01-17-2019, 12:46 AM
Hello everyone, It's really an odd coincidence I found this thread just as I'm having some trouble in re-building one of these.
For such a simple circuit, I shouldn't be having any trouble but, it seems I am.
My plan was to change the electrolytic and selinium rectifier in my GD-1B.
On the way, I installed a grounded power cord and did some corrections to the original wiring.
So, I've spent quite a bit of time trying various schemes (I was going to try re-stuffing the original cap, but abandoned that idea.)
and figuring out ways to mount the new components.
So, here I am, everything complete, I've double, triple, quadruple checked all my wiring, tested all resistors, and everything looks great, but.....
My measured voltages are way too high!
I'm taking measurements with the variac at 50 VAC, which goes through the transformer, rectifier, supply caps.
On the schematic, it shows the output from the supply caps to be 95 VDC, then passing through a 6800 ohm resistor to arrive at pin 1 of the tube with 45-52 VDC.
So even though I'm not applying full voltage (50 VAC), my pin 1 voltage is 60 VDC and follows any change I make in line voltage.
I considered that I'm using a silicon diode to replace the selinium rectifier, but there is a 1K resistor between the 20 Mfd caps and adding another 100 ohm resistor seems like a drop in the bucket when the tolerance of that 1 K resistor is considered.
I've been measuring and inspecting everything I can think of and can detect no shorts or parallel circuit paths to explain the lack of a voltage drop across the 6800 ohm resistor, which I measure at 7100 ohms.
I'll try taking a few pics tomorrow,
If anyone has any ideas about what I could check, I'm all ears.
Thanks,
Robert
For such a simple circuit, I shouldn't be having any trouble but, it seems I am.
My plan was to change the electrolytic and selinium rectifier in my GD-1B.
On the way, I installed a grounded power cord and did some corrections to the original wiring.
So, I've spent quite a bit of time trying various schemes (I was going to try re-stuffing the original cap, but abandoned that idea.)
and figuring out ways to mount the new components.
So, here I am, everything complete, I've double, triple, quadruple checked all my wiring, tested all resistors, and everything looks great, but.....
My measured voltages are way too high!
I'm taking measurements with the variac at 50 VAC, which goes through the transformer, rectifier, supply caps.
On the schematic, it shows the output from the supply caps to be 95 VDC, then passing through a 6800 ohm resistor to arrive at pin 1 of the tube with 45-52 VDC.
So even though I'm not applying full voltage (50 VAC), my pin 1 voltage is 60 VDC and follows any change I make in line voltage.
I considered that I'm using a silicon diode to replace the selinium rectifier, but there is a 1K resistor between the 20 Mfd caps and adding another 100 ohm resistor seems like a drop in the bucket when the tolerance of that 1 K resistor is considered.
I've been measuring and inspecting everything I can think of and can detect no shorts or parallel circuit paths to explain the lack of a voltage drop across the 6800 ohm resistor, which I measure at 7100 ohms.
I'll try taking a few pics tomorrow,
If anyone has any ideas about what I could check, I'm all ears.
Thanks,
Robert
