14-02-2013, 09:05 PM
The AF239 is not a good choice - I've no idea why it's so frequently recommended as a sub for the AF117. It has hardly any HFE, and far too much bandwidth. Use an AF127 if you can find one, or an AF117 that's had its tin whiskers "bopped".
Given the old engineering adage "amplifiers oscillate; oscillators don't", it's best to stick to the original design where possible.
Once done, if it doesn't work, there are a couple of things to check:
I've had instances of the oscillator failing that were caused by the main decoupling electrolytic on the AM IF PCB. It either leaks heavily, bringing down the HT, or it goes high-ESR or O/C. Any of which will cause problems.
Also, check the service manual for details of oscillator problems in early models.
Good luck,
Mark
Given the old engineering adage "amplifiers oscillate; oscillators don't", it's best to stick to the original design where possible.
Once done, if it doesn't work, there are a couple of things to check:
I've had instances of the oscillator failing that were caused by the main decoupling electrolytic on the AM IF PCB. It either leaks heavily, bringing down the HT, or it goes high-ESR or O/C. Any of which will cause problems.
Also, check the service manual for details of oscillator problems in early models.
Good luck,
Mark
