Predicta 17" HV question

[Don Lindsly]

Fold over at the pix bottom indicates over drive. Back off until the fold over disappears, then measure the 6DR7 grid for proper wave form amplitude. If OK, then the output transformer and yoke are all that are left.

I measured the VOT primary on a working test chassis using three different meters. They all indicate 415 ohms within a few ohms. That is no guarantee yours is good.

You can temporarily clip yoke lugs Y5 and Y6 together to eliminate the thermistor.

[Phil Nelson]

Here's a waveform from pin 3 of the 6DR7.



I didn't see any vertical foldover when that was taken. The height control was fully CW, giving maximum height.

A while back I measured resistance of the vertical yoke windings from the plug. Sams says each winding should measure 18 ohms and the thermistor 4 ohms cold. I measured 18 ohms between pins 10-11, and 25.2 ohms between pins 9-10 which includes the thermistor. So my thermistor measures 7.2 ohms cold, I guess. No idea what it measures when hot, but at least it doesn't look shorted or open.

I see lugs Y-5 and Y-6 on the Philco schematic. I assume you pull the yoke to get access to them.

Phil Nelson

[DaveWM]

cant tell the PP from that scope pic. can you check the cathode current on the vert out, per sams 30ma.

[Phil Nelson]

I replaced the most recent scope pic with one that's easier to read. Refresh this page in your browser to make sure it shows up. The vertical volts/cm control is set to 5 volts/cm and the horizontal sweep time/cm control is set to 2 ms. The probe is at 1x.

I got out my HP 428B milliammeter to measure the current on the cathode (pin 9). Looks like about 38-39 ma. (The meter's set to 100 ma scale and the needle points to about .38-.39.)



I also tried measuring the old fashioned way by bridging a .1 mfd cap on the cathode connection and putting my probes across the cap leads. With my old Triplett analog meter, the reading looked like 36-38 ma, in the same ballpark.

If I interpret any of this correctly, it seems that cranking the Height control all the way up sends both the grid voltage and the cathode current too high, yet I'm still not getting full vertical deflection.

While doing all this, a couple of times I heard a distinct snap from the yoke area. Maybe it's time to remove the yoke and look at that thermistor.

Phil Nelson

[DaveWM]

if I read that right then the PP is only 30v on pin 3 sams shows 140v. Unless I am just reading it wrong, is the 5v for each square or each division inside each square in which case it 150v

so now you need to scope pin 6, should be 140v pp same shape.

[Phil Nelson]

According to my scope manual, that vertical setting is 5 volts per square, so 6 squares in the previous photo would equal 30 volts, but I don't use a scope to measure voltage, so don't count on me having it set up correctly for that.

I turned down the Height control to reduce the pin 3 grid voltage to around -20V as given in the manuals and connected my DMM as well as the scope. This photo shows the voltage reading and waveform at pin 3 (compare to waveform W8 in the Sams manual):



Without changing the Height control or scope settings, I switched the probes to the pin 6 plate (compare to waveform W7 in Sams):



Pin 3 voltage is about -21V (Sams says -20V) and Pin 6 voltage is about 131V (Sams says 115V). The Philco manual calls for 130V on pin 6, so the measured 131V is good enough for me.

Executive summary: on that plate and grid, the waveforms match the models and the voltages look OK, but the height is still badly deficient -- about 2 inches high. Great picture, otherwise.

Phil Nelson

[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.