Kustom Quad 100DFX Nasty modulation when Bass is dialed in on any channel

[Hawk]

Hi!

Well, no matter what channel I use in this amp, if I crank the bass on any channel the amp modulates on and off rapidly. I can crank the amp without bass and all sounds good but once I dial the bass in in there is a big modulating problem--like a loud bap,bap, bap sound that continues for quite some time until the sound dies out.  Power supply unable to keep up with low frequencies? Any thoughts? I replaced the output IC but same problem....anyone experienced this before? Suggestions? Thanks.

I also ran the amp through another speaker and the same problem exists.

Also hear clicking sound from power amp when this happens--relay?

[galaxiex]

Bad cap somewhere. How old is it?

[Hawk]

Okay maybe I should be embarrassed here, but I'd been running this amp on a bulb limiter. I removed the bulb limiter and modulation gone!....amp plays great. Anyone care to comment on why this would have happened? Guess the reduced voltage on rails produced incorrect voltages on the components and the amp didn't function as designed, although it produced some good sounds except for the bass modulating/clipping? ..... I remember the same thing happened with a Peavey Renown (after replacing output transistor) the amp sounded crummy and I thought I hadn't fixed it until I removed the limiter and then the amp sounded good    

[galaxiex]

Yep, that'll do it.

[Enzo]

The bulb is there to save the fuses and the amplifier if there is a major fault.  Once the major fault is corrected, get rid of the bulb.  It was never intended that you operate the amp on the bulb.

The more current the amp draws, the more the light lights up.  That means the voltage across the light is increasing.  And that voltage comes out of the voltage the amp wants to operate on.  So the louder you play the amp, the more its mains voltage sinks, and bass notes take a lot more power than treble notes.  So when you turn up the bass, every note drops your mains voltage to the amp, which reacts unfavorably

[Hawk]

Thanks Enzo :cheesy:

[Hawk]

So according to manual this amp had an 8 ohm output impedance, but it has two 8 ohm speakers...how are these to be hooked up? Parallel 4 ohms, series 16...I must be missing something...the celestion G12N-65 are 8 ohms each according to spec sheets in net...hmmm

[g1]

What do the speakers measure each with your ohm meter?
Each model of Celestion is usually available in several impedances, the spec sheet you saw may be for the 8 ohm version.

[Hawk]

3.7 ohms with Meter, so 4 ohm impedance?

[g1]

Yes, they are 4 ohm speakers.
Wire them in series for your 8 ohm load on the amp.

[Hawk]

Great, thanks g1!

[Hawk]

All of a sudden the digital effects aren't working. The adjustable programming knob clicks into position as turned but it doesn't stop at 7 and 5 o'clock and it just keeps turning (is it designed that way?). Also the large IC with all the pins is quite hot--is that normal?

[Hawk]

Okay so for the heck of it I plugged a foot switch and bingo!!! Digital effects works! But when I unplug and try to turn on the effects using the panel pushbutton: nothing! There a number of pins on these little black pushbuttons. How do I measure these pins to see if this pushbutton is defective. Does this sound like the problem? I reflowed the solder joints and they look good. Thanks!

[Hawk]

Here's the schematic....

[Hawk]

Perseverance...broken solder trace... repaired. Pushbutton and effects back in action! :cheesy: