Fisher BA-6000 amplifier - need help!

[Tim]

Hi,

I have a Fisher BA-6000 amplifier on the bench that has a RF problem.
Both channels have an approx. 2Vrms, 21Mhz sine wave imposed on them
and I can't find the source, or otherwise get rid of the noise. This
amp uses discrete transistors up to the STK0100II output device. One
was STK definately bad, I replaced both. I have replaced most of the
caps(e-lytic, ceramic and film), still waiting on delivery of the
rest. Some of the original transistors were replaced with cross ref'd
transistors by a previous tech (it had RF problems then too). Could
this be causing a problem? I have not hooked a load to this amp yet.
Is it possible that the oscillation will go away with a load attached?

Any ideas?

Please reply to: tim...@teleweb.net

Thanks,

Tim

[Peter Gottlieb]

Yes, you should have a load attached.

No, it is not normal for the beast to act as an oscillator.

"Tim" <ti...@excite.com> wrote in message
news:fbf360ad.02110...@posting.google.com...

[Sofie]

Tim:
Could be power supply decoupling problems..... try installing 0.1uf (104)
or 1.0 uf (105) disc bypass (to ground) caps right where each STK chip
obtains B+ and B- supply voltages..... also install some bypass caps across
the 2 big electrolytics..... watch your voltage ratings.
..... also disconnect the preamp feed to the drivers to verify that the
oscillation is not coming from the preamp.
--
Best Regards,
Daniel Sofie
Electronics Supply & Repair
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

"Tim" <ti...@excite.com> wrote in message
news:fbf360ad.02110...@posting.google.com...