02-05-2012, 07:06 PM
Personally, I'd use some series resistance in addition to the capacitor - say 10K. Even a relatively small capacitor like 100n is a dead short circuit briefly :-)
Happily, the transistors in this circuit are pretty generic - the small-signal devices are JEDEC BC548/558 as far as I'm concerned. The output pair are pretty common - I've used them a lot over the years - and should be still available, but if not, I'm sure they'll be easy to come by second hand...
My first function generator was a Wavetek 162, and works in much the same way, only it has rather more bells and whistles, such as the ability to stop the generator, and output single cycles or tonebursts. As it goes to 30MHz, the transistors used are somewhat more specific! I think I was 16 when I was given this, and looking back, fixing the amplifier was quite an achievement!
Function generators aren't very good at producing low distortion sine waves, but they are so good at everything else that everyone needs one IMHO. For audio work, you really need something that's Wien bridge or state-variable based - although these days it's all gone digital (boo :-()
David, see post #16 - we've got triangle and square, so it's probably the output amp that's at fault. Personally, I'd dive in and change all the transistors (there's not many) and hope for the best. Although if I had plenty of time, I'd attack it more methodically, as it's always more satisfying to chase a fault logically
You're right that there is no protection here beyond the 50 ohm output impedance. I think I'd add 1n4148 diodes connected between output and each rail - these wouldn't offer all that much protection, but it's better than nothing. Bigger diodes like 1n400X would be better, but you'd have to check that the shunt capacitance isn't too high - I guess it would be fine here, but assume nothing.
Good test kit is always worth fixing, and especially if you can learn something along the way
All the best,
Mark
Happily, the transistors in this circuit are pretty generic - the small-signal devices are JEDEC BC548/558 as far as I'm concerned. The output pair are pretty common - I've used them a lot over the years - and should be still available, but if not, I'm sure they'll be easy to come by second hand...
My first function generator was a Wavetek 162, and works in much the same way, only it has rather more bells and whistles, such as the ability to stop the generator, and output single cycles or tonebursts. As it goes to 30MHz, the transistors used are somewhat more specific! I think I was 16 when I was given this, and looking back, fixing the amplifier was quite an achievement!
Function generators aren't very good at producing low distortion sine waves, but they are so good at everything else that everyone needs one IMHO. For audio work, you really need something that's Wien bridge or state-variable based - although these days it's all gone digital (boo :-()
David, see post #16 - we've got triangle and square, so it's probably the output amp that's at fault. Personally, I'd dive in and change all the transistors (there's not many) and hope for the best. Although if I had plenty of time, I'd attack it more methodically, as it's always more satisfying to chase a fault logically
You're right that there is no protection here beyond the 50 ohm output impedance. I think I'd add 1n4148 diodes connected between output and each rail - these wouldn't offer all that much protection, but it's better than nothing. Bigger diodes like 1n400X would be better, but you'd have to check that the shunt capacitance isn't too high - I guess it would be fine here, but assume nothing
.Good test kit is always worth fixing, and especially if you can learn something along the way
All the best,
Mark
