Hartke HA-3500 Troubleshooting - Bass Amp Crackles & Pops

[davidm]

Seems like there are quite a few knowledgeable techs here and I need some help.

I've been troubleshooting a friend's Hartke HA-3500. The amp crackles and pops quite loudly and visibly (I can see it moving the speakers from time to time). There is no volume change and it happens whenever he plucks a string. The noise is 'distinct' from the bass sound itself.

The pre-amp section is turned to 12 o'clock and the master output is set to 9 o'clock. I've ruled out a tube problem by bypassing the tube pre-amplifier. The problem persists whether I am running through the tube OR solid state pre. Likewise, the compressor, EQ and filter sections have no bearing on the problem (although you can slightly reduce this noise with selective EQ).

I've ruled out the speaker cabinet, the input and output jacks, bass, cables, etc. A quick measure of the speakers also yield the right resistance/impedence when the amplifier is not connected.

I'm pretty handy with a soldering iron, multimeter, reading schematics, etc. But I'm not familiar with what could cause this problem. Any tech advice for where to begin?

There is a schematic for the amp here.

[Enzo]

So the noise ONLY accompanies a string pluck? It does NOT make noises when just sitting there on?

Plug a cord into the efects send and connect that output to the input of some other amp for a listen. is the noise on that signal?

Explore the power suplies. The preamp runs on +/-15VDC. Are both present at the ICs and clean? The power amp runs on I don't remember what, but it will be two supplies of 40-50-60-70 volts. Look on the connector to the power amp module - measure there. Are both present, clean, and up to about the same voltage, ignoring polarity?

[oc disorder]

I have found some of these to be riddled with dry joints particuarly after they have been on the road for a while.Mainly on the preamp board.. the usual
stuff the bigger or taller components ie regulators, capacitors etc.
Also one of the NJM 2068 for memory the first one IC 102.
To add to the confusion there are different versions of the boards and the schematics.Try some freezer spray and or heat on that IC and see if the noise increases.If your friend is an adept "slapper" thats what I would suspect.
Had one once and the guy insisted there was an annoying background noise when he played. Had it running all day and couldn't find it.
He finally came up and had such a strong right hand his bass was producing
enough umph to light up a casino. Had stressed that IC and was fine after replacement.
But still check out the power supplies etc first as Enzo suggested.