My 475


Rey
 

Hi guys, my first post here looking for answer I found this place. I'm looking for a bit of guidance, my 475 just quit it was working fine. Both channels are gone, no beam find. Every switch light works, the screen lites up. But no trace out of either channel. I'm a hobbiest and I understand that electrical things do weird things. But any ideas and/or pointers would be appreciated. I already downloaded the schematic.
Thanks


 

My inbuilt autoresponder says: "Check the low voltages" and report back. The voltages are interdependent.
Highly likely you'll find a defective/shorted (Tantalum) capacitor.
Use dwg11 in the service manual as a reference.

Raymond


Paul Amaranth
 

Check all of the LV power supplies. That's always the first stop. Get the
service manual and make sure all of the supplies are within spec for voltage
AND ripple. Nothing works if the power supplies aren't right.

Paul

On Mon, Jul 05, 2021 at 06:48:42PM -0700, Rey via groups.io wrote:
Hi guys, my first post here looking for answer I found this place. I'm looking for a bit of guidance, my 475 just quit it was working fine. Both channels are gone, no beam find. Every switch light works, the screen lites up. But no trace out of either channel. I'm a hobbiest and I understand that electrical things do weird things. But any ideas and/or pointers would be appreciated. I already downloaded the schematic.
Thanks







!DSPAM:60e48921145828472026532!
--
Paul Amaranth, GCIH | Manchester MI, USA
Aurora Group of Michigan, LLC | Security, Systems & Software
paul@AuroraGrp.Com | Unix/Linux - We don't do windows


Tom Lee
 

Welcome!

Standard procedure is to check the supplies (there are several in that scope). A quite common problem is a shorted "teardrop" tantalum capacitor, but a few quick measurements will tell you a lot.

If you've downloaded the manual, you should also take a look at the troubleshooting chart. The manual is incredibly well written.

Good luck!

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 7/5/2021 18:48, Rey via groups.io wrote:
Hi guys, my first post here looking for answer I found this place. I'm looking for a bit of guidance, my 475 just quit it was working fine. Both channels are gone, no beam find. Every switch light works, the screen lites up. But no trace out of either channel. I'm a hobbiest and I understand that electrical things do weird things. But any ideas and/or pointers would be appreciated. I already downloaded the schematic.
Thanks




Dave Peterson
 

Just scanned the 475 SM online doc. I didn't find a diagnosis chart - is that not in the online 475 manual?

Wanted to throw this in the suggestion box - after checking and verifying the LV supplies: check F1318 at the rear of the main board - near the transformer. This drives the HV oscillator and the tantalum 47uF cap on the other side of the fuse is a somewhat common short. It's a relatively easy fix. Don't bypass the fuse if it's out, check for a shorted C1318. That'll take down the CRT without affecting the LV supplies and other indicators.

Dave

On Tuesday, July 6, 2021, 9:47:36 AM PDT, Rey via groups.io <cis181ellin=yahoo.com@groups.io> wrote:

Hi guys, my first post here looking for answer I found this place. I'm looking for a bit of guidance, my 475 just quit it was working fine. Both channels are gone, no beam find. Every switch light works, the screen lites up. But no trace out of either channel.  I'm a hobbiest and I understand that electrical things do weird things.  But any ideas and/or pointers would be appreciated. I already downloaded the schematic.
Thanks


Tom Lee
 

Dagnabit -- you're right, Dave.  I'm apparently thinking of a chart in some manual for another model.

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 7/6/2021 10:52, Dave Peterson via groups.io wrote:
Just scanned the 475 SM online doc. I didn't find a diagnosis chart - is that not in the online 475 manual?

Wanted to throw this in the suggestion box - after checking and verifying the LV supplies: check F1318 at the rear of the main board - near the transformer. This drives the HV oscillator and the tantalum 47uF cap on the other side of the fuse is a somewhat common short. It's a relatively easy fix. Don't bypass the fuse if it's out, check for a shorted C1318. That'll take down the CRT without affecting the LV supplies and other indicators.

Dave



On Tuesday, July 6, 2021, 9:47:36 AM PDT, Rey via groups.io <cis181ellin=yahoo.com@groups.io> wrote:
Hi guys, my first post here looking for answer I found this place. I'm looking for a bit of guidance, my 475 just quit it was working fine. Both channels are gone, no beam find. Every switch light works, the screen lites up. But no trace out of either channel.  I'm a hobbiest and I understand that electrical things do weird things.  But any ideas and/or pointers would be appreciated. I already downloaded the schematic.
Thanks









 

Rey,

When you say that "the screen lites up" what do you mean by that? Are you saying that you have two straight lines, but when you apply a signal to either input channel you don't get any vertical deflection of either trace?

As other's have said, the first step in any diagnosis like this is to check the power supply voltages. This is easy to do on the 475 as there are labeled test points on the main interface board for each power supply rail.

I will hold my own speculation about what the fault might be until after we find out what the power supply voltages are.

-- Jeff Dutky


Bill
 

You have received some great feedback. Be sure to follow the recommendations.
Bill


Rey
 

Hi, by that I mean all the selection switch lights are on or off as I change the selection. I was checking all the voltages and the only thats off, is the the 50v unreg which reads 69,6 V


 

On Tue, Jul 6, 2021 at 10:53 PM, Rey wrote:


I was checking all the voltages and the only thats off, is the the 50v unreg
which reads 69,6 V
That's fine, it's just the unregulated voltage, straight from transformer secondary, rectifiers and buffer caps that the regulated 50V is derived from in the 'scope.

Raymond


Rey
 

Hi Dave,
Thanks for the advice, check the F1318 and is fine. So now I'll order a couple of tools and some caps, as soon as they are in I'll continue. Mean while I'll re read the troubleshooting guide from tektronix also read more on 475. As far as visible goes nothing looks or smells bad. The story will continue, as a side note I finally paid attention to the tag in the back of the scope. Came from the navy, "fleet numerical weather central" go navy.

rey


 

Rey,

Sorry to belabor this point, but I am legitimately confused: are you saying that there is absolutely no image on the CRT?

If that is the case, I would begin to suspect the HV supply. You will need an HV probe to check that as it runs at 2500 V.

I wonder, however, if there is a failure mode of the Z-axis amplifier that could completely blank the display. I only say this because I've had a 475 with trouble in the Z-axis amp, not because anything specific about your case suggests it. Also, the Z-axis amp would be easier to diagnose than the HV supply, but that's also a terrible reason to go down that road.

Finally, you could apply a signal to the vertical inputs and trace the signal through the attenuators and vertical pre amps to the channel switch. The fact that you have lost both signals at the same time suggests that the failure is after the channel switch. It could be as simple as dirty contacts on the channel switch IC's pins, or the channel switch IC might be dead. I'll take a look through the schematics to see what else might interrupt the signal between the channel switch at the CRT.

-- Jeff Dutky


Rey
 

Correct no image on the crt or even using the beam find. But all the other switching(lights) on the face panel work. I'm trying to figure it out, and I love all the input and help from you guys. I been out of the electronics game forever, but I'm smarter than the average bear. The tektronix troubleshooting manual, stated about injecting a - 10-15V for about 10" using the EXT z input, thats my next step. By the way is a pretty cool manual, very clear and step by step still lots of scope lingo but I'll keep at it.


Mark Vincent
 

Rey,

I had a Z problem that was one coax connector that needed to be moved up and down in its socket a few times. I had all other voltages correct, nothing on the screen. After that one coax was moved to make contact, it worked. I move connectors in and out while having 90% IPA on it to help clean the pin and socket. I have seen enough of these look clean yet were not making contact. Of course these are done one at a time so no mistake in where that wire/cable goes.

Checking the main power supply voltages is always the best place to start. Those should be adjusted, if necessary, to the specs. The high voltage can be checked by putting the back of your hand near the hv wire insulation. You will not get shocked this way. The presence of high voltage will make the hairs move. A crackle sound at turn-on is another way to know the voltage is there provided that sound is made. No danger of that sound.

Check the oscillator transistor for the high voltage. With the fuse being good, it could have opened internally. I know this is unlikely but it would confirm that transistor. Another way is to see if you have a-c voltage at the collector of the transistor. If so, you should have the high voltages and heater voltage. You should see about 50-60VAC p-p at the collector.

Jeff is right about tracing the signal until you lose it and possible dirty contact(s).

Mark