Author Topic: 475A Sweep  (Read 9488 times)

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Offline SoundTech-LG

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475A Sweep
« on: November 21, 2014, 08:32:59 pm »
Hi,

Normal, non- delayed sweep has a strange 60Hz blank spot. If I trigger on LINE, at 20ms/ it really becomes stable, and very obvious. The -8 line seems contaminated with a small bit of it in the form of a pulse, but not into the ripple "out of spec" range. I'm sure it's a bad cap in Z-Axis, Horiz Amp or somewhere there. Anyone seen this?
 

Offline oldway

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Re: 475A Sweep
« Reply #1 on: November 22, 2014, 06:22:26 am »
Ripple of +15V unreg ok or not ? (on fuse F1318)
Waveform at TP1364 ok or not ?
Replace C1374 1µF 100V
 

Offline SoundTech-LG

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Re: 475A Sweep
« Reply #2 on: November 26, 2014, 05:56:38 pm »
Well that's definitely the right circuit! It got worse since I started probing around, or something was marginal, and now it's completely fried. Now no trace at all unless Beam Finder pushed, and CRT bias tweaked up a bit. DC level at TP1364 is only 6 volts. I'll be back at this some more. Thanks for steering me right in there!
 

Offline SoundTech-LG

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Re: 475A Sweep
« Reply #3 on: November 27, 2014, 02:02:22 am »
Found both Q1338, and Q1332 were shorted/open Q1338 is Tek Selected part. I swapped in some TO-92's I had laying around MPSH10, and MPSH81 (different basing).
No change after that. No signal at Q1344. Bummed this is not a simple fix. Putting it aside for a bit. My 465B is still working.
 

Offline wine+dine

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Re: 475A Sweep
« Reply #4 on: December 07, 2014, 05:49:10 pm »
Hi - 60 Hz sounds suspicious ... did you make sure your supply voltages are clean, especially the (5V?) logic supply?
I recently had a sweep problem caused by a bad filter cap on +5 which was kind of "pulsed DC".
 

Offline SoundTech-LG

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Re: 475A Sweep
« Reply #5 on: December 08, 2014, 11:28:36 pm »
Well you're partly right. Turned out to be the +60V unregulated filter cap. Unfortunately it all went wrong. I used a SAMXON cap I had laying around. Really bad move. It worked good for about 1/2 hour, and then it did in almost the entire scope. the cap almost burst, but it sent AC throughout.  I finally have it about 85% functionality back now after replacing at least 8 transistors and the sweep mode IC. Not even sure it was worth my time. Still going on it though.
 

Offline tautech

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Re: 475A Sweep
« Reply #6 on: December 08, 2014, 11:36:43 pm »
Well you're partly right. Turned out to be the +60V unregulated filter cap. Unfortunately it all went wrong. I used a SAMXON cap I had laying around. Really bad move. It worked good for about 1/2 hour, and then it did in almost the entire scope. the cap almost burst, but it sent AC throughout.  I finally have it about 85% functionality back now after replacing at least 8 transistors and the sweep mode IC. Not even sure it was worth my time. Still going on it though.
You might be 1/2 right blaming the cap, if it was old it should have been reformed before use.
Gently bought up to working voltage over time with reduced current load.

This is where you need good gear to test caps.
Lesson learned.
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Offline SoundTech-LG

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Re: 475A Sweep
« Reply #7 on: December 09, 2014, 04:22:53 am »
Yup. Shoulda' known it was crap. I had to replace the rect. too. Right now, the main symptom is no free run sweep in the AUTO mode, when signal is removed. Triggers ok, and sweeps, but without a signal it acts like it is in the NORM mode.
Still chasin' this sucker!
 

Offline johansen

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Re: 475A Sweep
« Reply #8 on: December 09, 2014, 04:59:15 am »
Yup. Shoulda' known it was crap. I had to replace the rect. too

this is where blaming the cap doesn't pass the smell test.
 

Offline SoundTech-LG

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Re: 475A Sweep
« Reply #9 on: December 09, 2014, 03:29:25 pm »
It smelled fine before the SAMXON crap blew. So the stink you're smelling must be somewhere else...
 

Offline johansen

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Re: 475A Sweep
« Reply #10 on: December 09, 2014, 07:59:50 pm »
no need to get pissy about it.

did the cap fail shorted?

i have a 475 scope. i watched a rectifier burn out in front of my eyes after replacing one of the others.

furthermore the new cap with a much lower esr and higher capacitance pulls higher current pulses through the rectifier.. which almost explains why there seems to be a history of the diodes failing in those scopes after a recap.
 

Offline SoundTech-LG

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Re: 475A Sweep
« Reply #11 on: December 09, 2014, 08:06:56 pm »
Well, you are absolutely correct. The originally installed factory rectifiers were crap, and so was the SAMXON crap. Combine those you get a quick flash of fire and stink as the AC runs down the 60V unregulated line frying all the way, and yes, NO REASON to get pissy, or smelly about it. Blaming doesn't help.
 

Offline oldway

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Re: 475A Sweep
« Reply #12 on: December 10, 2014, 11:24:24 am »
When I worked in TV repair, we used to check whether the electrolytic capacitors that we replaced were warming or not.
If warming, it was urgent to unplug the unit before they explode.
 

Offline SoundTech-LG

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Re: 475A Sweep
« Reply #13 on: December 11, 2014, 10:11:35 pm »
No free-run in AUTO, but also TIME/DIV Variable knob shortens sweep length down to about an inch of the left side of CRT when all the way from the CAL position. Not sure if that is another problem, or part of the same problem. Had to take some time off from this.
 

Offline tautech

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Re: 475A Sweep
« Reply #14 on: December 11, 2014, 11:19:08 pm »
Post an image so we all can be sure of what you see.
I wonder if your latest hurt of the 475 might have you chasing all sorts of damage arising from that blow up.
Have you considered getting another 475?
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Offline SoundTech-LG

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Re: 475A Sweep
« Reply #15 on: December 12, 2014, 07:21:48 pm »
Yes. All of the above has been fully implemented. I will be comparing the new to the damaged. Will send some photos too. Worst case...  I'll have a parts unit.
« Last Edit: December 12, 2014, 07:24:21 pm by SoundTech-LG »
 

Offline SoundTech-LG

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Re: 475A Sweep
« Reply #16 on: December 17, 2014, 04:10:45 am »
Progress...

Full Free Run sweep in AUTO mode now. CR647 was leaky at Pin 8 of U600 sweep control IC. Not easy to find, having the buddy unit was almost essential as the reverse bias measurement had been made at least twice before I was able to compare. Interesting to note that the 475, as opposed to the "A", does not use CR647 there, instead, it uses VR647 a Zener to ground. I think this is an improved protection for U600, and CR647 sacrificed itself, and saved U600.

Still have the weird problem of the VAR Time/Div red knob shortening (reducing gain) of the sweep, when out of CAL position. Not sure if maybe some resistors around that pot changed value. Setting it aside again, because inspecting the timing switch board for the third time is irritating.

It does not seem immediately obvious why the Time/Div unCAL knob function would now be controlling Horizontal gain, as well as Frequency of the sweep. I'm sure it will become clearer. Lots of resistors in there.
« Last Edit: December 17, 2014, 03:59:33 pm by SoundTech-LG »
 

Offline SoundTech-LG

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Re: 475A Sweep
« Reply #17 on: December 17, 2014, 05:12:13 pm »
2 more things...

First, I found a link to the 475A Service Manual. Extremely hard to find online, stumbled across it.
http://docmesure.free.fr/manuels/Tektronix/Serie%20400/TEK%20475/TEKTRONIX.%20475A.%20SCOPE.%20INS.%2014678.pdf

Secondly, looks like an error in the 475 manual for the Var. Time/Div pot. voltages. Cannot find the source for +25 Volts. Otherwise that circuit looks pretty similar to the 475A.
 

Offline SoundTech-LG

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Re: 475A Sweep
« Reply #18 on: December 17, 2014, 06:58:16 pm »
Tek describes most of the circuitry, and gives quite good troubleshooting advice on most of the scope, but the Timing Switch, and its's connections to the Sweep Generator circuit are areas that are not well documented, and as convoluted as it is, probably with good reason...
 

Offline tautech

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Re: 475A Sweep
« Reply #19 on: December 17, 2014, 07:12:15 pm »
Tek describes most of the circuitry, and gives quite good troubleshooting advice on most of the scope
Have you got the Tek troubleshooting guide?
https://www.eevblog.com/forum/repair/repair-documents-and-links-sticky-me-please-mods/msg477909/#msg477909
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Offline SoundTech-LG

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Re: 475A Sweep
« Reply #20 on: December 17, 2014, 07:20:06 pm »
Yes I do, found some errors in there as well.

Here's another. R985 to +50 Volts is shown as 6.2 OHMS it is not. It is 6.2K ohms. Some signal flows added for fun.
 

Offline tautech

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Re: 475A Sweep
« Reply #21 on: December 17, 2014, 07:29:33 pm »
Quote
It does not seem immediately obvious why the Time/Div unCAL knob function would now be controlling Horizontal gain, as well as Frequency of the sweep
It shouldn't.
Wonder if as the rising amplitude of the sweep waveform is applied to the horizontal output amp, something is amiss there.
Have you checked linearity of the sweep stage by stage to the CRT plate outputs?

EDIT
Emphasis added
« Last Edit: December 17, 2014, 11:19:09 pm by tautech »
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Offline SoundTech-LG

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Re: 475A Sweep
« Reply #22 on: December 17, 2014, 10:22:25 pm »
It???  :-// What is it?


I seriously doubt the gain of the Horiz. amplifier is being changed by the Tim/Div unCAL pot, but stranger things have happened... ya' never know. I did check the amplitude of the sweep signal as it enters the input to the Horiz amp, and it is changing amplitude there as the control is turned. That pretty much eliminates the H. amp. The timing, and gain calibration are spot on, unless that control is turned out of CAL position. Compared to the leaky diode find :phew:, this should be fun, and easier to find. We'll see.
 

Offline SoundTech-LG

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Re: 475A Sweep
« Reply #23 on: December 18, 2014, 05:45:12 am »
Found another leaky diode. Did absolutely nothing to help the gain issue with the VAR Time.Div pot.
I was wrong about the sweep changing frequency with the pot. , it only looked that way. It should be changing the sweep frequency, but instead it attenuates the Horizontal gain,  :wtf: shortening the sweep. Not all that much fun so far. Looked, and checked at the timing switch board for the third time, but found nothing on there.
 

Offline tautech

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Re: 475A Sweep
« Reply #24 on: December 18, 2014, 06:54:23 am »
I was wrong about the sweep changing frequency with the pot. , it only looked that way. It should be changing the sweep frequency, but instead it attenuates the Horizontal gain,  :wtf: shortening the sweep.
The horizontal CRT plate output amps don't care how short the sweep is, they will faithfully reproduce it.
That's what a change in timebase does.

Then the trace is blanked, then sweeps again.

Does the service manual have waveforms and test points?
Do your results match these?
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