20-07-2015, 08:33 PM
Hi,
A few days back, we mentioned the SMG 1 - lovely bits of kit, they are. This reminded me that I also had an SMG 40 up in the attic. When I acquired it, I was unable to find any service data, and as it had a fault label attached, I put it to one side. However, a quick search revealed that a service manual was now available. But, as it turns out, not much else. So, galvanised by the discovery of the service data, I decided to check it out, taking lots of pictures as I went. Ultimately, I should put these onto my own site - which is home to plenty of obscure things - but for now, GVR can have the exclusive
So, the SMG 40 is an instrument for lining up FM stereo decoders. It generates the signal that is applied to an FM transmitter - containing the M signal (L+R) in simple base-band form, and the S signal (L-R) encoded on a double-sideband, surpressed carrier at 38kHz. Finally, the 19kHz pilot tone is added - this, of course, is needed to allow the receiver to recreate the 38kHz carrier that was suppressed at the modulation stage. So, quite a complicated task.
The older SMG 1 has a built in FM modulator, tuned to 100MHz (but variable either side of that if required). This unit does not. However, I should be able to take the "composite" output and apply it to either of my FM generators...
The other main feature is pre-emphasis. As you might know, we boost the treble at the transmitter. The radio receiver cuts the treble by an equal amount to restore the frequency response, but at the same time, background noise is also reduced. Most analogue mediums use pre-emphasis - indeed, it's an option on CDs, believe it or not. When testing a receiver to see if it has a good audio frequency response, the modulator needs to have the same pre-emphasis curve as a broadcast transmitter, but both my generators lack this option. So this feature is ideal. In Europe, we use the "50us" standard; 75us is used in the US. The SMG 40 also offers 25us, which apparently might be used by "FM Dolby", according to the manual. Not something I've come across, BTW.
The unit contains an internal oscillator (Wien bridge, very respectable distortion, good range of frequencies). Naturally, you can apply an external signal on a pair of BNC sockets on the rear panel. Which brings me on to a minor moan - this unit has a large number of sockets on the rear, and that's a pain. Fine if the unit is placed on the bench, but I've got limited bench space so have a lot of stuff on shelves. Getting to the back panel is really awkward. But I guess that's the price to pay for the compact dimensions (approx 12" wide, 3" high and 9" deep).
Anyway, here is the front panel view (yes, I'd already cleaned it up before putting it into storage):
As you can see, the controls are grouped logically. The top row deal first with the modulation source; external or internal. And the 19kHz pilot tone can be switched on and off, and varied in level. The bottom row deal with the function, and the pre-emphasis. The function selection is actually slightly more complex than it first appears, and reference to the manual is essential. Basically, the internal oscillator goes to the right channel, and the external source goes to the left. If you wish to have a mono signal carrying the oscillator, you select L=R. If you pick L&R, then the left source comes from the left input socket on the rear, and the right will come from the oscillator. Unless you press "EXT", in which case the right source comes from the right input socket. Like all things, you get used to it, but it's confusing at first.
Finally, there is a level meter that shows the peak amplitude of the composite signal, and a variable output on the front panel. A fixed version of this appears on the rear.
Yes, the lumps on the top panel line up with the feet, allowing stacking.
Quite busy!
Ignoring the obvious, the first BNC is the 19kHz pilot tone. This isn't as heavily filtered as you might think, so it looks like a sine wave, but has some "lumps". No matter; it's there to lock your 'scope or other gear during investigations.
The Mod Sync socket is a copy of the internal oscillator - again primarily to lock other measuring gear.
The DIN socket provides power and the composite signal - it's intended to connect to an external FM modulator.
The Composite BNC is a fixed-level version of the signal that is available on the front panel.
SCA In is an interesting socket. Basically, the last amplifier stage is a summing amplifier - one input is the composite signal, and the other input is this. I imagine that the RDS carrier could be applied here.
Finally, the aforementioned left and right inputs.
Next post: tear-down!
A few days back, we mentioned the SMG 1 - lovely bits of kit, they are. This reminded me that I also had an SMG 40 up in the attic. When I acquired it, I was unable to find any service data, and as it had a fault label attached, I put it to one side. However, a quick search revealed that a service manual was now available. But, as it turns out, not much else. So, galvanised by the discovery of the service data, I decided to check it out, taking lots of pictures as I went. Ultimately, I should put these onto my own site - which is home to plenty of obscure things - but for now, GVR can have the exclusive
So, the SMG 40 is an instrument for lining up FM stereo decoders. It generates the signal that is applied to an FM transmitter - containing the M signal (L+R) in simple base-band form, and the S signal (L-R) encoded on a double-sideband, surpressed carrier at 38kHz. Finally, the 19kHz pilot tone is added - this, of course, is needed to allow the receiver to recreate the 38kHz carrier that was suppressed at the modulation stage. So, quite a complicated task.
The older SMG 1 has a built in FM modulator, tuned to 100MHz (but variable either side of that if required). This unit does not. However, I should be able to take the "composite" output and apply it to either of my FM generators...
The other main feature is pre-emphasis. As you might know, we boost the treble at the transmitter. The radio receiver cuts the treble by an equal amount to restore the frequency response, but at the same time, background noise is also reduced. Most analogue mediums use pre-emphasis - indeed, it's an option on CDs, believe it or not. When testing a receiver to see if it has a good audio frequency response, the modulator needs to have the same pre-emphasis curve as a broadcast transmitter, but both my generators lack this option. So this feature is ideal. In Europe, we use the "50us" standard; 75us is used in the US. The SMG 40 also offers 25us, which apparently might be used by "FM Dolby", according to the manual. Not something I've come across, BTW.
The unit contains an internal oscillator (Wien bridge, very respectable distortion, good range of frequencies). Naturally, you can apply an external signal on a pair of BNC sockets on the rear panel. Which brings me on to a minor moan - this unit has a large number of sockets on the rear, and that's a pain. Fine if the unit is placed on the bench, but I've got limited bench space so have a lot of stuff on shelves. Getting to the back panel is really awkward. But I guess that's the price to pay for the compact dimensions (approx 12" wide, 3" high and 9" deep).
Anyway, here is the front panel view (yes, I'd already cleaned it up before putting it into storage):
As you can see, the controls are grouped logically. The top row deal first with the modulation source; external or internal. And the 19kHz pilot tone can be switched on and off, and varied in level. The bottom row deal with the function, and the pre-emphasis. The function selection is actually slightly more complex than it first appears, and reference to the manual is essential. Basically, the internal oscillator goes to the right channel, and the external source goes to the left. If you wish to have a mono signal carrying the oscillator, you select L=R. If you pick L&R, then the left source comes from the left input socket on the rear, and the right will come from the oscillator. Unless you press "EXT", in which case the right source comes from the right input socket. Like all things, you get used to it, but it's confusing at first.
Finally, there is a level meter that shows the peak amplitude of the composite signal, and a variable output on the front panel. A fixed version of this appears on the rear.
Yes, the lumps on the top panel line up with the feet, allowing stacking.
Quite busy!
Ignoring the obvious, the first BNC is the 19kHz pilot tone. This isn't as heavily filtered as you might think, so it looks like a sine wave, but has some "lumps". No matter; it's there to lock your 'scope or other gear during investigations.
The Mod Sync socket is a copy of the internal oscillator - again primarily to lock other measuring gear.
The DIN socket provides power and the composite signal - it's intended to connect to an external FM modulator.
The Composite BNC is a fixed-level version of the signal that is available on the front panel.
SCA In is an interesting socket. Basically, the last amplifier stage is a summing amplifier - one input is the composite signal, and the other input is this. I imagine that the RDS carrier could be applied here.
Finally, the aforementioned left and right inputs.
Next post: tear-down!
