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Help please with Tek 545A Oscilloscope
Help please with Tek 545A Oscilloscope
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Old 9th December 2018, 02:43 PM   #1
Jazid is offline Jazid  United Kingdom
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Default Help please with Tek 545A Oscilloscope

HI all,
I have one of these venerable instruments that has been working ok up until yesterday. I started it (been a few months since it was last on) and all was good, it settled down fine but after maybe an hour there were a couple of flashes and some sharp cracks from the right hand side before I got to the off switch. No obvious magic smoke but there is a rather large fan operating in it...


I let it discharge and had the sides off, visually nothing seems obviously amiss. Tested a couple of the electrolytics and they seem fine and there's no bulges or evidence of gassing out; on one there is evidence of slight venting but I've had it up to full voltage for a fair few minutes and it seemed to be happy enough, no leakage, measures right, and didn't heat up.


The question is where should I start on this? There are a power of a lot of components in there, approx 80 valves, maybe 300 capacitors, anyones guess for the resistors. Anyone fixed one of these or know what tends to fail, or any idea where might be the best place to start looking? I'd need a month of Sundays to have any chance of fixing it otherwise with my limited spare time :-(



All help gratefully received.
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Old 9th December 2018, 03:31 PM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Check the EHT supply has not flashed over. It is 8.5kvolts and extremely dangerous!
V832,842 and 852 are the tripler rectifier diodes. There is a capacitor on the business end, that would be my first port of call.
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Old 9th December 2018, 04:59 PM   #3
Jazid is offline Jazid  United Kingdom
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Thanks for the reply Jon, appreciated. I don't have experience of such voltages, and beyond knowing flashovers can occur don't really know what they might look like after the event. As a result I've added a pic below to show what the EHT compartment looks like. The rear side of the 500pF 10kV is blackened but not unduly so, it reads 430pF when tested at 10kHz in situ. The trafo looks like it's got quite hot as well, no idea if this is unusual though?


Click the image to open in full size.

Click the image to open in full size.
Click the image to open in full size.

Last edited by Jazid; 9th December 2018 at 05:10 PM.
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Old 9th December 2018, 05:55 PM   #4
jackinnj is offline jackinnj  United States
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Help please with Tek 545A Oscilloscope
The TekScopes user group always has the answers.

You might know that the ceramic strips can be difficult to solder, and if they get dirty shouldn't be cleaned with anything abrasive.
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Old 9th December 2018, 06:43 PM   #5
dhaen is offline dhaen  Europe
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Exclamation Disconnect mains first

Then I'd start by brushing all the dust from the HV points. Then re-power to determine if the HV is faulty or whether the flashover just blew up something else.

You can also check whether the HV rectifiers - the 5 valves/tubes heaters are lighting up. They might be quite dim. If they're alight then HV is probably OK.

As Jack says, those ceramic insulation blocks are fragile. Supposedly you should only use silver loaded solder otherwise the plating can come away from the ceramic. At least that's what those old Tek manuals used to say - in-between the cartoons.

Another note of caution: there is probably a +ve HV supply for the CRT final anode and a lower -ve HV supply for the cathode. Most scopes used this system so that the deflection plates could vary relative to zero volts.
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Old 9th December 2018, 10:54 PM   #6
Jazid is offline Jazid  United Kingdom
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Thanks guys! So does it look like there has been a flashover or is that not clear/evident from the pics? Or could it have been eg. a resident spider who took an unfortunate turn? No sign of fried arachnids here, just trying to learn as I go.

Also if I clean and fire it up will heat dissipation be OK inside with the sides off and the fan not functioning optimally?
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Old 10th December 2018, 05:21 PM   #7
Jazid is offline Jazid  United Kingdom
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Hi,
Well I've bunked off work for the rest of the afternoon and cleaned the EHT area and surroundings, squirted a bit of contact cleaner on the rotary switches, and re-soldered the caps and wires with Cardas eutectic which I read is 3% silver or thereabouts. I use a Metcal which delivers heat pretty fast so I am hoping that my soldering, whilst not always Tektronics grade, will be adequate enough.
I plugged it in and fired it up and to my surprise I have a functioning display and indeed a very nicely functioning display, on the amplitude calibrator it is completely stable, much better than before. It has been on so far for maybe 50+ minutes without bangs so fingers crossed, and whilst about my work I noticed a suspicious scorch mark which in on the pic below - do you think this might be evidence of that flash?
.
Click the image to open in full size.



. However it brings me inevitably to a couple of other issues which I ask your indulgence with below;


1. It has the CA plugin and it appears only the B horizontal timebase is working. Channel B works with the Channel A timebase but there's a blank screen with its own timebase.


2. There is a major discrepancy between the amplitude calibrator and the Channel A which I knew was outstanding but hadn't ever addressed up to when it went bang as I tend to use my other scope (Tek 340A). The 545A calibrator is not too accurate currently - the 5 V P-P measures around 5.14 on the 340A, but on the 545A it measures as approx 1.7V P-P . It is spot on through the B channel.


Any ideas here please kind souls?
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Old 10th December 2018, 07:36 PM   #8
dhaen is offline dhaen  Europe
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I don't think there are any quick answers to the problems you describe (but I'd like to be proven wrong).
If you haven't got one, get a manual. There are online sites that have them free. At worst you'll have to buy one for a few bucks.
These scopes aren't rocket science. With respect for the high voltages you'll learn a lot following the cal signal through. Something will show as amiss. Maybe even check that both Y deflection plates are being driven.
As for the A timebase, check that the trigger signal is getting there.
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Old 10th December 2018, 08:06 PM   #9
Jazid is offline Jazid  United Kingdom
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Thank you, I shall do my reading up for the cal signal. I have a manual luckily, with parts and annotations for the subs carried out on that particular model. Phew! I checked the basic regulatd voltages and they are mostly spot on, the worst are the 100V which runs at 97.9 and the HV which should be 1350 but reads 1380. Does that seem about acceptable to you?
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Old 10th December 2018, 08:19 PM   #10
dhaen is offline dhaen  Europe
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You are smart for checking the supply voltages first. Many so-called engineers forget the basics.
The difference in HV will cause a very minor change in size of the display of about 2% but is probably within normal tolerance.
What I meant was to apply the cal signal and trace it through the Y channel.
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