22-04-2012, 06:26 PM
A few comments re the Bridge Paul:
The three resistors and three caps on the tagstrip under the chassis to the right hand side are of course the 'standards' by which any external resistor or cap under test is measured. Thus, the accuracy of the bridge depends upon the accuracy of the value of those six components. The resistors are carbon rod types and as they're tipped with gold at one end, that signifies that when new, those resistors had a tolerance of +/- 5% so weren't close tolerance for use in a piece of test gear. (Oddly, we might reasonably expect gold to signify the closest tolerance but it doesn't - grey is the most accurate - +/- 0.05%). The colour code of the resistors is the old 'body, tip dot' code so for example, the one with brown body, black tip and green dot is 1 MegOhm. The other (brown/black/orange) will be 10K and the lowest value resistor will be 100R. Even when new, the 1 Meg resistor could have had a value which would have been +/- 50k, ranging from 950K to 1.05 Meg. With the passage of time, it's quite likely that all three will have drifted high in value, so it might be worth checking them, if only out of curiosity. As to the caps, they're 100pF, .01uF and 1 uF. The two lower values look like silver mica ones, and will most likely still be bang on spec, but the 1uF is likely to be well out of spec as well as being poorly.
You'll see that the range selector switch has an 'external' position which switches out all of the internal res/caps to enable you to match two caps or resistors to compare them.
With old test gear, there are two approaches - conservation versus restoration.
By the standards of the day, the test gear - whatever it is and whoever made it - Avo, Hunts, Taylor et al, it was adequate for most purposes in which it had to earn its keep - radio repair workshops and later, TV, but by today's standards, in terms of accuracy it falls far short of even the most modestly priced multimeters, so other than the enjoyment of using old test gear to repair old radios, there is no logical argument for using it, and arguably, it should be conserved (ie, left alone) rather than restored. However, many collectors do like to restore test gear and put it back into working order, and if for example, you were to replace those carbon rod resistors with modern 1% ones, it will have a better accuracy than when it was first made, but at the same time, that destroys the originality of a piece of equipment that you're unlikely to ever want to use.
I'm not arguing the case either way - each of us has to decide whether we want to conserve a particular piece of test gear, or to restore it.
Certainly if you do intend to use the bridge, I think that those resistors, and possibly the 1 uF cap will have to go, but that depends on what you find when you measure their values. As things stand, it would make no sense to measure a resistor or cap on the bridge if the standard resistors and caps are out of tolerance because if the component under test is itself close to tolerance, the bridge will say otherwise.
IMHO, the scales are tilted more towards conservation rather than restoration with test gear than with radios, because of course most of us do wish to restore radios to working order, and to use them, and many of us prefer the sound to that of modern transistor offerings or DAB (let's not go there!).
Just a few thoughts - others may see things differently, which is fine by me.
The three resistors and three caps on the tagstrip under the chassis to the right hand side are of course the 'standards' by which any external resistor or cap under test is measured. Thus, the accuracy of the bridge depends upon the accuracy of the value of those six components. The resistors are carbon rod types and as they're tipped with gold at one end, that signifies that when new, those resistors had a tolerance of +/- 5% so weren't close tolerance for use in a piece of test gear. (Oddly, we might reasonably expect gold to signify the closest tolerance but it doesn't - grey is the most accurate - +/- 0.05%). The colour code of the resistors is the old 'body, tip dot' code so for example, the one with brown body, black tip and green dot is 1 MegOhm. The other (brown/black/orange) will be 10K and the lowest value resistor will be 100R. Even when new, the 1 Meg resistor could have had a value which would have been +/- 50k, ranging from 950K to 1.05 Meg. With the passage of time, it's quite likely that all three will have drifted high in value, so it might be worth checking them, if only out of curiosity. As to the caps, they're 100pF, .01uF and 1 uF. The two lower values look like silver mica ones, and will most likely still be bang on spec, but the 1uF is likely to be well out of spec as well as being poorly.
You'll see that the range selector switch has an 'external' position which switches out all of the internal res/caps to enable you to match two caps or resistors to compare them.
With old test gear, there are two approaches - conservation versus restoration.
By the standards of the day, the test gear - whatever it is and whoever made it - Avo, Hunts, Taylor et al, it was adequate for most purposes in which it had to earn its keep - radio repair workshops and later, TV, but by today's standards, in terms of accuracy it falls far short of even the most modestly priced multimeters, so other than the enjoyment of using old test gear to repair old radios, there is no logical argument for using it, and arguably, it should be conserved (ie, left alone) rather than restored. However, many collectors do like to restore test gear and put it back into working order, and if for example, you were to replace those carbon rod resistors with modern 1% ones, it will have a better accuracy than when it was first made, but at the same time, that destroys the originality of a piece of equipment that you're unlikely to ever want to use.
I'm not arguing the case either way - each of us has to decide whether we want to conserve a particular piece of test gear, or to restore it.
Certainly if you do intend to use the bridge, I think that those resistors, and possibly the 1 uF cap will have to go, but that depends on what you find when you measure their values. As things stand, it would make no sense to measure a resistor or cap on the bridge if the standard resistors and caps are out of tolerance because if the component under test is itself close to tolerance, the bridge will say otherwise.
IMHO, the scales are tilted more towards conservation rather than restoration with test gear than with radios, because of course most of us do wish to restore radios to working order, and to use them, and many of us prefer the sound to that of modern transistor offerings or DAB (let's not go there!).
Just a few thoughts - others may see things differently, which is fine by me.
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'
