Re: DRRI Oscillation

JP schrieb:
On Thu, 13 Nov 2008 09:23:35 -0600, "Stephen Cowell"
<scowell@xxxxxxxxxxxxx> wrote:

"JP" <JP@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote
It is a Switchcraft jack and also I placed a star ground on the
chassis there for all the jacks and the ot in an effort to find the
problem. I also placed one uner the input jacks and the clean ground
is used for the reverb jacks also. I have a star ground right by the
pt for the ac in green wire, circuit board (p2p) ground and several
other things. The pots have a ground buss grounded to the chassis
within an inch or so fo the main star ground and in a couple of other
places along the buss - all added post problem. I';; get some pics up
in a day or two...
Fender amps were not designed for star ground... there's a method
to Leo's madness; the 'strip' or buss ground starts close to the PT,
with each section (in decreasing voltage) taking its ground from
the buss. This puts big ground return swings (i.e. finals) closer to
the PT and chassis solder ground, with preamp grounds further down
the brass buss plate. The idea is to let ground currents circulate
without having them pass through sections that could be influenced
by them; examine a Fender layout for more. When you toss the
circuit card, you commit yourself to re-creating the original Fender
ground scheme... ad-hoc star ground setups are not going to
be easy to do. Follow any information you can from Hoffman...
it's his board you're putting in.

Make sure your feedback wire is connected... it's not a
trace on the board, there's a wire that has to go from the
OT/ext speaker jack to the 820 resistor.
I checked this and on the scope, it makes the sine wave smaller when
this is connected...goes to the speaker jack on this design. Then to
the board/resistor
OK... the signal would get smaller with it connected to ground,
too... make sure that you're connecting it to tip.

(looking at DR schem AB763)

http://www.schematicheaven.com/fenderamps/deluxe_reverb_ab763_schem.pdf

You say you have no squeal with just the PI-final setup... we can work from
here. First let's test the final amp for decent output.
I'll do this in a few minutes and get back...

Inject your tracer signal into a grid of the PI, any grid will do...
most folks use the normal phase grid, the top tube, at
the junction of the .001 and 1M. You should get speaker
noise when you do this...
I am hooked up to an 8 ohm power block resistor with a terminal strip
and the scope output setup with a resistor as on Lee Jackson's web
site but can do this easily.

put in 5 or 6 volts
Not sure how many volts my little signal generator puts out..will
check
OK... this is the first thing about owning a 'scope, seeing
what your generator output looks like, and how big/small
you can make it.
__
Steve
.


My "signal generator" will make abot 1.5v in sine wave mode and about
3.5v in square wave mode...so, I hooked up the speaker again, clipped
it onto the input of the PI tube...got a tone...turned it up ...got
the same tone but louder...switched it over to square wave and the
tone got noticeably louder...but it doesn't really sound like it is
oscillating when doing this...how do I check the voltage - clip onto
the speaker out jack?
Should I start to put the tubes back in and do the same thing to
each one and see if I can get iot to oscillate to determine which
stage is acting up?

Hi JP,

you'd like to have the most possible parts of the circuit to be in the same condition as with the oscillation occuring.

It just 'couples in' somewhere in the circuit. Think analog to positive feedback...

sometimes 'cruising' over/above the amp with a high impedance probe - with a scope set to high sensivity - tells a lot about parasitic 'pollution' inside...

unfortunately not all; then it would be too easy.

the last 2000S on the bench cost me hours...

regards

Jochen
.

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