Tektronix 547 Oscilloscope reserection

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Hello fellow dyiaudio enthusiasts, I thought I would share a photo of my latest fun electronics project, a Tektronix 547 dual channel scope. This scope was made in late in 1968 I believe with an unknown history of how long it had been setting without use. I tested all the tubes (guessing about 35 or so) and found that 6 needed replaced. After sourcing the tubes, I defeated a delay HV relay so it could be closed with an external bench supply before any line voltage was applied. This was done to allow me to reform as many of the capacitors in the system as possible using a variac and light bulb. I progressively brought up the line voltage over a few hours and observed no unusual signs of anything heating up or drawing excessive current. I also wanted to share that my visual inspection prior to starting the reforming process resulted in finding no capacitors that appeared to be in trouble. All electrolytics looked brand new, dry and not deformed. All tubular capacitor were high quality plastic film or Tektronix custom made. Long story short, the scope came alive but did not function correctly until I found two bare wires that were touching (shorted together) in the CA plug in vertical amplifier. Once the wires were cleared of the short and a few pots and switches cleaned the scope began working properly. I was very thrilled and excited to find a bright crisp and sharp display. Keep in mind that this scope is 48 years old. I have now been running it for the most part of a week and all is great! For what its worth, I spoke to an Engineer who works at the Tektronix Instruments Museum and he said that not replacing the caps did not surprise him at all. They have had very good results in bringing up old Tektronix instruments for the museum this way and only replacing actual failed components. Moderators, please feel free to move this post to another part of the forum if it is better suited there! Thanks!
 

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The Tek 547 is one of my favorite old scopes. I used one for years and still have it, but it's retired to a corner of the shop and no longer used. I also have a 585A somewhere. (Lots of bugle boy 6DJ8s in there) I now use a Tek 7904.

After many years of service my 547 developed a strange problem. After a couple hours of operation the high voltage on the CRT slowly went down and the trace would expand horizontally as a result. Eventually the trace would fade out. Turning it off and letting it cool would restore normal operation for a time. This seemed like a simple case of thermal overload from something in the HV power supply. Turned out to be the HV transformer itself. But it wasn't getting overly hot.

If you study the schematic there is a capacitor across the HV transformer primary. But the cap is shown in dotted lines because there is no physical capacitor there. It refers to the internal distributed capacitance of the winding itself. Placing a 1000pfd mica cap across the primary helped somewhat but was not a cure all. The HV oscillator would start running at a lower frequency at first and slowly increase as it warmed up. The HV regulation circuit kept things going as long as it could but after several hours it wouldn't. That's when I decided to retire it. But it was a great instrument that served me many years. The 500 series is what made Tek's reputation as a great company. The 454A was also very good. But after that plastic parts became the rule and that was the beginning of Tek's fall from grace in my opinion.

BTW, get yourself a 1A1 plug-in if you can.
 
HollowState and Conrad Hoffman, I am sorry to hear that both of your scopes are experiencing the humidity related HV problem. This is a somewhat common problem with some of these older 5xx series scopes. A friend of mine who works at the Tektronix Instrument museum knows of a process that has shown success with fixing the humidity problem. The process uses a vacuum pump to remove the humidity from the transformer and then the HV transformer is resealed with bees wax. If either of you want more info on this process I could likely get it. Mickeystan
 
. Unfortunately my 545 also has the fade-out problem every now and then. Keeping the scope in a very low humidity environment seems to have helped a lot. How did you establish it was the HV transformer?
By using well established servicing techniques and the process of elimination. After checking capacitors, rectifiers, resistors and even substituting the CRT the transformer was the only thing left. In my case I question the humidity because, even though my shop is in the basement, I run a dehumidifier in the summer. Mine seemed to be more time related, although it was many years ago.

Mickystan,
Thanks for your offer to check up on the fix. As I remember the HV transformer windings were encapsulated with a hard rubber or perhaps plastic casing of some kind. They were not exposed as many earlier and also newer types were. Also I don't plan on using it again because it's too old and lacks bandwidth by today's standards. I do more then just audio work with a scope.
 
You could try sourcing a transformer from an older 500 series scope, these are likely not potted in epoxy but beeswax and seem to last forever.

The majority of the 500 series transformers use the same +8650V and -1350 cathode voltages, therefore most transformers can be interchanged between models.

Models to look for (from the top of my head) are the 535a 541 and 543.

By the way, there are a couple of 535A transformers on ebay.

As for the tread, I have 585A 545A 545B and a 535A all in relatively good working order, and a couple of 543's that seem to have an array of different problems.
 
My own 547 (1A1 plugin) has been in daily service for over 20 years. I believe I've changed two tubes. Before coming to me it was a line scope,and ran almost continuously.Absolutely bulletproof,and sensitive enough to pick up a fart on Mars.My all-time favorite scope,and heats the shop in the winter too:D

I also have a 515A,535,545B and a 2235 for road work.They are addictive...

Other than size,weight,heat and fan noise (none of which I care about) I don't think there's a better machine.Or one that will ever last as long.
 
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