Re: 475 questions
Hi Bruce:toggle quoted messageShow quoted text
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!
On Wednesday, May 20, 2020, 09:53:20 PM PDT, ciclista41 via groups.io <firstname.lastname@example.org> 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!