08-04-2015, 01:13 PM
Though the DM70 'magic eye' shines brightly on the display, no null occurs on any of the capacitance or resistance ranges with a component under test, so I've now got round to furtling with it!
I've replaced the two Hunts caps, and two electrolytics I referred to in post #18, which I'd hoped might produce an improvement but it hasn't. All of the transistors test OK, and the oscillator is running (I can even hear it!). I'd wondered if the two diodes in the voltage multiplier might be defective, but I doubt it, because at the junction of C3/D1, the voltage is a respectable 84 Volts, which - after D2, is raised to 118 Volts. The voltage on the anode of the DM70 is 92V, and the grid is minus 7.8 Volts, which doesn't vary when the Balance Control knob is swung round with a component under test. Those Voltages seem about right to me, and suggest that the two diodes in the Voltage multiplier which lift the Voltage from 9V aren't suspect. The range switch looks fine, and I've cleaned the contacts with Servisol.
I don't really understand the operation of the circuit, though I do understand the 'Wheatstone Bridge' principle, in which - when all four arms of the bridge are in balance, no current will flow across the bridge. I assume that in the circuit when the bridge is in balance with a component under test, a negative voltage is presented to the base of TR2, which TR2 & TR3 then amplify to make the Voltage on the grid of the DM70 go more negative, thus causing the 'exclamation mark' display to shorten to give an unmistakable 'null'.
To make the circuit more understandable (to me anyway!) I've redrawn it without the range switchery and have shown just what I've termed a 'Standard Resistor' - that being the one for the range which suits a resistor under test. (I could just as easily of course, shown a 'Standard Capacitor' and shown a 'capacitor under test' at the test terminals).
Firstly, I'd be grateful if anyone who has a working Nombrex Bridge could confirm that yes - there is an unmistakable null when the bridge is in balance with a component under test, causing the DM70 display to shorten almost to nothing. Secondly, any thoughts as to where to look next please?
I try not to just pull components out on a lucky dip basis to test them in the hope that I might come across one that's duff. The four caps I replaced were likely suspects, the diodes and transistors seem OK, and short of taking the rest of the components out one at a time to test and replace if found faulty - methodical though that might be - I'd prefer a more rational approach.
Waffled and dribbled 'Confused, of Cottingham'!
I've replaced the two Hunts caps, and two electrolytics I referred to in post #18, which I'd hoped might produce an improvement but it hasn't. All of the transistors test OK, and the oscillator is running (I can even hear it!). I'd wondered if the two diodes in the voltage multiplier might be defective, but I doubt it, because at the junction of C3/D1, the voltage is a respectable 84 Volts, which - after D2, is raised to 118 Volts. The voltage on the anode of the DM70 is 92V, and the grid is minus 7.8 Volts, which doesn't vary when the Balance Control knob is swung round with a component under test. Those Voltages seem about right to me, and suggest that the two diodes in the Voltage multiplier which lift the Voltage from 9V aren't suspect. The range switch looks fine, and I've cleaned the contacts with Servisol.
I don't really understand the operation of the circuit, though I do understand the 'Wheatstone Bridge' principle, in which - when all four arms of the bridge are in balance, no current will flow across the bridge. I assume that in the circuit when the bridge is in balance with a component under test, a negative voltage is presented to the base of TR2, which TR2 & TR3 then amplify to make the Voltage on the grid of the DM70 go more negative, thus causing the 'exclamation mark' display to shorten to give an unmistakable 'null'.
To make the circuit more understandable (to me anyway!) I've redrawn it without the range switchery and have shown just what I've termed a 'Standard Resistor' - that being the one for the range which suits a resistor under test. (I could just as easily of course, shown a 'Standard Capacitor' and shown a 'capacitor under test' at the test terminals).
Firstly, I'd be grateful if anyone who has a working Nombrex Bridge could confirm that yes - there is an unmistakable null when the bridge is in balance with a component under test, causing the DM70 display to shorten almost to nothing. Secondly, any thoughts as to where to look next please?
I try not to just pull components out on a lucky dip basis to test them in the hope that I might come across one that's duff. The four caps I replaced were likely suspects, the diodes and transistors seem OK, and short of taking the rest of the components out one at a time to test and replace if found faulty - methodical though that might be - I'd prefer a more rational approach.
Waffled and dribbled 'Confused, of Cottingham'!
Regards, David.
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
