Author Topic: Tektronix 545A CRT Vertical Fualt  (Read 488 times)

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Offline TekHead

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Tektronix 545A CRT Vertical Fualt
« on: September 24, 2021, 08:24:15 pm »
Hello, I'm back to working on my old Scope. I Made a lot of progress from when I last touched it.

(Preface)
I ended up reseating every tube, Inspecting the top to see if it turned white and dusted it (SilkScreen is still intact). I went back to my last forum post and read all the suggestions and Replies that I got. I started with checking the -150v rail with my multimeter and adjusted it until it was 150 and all the other rails where Fine, Next I adjusted the high voltage to 1350v. I got my other scope set up for reading high voltage and checked chassis grounding. I managed not to blow up my other scope and did some troubleshooting of the power supply. The Test Point (6.3 Not under ELEV .225 bracket) has ripple (+/- 10V). (Other then that everything works except my issue)
(TLDR. Rail Voltages are in Range (-150,100,225,350,500,-1350), Rails has Ripple (+/- 10V)).

(Repair Help)
The issue that I'm having is that the vertical of the CRT Is causing the Trace to go off the CRT. I thought that there was something wrong with the CRT Because there was just an Moving smudge. one time I started the Scope and the trace started at the bottom of the CRT as an smudge then an trace on the screen that started to move up the screen and then going off the screen at the top. Turing into another smudge.

After that I unplugged the upper and then the Lower CRT deflection plate (One at a Time) and got a Good trace with the horizontal sweep working Fine along with the timebase. The issue is that I don't know why the vertical don't work properly causing the trace to go off the CRT.

Everything on the Surface seems fine (Minus the Ripple) but I have some suspects I think that either the vertical amplifier or the type CA Plugin is faulty. both look visual fine (Tubes aren't cracked, etc) the issue is that none of the pots (Vertical Position & Vertical Position Range control on the ca Plugin) have no effect. The other two pots on the vertical Amplifier have no effect either (GAIN ADJ, DC SHIFT).

TLDR. Rail Voltage is fine, (+/- 10V) Ripple On Rials, Visual Inspection is Fine. CRT Vertical is Faulty making the trace to go off the CRT. I Unplugged the Vertical Deification Plates and everything was Fine. Vertical POT's have no effect on trace Position, Vertical Amplifier and CA Plugin are my main culprits.

If anyone has any suggestions/ideas/help then it is greatly appreciated. Thank you in Advance! (Images 1,2,3 are what happens on startup  and #4 is with the vertical deflection plates Unplugged)
« Last Edit: September 24, 2021, 10:14:12 pm by TekHead »
 

Offline TekHead

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Re: Tektronix 545A CRT Vertical Fualt
« Reply #1 on: September 24, 2021, 10:13:19 pm »
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Online med6753

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Re: Tektronix 545A CRT Vertical Fualt
« Reply #2 on: September 25, 2021, 04:46:01 am »
Your vertical amplifier is out of balance. With power on pull the CA plug-in. The vertical amplifier should automatically balance at the center line. If it does the CA plug-in is at fault. If not then the vertical amplifier itself is at fault. A balanced vertical amplifier should have approx +310V on each vertical deflection plate. If one plate measures much higher than the other then that's why the trace is off screen. Start with that and post results. And as always use caution around high DC voltages.   
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Offline TekHead

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Re: Tektronix 545A CRT Vertical Fualt
« Reply #3 on: September 26, 2021, 01:42:10 am »
So I have a Bad Update. I Unplugged the CA Plugin and there was Just a smear on the screen. I also checked and the Plate voltages and they are not the same I think the Vertical amplifier is the issue.

Now for the Bad Part. The High Voltage Section Let the Magic Smoke Out, the 1350v is now 300~ I did not really get to see where the smoke came from specifically because I had the H.V. shield in place. I did some research and found out some stuff. My Scope has a beeswax coated transformer that I don't think is arching (I turned off the lights and saw nothing). the H.V. adjust Pot seems to make no difference. There are 5 tubes next to the transformer (Listed as Supply Rectifiers). they are still glowing and nothing looks cracked. Nothing seems off. (Except for the missing 1000V).

Is there any common failure points for the H.V. supply?.
« Last Edit: September 26, 2021, 01:44:21 am by TekHead »
 

Offline TekHead

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Re: Tektronix 545A CRT Vertical Fualt
« Reply #4 on: September 26, 2021, 05:44:54 am »
So I Have a Question about Tubes (I don't know a lot about the way Tubes Fail). On the Hv Power supply section there are Vacuum tube rectifiers. in the photo I took there is one tube that looks fine but the rest seem to be getting progressively darker. Have they failed? and if so what can I Replace them with?. Thanks in advance (Sorry for the glare).
« Last Edit: September 26, 2021, 05:51:59 am by TekHead »
 

Online med6753

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Re: Tektronix 545A CRT Vertical Fualt
« Reply #5 on: September 26, 2021, 07:27:44 am »
If there was smoke with careful examination you should be able to see what component let the magic smoke out. Yes, the HV transformer in the Type 545A is wax impregnated and not prone to failure unless severely overloaded. Those HV rectifiers are 5642 vacuum tubes and readily available on Ebay. The 3 that are darker appear to be a different maker than the other 2 and it could be just a darker getter material. If the filaments light up and there's no flash over within the tube they are probably OK.

Someone has been in there before. Some Gorilla who doesn't know how to solder. Those joints are absolutely shitty. I suggest closely checking over the HV area and cleaning up those solder joints. Sharp points like those in that picture can invite flash over and corona discharge.

And make absolutely certain before you go poking around in there to discharge the HV lead to the CRT. 
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Offline TekHead

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Re: Tektronix 545A CRT Vertical Fualt
« Reply #6 on: September 26, 2021, 10:34:02 am »
So I ended up Taking out the H.V. Power supply Cleaning it/checking it up and putting it back in. The good News is that I did not make the issue worse. the bad news is that it did not fix the issue. My next suspect it That either the Red capacitor is faulty or the resistor is faulty. (Yes the resistor is that black in real life). The resistor is 3.6 Mega Ohm. The resistance is not listed on the schematic. (The Capacitor is an open Circuit(Break), No Capacitance).

Do you know what the resistance of the indicated resistor is supposed to be? and is the Capacitor bad if it has no resistance and no Capacitance?
Thanks in Advance
« Last Edit: September 26, 2021, 10:46:30 am by TekHead »
 

Online med6753

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Re: Tektronix 545A CRT Vertical Fualt
« Reply #7 on: September 26, 2021, 11:23:59 am »
Don't know where you got that schematic but it is incomplete. Go to TekWiki and download a better manual.

That is R836 and it should measure 1 MEG, not 3.6 MEG. Looks like you found your burning culprit. The capacitor is C836 and should be 500pF.
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