22-08-2019, 06:24 PM
Worked on this most of today.
1) The lack of tracking on 3 / 10 /30 /100 Grid voltage range:
I realised there is a 15nF cap to decouple the gm measuring Osc signal on to grid voltage. It was on the mA/V gain switch and a Hunts! Leaky, the only Hunts in the entire machine. Doesn't look like a "waxy". Plastic sleeve. Replacing that with a Polyester(metpoly) fixed that.
2) Calibration of AC mains.
In the absence of a variac and/or stable 240V AC the solution is to measure AC on the transformer used for Anode, Screen & Grid. It's fed from taps on the Heater transformer that has the switch and fuseholder. So put fuse in holder that allows front panel switch to be closest to mid point. In theory the 230V place, but my friend's house is rural and voltage drops a lot. It needs 220V position though here is 230V or a little more. Then adjust RV3 so left meter needle as at Cal while watching mains across the transformer kept at 240V by front panel switch. The 60uF cap isn't in circuit during normal test.
3) Calibration of Grid supply.
You need an AVERAGE reading meter. Analogue works by default, a DMM might work. A True RMS Fluke will give "wrong" reading! With 240V on the Grid/Anode/Screen transformer (see point 2), adjust RV2 for 52V where it feeds the resistors on the 3/10/30/100 range switch. Both sides of the pair of 2M2 resistors are handy. Then on 100V range put cursor of RV4 pot so when at "5" there is 26V. The cursor will slightly go past 0 and a good bit past 10. Then as you change switch you DO get 0.78, 2.6, 7.8 and 26.
While we were doing this the voltage dropped to 47 V approx and would not adjust! Intermittent connection on the wiper of the pot. Replaced the pot.
Note that RV4 grid voltage on front panel must be 10K 1% or better otherwise the calibration won't work. The pairs of resistors on 3, 10 and 30V ranges in conjunction with RV4 always give a 10 K Ohm load to the grid supply. On the 100V range the load is the 10K pot. Why is it -26V when scale is -50V, because the Screen and Anode are half wave raw voltages, no smoothing. So the Grid voltage on the "Test" position is negative half wave. Hence the -26V AVERAGE reading on a meter is equivalent to testing the valve at -50V. The calibration of the scales isn't the actual average Anode current a test meter will see, but the equivalent. So no actual voltage or current reading will match the knobs or readings. A true RMS meter is even more misleading than an average reading meter and can't give the readings specified in the Service Manual.
Next visit I will add the connector for the Tektronix curve tracer. The AVO VCM163 will provide valve bases, screen voltage, g3 voltage (if wanted and not 0V) and heater supply. The Tektronix has a Depletion J-FET mode, so bias is up to 0 to -20V and HT is up 1500V at loads of Watts! The Tektronix had dodgy switches, which operating and cleaning has mostly sorted, so I've managed to test n-jfets, enhancement MOSFET/VFET and regular bipolar NPN on it. It's very heavy!
1) The lack of tracking on 3 / 10 /30 /100 Grid voltage range:
I realised there is a 15nF cap to decouple the gm measuring Osc signal on to grid voltage. It was on the mA/V gain switch and a Hunts! Leaky, the only Hunts in the entire machine. Doesn't look like a "waxy". Plastic sleeve. Replacing that with a Polyester(metpoly) fixed that.
2) Calibration of AC mains.
In the absence of a variac and/or stable 240V AC the solution is to measure AC on the transformer used for Anode, Screen & Grid. It's fed from taps on the Heater transformer that has the switch and fuseholder. So put fuse in holder that allows front panel switch to be closest to mid point. In theory the 230V place, but my friend's house is rural and voltage drops a lot. It needs 220V position though here is 230V or a little more. Then adjust RV3 so left meter needle as at Cal while watching mains across the transformer kept at 240V by front panel switch. The 60uF cap isn't in circuit during normal test.
3) Calibration of Grid supply.
You need an AVERAGE reading meter. Analogue works by default, a DMM might work. A True RMS Fluke will give "wrong" reading! With 240V on the Grid/Anode/Screen transformer (see point 2), adjust RV2 for 52V where it feeds the resistors on the 3/10/30/100 range switch. Both sides of the pair of 2M2 resistors are handy. Then on 100V range put cursor of RV4 pot so when at "5" there is 26V. The cursor will slightly go past 0 and a good bit past 10. Then as you change switch you DO get 0.78, 2.6, 7.8 and 26.
While we were doing this the voltage dropped to 47 V approx and would not adjust! Intermittent connection on the wiper of the pot. Replaced the pot.
Note that RV4 grid voltage on front panel must be 10K 1% or better otherwise the calibration won't work. The pairs of resistors on 3, 10 and 30V ranges in conjunction with RV4 always give a 10 K Ohm load to the grid supply. On the 100V range the load is the 10K pot. Why is it -26V when scale is -50V, because the Screen and Anode are half wave raw voltages, no smoothing. So the Grid voltage on the "Test" position is negative half wave. Hence the -26V AVERAGE reading on a meter is equivalent to testing the valve at -50V. The calibration of the scales isn't the actual average Anode current a test meter will see, but the equivalent. So no actual voltage or current reading will match the knobs or readings. A true RMS meter is even more misleading than an average reading meter and can't give the readings specified in the Service Manual.
Next visit I will add the connector for the Tektronix curve tracer. The AVO VCM163 will provide valve bases, screen voltage, g3 voltage (if wanted and not 0V) and heater supply. The Tektronix has a Depletion J-FET mode, so bias is up to 0 to -20V and HT is up 1500V at loads of Watts! The Tektronix had dodgy switches, which operating and cleaning has mostly sorted, so I've managed to test n-jfets, enhancement MOSFET/VFET and regular bipolar NPN on it. It's very heavy!
