Predicta 17" HV question

[DaveWM]

the neg voltage readings don't tell the whole story since its derived from the neg bias of the HOT via the voltage divider. in this case I would try to figure out why the PP is not correct.

[DaveWM]

you should be able to calibrate the scope if it has a test point on the scope, generally a .5v square wave. then you adj the center of the voltage knob to make if fill one square when its set to 500mv. OR your can just use a 9v battery, set the scope to DC and see if you get apporx 2 squares of rise when set to 5v

[DaveWM]

can you confirm the resistance readings on the 6DR7 esp the pin 7

[Phil Nelson]

Here are the 6DR7 resistance readings.

1: 406 ohms (measured from 275V B+ source)
2: 1.97 megohms
3: 1.97 megohms
4: .1 ohm
5: zero
6: 1.68 megohms (measured from 275V B+ source)
7: 67.3 K ohms
8: 51 K ohms
9: 217 ohms

The resistance on 6, 8 and 9 varies with control settings.

The R63 thermistor is behind the yoke. I shorted it out with a jumper and tried the set. No change in height.

Phil Nelson

[Phil Nelson]

I checked the scope's vertical calibration using its native signal (jack at upper right), a 0.5V square wave. With the Chan B vertical control set to 0.1V/cm, the trace is about 5 squares high, so 5 x 0.1V = 0.5V. Things add up.



Then I changed the Chan B control to 5V/cm and checked pin 3 of the 6DR7 with a couple of meters (an old Voltohmyst VTVM and a new Fluke DMM). I left the photos full size so you can read the dials & meter settings. The meters show -20V to -21V, and the scope seems to agree (4 squares x 5V = 20V). Things still add up.

http://antiqueradio.org/art/6DR7Pin3...ystVoltage.jpg
http://antiqueradio.org/art/6DR7Pin3FlukeVoltage.jpg

I left the vertical control unchanged -- still at 5V/cm -- and then checked pin 6. The meters agree, both showing around 130V. The waveform has the right shape, but if you count the squares, they don't add up to 130.

http://antiqueradio.org/art/6DR7Pin6...ystVoltage.jpg
http://antiqueradio.org/art/6DR7Pin6FlukeVoltage.jpg

So, my VTVM gives the same readings as my DMM, for what that's worth. And my scope can give sensible voltage readings for some test points, but not pin 6 for some reason. I'm using a plain vanilla scope probe, 1x. Possibly I'm overlooking something obvious, or I don't have the right equipment to learn much more from measuring these points.

Phil Nelson