Should I be concerned w/these John E. Fast caps (as in leaky)?

Early stage of restoring a Tektronix 585A oscilloscope manufactured about 1964. These caps are abundant in the LV and HV power supplies. When I've restored older Tek scopes, they often had Sprague BBees or BBoD paper caps in these same positions, leaked like sieves and had to be replaced. The manual says these are 'MT' caps (OTOH, 'PTM' described the older sprague caps which I suspect was Teks way of indicating paper caps.).

These look different and I've never encountered them before. Typical values are .01, .001 and .047 in the 400vdc or 600vdc rating. Pic (link below) is from the HV oscillator/power supply section.

Anyone have any experience w/these? Are they paper, poly or what? Worth giving a shot as-is?

http://antiqueradios.com/gallery/v/temp ... d.jpg.html

Thanks!

In modern terminology, MT might be Metallized Terylene (Mylar). Easy enough to find out: check one for leakage. Fast generally made excellent capacitors.

Agreed. I've seen them often in other oscilloscopes, and Heath provided them for use in their modulation monitor scope kits.

Any cap can go bad over time, however. Best to test.



L

Agree with the others that those caps are very high quality.

If the scope belonged to me, I would not embark on shotgun re-capping without testing a few of the caps for leakage on a high voltage insulation tester. Chances are that most of those caps are just as good as the ones that would be used for replacements. I'd just troubleshoot the circuits and replace any caps or other components that actually turn out to be bad.

On the other hand, the brown ones from the 1950s are no better than Black Beauties. Seeburg jukeboxes used them a lot.

Make sure to use silver bearing solder if you do change parts on those ceramic
turrets. Otherwise the metal to ceramic bond with fail.

Pete

Thanks everyone for the input!

I tested one of these when I was reforming an electrolytic to 350V in an isolated circuit. In summary, no leakage (and the electrolytic came back fine too). If it had any leakage - that would have shown up because the coupling cap blocks the dc path to ground. The cap has a 400V rating (it is C806 in a 585a).

I realize that's a small sample size - hopefully indicative of the rest. At this point, after settling all other issues, I'm probably going to power up the scope and troubleshoot from there. Paying close attention to the power supplies along the way.

Researched the Tek coding a little more. 'PTM' may represent 'Paper Tubular, Moulded' based on input from another poster. So 'MT' may mean 'Moulded Tubular' in Tek speak. Indeed upon close examination, the John E. Fast cap has a moulded plastic outer case along it's axial body. The dialectric inside still remains a mystery.

Thanks again and best regards!

Update: The John E. Fastcaps, circa 1964, discussed above work just fine.

As mentioned, these are used in both Low/High Tek 585a scope power supplies. I got it up/running (with some effort) and the supplies are regulating just fine and in spec. And yes, good pointer, I do use silver solder (2% from RS works ok). Now I have to troubleshoot a 'no sweep' issue but that's another story!

So again - I appreciate all the input.

BTW - I've either forgotten or there's a new method for including photos in ARF post. Could someone point me to the current instructions.

Just follow the menu boxes in the reply window: choose your photo on your computer (must be 800 x 800 pixels or less), upload it, place inline. Way easier than most other forums.

I don't know how many models Tektronix did this for, but whenever we get one in the shop I like to pop the side panels off and look for the little spool of solder they provided just for using on those ceramic circuit tie-points. They are pretty rare after all these years, I think I've only ever seen three or so in over thirty years.



You can see the little spool snapped into the back horizontal panel by the right rear upper corner.



In case you think Peter was just promulgating an urban legend (or if you lost your service manual), Tek themselves gave you a warning about using the correct solder right on the chassis of the scope!

--Bob