Re: TEK514 needs new 5V4G tube

Trevor
 

Steve-
Looking at the schematic I downloaded for the 514, there are five capacitors in the power supply that are two section 20 micro-farad cans, and these all should be tested, reformed, or replaced.  A sensible approach for testing and reforming is to use a separate laboratory bench power supply with a current limiting series resistor.

Be careful with C210, as it has its outer aluminum can (which is the negative capacitor lead) connected to the +225 volts supply. This is common practice in Tek tube scopes, and the case of a capacitor at elevated potential is protected with an insulating blue paper or clear plastic sleeve.

Protective sleeves are also used with capacitors in the negative supply section, as with C216 where the positive capacitor lead is grounded.


Newer 500 series scopes typically have a thermal time delay relay, so the scope's plate voltages are delayed 30 seconds or so, allowing tube filaments to warmed up first. The 514 does not have a delay relay, but the 5V4 rectifiers, with their indirectly heated cathode will yield a desirable slow plate voltage ramp-up. Replacing the 5V4's with solid state rectifiers has some appeal, but note that the slow ramp-up will no longer occur.

Note also, there are three 5V4 tubes in parallel. Ideally, they should share the current equally, but if one tube is near end of life, the other two will need to work harder. Verifying that the three rectifiers are sharing the load equally presents a challenge. Perhaps your infrared thermometer would work. A tube tester would help, if you have access to one. A riskier approach is to remove just one 5V4 at a time, making two remaining rectifiers do the work of three, but just for brief testing. Observe the voltage on C212 to see how good the two installed 5V4s are.


Vacuum tubes typically have an area inside the glass envelope that is shiny and mirror like, which is the "getter flash". Faulty tubes can sometimes be identified by this flash being white, meaning the tube's vacuum has been compromised. You should visually compare the three 5V4s for other differences, such as sputtered metal debris. Test with a DVM for dead shorts too of course.

Work slowly and methodically, and not when tired. The power supply voltage are dangerous. Besides shock hazards, flying molten metal from accidental short circuits can cause eye injury.
Trevor

On Tuesday, January 28, 2020, 6:57:26 AM EST, David Holland <david.w.holland@...> wrote:

On Mon, Jan 27, 2020 at 10:05 PM Chuck Harris <cfharris@...> wrote:

A hack saw works fine too... if you are careful.
If you completely remove the cap (I usually do), a copper tubing
cutter works wonderfully....

One of these things:
https://www.harborfreight.com/tubing-cutter-40913.html   (Yeah, I
know, Harbor Freight, but they did have a convenient picture.)

David

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