schematic. If you can scan the power supply section and email it, I will try to help.
[email protected]

JS
Posts: 19
Joined: Fri Aug 16, 2002 1:01 am
Contact:

Re: Tektronix 485 scope repair help needed

Post by JS » Thu Feb 05, 2004 7:18 pm

I have a Tek 2215 with a similar problem. The ticking sound is the inverter starting up and shutting down. This is what it should do if there is an overload. My problem is also intermittent. I went into it once before and found that everything checked OK as far as values, remelted a few solder pads and put it back together. It ran, and was OK for a few months and now the ghost is back. I think the load side of the supply will have to be isolated and a good clean load applied. This will tell if the problem is in the PS or other circuits. Good luck and if you nail it, post it please. John S.

User avatar
Externet
Posts: 1864
Joined: Fri Feb 01, 2002 1:01 am
Location: Mideast USA
Contact:

Re: Tektronix 485 scope repair help needed

Post by Externet » Thu Feb 05, 2004 8:32 pm

Hi Theremin.
Today I fixed a 485. The symptom was no voltages. A shorted tantalum capacitor on the +15V line at the vertical board caused the whole power supply to self-protect.
There is a few pages in the manual on how to troubleshoot specifically the power supplies, try to follow it. The best procedure is isolating the loads. If that does not revive it, then look for a shorted electrolytic can type capacitor, those vertically mounted.<p>John S:
The 22xx series have a switching power supply that proved a nightmare for me. I have 2 in queue for repair, and are driving me nuts. Tektronix specializes in unreliable underated complex power supplies in their scopes.
The schematic for the 2215 may also be wrong; there is a missing trace in the 43V preregulator that prevents understanding the circuit. One I could repair had an open 10µF electrolytic, it's at the bottom center of that 43V schematic, located near the edge of the board.
Miguel
- Abolish the deciBel ! -

JS
Posts: 19
Joined: Fri Aug 16, 2002 1:01 am
Contact:

Re: Tektronix 485 scope repair help needed

Post by JS » Thu Feb 05, 2004 10:00 pm

JS here, Thanks for the info about the 2215. Yes, the power supply is a dog to work on and TEK revised it at least once. The manual does include the changes. I'll have to get back to this one later. Thanks all.JS

rshayes
Posts: 1286
Joined: Tue Mar 04, 2003 1:01 am
Contact:

Re: Tektronix 485 scope repair help needed

Post by rshayes » Fri Feb 06, 2004 3:15 am

Most switching supplies use a bridge rectifier to rectify the AC power. The negative output of this rectifier is usually the common reference for the primary side circuits of the switching supply. It is not ground.<p>For one-half cycle, this lead is about one diode drop away from the neutral power lead, which is supposed to be at ground potential. One the other half cycle, this lead is about one diode drop from the hot power lead, which reaches a peak value around 177 volts.<p>Most AC powered oscilloscopes have the ground side of the input jack connected back to the safety ground (green wire) of the power source. If you connect this lead to a line powered switching supply, you will probably blow the fuse in the switching supply. If you are lucky, this may be the only damage.<p>One solution to this problem is to use an isolation transformer on the input to the switching supply. This will allow connecting the common lead of the primary side circuits to ground.<p>Most digital voltmeters are battery operated and well insulated and thus do not have this problem. However, troubleshooting a switching supply with only voltage readings is difficult at best. Eventually, you will probably need to see waveforms, which will require an oscilloscope.

Dean Huster
Posts: 1263
Joined: Wed Dec 05, 2001 1:01 am
Location: Harviell, MO (Poplar Bluff area)
Contact:

Re: Tektronix 485 scope repair help needed

Post by Dean Huster » Fri Feb 06, 2004 3:34 pm

"Tektronix specializes in unreliable underated complex power supplies in their scopes."<p>Sorry, Miguel, no hostility intended, but that statement, at least on all Tek scopes manufactured prior to 1985, is pure hogwash. After six years in U.S. Navy PMEL and six years on the bench with Tek, power supply problems were few and far between, ESPECIALLY with the switchers. I've found the switchers in the 485, 7704A, 7904 and other models, all of which have similar circuitry, to be one of the most reliable designs I've ever seen. It's the simple fact that they "self-protect themselves" that they are so reliable. That self-protection is in essence an electronic fuse. 99% of the time, the fault is in some shorted bypass cap somewhere that hogs down on one of the regulated or raw supplies, and the switcher going in to the self-protecting "ticking" mode, protects not only the switcher, but all the rest of the circuitry in the scope from damage. There really isn't much to one of Tek's 1976 SMPS supplies, with most of the electronics contained in a Tek-made control chip that monitors all the supplies for balance, checks for current overload in the primary and watches out so that the supply coincides it's operation with zero crossing of the mains power.<p>Whenever you happen across one, pick up a Tek 7504 or 7704 and compare its weight with that of a 7704A. The "A" is no more than half the weight of the "non-A" and far more reliable. If you want to rag on a Tek SMPS supply design, pick on that of the 434/432 prior to s/n B500000 -- now there was a design (all discrete) from the pit that was later replaced by the newer design after s/n B500000.<p>It's pretty hard to legitimately complain about this particular Tek SMPS design considering how many models used the design, how many were made of each of those models, the conditions under which many of the individual scopes are used and how few problems the SMPS itself actually ever has.<p>If you want to fault Tek for one particular problem, it's their use of transistor and IC sockets and PCB interconnects. Intermittents in Tek solid state scopes made prior to the 2200-series manufacturing philosophy are the most common problem you'll find in Tek scopes. These are problems caused by the very features that have long made Tek scopes easy to work on and troubleshoot. Trouble is, it was that convenience for service personnel that caused most of the problems in the first place!<p>The O.P. originally contacted me directly with the problem and here was my answer, hastily written and I'm sure fraut with typos and other errors!<p>The 485 uses the SMPS that's also used in the 7704A and 7904, a simple (in part count) yet complex (it does lots of things with that control chip) in design. The "ticking" you're hearing isn't the high voltage. It's the switcher. What's happening is that the supply is trying to come up but the control chip is finding an imbalance on the supply indicating (most likely)
that one of the supplies is shorted. The "ticking" is a burst of a few cycles of the normal switching frequency that occur before the switcher decides that there's a fault and shuts down to try it all over again, at a rate of about once every half- to one second.<p>What you can do is get another scope, set to dc coupling and the V/DIV set so that the normal supply voltage on the 485 that you're going to check will remain on-screen (e.g., for a +8v raw supply, you'd set the scope for 2v/div) and adjust the position so that the trace is at the bottom of the screen for a positive supply and the top of the screen for a negative supply. Connect the scope to the RAW supply of the regulated supply you want to check (usually just off the rectifier or on the first filter
capacitor). Turn on the 485. If the supply is working like it should, you should see the voltage pop up anywhere from 50% to 100% of it's normal value. If the supply is shorted, the trace will barely move.<p>If all the raw supplies look like they're trying to come up, make the same test with the regulated supply outputs and see what they do, again looking
for one that isn't trying to come up very far.<p>Alternately, you can use an ohmmeter with the scope off to see if either a raw supply or a regulated supply resistance looks too low for the voltage. Trouble is, this doesn't often tell you much.
<p>Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

User avatar
Externet
Posts: 1864
Joined: Fri Feb 01, 2002 1:01 am
Location: Mideast USA
Contact:

Re: Tektronix 485 scope repair help needed

Post by Externet » Fri Feb 06, 2004 8:31 pm

Hi Dean.
I do not see hostility. That is what a forum is all about, the beauty of it, the interchange of opinions in all directions.
Thanks for your comments and advice; consider me spanked.
I have a 434 and a 2265 to play at home and I am happy, and the entire world respects Tektronix. But with all that fame, still believe Tektronix power supplies stink. And those were the precise words I told to someone from Tektronix who called me at work 2 years ago to ask for my opinion about their products.
I do not fix oscilloscopes for a living, but when work gets slow, I choose to fix the ones at my workplace as a brainwash, fixing scopes is challenging, fun, relatively easy and personally rewarding; and 9 of 10 have power supply problems: that is meaningful. The rest of the circuitry is beautiful fine engineering.
I repaired 7 in the last 2 weeks; but these 2235A are hellish.
Regards,Miguel<p> :) :)
- Abolish the deciBel ! -

Dean Huster
Posts: 1263
Joined: Wed Dec 05, 2001 1:01 am
Location: Harviell, MO (Poplar Bluff area)
Contact:

Re: Tektronix 485 scope repair help needed

Post by Dean Huster » Fri Feb 06, 2004 10:15 pm

Miguel, I'll agree that once Tek started manufacturing the 2200-series and later models, the supplies weren't as easy to fix and had more problems than the old tried-and-true design of the 7704A. The entire concept of the 2200-series is a very interesting story for which the U.S. manufacturing sector owes Tek a lot of credit for saving the low end of the U.S. scope market, no thanks to the other U.S. manufacturers who took the "'not I,' said the duck route. The things that Tek did with the 2200-series to make them low cost without much performance/engineering compromise is really interesting stuff.<p>In some ways, I'm sorry you have that 434, for I consider it Tek's poorest design from a mechanical engineering standpoint. The knob skirt lights are one of the most likely components to fail and are one of the most difficult in the scope to replace, requiring you to nearly gut the instrument. If you ever have to replace them, replace them (ALL of them at the same time) with LEDs and find the appropriate series current-limiting resistor that they installed on the PCB and replace it with the appropriate one for the LED you choose.<p>Most importantly, take the cover off the back of the instrument, get a #2 Posidriv screwdriver, and crank down as hard as you can on every screw that secures the back casting to the frame rails, ESPECIALLY the one behind the line fuseholder. If that danged thing gets loose, it plays havoc with the mains connections -- been there, done that more than once. I have yet to see a 434 that I got in the Service Center for first time service that didn't have loose screws back there.<p>If you ever have to remove an PCB, consider replacing the screws that secure the board with steel 3-48 screws so that a magnetic screwdriver can help you with those that are hard to reach. Tek really screwed up when they veered from their normal use of steel 4-40 PCB mounting screws .... don't know what they were thinking.<p>If you ever have any gain inaccuracies, the first thing to blame are those little low-value, carbon composition resistors in each vertical preamp. Tek normally did most of the v/div changes using attenuator modules that were switched in and out; the 434 does gain switching for two decades and then kicks in a divide-by-100 attenuator for the next six gain switches. If you replace one or more of those resistors to fix a gain problem, I'd suggest replacing them all with the nearest standard value metal film resistor for better stability. About the only resistor that has worse stability than a carbon composition resistor is a thermistor!<p>On the subject of carbon comp resistors, anyone who has a 2213 or 2215 (not 2213A or 2215A) that they can no longer focus, the like culprit is a string of 510K ohm, 1/2-watt carbon composition resistors in the focus portion of the CRT circuit. Replace them all with the same value/size carbon film resistors and the problem will be solved. For some reason, those original resistors that Tek used will shift in value, always going high to values anywhere from 600K, to 800K, to 1.4M to greater than 20M! Weird. Don't know if it was a major batch problem or not, but I've fixed a bunch of those scopes with that exact problem -- the focus control doesn't have enough range to focus the beam.<p>Anyway, back to the 434. The 434 is a light, sweet little split-screen bistable storage scope that from an operator's standpoint, is a lot of fun to use and quite versatile. It's a shame they never made a similar model with delayed sweep and 100MHz bandwidth without the exotic storage capabilities of the 464 or 466 in the storage department.<p>Maybe my hints for the 434 will soften the coarseness of my previous post!<p>Dean<p>[ February 07, 2004: Message edited by: Dean Huster ]</p>
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

User avatar
Externet
Posts: 1864
Joined: Fri Feb 01, 2002 1:01 am
Location: Mideast USA
Contact:

Re: Tektronix 485 scope repair help needed

Post by Externet » Sun Feb 08, 2004 4:37 pm

Hello Dean.
You do not have to soften any non-existing coarseness. Real opinions belong to real life.<p>Thanks for your list of suggestions above; it is now printed and filed in the event the Tektronix 434 fails some day. I bought it in shiny sparkling dead condition for $20 three years ago, fixed its... POWER SUPPLY and is happily working since.<p>My permanent respects,
Miguel :)
- Abolish the deciBel ! -

[email protected]
Posts: 2
Joined: Tue Feb 03, 2004 1:01 am
Contact:

Re: Tektronix 485 scope repair help needed

Post by [email protected] » Wed Feb 11, 2004 2:54 pm

hi Miguel,<p>you are right, removing the vertical board the scope comes up fine and all voltages present. I checked the tantalums in circuit (about 15)<p>Any tips on checking caps in circuit? My cap checker only measures to 20uF and shows open when i connect the test leads together so if its over 20uF or shorted it won't show up with the tester.<p>Tony

User avatar
Externet
Posts: 1864
Joined: Fri Feb 01, 2002 1:01 am
Location: Mideast USA
Contact:

Re: Tektronix 485 scope repair help needed

Post by Externet » Thu Feb 12, 2004 8:27 am

Hi Tony.
The troublemaker tantalum filter capacitor in that board is centered at the edge of the board; I do not have the schematic with me now, but remember being elusive at extreme left on the schematic. Replace ONLY that one first and try. You can install a plain electrolytic instead. Post findings.
Miguel
- Abolish the deciBel ! -

Dean Huster
Posts: 1263
Joined: Wed Dec 05, 2001 1:01 am
Location: Harviell, MO (Poplar Bluff area)
Contact:

Re: Tektronix 485 scope repair help needed

Post by Dean Huster » Fri Feb 13, 2004 9:16 pm

I'd suggest installing the aluminum electrolytic (nearly any value that's even close will do) as a substitute for testing only. The final fix should be replacement with another tantalum. Tantalums handle higher frequencies and transients better than the aluminum electrolytics and in a 350MHz scope, that's important.<p>As a matter of fact, just for finding the bad cap, just disconnect one end of the suspect and turn the scope on to see if the supply comes up. Won't hurt anything and there'll be less posibility of damaging the board with swapping the cap out a couple of times with a temporary replacement. Tantalums are harder to find in this world. If you need one, just let me know the value and the voltage. I have millions of them. OK. An exaggeration. Thousands.<p>Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

User avatar
Edd
Posts: 885
Joined: Sun Jan 27, 2002 1:01 am
Location: Dallas Tx
Contact:

Re: Tektronix 485 scope repair help needed

Post by Edd » Sun Feb 15, 2004 1:14 am

Tony:<p>Relevant to your comment:
<<My cap checker only measures to 20uF and shows open when i connect the test leads together so if its over 20uF or shorted it won't show up with the tester.>><p>I have a little mod for you….or other owners….that might make that unit quite a bit more functional for you on its cap ranging.
That units 20 ufd top cap range sort of reminds me of the specs on multiple function DVM’s that entice you to their units with all sorts of extra features, all the way up to temp probes or…….checking your pools Ph !
I carry a portable 3¾ Digital LCR unit with me off site ,out of the lab, which usually suffices for my needs, as I am usually more interested in L constants and capacitance within the 1µfd-100µfd range. On my particular unit, the capacitance range tops out at 200µfd. What I have done is taken a male banana plug and soldered to a 10 Ω resistor which then terminates to a female banana jack at the other end, thus leaving the resistor enclosed in the plastic shell that is between the two connectors. Ultimately, this places the res in series with the instruments positive test lead. When I have need to confirm a cap value above that 200µfd value, all the way up to~4700µfd, I just wait ‘til last to test those high values and then pop out the positive test lead and plug this adapter in line. I then take a reading and compare that figure to a correlation chart that I have placed on the back of the instrument. Those readings for common different EIA values of capacitance, were previously ascertained by test of confirmed values and then listed on the chart as the reference readings.
Since your readings and the resistance would exhibit a variance for the 20µfd range, I had suspected >10Ω but<100Ω and opted for a 47Ω res as a test value and found that the readout was:
33µfd----15.3
47µfd----13.5
100µfd--9.95
150µfd--7.95
220µfd--6.5
330µfd--5.95
470µfd--5.21…for tested capacitor values /vs/ the meter readings
[These were just for a quick example of the reading variances, make your own chart custom to your specific equipment and selected R value]<p>Therefore you can see with that value you could have an extension of on up to ~470µfd using the modified mode. With a lower, ~33Ω value there might be enough reading differentiation to stretch on up to the 1000/2200µfd end.
(Forget the thought of a precisely selected R value to extrapolate an even decade shifting of the readout, for , as the numbers show, it is a reverse logarithmic scaling)<p>Overall, you would potentially have a much more useful cap meter function.<p>73's de Edd
[email protected] .........(Interstellar~~~~Warp~~~Speed)
[email protected].........(Firewalled-Spam*Cookies*Crumbs)<p>#6 ON THE LIST OF THE TOP 12 THINGS YOU DON'T WANT TO HEAR FROM TECH SUPPORT
<<"We CAN fix this, but you're gonna need a butter knife, a roll of duct tape and a car battery.">>
:D :D :D

Post Reply

Who is online

Users browsing this forum: No registered users and 36 guests