22-01-2012, 10:54 AM
I hadn't realised how long it's been since I reported. In mitigation it has been a bit brisk in the garage. Things have warmed up a bit - even Col isn't complaining - so ....
Having proved that the af sections work via the Gram sockets I figured it was time to reinstall V1 and see what was what. In a nutshell there were two problems. First there were some classic crackles at various parts of the tuning indicating that the plates were catching somewhere. Second, the Volume Control was still doing the same thing, that is the volume increases until the Reaction kicks in and then it goes bananas.
So, tuning gang first. Actually both problems require the removal of the Left hand cover plate, and as I've done it before it was no big deal. A couple of the split end plates looked a bit close so I bent them carefully, bearing in mind that Col managed to snap one off. That, plus a blast of compressed air and some Servisol, mostly cured it except right at one end. So far so good.
Now the Volume control - see attached. As far as I can tell from the circuit and the description it's supposed to work as follows: R12 and C20 are ganged. Minimum volume is maximum R12 and no Reaction, ie maximum negative bias on V1. As the knob is turned R12 reduces to zero and then stays there - there's a solid piece of track after the wirewound section - and C20 is rotated. As far as I can tell the resistance of R11 plus R12 does just what it says on the tin. The bias on V1 varies from about -9 to -2, which seems reasonable. After that the volume from the speaker drops and there's lots of whistles, indicating I think too much Reaction. One other potential problem was the singed resistor. I believe that it's R10, which should be 10k. However, it reads about 40k. On the other hand the Screen voltage, and the Anode voltage for that matter, on V1 are about right, so I left it for the moment. The data does say that there were various modifications round that area so it may actually be the right value. At that stage I thought I'd just have a look to see what was oscillating. I glanced down at my trusty 455 only to find it blank. The fuses are OK so I've brought it up to the house to thaw out.
The Pye information says that the whistling problem may be caused by a low value C18. Well, at least that's pretty easy to get at so out it came and a modern one was tacked in its place. The best I can say is that it made a marginal improvement. The other interesting factor about it is that whereas the diagram says it should be 25u it's marked 10. Presumably that's big enough. There is a code on it which may imply 1934, which would be OK.
So, it could be the 'Volume Assembly', or at least the Capacitor half of it. It's quite difficult to see exactly what's happening as it's solid dielectric. I've given it a blast of air and Servisol which hasn't made one iota of difference. My enthusiasm for taking it to bits is very low.
I think at this stage the best bet is a careful measurement of all Rs and Cs around that area.
One last comment; I don't know the state of V1. It actually is a Mazda AC/SG which is right - the others aren't - and it's voltages and currents look reasonable, but who knows. I've been looking for a spare but they seem pretty thin on the ground.
The sun is actually shining and my Weather Station says 10.3 outside, but being Sunday I expect I'll be hauled off to 'do things'
Alan
Having proved that the af sections work via the Gram sockets I figured it was time to reinstall V1 and see what was what. In a nutshell there were two problems. First there were some classic crackles at various parts of the tuning indicating that the plates were catching somewhere. Second, the Volume Control was still doing the same thing, that is the volume increases until the Reaction kicks in and then it goes bananas.
So, tuning gang first. Actually both problems require the removal of the Left hand cover plate, and as I've done it before it was no big deal. A couple of the split end plates looked a bit close so I bent them carefully, bearing in mind that Col managed to snap one off. That, plus a blast of compressed air and some Servisol, mostly cured it except right at one end. So far so good.
Now the Volume control - see attached. As far as I can tell from the circuit and the description it's supposed to work as follows: R12 and C20 are ganged. Minimum volume is maximum R12 and no Reaction, ie maximum negative bias on V1. As the knob is turned R12 reduces to zero and then stays there - there's a solid piece of track after the wirewound section - and C20 is rotated. As far as I can tell the resistance of R11 plus R12 does just what it says on the tin. The bias on V1 varies from about -9 to -2, which seems reasonable. After that the volume from the speaker drops and there's lots of whistles, indicating I think too much Reaction. One other potential problem was the singed resistor. I believe that it's R10, which should be 10k. However, it reads about 40k. On the other hand the Screen voltage, and the Anode voltage for that matter, on V1 are about right, so I left it for the moment. The data does say that there were various modifications round that area so it may actually be the right value. At that stage I thought I'd just have a look to see what was oscillating. I glanced down at my trusty 455 only to find it blank. The fuses are OK so I've brought it up to the house to thaw out.
The Pye information says that the whistling problem may be caused by a low value C18. Well, at least that's pretty easy to get at so out it came and a modern one was tacked in its place. The best I can say is that it made a marginal improvement. The other interesting factor about it is that whereas the diagram says it should be 25u it's marked 10. Presumably that's big enough. There is a code on it which may imply 1934, which would be OK.
So, it could be the 'Volume Assembly', or at least the Capacitor half of it. It's quite difficult to see exactly what's happening as it's solid dielectric. I've given it a blast of air and Servisol which hasn't made one iota of difference. My enthusiasm for taking it to bits is very low.
I think at this stage the best bet is a careful measurement of all Rs and Cs around that area.
One last comment; I don't know the state of V1. It actually is a Mazda AC/SG which is right - the others aren't - and it's voltages and currents look reasonable, but who knows. I've been looking for a spare but they seem pretty thin on the ground.
The sun is actually shining and my Weather Station says 10.3 outside, but being Sunday I expect I'll be hauled off to 'do things'
Alan
