Predicta 17" HV question

[miniman82]

Use the 10x probe setting, it's likely throwing off your readings.

[DaveWM]

to further illustrate the point of scope reading for voltage compared to meters, read the test point with you meter, should be 2.5v not 5v PP as on the scope. the square wave duty cycle will avg out on the meter between 0 and 5v pp. since its duty cycle is the same on and off, the meter will just avg and give you 2.5. Sine waves are different scope PP will be 1.41 IIRC more than the meter. This is why you get 165 vdc out of a 120vac supply, the PP of the 120vac is actually higher, and when rectified and filtered it shows up.

when reading odd shape waves it gets really wacky. So sams gives both the meter voltage AND the PP to make it easy to see whats going on. I dont know the input impedence of modern DMMs prob as high as a VTVM, but I still prefer the VTVM again due to how odd shaped waved may effect the DMM.

I do use the scope for reading DC voltages, like B+, you can use the DC setting to check voltage, and then look at the line for ripple. IF the DC is correct you can switch to AC and take a closer look at the ripple.

[Phil Nelson]

If the meters are to be trusted on pins 3 and 6, then I'll quit fretting about them, since the meter readings match the specs and the waveforms are shaped correctly.

The final question concerns pin 7, the other grid, where the waveform looks correct (per Philco manual) but the voltage looks low, less than -1V where the manuals call for -3.2V to -3.5V.

http://antiqueradio.org/art/6DR7Pin7...ystVoltage.jpg
http://antiqueradio.org/art/6DR7Pin7FlukeVoltage.jpg

As I understand it, the pin 7 grid returns feedback from the pin 1 plate. I have replaced the K5 couplate, which does the returning.

I also replaced capacitor C43 (.0015/1KV) by wiring two .00033/630V caps in series to make one .00165/1260V. Since then, I got a ceramic .0015/1KV cap which I could substitute for C43 if the difference between .0015 and .00165 seems that critical. (Only a 10% difference, right?)

On that part of the tube, the plate voltage on pin 6 is correct and the cathode voltage on pin 8 is in the ballpark (actual 6.5V; Sams 7.5V).

Beyond that, I'm back my original question whether the output transformer is bad. I ordered a replacement from Moyer, so I'll soon be able to test that by substitution.

Regards,

Phil Nelson

[DaveWM]

I also replaced capacitor C43 (.0015/1KV) by wiring two .00033/630V caps in series to make one .00165/1260V. Since then, I got a ceramic .0015/1KV cap which I could substitute for C43 if the difference between .0015 and .00165 seems that critical. (Only a 10% difference, right?)


Phil Nelson[/QUOTE]

2 .00033 in series would be .000165 not .00165 off by a factor of 10x that would certainly cause problems. if that is the case it point to the reason why I alway check what comes out to what goes in on my old cap tester. even leaky caps will generally give a good enough value check to compare to new caps going in.

[DaveWM]

oh and would not trust the series 630's as a replacement for a 1000 anyway, that cap has a tough job dealing with all kinds of spikes etc... not sure if the voltage would evenly distribute across a series connection.

[Phil Nelson]

Dang, I will go back and look at those caps to see if I really installed the wrong values or just mistyped the value now from bad memory.

Thanks,

Phil "I was told there would be no math" Nelson

[ChrisW6ATV]

Here is a Mouser .0015/1600V film capacitor:

http://www.mouser.com/ProductDetail/...71iJrRleNpY%3d

I used that one or a similar value just now in my CT-100.

[Phil Nelson]

Ah, under the mysterious "snubber" category. When I searched in the usual categories, everything of that value was unstocked, with 20+ week lead times. Or maybe I was too restrictive in the voltage rating. Searching their website is kind of a black art.

Phil Nelson

[DaveWM]

shoot, I was really hoping you had .000165 in there. I doubt the new cap will solve the problem. I know you have tried a new tube as well...

I am about out of suggestions, other than just going over your work with a fine tooth comb. Yes the snubbers are the place to find HV caps.

You have tried adjusting the lin and height as they interact.

[ChrisW6ATV]

Originally Posted by Phil Nelson

Searching their website is kind of a black art.

You summed it up well there, Phil. I only knew how to find those easily this time from all the tinkering a couple of weeks ago when I first needed to find one or more over-1000-volt capacitors. I think I clicked "capacitors", then "film" (which has sub-categories as you mentioned but I ignored them), then the value, then different voltages until I found some in stock. At that point I removed the value filter and found they stock a bunch of those CDE caps in different values. They fit right in with other orange, red, and brown dipped capacitors.

[kvflyer]

Good on 'ya Mate!

[bandersen]

I'm glad your perseverance payed off.

[DavGoodlin]

Super-techs! Congratulations on overcoming vertical challenges!

[old_coot88]

BOO-YAH!! Houston we have vertical!

[Big Dave]

In the couplates I made, On the one shown, I divided the resistance by 2. Then, I divided the capacitance by three. That value came out to about 0.0012. I found that using 0.001 caps worked fine. I found the values aren't that critical. Then again, Predictas are weird. Anything goes. Nice work, Phil.