Products > Test Equipment
Blew Up My First Oscilloscope!!!
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Gyro:
The USP of the Rifa caps is higher dV/dT rating than Polypropylene, that's why they still use them.
Calambres:
Someone should start a database of appliances using those infamous RIFA caps!
tautech:

--- Quote from: Calambres on February 28, 2019, 07:43:03 pm ---Someone should start a database of appliances using those infamous RIFA caps!

--- End quote ---
You'd be amazed how large it would be, they were used in a very wide range of mains powered gear.
Zenith:

--- Quote from: JohnPen on February 28, 2019, 06:44:43 pm ---High value Resistors with high voltages across them used to be a problem with the old CRT TV years ago.  The focus potential used a chain of high value carbon resistors, often in the Megohm range, and they used to shift dramatically in value as they aged.  Sometimes down by several Megs other times similarly upwards.  High voltages across resistors, particularly the carbon based ones,  are definitely not very good for them. :(

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It's often forgotten that resistors have a voltage rating which you can exceed without exceeding the wattage, and if you do you alter the behaviour of the resistor. Carbon composition resistors tended to drift high after a few years anyway

I've seen special high voltage resistors in old scopes, which I think were quite long with a spiral carbon film track. They weren't that reliable, but people tried to replace them with the porcelain bodied 1/4 watt carbon composition resistors which died very quickly. Farnell and others used to stock modern metal glaze resistors rated for 3kV or so, I don't know if they still do. I never had any problems when I used them as replacements
David Hess:

--- Quote from: duak on February 28, 2019, 06:23:09 pm ---Some 2200 series scopes also have problems with some 470K or so 1/4 W resistors in the focus circuit.  If the the scope is apart, look for a group of  resistors with a bit of space around them.  The PCB itself might also look a bit stressed.  If I remember correctly, Tek used 5 or so of them in series to handle a 1 KV or so potential.
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The 2213/2215 used carbon composition resistors in the focus resistor chain.  The 2235 and 2213A/2215A, which are really a different but related series from the 2213/2215, also used carbon composition resistors in the focus resistor chain.  Oddly enough the 2236, which was released at the same time as the 2235, uses high voltage film resistors instead (or at least the service manual says) and I think all of the later instrument do also.


--- Quote from: Zenith on February 28, 2019, 07:53:23 pm ---I've seen special high voltage resistors in old scopes, which I think were quite long with a spiral carbon film track. They weren't that reliable, but people tried to replace them with the porcelain bodied 1/4 watt carbon composition resistors which died very quickly. Farnell and others used to stock modern metal glaze resistors rated for 3kV or so, I don't know if they still do. I never had any problems when I used them as replacements.
--- End quote ---

I did some research on this in connection with the 2235 series several years ago and found evidence that the high voltage film resistors Tektronix used to replace the carbon composition resistors in the high voltage circuits eventually became the Vishay VR25 and VR35 series of high voltage film resistors which have good pricing and availability now.
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