21-12-2015, 04:01 PM
Thanks for reporting back with your success.
For those who are a bit hazy on the function of decoupling caps here'a very brief intro. The power rails in a radio etc are at different DC voltages to the 0V rail. However they are are all meant to be at zero AC voltage. The decoupling caps act as something close to a short circuit for AC (ie signals, such as audio or RF) to ensure that that (for example) the HT rail is at the same AC potential as the chassis.
Without decouplers you can have all sorts of unintended signal paths via the HT rail etc. The effect of this can be unpredictable. Oscillation at anything from motorboating to UHF is a common effect. Odd effects as you tune across a waveband. Almost anything.
Decouplers are not perfect. They have stray inductance and are not truly zero impedance. It's the designer's job to select suitable components. Some circuits can be very critical about their decoupling. If you've ever designed with big, modern, fast digital chips you'll know that decoupling can be a serious headache. Vintage kit is a lot simpler but there's still the odd set that's too fussy for its own good.
For those who are a bit hazy on the function of decoupling caps here'a very brief intro. The power rails in a radio etc are at different DC voltages to the 0V rail. However they are are all meant to be at zero AC voltage. The decoupling caps act as something close to a short circuit for AC (ie signals, such as audio or RF) to ensure that that (for example) the HT rail is at the same AC potential as the chassis.
Without decouplers you can have all sorts of unintended signal paths via the HT rail etc. The effect of this can be unpredictable. Oscillation at anything from motorboating to UHF is a common effect. Odd effects as you tune across a waveband. Almost anything.
Decouplers are not perfect. They have stray inductance and are not truly zero impedance. It's the designer's job to select suitable components. Some circuits can be very critical about their decoupling. If you've ever designed with big, modern, fast digital chips you'll know that decoupling can be a serious headache. Vintage kit is a lot simpler but there's still the odd set that's too fussy for its own good.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
