Hartke 1400 intense feedback at higher volumes

[Mr_bibbles]

Arrrrg! Got a Hartke 1400 on my bench right now, causing me no end of headaches. Not very familiar with SS gear, and have never expereinced the following:

When you dime the tone settings and increase the volume to about 80%, the speaker starts to vibrate violently, creating a "BLAATTTT" until it's turned down. With a DC meter across the speaker terminals, DC offset is about 0, but when the speakers hits the threshold it goes up to about 1v.

The Strangest symptom is that when sending a signal from the preamp, it will cause speakers in the chained amplifier to vibrate too, BUT ONLY WHEN THERE IS A SPEAKER PLUGGED INTO THE HARTKE AS WELL. Additionally, when I reverse this and send a signal into the 'return' , there is no vibration.


So, the problem seems to be in the preamp section, but maybe is interacting with the output stages somehow?? NFB?? Educate me, baby.

[Jazz P Bass]

I do not see a feedback path back to the Send jack.

HA-1400.pdf

There are two ic's in between the Input & the Send jacks.
I would start there by monitoring exactly what is going on at the output pins of those ic's.

You may want to flip the PCB over to inspect the factory solder job.
If typical, it will need some attention.
You may simply have a bad ground connection.

[Mr_bibbles]

I do not see a feedback path back to the Send jack.

[ATTACH]40944[/ATTACH]

There are two ic's in between the Input & the Send jacks.
I would start there by monitoring exactly what is going on at the output pins of those ic's.

You may want to flip the PCB over to inspect the factory solder job.
If typical, it will need some attention.
You may simply have a bad ground connection.

Thanks Jazz! Dead on. There was indeed a poor factory solder job, causing a bad ground connection off of the first IC. Thank you very much! Can you share a little more about feedback paths and how you were able to call this one?? I'm guessing without a loop to absorb feedback, it will travel through the amp into the speaker? Any further knowledge would be much appreciated.

[Enzo]

You may not recognize it on the drawing, but the powr supply is feedback path. Especially if your power amp is motorboating. The large pulses spike the power supply, causing it to drop momentarily, which affects the preamp. Thus your pre out is affected only when a speaker is connected. Without the speaker load, there are no current spikes in the output.

[Mr_bibbles]

You may not recognize it on the drawing, but the powr supply is feedback path. Especially if your power amp is motorboating. The large pulses spike the power supply, causing it to drop momentarily, which affects the preamp. Thus your pre out is affected only when a speaker is connected. Without the speaker load, there are no current spikes in the output.

Excellent information! Very succinct and to the point, thanks a lot Enzo.

-Mr. B