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Old 27th Jan 2019, 2:39 pm   #1
Diabolical Artificer
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Default Tektronix 585A issues.

A few days ago the above turned off/stopped working. I found the mains fuse blown, so after checking the mains tfmr, and a thorough visual check inside and a quick check of the main filter caps I powered it up with a lower value mains fuse in - 1A fast type instead of 4A slo blo, smoke issued, SW off. R670 was toast, looked for shorts, found C671 shorted, replaced both, these on the +100v rail.

Scope now powers up but as soon as the delay period is over, EG 30 seconds and K601 latches, fuse blows. Have checked every rail for shorts, checked R's in V dividers as per Tek manual, checked res to ground of rails, checked the cap across K601 and K601 coil res, checked the voltage regulator valve for short and checked all big smoothing caps. Removed plugin. Checked all diodes.

Stumped, any ideas? Andy.
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Old 27th Jan 2019, 3:50 pm   #2
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Default Re: Tektronix 585A issues.

hello Andy,
I think that must be one of the electrolytics, a isolation fault inside.
This must not be defective in test with ohm meter, but when the high voltage is present it do short.. How you have tested them, and are you very shure you have tested them all?

the area of +325V is the most powering there, it can blow out the general fuse.
The best idea is, to use a tubes PSU in series with a bulb of 20 Watts and check one by one the disconected electrolytics. One of them will be bad and the light goes ON ..

Martin
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Old 27th Jan 2019, 6:12 pm   #3
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Default Re: Tektronix 585A issues.

Thanks Martin, I checked them with a DMM, will check at WKG V.

A.
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Old 31st Jan 2019, 7:33 am   #4
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Default Re: Tektronix 585A issues.

Checked most of the big caps and whilst they're all leaky, some over 1mA, there are non that are SC at wkg voltage so I suspect it's something further downline. The problem is that the fuse blows after 20/30 secs so it's hard to check voltages etc in that time, I'm going to have to see if I can disconnect the TB, CRT PSU etc in order to confirm the various voltage regulators are working ok. It's a bit of a nightmare though as this is a complex scope with a wiring loom more complex than a 737. Wire colour code isn't given on the schematic.

Will investigate further, Andy.
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Old 2nd Feb 2019, 4:46 pm   #5
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Default Re: Tektronix 585A issues.

Step one is the -150V stage.
give that a fresh cap, chck the stabi tube ( 85A2 mostly) , and the tubes there.
This -150V is the reference voltage for all other psu voltages, if there is a problem the complete scope is out of control.

Martin

( I have also a 585A )
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Old 10th May 2019, 3:05 pm   #6
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Default Re: Tektronix 585A issues.

Dropped back on this in the last few days and have narrowed things down. As all is well until K601 latches I check for shorts at each relay contact but found none, however with the wire at D672, the K601-4 SW that connect to D702 A/B the main fuse strays intact, so the problem maybe in the circuits around V664, V667A, the +100v regulator.

What is the purpose of connecting the 140v and dual 114v windings, anywhere I should look? One indicator may be that C671 had shorted. C670A checks ok as far as SC is concerned, that is without power applied. I tested for a SC from the top of R667 to ground, again for a SC, none found.

I was going to check the 6080's and driver valves for htr/cathode shorts next.

Andy.
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Old 10th May 2019, 4:49 pm   #7
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Default Re: Tektronix 585A issues.

Do you have a valve tester, Andy?
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Old 11th May 2019, 8:00 am   #8
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Default Re: Tektronix 585A issues.

A homebrew one and a big HV PSU so I can sort of test valves. I was thinking this fault is unlikely to be a valve as the fault presents as a dead short.

Andy.
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Old 11th May 2019, 8:18 am   #9
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Default Re: Tektronix 585A issues.

Another thing to check is the rectifier diodes. I have not heard of one going short or open in this sort of scope - but as they are early silicon it could be worth a look.

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Old 11th May 2019, 10:46 am   #10
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Default Re: Tektronix 585A issues.

You need to check each leg of D672 for short circuit.

Mike.
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Old 17th May 2019, 6:10 am   #11
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Default Re: Tektronix 585A issues.

All diodes check ok Mike.

Still plugging away at this, going through fuses like mad, luckily I have some Russian NOS types, boxes full.

Anyroad, tested 6080's and all 6AU6A's out of the PSU, they're all fine, spent yesterday disconnecting CRT (325v unreg to CRT) and C679 as well as testing the latter. Concluded it has to be something attached to or the 100v PSU is at fault.

There's a dead short in there somewhere, mains fuse vapourises, but where? Tracing where all the wires go is what takes time, slowly slowly catchee monkey an all that.

Andy.
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Old 17th May 2019, 8:47 am   #12
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Default Re: Tektronix 585A issues.

Unplug K600. Then hotwire the -150V, then the +100 etc. That will tell you exactly which supply line is responsible, and narrow the search.

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Old 17th May 2019, 11:21 am   #13
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Default Re: Tektronix 585A issues.

Quote:
Originally Posted by Diabolical Artificer View Post
Wire colour code isn't given on the schematic.
Hello Andy, I don't know if you've worked it out but I believe the HT Supplies are colour coded as below;

-150V Brown with Black tracer
100V White with Brown tracer
225V White with Red and Brown tracers
350V White with Orange and Green tracers
500V White with Green and Brown tracers

We only had one 585A in the department where I worked, but IIRC you can see the colours at the HT test points at the rear of the top deck on the RHS, immediately in front of the EHT box.

Incidentally I've always thought the insulation was a bit thin, however I've not heard of any issues due to this.

Regards
Mike A.
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Old 17th May 2019, 1:28 pm   #14
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Default Re: Tektronix 585A issues.

Quote:
Originally Posted by Diabolical Artificer View Post
What is the purpose of connecting the 140v and dual 114v windings
Andy.
The HT voltages are obtained by adding together the DC from the 140V bridge with the 114V bridge. From what you describe I assume yours is the earlier version that switches this link?

I assume the K601 drops in after the heaters have warmed up. This would cause a huge surge that might blow the fuse particularly if the scope is left unused for a while and the electrolytics need 'reforming'. It is possible they moved the switch to the +325V line in later versions for this reason.

You can test and reform capacitors in circuit. With the mains off and the plugin removed, use your high voltage DC supply through a series resistor to each of the PSU lines and monitor the current. You should not exceed the working voltage of the HT line and should keep adjusting the resistor to limit the current to say 10mA. Lots of 2W resistors are best as there will be plenty of heat to dissipate. The electrolytics should slowly reform and the HT voltage gradually increase. If the voltage refuses to increase you should investigate.
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Old 18th May 2019, 8:06 am   #15
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Default Re: Tektronix 585A issues.

"Unplug K600. Then hotwire the -150V, then the +100 etc. That will tell you exactly which supply line is responsible, and narrow the search." I've been doing this Craig, but not by unplugging K601, the main problem is finding the interconnects, there isn't a handy PSU PCB with headers, I'm narrowing it down, albeit slowly.

"Hello Andy, I don't know if you've worked it out but I believe the HT Supplies are colour coded as below;" I hadn't Mike, thanks for that, that'll help sort things out.

"The HT voltages are obtained by adding together the DC from the 140V bridge with the 114V bridge" Thanks PJL, looking at the schematic again it's obvious. I've found one SC electrolytic and the others will need replacing, though ripple isn't bad. The scope was in weekly use so the electrolytics should have been formed, I have a cap reformer, thanks for the info.

" It is possible they moved the switch to the +325V line in later versions for this reason." There is a note on the schematic "K601-4 relocated S/N 9240" Not sure what S/N etc means.

Am slowly getting my head round this, it being a complex beasty it takes some time to map things in the old noggin. Thanks for the replies, Andy.
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Old 18th May 2019, 8:22 am   #16
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Default Re: Tektronix 585A issues.

Hi, S/N etc means that particular change was implemented from that specific serial number onwards. The serial number is on a small plastic insert on the front panel. I would check each individual diode in the various HT bridges for a dead short. Ross
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Old 18th May 2019, 8:51 am   #17
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Default Re: Tektronix 585A issues.

Quote:
Originally Posted by n_r_muir View Post
I would check each individual diode in the various HT bridges for a dead short.
The fuse only blows when the relay kicks in so I don't think it can be the bridge rectifiers.

How confident are you that you are using the correct fuse type? I would expect the surge to kill a fast blow fuse.
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Old 19th May 2019, 7:30 am   #18
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Default Re: Tektronix 585A issues.

Thanks, will check SN. Checked all diodes and replaced two that looked manky even though they tested ok.

For testing I'm using an under rated 3A, as I have about 100, it should be a 4A Slo , however I'm running it up on a variac at about 180v, the fuse element vapourises indicating a SC. I've tried a 5A Slo periodically, still blows.

As I said previously if I cut the link at D762 C by un soldering a wire link, K601 latches, fuse don't blow but R700 gets V hot and starts smelling, therefore it has to be the 325v unreg/225v PSU or the 100v PSU, though one would think the former.

I then unsoldered the 325v from the CRT and also C769 A/B and C, still blew.

Andy.
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Old 20th May 2019, 10:17 am   #19
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Default Re: Tektronix 585A issues.

If R700 gets hot when you have removed the link that is made with K601-4, the most likely explanation is the mains transformer has a short.
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