Re: New member introduction and several Tek 545A questions.

Dave Wise
 

The 545A has the time delay relay that's common to all the 500 series "convertible" mainframes. A variac will not bring up anything slowly except the tubes' heaters. At some point the relay will pick, and at that point the filter caps will get the full jolt of whatever the voltage is at the time.

On the other hand, Tek used some exceptional filter caps in these models, and I have never had to replace one.

I respectfully disagree with Dan about the "bumblebee" paper caps. Shoot on sight.
On the other hand, the 545A uses newer caps, and you may be able to pick and choose - if you are a seasoned technician thoroughly familiar with the instrument.
I have the impression this is not so, but figuring it out is a good way to get familiar if you want.

Dave Wise

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...]
Sent: Friday, October 13, 2017 2:06 PM
To: TekScopes@...
Subject: Re: [TekScopes] Re: New member introduction and several Tek 545A questions.

Hi Robert,   Not sure if you've received any more replies from to your post.  I have 545 and there are also a few others here who can be of assistance in your repair.
  Somewhere I have a "treatise" for those starting out on 500 series scopes based on my experiences fixing them.  I'll post if I can find it.
  I think the 545 is one of the scopes that can benefit from a "slow start" with a variac to slowly energize the caps and re-form them.   It's not clear to me that it's really all that beneficial a process though.  At this point they are either good, or totally shot.  I've never bothered doing that.  If you do however, pull the time delay relay out so you don't provide voltage to the rest of the circuitry until you are up to full voltage and the main power supply runs without blowing fuses.  
  I would not plan on pulling anything until you know if it works or not, and that includes bumble bee caps.   My scope ran fine for years with dead or leaking BB caps.  In a lot of cases there's a resistor in parallel with a cap so a little leakage isn't going to kill the circuit operation.  There's one exception and that's the BB cap feeding the Vertical Signal Out connector on the front panel from the cathode follower preceding it.  V1223A is connected between +500V at the anode and +225V on the cathode, so any leakage there has the potential to electrocute the user.  Replace that one!
  I've been using yellow jacketed polypropylene capacitors (from Mouser I think) to replace bumble bees.  Just about any modern capacitor is better and smaller than the originals, so I just bought a whole bunch in all the values I need at 630V rating and use those for replacement.  
  I didn't get the picture of the HV area.  I bet it's ok though.  Probably dust, ionized by the HV section and stuck to all surrounding surfaces over the years.  I wouldn't touch the HV section till you know there is a problem there.  You can possibly make it worse by creating discharge paths by cleaning it, so clean only if you see sparks or arcing.   I'll leave it to others to comment more on cleaning the scope.  I would guess that you can wash it with water and mild detergent, but be prepared to blow dry the scope and keep water out of places where it won't come out, like the main transformer.   It would be best if you could bake the entire scope after a bath to dry it out.  I've done this with plug-ins, but then again I have a vacuum oven in which it will fit.   Using distilled water is pointless if you are going to use soap - it won't be distilled anymore.
  Dan

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