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475 questions
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Hi, all! First post here. As I'm sure will soon become obvious, I am new to electronics, though I have completed a few minor repairs and projects over the years. I have never used an oscilloscope. I am retired, and have lots of time to pursue this new hobby. I just bought a couple of non-working (according to the seller) scopes--a Philips PM 3218 (irrelevant, I know) and a Tektronix 475 Mod. OS261C Opt. 4, 7 Ser.#B287863. I say according to the seller, because I have been afraid to power up the 475 until I know more about it and its condition. I don't think the seller had had it on for quite some while, either. My research so far has told me that the 475 is the one perhaps more worth bringing back to life. I haven't read every post referencing the 475, but I have read a LOT of them. Also, I have the 070-1832-00 Tektronix 475 Service Instruction Manual in .pdf. Anyway, here are some questions that I hope one or some of you will be able and willing to help me out by answering:
Why are there three power settings on the back by the fuse that overlap? I measure ~ 118V at my wall receptacle, so it appears that any of the settings will work equally well. Someone posted that the test point ohm readings should be the following (my repeated results with two different analog meters follow in parentheses): (test point = expected ohm reading) 110 = 11K (18 climbing from < 10) 50 = 2.7K (2.7) 15 = 63 (0) 5 = 46 (0) -15 = 480 (19) -8 = 32 (0) UNREG 50 = 14K (7.5) 105 = 12K (16 rising from < 10) no test post found to match the others, but I was able to find the trace with that label and take a reading from its junction with a large cap lead. It would appear that there is so much wrong based on these readings that maybe the $20 I paid was no bargain. However, I once diagnosed a Peugeot 504 Diesel electrical problem that was extensive and affected almost every electrical circuit. I found one melted trace in the instrument panel that, once bridged with a short piece of wire, solved every electrical problem in the vehicle. So, I remain optimistic that perhaps a single capacitor (or a few) might correct much of this. On the other hand, I've also read that some key parts may no longer be available, and if it needs a new CRT, I'd guess it's a lost cause. What would anyone else surmise from these readings? Where did the expected readings come from? I found no such reference in my manual. Neither glass fuse that I found was blown, but under the aluminum "Warning High Voltage" panel, there was evidence of magic smoke having been emitted onto the white Sprague capacitors and the two glass neon bulbs DS1382 and DS1383. What the heck are those for under a panel? Those items, themselves, do not visually appear to be damaged. I see no other components that appear to be the source of the smoke, either, though. How does one access the large electrolytic capacitors? They are measurable from the bottom of the board, because they rise from that upward, sandwiched between the bottom and top boards and between the right board and the CRT. My manual provides directions for board removal, but as others have said, it is no easy process. I'm hoping there's a shortcut. Unfortunately, my ESR meter does not measure capacitors with such a high capacitance rating. Also, it does not measure capacitors with such a low capacitance rating as those Spragues under the HV panel. I've read over and over that the tantalum caps should be replaced, but nearly all those I can read show themselves to be within spec. when read in-circuit with my ESR meter. Do I need to tear this whole thing apart to replace all the questionable caps? I'm mechanically inclined, so I'm not necessarily averse to it if so. Finally, I just read that there are white dummy resistors (I count six that appear to meet this description) that can be unsoldered to isolate portions of the board so as to facilitate diagnosis. Where can I find more information about these? Thank you very much in advance! Bruce |
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Hi Bruce:
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I have found that resistance checks are not a good measure of the reliability of any instrument.? Here is what I would do.? Get a DC voltmeter turned on, connect the ground lead to the chassis and put your hot lead on the +50 TP and turn the scope on.? Look immediately at the voltmeter if it says 50V great, power off immediately.? Next move your probe to the next lower voltage and repeat test, continue until all the low voltages have been tested.? Next turn the intensity up and look for a beam or light on the screen,? If you see a crazy collection of lines on the CRT you probably have an open filter cap in the low voltage circuit.? Try putting a 200uF cap from the center terminal to ground of each filter cap.? What ever you do DO NOT REMOVE THE MOTHER BOARD.? It is a pure PITA to get out and even worse to get back in.? Good luck! Reed Dickinsonreed714@... On Wednesday, May 20, 2020, 09:53:20 PM PDT, ciclista41 via groups.io <ciclista41@...> wrote:
Hi, all!? First post here.? As I'm sure will soon become obvious, I am new to electronics, though I have completed a few minor repairs and projects over the years.? I have never used an oscilloscope.? I am retired, and have lots of time to pursue this new hobby.? I just bought a couple of non-working (according to the seller) scopes--a Philips PM 3218 (irrelevant, I know) and a Tektronix 475 Mod. OS261C Opt. 4, 7 Ser.#B287863.? I say according to the seller, because I have been afraid to power up the 475 until I know more about it and its condition.? I don't think the seller had had it on for quite some while, either.? My research so far has told me that the 475 is the one perhaps more worth bringing back to life.? I haven't read every post referencing the 475, but I have read a LOT of them. Also, I have the 070-1832-00 Tektronix 475 Service Instruction Manual in .pdf. Anyway, here are some questions that I hope one or some of you will be able and willing to help me out by answering: Why are there three power settings on the back by the fuse that overlap?? I measure ~ 118V at my wall receptacle, so it appears that any of the settings will work equally well. Someone posted that the test point ohm readings should be the following (my repeated results with two different analog meters follow in parentheses): (test point = expected ohm reading) 110 = 11K (18 climbing from < 10) 50 = 2.7K (2.7) 15 = 63? ? (0) 5 = 46? ? (0) -15 = 480? (19) -8 = 32? (0) UNREG 50 = 14K? (7.5) 105 = 12K? (16 rising from < 10) no test post found to match the others, but I was able to find the trace with that label and take a reading from its junction with a large cap lead. It would appear that there is so much wrong based on these readings that maybe the $20 I paid was no bargain.? However, I once diagnosed a Peugeot 504 Diesel electrical problem that was extensive and affected almost every electrical circuit.? I found one melted trace in the instrument panel that, once bridged with a short piece of wire, solved every electrical problem in the vehicle.? So, I remain optimistic that perhaps a single capacitor (or a few) might correct much of this.? On the other hand, I've also read that some key parts may no longer be available, and if it needs a new CRT, I'd guess it's a lost cause.? What would anyone else surmise from these readings?? Where did the expected readings come from?? I found no such reference in my manual. Neither glass fuse that I found was blown, but under the aluminum "Warning High Voltage" panel, there was evidence of magic smoke having been emitted onto the white Sprague capacitors and the two glass neon bulbs DS1382 and DS1383.? What the heck are those for under a panel?? Those items, themselves, do not visually appear to be damaged.? I see no other components that appear to be the source of the smoke, either, though. How does one access the large electrolytic capacitors?? They are measurable from the bottom of the board, because they rise from that upward, sandwiched between the bottom and top boards and between the right board and the CRT.? My manual provides directions for board removal, but as others have said, it is no easy process.? I'm hoping there's a shortcut.? Unfortunately, my ESR meter does not measure capacitors with such a high capacitance rating.? Also, it does not measure capacitors with such a low capacitance rating as those Spragues under the HV panel. I've read over and over that the tantalum caps should be replaced, but nearly all those I can read show themselves to be within spec. when read in-circuit with my ESR meter.? Do I need to tear this whole thing apart to replace all the questionable caps?? I'm mechanically inclined, so I'm not necessarily averse to it if so. Finally, I just read that there are white dummy resistors (I count six that appear to meet this description) that can be unsoldered to isolate portions of the board so as to facilitate diagnosis.? Where can I find more information about these? Thank you very much in advance! Bruce |
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Leanna L Erickson
Test points
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Equipment must be energized. PS caps Can be a bear to replace without removing board. Been there, done that. A hard copy of SM works out best. Good Luck, Keith, Wayzata, MN I have brought back several. On May 20, 2020, at 11:53 PM, ciclista41 via groups.io <ciclista41@...> wrote: |
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Thanks for the helpful reply, Reed!
Do you think it's safe to turn the scope on with all those test points showing as shorted? I don't want to destroy anything more than already possibly has been, and in my admittedly limited experience, a short means things go POP! I'm going to put a 100 watt incandescent light bulb in series with the hot line of my electrical receptacle as a voltage limiter to reduce the likelihood of frying anything in the few seconds it will take to do the measurements as you suggest. I like the idea of using the 200 μF capacitor to see if it resolves some problems...or are you suggesting adding them in parallel with the filter caps that are so inaccessible as a permanent fix? |
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Thanks Keith! I'd like to get a hard copy if I'm keeping the scope, but if I discover it's beyond worth reviving, I don't want to make the investment. It's encouraging to hear that you've brought back several! Sounds as if you actually did replace those big caps without removing the board. I'd like to know how to do that, unless it's more trouble that removing the board to do it!
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Leanna L Erickson
PS capacitors
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Figure on replacing some of the full wave bridge rectifier, cut them out. Use a Weller old style soldering gun. Use solder wick or sucker to remove a lot of solder, gently and forcibly use a forceps or similar device and pull, rocking back and forth. Note the connectivity of associated traces. You have to maintain continuity of the ground lug connection. Replacement of cans will use something other than the cans, you may need to be creative. Same routine appies to the 465. Keith On May 21, 2020, at 9:26 AM, ciclista41 via groups.io <ciclista41@...> wrote: |
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Thanks, Keith! Appreciate the rectifier replacement info!
Now that I look again at the large caps, it appears to be relatively easy to replace them without removing the board. Don't know what I was thinking. Since they are electrolytic, I'll just have to mark them for polarity and location so I get it right with new ones. I can solder long leads on the new caps to make it easier to fish them up through the holes in the board, then trim them off again once they're through. Even if smaller, I think I can shove them up in there and get the leads to go where they belong, and the bottom side of the board is easily accessible for soldering. I have a Weller gun 140/100 watt depending on trigger position. I also have a Yihua 8786D. Are you saying the Yihua won't have enough thermal mass for the job? Paul of Mr. Carlson's Lab on YouTube says the Weller is great if you replace the tip with a short piece of bare #14 copper. Will even solder to a chassis with that setup. I'd have thought the lack of control compared to the Yihua would disqualify the Weller for the rectifier job, though. |
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Hi Bruce,
Please note that your +15V, +5V and -8V zero ohms measurements can *not* be due to shorted big filter caps. Also the -8V regulator needs the +15V and +5V to be up. So I think the "one at a time" approach as suggested won't be of much help. You probably used a DMM for measurements. Large leakage currents might look like a short with a DMM. I would recheck for "true" shorts using an external DC supply with voltage 0.5 V (below a diode voltage drop) and current limiting. Albert |
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Leanna L Erickson
Cap failures may have take the rectifiers out anyway.
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I am not familar with the Yihua. On May 21, 2020, at 11:40 AM, ciclista41 via groups.io <ciclista41@...> wrote: |
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Bruce,
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Did you try reversing the probes on your ohmmeter? There are reverse biased diodes on each of the regulated supplies outputs, (excepting +110v), that may be forward biased by your ohmmeter and appearing as a short. Dave Someone posted that the test point ohm readings should be the following (my |
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Hi Albert,
Thanks for replying! I only used an analog multimeter to take the readings, and I verified them with another analog, so am confident in the values I got. However, the value of resistance readings has been called into question, anyway. I put together a voltage-limiter (150W incandescent bulb in series with the hot lead of my wall receptacle, which greatly increased my confidence that turning the scope on would not do further damage. So far, so good! With the scope turned on, the filament glows dimly, so I probably should run a 200W bulb, but I don't have one handy. Knowing that the scope would run without smoke or pops, I took readings with the voltage limiter and then plugged directly into the wall, with the latter readings in parentheses. Yes, a larger bulb is in order! Checking voltages at the same test points that I got the resistance readings from, I got: 110 = 87 (91) 83% 50 = 37.5 (40) 80% 15 = 10 (10) 67% 5 = 3.5 (3.8) 76% -15 = -14.2 (14.5) 97% -8 = -5.7 (6) 80% UNREG 50 = 40 (47) 94% 105 = 7.5 (9.5) 9% The scale intensity seems to work fine, but I get no trace at all. I put my probe in Ch. 1 and put the negative to chassis ground and positive to the calibrator, but still got nothing. Pushing the beam finder button didn't help, either. Same story with Ch. 2. All indicator lamps on the front seem to be working fine. Fan works fine. Nothing seems to be getting hot. Time to start desoldering, I guess. |
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On Thu, May 21, 2020 at 11:40 AM, <ciclista41@...> wrote:
I have a Weller gun 140/100 watt depending on trigger position. I also have aBe very careful using that Weller 110/140 as Mr. Carlson describes. That thing will put a lot of heat into the board and traces. I use my Weller WES51 and Vacuum solder station to remove these often. No need to stress these old boards any more than is absolutely necessary. It may have already been mentioned, but remember that the cans of these caps also connect the various ground points on that board. You will need to provide some means of reconnecting these points together when using standard radial caps. I personally use those little round PC boards that can be found on E-Bay. I also make some small brass bushings to fill the large holes on the board and provide a good solder connection. These make the job easier and they also provide the necessary ground connections as well. Just a suggestion. Here is the E-Bay # for these kits. . . . 273254508468 I have bought from this seller and have no other association with them. Just a satisfied customer. Good luck. -- Michael Lynch Dardanelle, Arkansas |
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Hi Bruce,
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I wonder if you would again read 0 Ohm now. Maybe some caps reformed when you powered the scope up. The combination of a slightly low +110V and a way too low +105/+160V is strange. The +110V should be pulled down by CR1489. This 475 seems to be your first scope. Otherwise the advice would be to check the wave forms in stead of only DC measurement. You could still do AC ripple measurements (check that your meters block DC). Normally ripple is in the mV range (except perhaps for +110V). You will probably find some very excessive values. Albert On Thu, May 21, 2020 at 09:38 PM, <ciclista41@...> wrote:
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Hi ciclista41@
You are bringing this old scope to life, and you correctly start by evaluating its power supplies. What is very useful to have for this step, is a variac (a variable autotransformer) that allows you to increase the AC supply voltage practically from zero This is a good precaution when not knowing what can be wrong, and what could burn out. Your scope has a linear regulator, meaning that after the input transformer and rectifiers, the unregulated supplies will rise proportionally with the AC you apply with the variac. If you are not in a hurry, you can then rise the AC in steps, like 25%, 50%, 75% or finer coming close to the 110 VAC, over a period of a day or so, to give an opportunity for the electrolytic capacitors to regenerate. While you come up to the nominal voltage 110 VAC, you can watch the voltage on the +50V, which is the "master" reference for the other supplies. You can check that at some point it stabilizes, regulates, and verify that it is 50 V If it does not reach 50 V, then you may have a failure in it, and this will be reflected in all the other supplies. The failure can be in the supply itself, or in its load (the rest of the instrument) I don't know how much you can disconnect the power supplies from the rest of the instrument, and if you can, this helps to limit the extent of the failure to the supplies themselves, and not to some short elsewhere. Ernesto |
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Hi Albert,
Yes, I re-read them just after turning the scope off, before my last reply, and I re-read them just now. All three are shorted regardless of the polarity with which I measured them. So it's surprising to me that the board doesn't pop, get hot, or release magic smoke when I run it. I was guessing I'd have lots of ripple with bad capacitors, but I really don't know if they're bad or how bad they are. Currently, no meter I have measures such high μF. Guess I'd better find one. Just a month or two ago, I was checking the start and run capacitors on my roof-mount air conditioner. We Arizona desert dwellers do not want the air conditioner to quit during our summers. That can be deadly if we don't move out temporarily until it can be fixed or fix it immediately. They're predicting 108°F for this coming Wednesday, so the season is just about here. Although all capacitors measured within spec, I still bought replacements to have on hand. The only one I could not check was the compressor start capacitor, because it is rated higher than my meter will go. Another reason to find a better meter. I bought two non-working scopes at once, so although the 475 is not necessarily my first scope, neither do I have another scope that works. I'm tempted to buy a new digital scope, but I might wind up disappointed in one that I could justify the expense for. Maybe not. I haven't done enough research to take that plunge yet. And if I get this one working, I will find it even more difficult justifying such an expense. |
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Not "if" but "when" you get it working. You're over half-way there!
Thanks, Barry - N4BUQ Hi Albert,<snip> I bought two non-working scopes at once, so although the 475 is not |
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Hi Ernesto!
Thanks for the excellent advice! I have been looking at new and used variacs with that in mind. However, after I built the incandescent bulb current limiter this morning, I felt that I could probably get away with turning on the scope, which I did without any (to me) known negative consequences. I let it run for several minutes. The bulb just glowed faintly. I would have preferred that it glow even more faintly or not at all, but I did not have a larger wattage bulb than a 150. My guess is that for this scope, 200W would be the ideal bulb. After seeing no ill effects, but all voltage readings low, I shut it off and replugged it into the wall receptacle. It ran fine (no trace, but nothing seemed to pop, get hot, or release smoke) for the half hour or so that I ran it. The voltage readings I took when it was plugged into the wall rose with the bulb removed, but did not reach spec. The measurements I got are recorded in my post #167295 below. My 50V didn't get above 40, so only 80% of what it should have been. Yes, I wish I knew how to isolate the problem(s) better. At this point, I think I'm just going to start changing out old capacitors and rectifiers. |
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Bruce,
Congrats on the scope! A few years ago I came in here barely knowing what a scope was, how it worked, or even what it was supposed to do. I had acquired a non working 475 from my father who had recently passed. It sat in the corner for a year with me not knowing what to do with it but not feeling quite right about tossing it out. After one day when my computer monitor quit and I was successful in reviving it with a couple of cap replacements I decided to go after the scope. The folks in this group helped me get mine going. What I'd suggest is reading through the service manual and studying the diagrams. I did have enough knowledge of reading diagrams and with help from the great people here I had my 475 working. So, if I can do it, you can do it. I would strategically test the power supply circuits and troubleshoot them in the order suggested by those here and the service manual. I wouldn't just throw caps in it just randomly. After I got my scope working I did replace some tantalum caps that were known for failure but only after I had it working and just for preventative maintenance. You always risk damage to the board any time you effect a repair. I'm not good enough at all this to be much more help but wanted to give you some confidence that even an extreme novice like me was able to fix my scope. It still works to this day...again thanks to everyone here. Good luck! John in Charlotte ________________________________ From: [email protected] <[email protected]> on behalf of ciclista41 via groups.io <ciclista41@...> Sent: Friday, May 22, 2020 1:23 AM To: [email protected] <[email protected]> Subject: Re: [TekScopes] 475 questions Hi Barry! I appreciate the positive words! I don't quite have that level of confidence, but since those who have gone before me in similar efforts have been successful, I guess it's not out of the realm of possibility for me, as well! Bruce |
Michael W. Lynch
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