Re: Tek 576 Curve Tracer Repair

Don Black <donald_black@...>

So, Tektronix not only supplied extra silver solder in their scopes, they also had extra magic smoke to allow for some escaping. That's pretty impressive.

Don Black.

On 13-Apr-13 9:12 AM, c1ferrari wrote:

Hello all!

This is a reply from Daniel Schoo regarding my "smokin' Tek 576"...I found it illuminating and hope others will benefit from seeing it:

Hi Sam,
You ran the plate current up too high for the tracer. It can and will supply currents well in excess of the dissipation rating of the series resistors. Welcome to the club. Forgive my attitude but I just had to laugh when I read your message. I think anyone who has operated a tracer for any length of time has smoked the series resistors. While it seems like you have done serious damage there is little to be concerned about. The resistors are quite robust wirewound types and will survive. Rest assured that isn't the first time your tracer has let out a little smoke and it will continue to operate as well as it did before. You didn't do any discernable damage.

The series resistors are located on the left side of the tracer behind the MAX PEAK VOLTS / SERIES RESISTORS switch. You take off the left side cover and inside you will find a brown Micarta insulation plate. the plate is attached with four nylon screws. Remove the screws and take off the cover. You will see several large 'dog bone' resistors lined up in a frame behind the switch. Those are the series resistors. As you can see they are not about to be hurt by a little overheating. I doubt that you will notice anything unusual about them. When you put the cover back on tighten the nylon screws carefully. They don't need to be tight, just snug and you can easily break them if you try to tighten them too much. Also they are probably a little brittle from age and even more easily broken. Do not substitute metal screws. The frame that they are attached to can have high voltage on it and metal screws would expose that voltage.

Allow me to quote from my article on the 576 curve tracer:


"The maximum rated plate current for the 350 volt range is 500 milliamps. At the nominal 350 maximum peak volts setting with the 220 watt series resistor of 140 ohms you can calculate the approximate maximum plate voltage at various currents using ohms law. Starting at zero plate current you would have about 400 volts on the plate plus or minus a few volts depending on your AC power line voltage. At 0 milliamps the drop across the 140 ohm series resistor is about 7 volts so you have 393 volts on the plate. At 100 milliamps the drop is 14 volts leaving 386 volts. At 200 milliamps the drop is 28 volts leaving 372 volts. At 500 milliamps the drop is 70 volts leaving 330 on the plate. The calculated plate voltages are higher than actual since there are more internal losses than just the series resistor. The maximum rated plate current for the 1500 volt range is 100 milliamps.

For the 1500 volt range the 220 watt series resistor is 3000 ohms. You would not ordinarily plot a family of curves on an audio tube with a 1500 volt plate supply so let's assume something more realistic. For the plate supply set to 600 volts the 50 milliamp voltage drop on a 3000 ohm resistor is 150 volts leaving 450 volts on the plate. At 100 milliamps the drop is 300 volts leaving 300 volts on the plate. At 200 milliamps you lose all 600 volts across the resistor, you are dumping 120 watts into it and burning it up. The curve tracer will actually supply a lot more current than it is rated for so you must pay attention to the settings and not allow the plate current to go much above 100 milliamps on the 1500 volt range or 500 milliamps on the 350 volt range."


--- In TekScopes@..., "c1ferrari" wrote:
> Hi all,
> I bought a 576 from eBay with a Tektronix cal from Dallas which expired 01-DEC-10. Everything was going well and I was learning how to use the instrument...until today.
> I was attempting to characterize 6C33C tubes when smoke began to emanate from the top array of ventilation ports on the machine's left side. Immediately, I attenuated the Variable Collector Supply and switched the machine off.
> I did not observe any POP or other sound in connection with this event.
> Now that I've recovered, somewhat, my equilibriium -- I'm writing the post to ask this august group if there is anyone, any resource in my neck of the woods -- San Diego, that can take a look at my beloved Tek 576 CT.
> Please, let me know.
> Very best regards,
> Sam

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