Tektronix 475, is this likely to be a straightforward repair job?

[tekfan]

Yes that microswitch is for the delayed time base. I remember having seen it in the 7B53A time base plugin. As far as i can remember when you pull the time/div knob that microswitch engages so the scope knows that it is in delayed time base mode.

As for the trigger it sounds as if you either have it set to line or are triggering from the other channel.
When the trigger mode is in normal the scope wont draw a trace until it sees a signal it can trigger on.
In auto mode it will always draw a trace even if there is no signal present and will trigger on the signal if it crosses the trigger threshold that is set by the trigger level knob.

For first time operation put the level control in the middle, trigger mode to auto, trigger coupling to AC and trigger source to CH1. You should get a stable trace if you connect the CH1 input to the calibrator.

In the manual there is a step by step guide to familiarize yourself with the scope. You might want to check that out.

I suggest that you don't disassemble the knob unless you absolutely must. If so firstly move the time/div knob very slowly so you can hear the microswitch click. If you can't then adjust the microswitch position. If even that doesn't help then remove the time/div knob from the front. There are some pins that grap a hole so that you can move the knob with the transparent skirt around it when the knob is pressed in.  Check that the metal parts are clean and then reassemble the knob.

It probably sounds strange but you'll see how it works and then you will understand. Putting it back together isn't such a problem. You should follow the manual for the 475 to get familliar how the time/div knob works.

[Rerouter]

also for the -15V supply, just make sure its not shorted to anything, the only thing it is meant to power is the active probe ports on the back of the thing,

http://imageshack.us/g/192/p9262865.jpg/
as for the rest, here are some reference images of my 475, the last one was to show the effect of bandwidth limitation to a friend, (channel attenuators havent been calibrated yet as i lack a trusted fixed amplitude waveform)

and yes i know they are far from perfect, but its just for reference

[Chris Wilson]

More great advice, thank you.

I see what the idea of the extra scaling is now, the knob doesn't turn through as many detents as there are scalings, the last 3 "missing" detents are not meant to be there, the scale "pointer" is the light, and that only covers the three uppermost ranges when a genuine Tek probe grounds the outer ring. I have that clear now, thank you. I have also found a manual for it, so can do some more meaningful voltage probings, i am still pretty sure the major issues are still mechanical, switch contact related. I think I have to bite the bullet and remove some boards to gain proper access. My digital camera and pen and paper should help me reassemble it OK, hopefully.

The horizontal display and the trigger mode push buttons seem to be where the problem lies. With a sine wave input of 1 khz from my generator I can always get an auto ranging display, switch gremlins permitting, but the trigger level knob does nothing. Very occasionally, and very briefly the "normal" button will give a flash of a display of the waveform, and I am pretty sure the trigger level knob position influences where this waveform starts on the screen, but it happens very fast and is gone. The only way to get close to a stable stationary display in auto mode is to use the hold off knob. The horizontal display and trigger mode push buttons have a mind of their own, when the board is out hopefully I can probe the switch contacts and confirm they are high resistance or open when they should be closed. I see no point in trying further testing until the switches work reliably, as it will just confuse me. Does that seem sensible?

I will ignore the -15 volts being potentially dirty for now. Thanks all.

[ModemHead]

Your picture shows "line" triggering, but yet the trigger light is not on. Either the lamp is burned out or the trigger circuit is not functioning or not receiving any input. Could easily be the trigger source switch.

Take your time, make copious notes and pictures. Note that you will have to desolder the trigger input BNCs to get the sweep/trigger board out.  I wouldn't waste much time with an ohm-meter, just clean everything. Twice. Some people don't like "contact cleaner" products, but I found a spray can of Caig De-oxit to be very useful. You have to control over-spray and allow more time for it to dry up than IPA.

While you're at it, the transistors and the few ICs are probably all socketed (they are in the 465). Some of the sockets may not be obvious, since they're little tiny eyelets installed on the PCB. Pull them up a milimeter or two, then push them back down to wipe the contacts a bit.

[Chris Wilson]

It's a hell of a coincidence that "messing" with the switches brings the trace back, and the board with signs of damp is the one on the same side of the scope as the trigger switches. I will have to grit my teeth and get the top board off so I can gain proper access. The bits of the contacts I can see on the slider switches controlled by the top right hand toggle switches via link rods, look horrible.  I have never seen the ready nor the triggered lights come on, so i will check the bulbs whilst i am in there. I am now putting some shelving up in my office as I am running out of work space.  I don't want to disassemble further until i have a clean and tidy area it can sit undisturbed whilst I take my time. I have not given up hope with it yet Thanks!

[Chris Wilson]

Boards are out, immediate issues are both triggering bulbs are rattling about in their hoods, the leads are broken off, as is the power on bulb. Can anything easily obtainable be substituted?

The main issue is the rather complex drum switch, the grey plastic thing like one of those musical instruments with pegs on a brass drum that plucks at different length strips on a sounding board, i forget what they are called. Anyway, I always thought it worked oddly, as pulling the knob just resulted in a squeaky raspy noise when turned. It's the drum in ModemHead's photo in this thread that does NOT have the cleaning paper under the contacts that I am speaking of. Not having used on of these scopes before I assumed it was turning a dry potentiometer. I have disassembled the drums and I assume the knurled bit of the shaft that enters the rear drum is supposed to be moulded or bonded into the back drum itself, so it can turn it? Well, it just pushes out from the front, revealing the knurled bit of shaft, and some plastic dust followed it. I assume the knurling is to give it surface area and texture to grip when it's moulded or glued into the rear drum  I reckon I have some suitable aircraft adhesive that will bond it back in again, so the shaft can turn the rear drum. As long as I have had the scope it's not turned the rear drum, it's been stationary. I am sure that depending where it stopped it may have caused strange effects? The push button switches all have low resistance when closed and infinity when open, so assuming they are still low resistance with a load across them I can give those a clean bill of health. Is there a better yet safe way to test the contacts with the switches still on the board?

The toggle switches look a nightmare as they are plastic welded onto the boards and the contacts themselves are not accessible. I need to sit down when I am not tired and have the circuit diagrams in front of me so i van pin out the contacts on the board tracks. I suspect these ARE less than 100% the copper coated spring strips are manky, they just don't look good at all. I will have to get some of the de oxidising cleaner that's been mentioned.

If I can re bond the shaft in the drum, and get the damned thing back together as it's meant to be there's still hope. My worry is phasing a little cam that sits between the two drums and operates against a spring loaded peg. It was loose, so I have no means of knowing where radially it should be locked up on the shaft. It allows continual rotation in one direction, but due to the shape of the camit locks the shaft when it is counter rotated. It's WHERE it should lock the shaft that may be an issue

It's miracle these things work at all, I had no idea there would be so much electro-mechanical stuff to go wrong. I am thinking there's a lot to be said for my USB scope right now

[ModemHead]

Sounds like you've got some serious mechanical problems there. I hope you can get it back together. It'll definitely be a repair to be proud of.

The back drum is for setting the B-sweep speed. When the knobs are locked, it will be identical to the A-sweep speed, but when you pull the knob out and turn it clockwise, the mechanical set-up will allow you to set the B-sweep higher. It allows for "zooming in" on portions of a waveform. When the display mode is set to "A lock-knobs", the B-sweep is not used.

You should be able to replace the lamps with LEDs and ballast resistors. I think the lamps run off the 5V supply.

[alm]

Have you checked if those bulbs aren't still a current item at distributors like Digikey? They still sell a fair number of incandescent indicator bulbs. LEDs may draw too much current, you'd have to check the circuit.

[grumpydoc]

Farnell sell a good range of indicator bulbs - something will probably match.

[ModemHead]

Tek P/N 150-0130-00: Incandescent bulb, wire leads, 5V 60mA, GE 2200DX

No hits on GE 2200DX, Chicago Miniatures 6833 looks like a match, probably others.

[Chris Wilson]

Thanks for the bulb info, will get onto this, but now have a need for a part. I tried to repair my timebase switch drum, but found the drum is internally damaged and
un-fixable.  So I am looking for a replacement.  It's  the rear grey plastic toothed drum that is engaged when you pull the timebase knob to select delayed sweep. I can supply a photo and serial number of scope if anyone can help. Would take the whole board if someone has one at a good price. Is there any reason that I cannot leave this drum manually rotated fully anti clockwise whilst I continue fault finding? Serial  number  is  102175. I do have the service manual but the parts listing  is not clear enough to cite a part number as it's a pdf copy, not original paper manual, sorry. But here is a photo of the offending part. Thanks!

[ModemHead]

I'm reasonably sure the Tek P/N for the entire cam assembly is 105-0363-00 (Fig 3, item 50).

Options may be: try to get the switch/cam assembly, try to get the entire timing board, or look for another 475 as a parts donor.

The eBay seller "fm-tx" had the entire board listed recently (item 400250111098).  Another seller "qservice_rhodes" has some 475 cam switch assemblies listed, but the part numbers don't match the one I found. The vertical attenuator has a similar-looking cam switch, possible confusion.  Emails to these sellers might yield positive results. I have no experience with either one but I have seen Qservice mentioned favorably on TekGroups.

As far as continuing the fault-finding in its current condition, I would rotate the back drum fully clockwise. It would be a valid configuration for the B-sweep to be faster than the A-sweep, but not slower.

[Chris Wilson]

Can't thank you enough, I did a search on Ebay and never found that. I have e-mailed the seller to see if he still has it, would make sense to buy the whole board. I'll see if he gets back to me. BTW I am in England UK, but he lists a reasonable shipping charge. Thanks again. Would it be sensible to just change the switch initially, if I find a whole baord, will changing the board mean a re calibration, or is this thing such a wreck it will need a calibration anyway?

[ModemHead]

I'm glad that was helpful.

I wouldn't worry about the calibration aspect too much, especially if this will just be a hobby scope. You're doing this for fun right? If you can get the vertical gain and sweep speed about right, it should be useable enough.

Think seriously about getting a parts-donor scope. It may not be any more costly than purchasing these hard-to-find parts.

[Chris Wilson]

Will do. I wonder if there's money to be made breaking scopes?  I KNOW there is in breaking engines and race cars It is just for fun I have a decent Fluke / Philips combi scope that was calibrated in late 2010. Thanks again.

[milesy]

hi guys i know this is a little bit of an old thread but you seem to have some experience with these scopes

my 475 is clipping a sine wave at both top and bottom, everything else seems to work ok except for the clipping, it happens with either 240v and also from a 12v transformer.

i would be fairly confident in saying the mains should be ok so im wondering if you have any ideas why this might be happening??

[grumpydoc]

t happens with either 240v and also from a 12v transformer.

I think you might need a better signal source 

The place to start is to check all the power supply voltages for value and ripple.

Manuals at http://bama.edebris.com/manuals/tek/475