Fender champion 30 dsp

[gbono]

Are these amps unusually noisy? This amp has a 60Hz buzz which increases with use of the volume control. I'm wondering if I'm also hearing the 45Hz square wave that is used to multiplex the two Fx pots. The noise seemed to improve when I plugged into the headphone jack though it still increase with use of the volume control.Champion 30 DSP .pdf

[Enzo]

These amps are made the same way all the other fender SS amps of the era were made. No reason for it to be more or less noisy than the others.

Is the hum present with nothing plugged into the input jack? And with a guitar, does turning the guitar volume control to zero affect the hum?

The volume controls the hum? That means the hum is coming from before the volume control. The clean channel control is called volume, the drive channel control is drive volume. Does this hum occur in the drive channel too, or only the clean? So does the drive volume also control the hum?


Are the input jacks the original jacks, not replacements? The original jacks look about like any other except they have a small point on their front surface. That point digs into the backside of the control panel part of the chassis. Common replacement jacks lack that feature, so there is no grounding to the panel.


The difference in hum between speaker and phones is probably more about the phones than the circuit.

[gbono]

Are the input jacks the original jacks, not replacements? The original jacks look about like any other except they have a small point on their front surface. That point digs into the backside of the control panel part of the chassis. Common replacement jacks lack that feature, so there is no grounding to the panel

Buzzinga! Thanks Enzo - probably wouldn't have seen this.

[Enzo]

Really? Cool.

If you have a common jack, and need to replace the similar ones with the point, my solution is crude but simple.. I take a couple inch long piece of thin bare solid wire, and solder it to the ground pin of the jack - the sleeve pin - on the underside of the board. I then bend the wire from there, up and over the barrel of the plastic bushing. Just a loop of thin wire wrapped once around the bushing. Now when I mount the board to the chassis, that hunk of wire is pinched between the face of the jack and the chassis, making contact with chassis, and doing the job the point would have done. Takes longer to explain than to do.

[gbono]

Okay fixed the buzz and now there are two other issues - the amp has a nasty, raspy, clipping, noise when you play short transients (choppy guitar chords) on the clean channel. You can hear this distorted signal in the headphone jack so I'm assuming preamp section as opposed to output. The Enzo fist bash on the cabinet does not change or generate the issue - maybe not a solder joint? I don't appear to hear this issue on the other non-clean channel and tone controls don't have an effect on the issue.

I'm going to scope the input signal chain through U1-B to U3-B - using a guitar to simulate the short transient signal. I prerecorded loop would be nice to use as an input signal - hands free to chop stick the chassis;-) Also need to check the power supply outputs (+-15V and +27V).

The other issue this amp has is since the baffle board is held on with two screws - you get a nasty vibration where the baffle sits in the channel at the bottom of the cabinet. You can dampen the vibration with a well placed "foot" but I need to add some speaker "caulking" to the bottom of the baffle. Two wood blocks and screws would have prevented the problem in production. I just fixed an old Gorilla (aka Pig Nose) baffle with the same issue.

Working on the cheap and cheerful....

[gbono]

The power supplies are all within spec. +25.5 and -25.7V. The supplies sag about -8% at rated output 24W into 8 ohms. You can see that the output is clipped when playing the amp at moderate volumes (2-3 on volume control - clean channel).

I checked the voltage at the V+ and V- of the LM3876 (U8) and the V- pin is at 0V and there is no continuity to V- (test point 4) from pin 4 of U8. The output is clipped with a guitar (no clipping observed at outputs of U1 or U3 though). The amp meets it rated output but it looks like either U8 is damaged or there is a cold solder joint or broken trace at pin 4 of U8. I have to pull the main board and check out the reason there is not V- on U8.