Author Topic: Need help with a dead Tek 475 scope  (Read 3887 times)

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Offline echap

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Need help with a dead Tek 475 scope
« on: December 12, 2015, 05:14:14 am »
Hey guys,

Bought a Tek 475 off ebay that was sold as parts.  I have one already, but I figured it would be good to have some spare parts.  For fun I decided to see if I could get it working.  Pulled the covers and fired it up and the voltages all seem fine at the test points, but no trace.  Found the HV lead arcing to the case, so I wrapped some Scotch 33 around it and got it to stop arcing.  But still no trace.  There was a fuse missing off the board right next to the HV lead plug, so I thought I had figured it out.  I could not figure out what size it was supposed to be, so I put a 3/4A fast blow fuse in.  the fuse holder has pretty large wire going to it, so I doubt that 3/4A is to big. But still no trace. 

Here is a pic of the fuse holder.




I  found R1372 was lifted off the ceramic mount under the HV shield and tacked it back on, but still no trace.

I think that someone has tried to repair it before, but gave up.  The HV lead looks like it might have been cut and had shrink tube installed over it, but was not insulated enough, as the HV lead was arcing thru the shrink tubing. 

I am pretty much a newb on this type of equipment, mostly working on old radios and what not, your basic capacitor changer  ;) 

Any ideas what to check next would be appreciated. 

It's in really good shape, like new on the inside.  Has an Amy sticker on the back that says it was from 1997.  Here's the sticker





« Last Edit: December 12, 2015, 09:40:57 pm by echap »
 

Offline mzacharias

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Re: dead Tek 475
« Reply #1 on: December 12, 2015, 12:50:34 pm »
"Thou Shalt Check Power Supply"

Rule number one. Seriously, Tek's are pretty notorious for bad power supply caps and the occasional bad bridge rectifier, etc. That is certainly where I would start.
 

Offline echap

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Re: dead Tek 475
« Reply #2 on: December 12, 2015, 09:28:16 pm »
I don't believe this is a power supply issue.  Power rails are as follows:

+110V = 111.3V w/ 3.1mv ripple
+50V = 49.9V w/ 1.9mv ripple
+15V = 14.97 w/ 1.9mv ripple
-8V = -7.97V w/ 1.7mv ripple
Unreg 50V = 60.00

As measured with my Fluke 87.  I measured the ripple with Fluke set to AC on the test ports and waited for the reading to stabilize.  They where higher when I first measure, but the settle down to the readings I provided above.

I used my 30kv HV probe to check the -2450V test port, it's pretty crude, but there seems to be at least -2KV on it.  It goes up toe 30Kv with an analog display, so it's not that accurate.

I have a good 475, so I know how to operate it.  I can test the cal bar(on the broken scope) and it is OK, I can set up the trigger for ch1 and ch2 on this one and the settings are about the same and the trigger light comes on, so it seems to be working, just no trace. 

The missing fuse in the pic above, which I am not sure what it does.

Any other ideas what to check are appreciated.

Thanks,
Eric

eta:

Been reading the Tek troubleshooting manual and it seems like the PS is acting like I would expect.  The low level for regulation is as follows:
110V rail drops out at 108V
50V rail drop out at 101V
15V rail drops out at 101V

The tap is set to the 120V setting (high)
Here are the pics on my other 475 scope:
110V rail at 120V input


110V rail at dropout (108V)


50V rail at dropout (sorry didn't get the trigger set right)


15V rail at dropout


« Last Edit: December 12, 2015, 10:17:29 pm by echap »
 

Offline PaulAm

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Re: Need help with a dead Tek 475 scope
« Reply #3 on: December 12, 2015, 11:18:20 pm »
If the power supplies look ok, and you have hv, there are a couple of things that might be going on.

Unblanking circuits may have a problem.

Your trace might be deflected off the screen in one of the H or V sections has a problem.

The CRT filament could be open

If you turn the intensity all the way up, do you see any kind of green glow on the screen?  You might have to do that in the dark.
 

Offline echap

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Re: Need help with a dead Tek 475 scope
« Reply #4 on: December 13, 2015, 07:03:49 pm »
If the power supplies look ok, and you have hv, there are a couple of things that might be going on.

Unblanking circuits may have a problem.

Your trace might be deflected off the screen in one of the H or V sections has a problem.

The CRT filament could be open

If you turn the intensity all the way up, do you see any kind of green glow on the screen?  You might have to do that in the dark.

I looked at it in the dark, and all was dark.  I popped the back off and measured pins 1 and 14 (either side of key?) and there is no continuity.  I measured the same pins when the scope is energized and have 5.9volts AC so I believe that I have an open filament.  Not to sure what I can do about that other than look for a new CRT.
 

Offline AF6LJ

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Re: Need help with a dead Tek 475 scope
« Reply #5 on: December 13, 2015, 07:12:05 pm »
These guys might have a CRT..
http://www.sphere.bc.ca/
Sue AF6LJ
Test Equipment Addict, And Proud Of It.
 

Offline PaulAm

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Re: Need help with a dead Tek 475 scope
« Reply #6 on: December 13, 2015, 07:53:24 pm »
There's a 475a on ebay now for 79.99 + shipping that at least has a working CRT.  You never know if the geometry is OK though.

Might check on the tekscopes group; somebody might have a CRT to sell.  Might be worthwhile just to get another parts scope given what you'd pay for the CRT.
 

Offline grumpydoc

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Re: Need help with a dead Tek 475 scope
« Reply #7 on: December 13, 2015, 08:05:13 pm »
These guys might have a CRT..
http://www.sphere.bc.ca/
It doesn't look like Sphere or qservice have one in stock.

ebay for a parts 'scope or a local ham meet will be your best bet.
 

Online T3sl4co1l

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Re: Need help with a dead Tek 475 scope
« Reply #8 on: December 13, 2015, 10:22:43 pm »
Heh.. I have an old, tired CRT still on hand.  Very dim even at full intensity, but better than nothing I suppose?  Might be worth a try rejuvenating it?

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 


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