"bg" wrote in message ...
wrote in message ...
Hi,
My Fender guitar amp has developed a fault and I'm pretty good working
with electronics, safety (discharging caps etc) and soldering, but I
don't have the circuit knowledge to work out which components are
faulty. Could someone here offer any help please ?
The symptoms are that it's making a horrible buzz on the 'drive' &
'more drive' channels, as well as letting some sound through on the
clean channel when the volume is at zero.
Here's the full schematic....
http://www.fender.com/support/amp_sc...xe_Schematic.p
df
And here's just the preamp section:
http://studentweb.eku.edu/justin_hol...HRDxpreamp.gif
Here's a Wav file of the noise it's making...
http://www.fileden.com/files/2006/6/...9/Amp_Buzz.wav
Be careful when listening to it, it starts off very quietly then
gradually gets loud - so don't turn up your speakers because you can't
hear it very well when it starts !
The increase in oscillation frequency is caused by me turning the
DRIVE knob from 12 down to 0. Turning the BASS knob from 12 to 0 also
has a similar effect. Adjusting any of the other controls does not
cause a change in frequency - only in volume / tone of the buzz.
This is happening *only* on the Drive channel, and gets nastier with
the MORE DRIVE button pushed.
The recording was done with nothing plugged into the amp.
With the amp on the clean channel, you can hear a little output from
the speaker with the VOLUME set to zero - which it never used to do.
Apart from this anomaly, the amp behaves fine on the clean channel
and
I used it for practice this week.
I've tried swapping positions of the preamp valves and also a
different set of power amp valves - no difference. I've also tried
resoldering R78 & R79, which apparently have a tendency to develop dry
joints, and all the big electrolytic caps.
I'm more than happy to give test point readings to anyone who can
help... the schematic gives TP voltages so hopefully that might be of
use.
Anyone ? Many thanks in advance - I really can't afford to take this
anywhere to be repaired.
That buzz is the classic case of motorboating which is usually caused by
a
decoupling capacitor gone bad. The concept is that your amp begins to
oscillate due to feedback, and the feedback path is in the power supply.
The power supply connects all of the different stages of the amp together.
It should be DC, but if a decoupling cap goes bad, the power supply will
have AC. Any high gain stage can amplify that AC which puts more AC on the
power supply which in turn gets amplified, and you get the idea, it
oscillates.
I would suspect any of the filter caps in the power supply, especially the
16 volt supply, in section A4 of the schematic. Check that the 16 volts is
16 volts(plus and minus 16 volts). It would be common for one of those
zeners to go bad or one of the 470 ohm 5 watt resistors to go bad which
would destroy the voltage regulation. The 16 volt supply powers the
opamps.
Test or replace all four of the caps in the 16 volt supply.
You also have four caps in the high voltage section right above the 16
volt
power supply. The later sections such as the 345 volt B+ would be used to
power the preamp tubes. If those caps open up, you probably would get
motorboating too.
bg
That is a pretty fair description of the problems that would normally be
associated with an unstable amp, but assuming that this example is the same
as the one of the same model that I had exhibiting *identical* symptoms,
then a power supply filtering / rail decoupling issue such as a bad cap, is
not what is causing it. It certainly wasn't on the one that came across my
bench. The rails remained very nearly as clean when the problem was
occuring, as when it wasn't. Bear in mind also, that the problem only
affects the dirty channel. You can drive the clean channel as hard as you
like, with any ancilliary control settings, without the amp exhibiting a
problem, which 99% knocks out the power supply, as a cause. As I said in my
earlier post, it eventually went back to Fender's own service department,
but the only comment that they made was that it was "a couple of resistors",
which probably means that they couldn't find the problem in any reasonable
period of time either, so just jammed a replacement board in it, and came up
with the 'couple of resistors' story ...
Arfa