I thought it best to start a new thread in the Test Gear section, so here goes:
The oscillator of this dinky little Bridge works 'cos I can hear it, and the magic eye works too (which the circuit refers to as a V1 - balance indicator'). I've checked the waveform on the scope (about 20V P-P) and the frequency on my trusty homebrew frequency counter, (which has its fortieth birthday next year, when I have my 75th!). It's 896 Hz - near enough 1kHz. Not important what it is - it has no bearing on the accuracy.
There are some holes in the PCB and someone has pencilled letters near them. At first sight I wondered if it's been got at and some components snipped out, but as the letters are 'A', 'G' and two of 'H', I think it probably signifies the anode, grid and heaters of the little magic eye. Originally the Bridge used the long since obsolete PP4 9V battery which had large (PP9 style) press stud ends, and I guess that the simplest thing would have been to have fitted smaller PP3 connections and use a PP3 battery, but instead, an external power socket has been fitted for a 9V battery eliminator. Maybe the current drain to feed the magic eye is a bit heavy on batteries? Looking at the circuit of the 'Type 32' - which I assume is very similar to the Type 62 - D1 & D2 look to me to be a voltage doubler which goes to the anode of the magic eye, though via a 2 M resistor. Also, as stated above, the waveform of the oscillator is about 20V p-p.
At first I was puzzled as to the role of the 'neon' as it didn't light up in any position, so it was obviously not an on/off indicator. The on looking at the circuit, I realised that the only position in which it is in circuit is in the 'L' position, when no range resistors/caps are in circuit for the bridge function. 'L'? Of course - Leakage! Shorting out the test leads causes it to light up. I guess that putting a Hunt's cap across the test leads would have the same effect!
Not sure what the control is which is marked 0 - 70 also marked 'PF%'. It's a 100 Ohm linear pot which only comes into circuit in the 1uF range and is in series with a 1uF non-electrolytic range cap.
I've attached a few pics, including the circuit of the 'Type 32' but if Rob does indeed have a copy of the 'Type 62' Bridge, I'd much appreciate it. (The '32' circuit shows 3 x 5uF electrolytics, but I can only see two on the PCB of this '62' Bridge). I'll start by replacing those, and a couple of smaller value Hunts (0.1 and 0.01uF).
I must say that it's nicely conceived and constructed, and aesthetically pleasing. I well remember Nombex instruments regularly being advertised in PW for some years.
The oscillator of this dinky little Bridge works 'cos I can hear it, and the magic eye works too (which the circuit refers to as a V1 - balance indicator'). I've checked the waveform on the scope (about 20V P-P) and the frequency on my trusty homebrew frequency counter, (which has its fortieth birthday next year, when I have my 75th!). It's 896 Hz - near enough 1kHz. Not important what it is - it has no bearing on the accuracy.
There are some holes in the PCB and someone has pencilled letters near them. At first sight I wondered if it's been got at and some components snipped out, but as the letters are 'A', 'G' and two of 'H', I think it probably signifies the anode, grid and heaters of the little magic eye. Originally the Bridge used the long since obsolete PP4 9V battery which had large (PP9 style) press stud ends, and I guess that the simplest thing would have been to have fitted smaller PP3 connections and use a PP3 battery, but instead, an external power socket has been fitted for a 9V battery eliminator. Maybe the current drain to feed the magic eye is a bit heavy on batteries? Looking at the circuit of the 'Type 32' - which I assume is very similar to the Type 62 - D1 & D2 look to me to be a voltage doubler which goes to the anode of the magic eye, though via a 2 M resistor. Also, as stated above, the waveform of the oscillator is about 20V p-p.
At first I was puzzled as to the role of the 'neon' as it didn't light up in any position, so it was obviously not an on/off indicator. The on looking at the circuit, I realised that the only position in which it is in circuit is in the 'L' position, when no range resistors/caps are in circuit for the bridge function. 'L'? Of course - Leakage! Shorting out the test leads causes it to light up. I guess that putting a Hunt's cap across the test leads would have the same effect!
Not sure what the control is which is marked 0 - 70 also marked 'PF%'. It's a 100 Ohm linear pot which only comes into circuit in the 1uF range and is in series with a 1uF non-electrolytic range cap.
I've attached a few pics, including the circuit of the 'Type 32' but if Rob does indeed have a copy of the 'Type 62' Bridge, I'd much appreciate it. (The '32' circuit shows 3 x 5uF electrolytics, but I can only see two on the PCB of this '62' Bridge). I'll start by replacing those, and a couple of smaller value Hunts (0.1 and 0.01uF).
I must say that it's nicely conceived and constructed, and aesthetically pleasing. I well remember Nombex instruments regularly being advertised in PW for some years.
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'
