Author Topic: Tektronix 475A repair attempt  (Read 1065 times)

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Offline whyrlpool

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Tektronix 475A repair attempt
« on: April 05, 2019, 02:15:48 am »
Snagged a 475A for spares/repair that looks like it lost a fight with a hammer.

Was labelled 110V and had no plug on the end of it, so I wired up a plug and switched it to 230V as I’m in the UK.
1.5A fuse on the back of the thing was blown out and blackened so I replaced it with a 3A fuse (smallest I had to hand, apparently 700mA fuses aren’t a thing).
It’s a rackmount unit so I coaxed it out of the casing and put it on my bench upside down so I could probe the power supply.
before powering it up I took a look at the insides and noticed the vertical section had all the jumpers disconnected (some ripped out), and one of the jumpers was misrouted, so I assume this has already had someone noodling about inside of it.
Once powered on all of the voltages are within tolerance except unregulated 50V which was quite high at 65V, but regulated 50V was within fine so I assume it’s not an issue as the unregulated line is only used for the “low line” sensor on the front.

It powers up, CH1 and CH2 uncal lights come on, and there’s no beam.

I hit the beam finder and I get an off-centre dot that I used the horizontal adjustment knob to move into the middle, but it was still off vertically.
Hooking up CH1/CH2 to the CAL output (1kHz 300mV pk-pk, only have a DC multimeter and it reads approx 300mV so I assume it’s working) and I still only get a dot with the beam finder. Oddly if I change the TRIG settings I can get a single dim vertical line to appear (see picture).

I made a new slider for the power switch today, as it had been lost in the melee, and turned the scope on after flipping it upright, lo and behold there’s a single bright dot in the centre of the screen, without needing to hit the beam finder!
 But when I do hit the beam finder it simply shifts the dot vertically by approx two scale divisions. I cant tell what the volts/div setting is currently as the dial is destroyed, but rotating the knob had no effect on the movement.
An accidental bump later and the dot goes, however the entire screen is slightly lit up. Repeated bumping can change between these two states.
With the now-upright oscilloscope, I noticed odd behaviour when adjusting the horizontal position of the beam. Turns out it’s not a dot but a line that changes length with the horizontal adjustment knob, video of this can be provided.
I’m lacking experience in debugging scopes, and having checked out the stickied post which has an excellent looking pdf on debugging tek scopes linked on sphere.bc.ca, I am led to believe there is most likely a problem with the horizontal section from the symptoms I had when the scope was upside down (no beam, dot on beam finder). However, the righted scope seems to have different issues which have me stumped (temperamental dot).
When I first took a look at it, one or two of the wires inside were disconnected and there was a solder joint snapped off that connected two grounds (CHB and the vertical section). Clearly this scope has been through hell, but does it sound like given a few weekends and some TLC it could be brought back to life?
I have pictures and video of pretty much everything with the scope so far, so I'll attach them once I figure out a sane way to embed them in this post.
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Offline PaulAm

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Re: Tektronix 475A repair attempt
« Reply #1 on: April 05, 2019, 02:55:37 am »
Well the first thing to do is to get a service manual; you can find free ones on the net that are OK.

The first trouble shooting step is to measure all of the low voltage supplies to make sure they're in spec.  DO THIS FIRST!  If the supplies aren't right it can cause all sorts of weird behavior and can save you hours of effort chasing ghosts.  A scope is handy to look at ripple but you can use a dmm on an AC range.  You should be seeing mV; anything more is a red flag (the actual specs are in the manual and vary a bit with the supply).
 

Offline whyrlpool

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Re: Tektronix 475A repair attempt
« Reply #2 on: April 05, 2019, 02:59:16 am »
I’ve skimmed the service manual and as stated in my first post, I checked the low voltage power supply already and it’s all within spec, except the unregulated 50V line which is 65V, but there’s no spec for that as far as I can see, and as it’s only used to check if the line voltage is too low it’s not an issue. I haven’t checked the HV supply, other than taking the cover off before I first turned it on to check for burn marks. I still have no clue as to how the fuse initially blew, but it was pretty blackened.
Not sure how to measure ripple with my crappy multimeter, but none of the voltages oscillated between values when I measured them, although this meter isn’t particularly fast updating.

I just don’t understand exactly why it behaves differently when it’s upside down
« Last Edit: April 05, 2019, 03:23:21 am by whyrlpool »
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Offline tautech

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Re: Tektronix 475A repair attempt
« Reply #3 on: April 05, 2019, 05:19:56 am »
The vertical trace is indeed unusual like maybe the X and Y plates wiring is reversed.

To check for ripple on a linear PSU just put your DMM in AC mode.
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Online bd139

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Re: Tektronix 475A repair attempt
« Reply #4 on: April 05, 2019, 10:31:51 am »
Wow that's pretty munted. Hope you didn't pay too much for it. I wouldn't even bother to repair that myself.
 

Offline med6753

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Re: Tektronix 475A repair attempt
« Reply #5 on: April 05, 2019, 11:23:09 am »
By the looks of the outside I'd say it's been dropped and there's probably irreparable internal damage to the CRT. Sorry to say but if that were mine it would be a parts mule and I'd buy one in much better shape.
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Online bd139

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Re: Tektronix 475A repair attempt
« Reply #6 on: April 05, 2019, 11:36:14 am »
My Tek parts mules have been in better nick!
 

Offline PaulAm

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Re: Tektronix 475A repair attempt
« Reply #7 on: April 05, 2019, 02:04:17 pm »
Since you're seeing something on the crt, the HV is probably working well enough.

Those missing knobs and controls aren't going to help anything.  For what it would cost to replace the missing knobs and controls you can probably buy another scope.

If you don't have another scope it's going to be tough to track the problem(s) down.
 

Offline whyrlpool

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Re: Tektronix 475A repair attempt
« Reply #8 on: April 05, 2019, 02:08:39 pm »
Thought as much regarding the HV side. As far as I can tell all the broken knobs still work other than 1 10k pot for the B/DLY’D trigger slope, so I’ll order one of those later. Mostly the physical damage to the front is cosmetic and I’m fine using any old knobs. I’ll take a closer look at the horizontal section tonight, take it out and clean everything up a bit and measure voltages where I can. Who knows, it might just be a transistor fell out of its socket or something.
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Online bd139

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Re: Tektronix 475A repair attempt
« Reply #9 on: April 05, 2019, 02:21:29 pm »
First step is disconnect the H and V deflection plates and see if you get get a focused dot on the screen. If you can't do that, fix the HT. Only then work on the amps.

As med said, there's a possibility the tube is futzed. I've had a couple of Tek tubes which were internally damaged and there's no fixing them.  Proof from my bin:

« Last Edit: April 05, 2019, 02:25:50 pm by bd139 »
 
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Offline Dacke

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Re: Tektronix 475A repair attempt
« Reply #10 on: April 05, 2019, 11:55:27 pm »
Just finished restoring a 475A a couple weeks ago that had multiple issues -  https://imgur.com/a/SH6hLrt ,  including just a dot on the screen/ no sweep,  unstable and missing voltage rails,  both channels did not react to input ( no deflection when touching the probe tip either),  delayed sweep was stripped out so it did nothing,  there was a laundry list of problems and it also looked like it had been dragged through the mud.   These things are a work in progress honestly.

Since the screen is just showing a dot,  first I would check Q1010,  a JFET in the sweep generator circuit.  If it has failed you can replace it with Q920 (it's the same JFET,  just not as critical) just for testing.  The unregulated 50 volt rail should be fine ,  mine was sitting at 67V last time I checked.  If you need me to look at anything on mine or take pics of specific areas just let me know...

Edit - Just now looked at the picture of yours,  wow.   I often wonder how that even happens,  it looks like it was dropped on it's face....
« Last Edit: April 06, 2019, 12:17:06 am by Dacke »
 

Online tggzzz

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Re: Tektronix 475A repair attempt
« Reply #11 on: April 06, 2019, 08:12:33 am »
As med said, there's a possibility the tube is futzed. I've had a couple of Tek tubes which were internally damaged and there's no fixing them.  Proof from my bin:

Different damage, but here's proof from my display cabinet :)
https://entertaininghacks.wordpress.com/2016/03/09/rescuing-a-broken-tektronix-465-crt/
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Offline med6753

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Re: Tektronix 475A repair attempt
« Reply #12 on: April 06, 2019, 12:52:20 pm »
First step is disconnect the H and V deflection plates and see if you get get a focused dot on the screen. If you can't do that, fix the HT. Only then work on the amps.

As med said, there's a possibility the tube is futzed. I've had a couple of Tek tubes which were internally damaged and there's no fixing them.  Proof from my bin:



Good advice. If you do get a focused dot in the center of the CRT chances are you got lucky on it's condition. However...do NOT leave that dot focused on the screen any longer than to just verify you have that or else you will burn the phosphor on the screen.
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Offline whyrlpool

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Re: Tektronix 475A repair attempt
« Reply #13 on: April 16, 2019, 09:44:32 pm »
The dot I get with the beam finder is focused and small, so there’s hope!
Just opened it up again and measuring AC volts across the rails
GND to 5V: 9.9-10V
GND to 15V: 31.6V
GND to -8V: 0V
GND to 50V: 107V
GND to 110V: 241V
GND to unreg50V: 142.5V

5V to GND: 0V
15V to GND: 0V
-8V to GND: 16.4V
50V to GND: 0V
110V to GND: 0V
Unreg50V to GND: 0V

(I’m using my multimeter on V~, shouldn’t the readings be the same regardless of polarity or is my trusty old multimeter fucked too?)

Gonna disconnect the H and V deflection plates now and see if I get anything
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Offline med6753

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Re: Tektronix 475A repair attempt
« Reply #14 on: April 17, 2019, 11:35:57 am »
The dot I get with the beam finder is focused and small, so there’s hope!
Just opened it up again and measuring AC volts across the rails
GND to 5V: 9.9-10V
GND to 15V: 31.6V
GND to -8V: 0V
GND to 50V: 107V
GND to 110V: 241V
GND to unreg50V: 142.5V

5V to GND: 0V
15V to GND: 0V
-8V to GND: 16.4V
50V to GND: 0V
110V to GND: 0V
Unreg50V to GND: 0V

(I’m using my multimeter on V~, shouldn’t the readings be the same regardless of polarity or is my trusty old multimeter fucked too?)

Gonna disconnect the H and V deflection plates now and see if I get anything

What kind of multimeter are you using? And it's set to AC volts and you're getting readings like that? Throw it in the garbage and get yourself a decent DMM. Across the DC voltage rails to gnd with a DMM set to ACV you should measure no more than approx 2.0mV ripple on a properly operating supply. But what's more important at this point is what is the DC voltage of each supply? That should be your starting point. 
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