Hartke A100 bass amplifier hum and crackle

[Kekke466]

Hello guys, I have a Hartke A100 (basically the same as the A70) and I am really stuck on this one.
The problem is that it hums very loud and the longer the amp is on, the louder the hum becomes. No controls seem to influence this.

The +15V and -15V rails check out.
I disconnected the power amp by lifting resistor R39.
No strange noises happen if the pre amp is disconnected, so I have a really strong feeling that the problem is in the pre amp section.

The first opamps IC1, IC2 are all good, audio signal goes until IC4 (JRC2068) which I already socketed and replaced but with no luck.
When probing, I have -13,25V DC at pins 5,6,7,8 of IC4. After this stage there is no decent audio signal that I can probe. This is the stage where it enters the EQ.

Any help on how I should proceed?

Link to schematic

[J M Fahey]

1) What DC do you have on IC4 pins 1-2-3?

2) pins 5-6 are connected to external parts, slider pots and then to IC5-6-7-8 so connector or wiring problems may cause a perfectly good IC4 gto mis bias or might kill a fresh one.

Shotgunning or focusing on possible "bad parts" alone is only half the picture.
Let's see your DC readings first, *all* pins, both with and without IC4 in its socket.

[Kekke466]

1) What DC do you have on IC4 pins 1-2-3?

2) pins 5-6 are connected to external parts, slider pots and then to IC5-6-7-8 so connector or wiring problems may cause a perfectly good IC4 gto mis bias or might kill a fresh one.

Shotgunning or focusing on possible "bad parts" alone is only half the picture.
Let's see your DC readings first, *all* pins, both with and without IC4 in its socket.

My DC readings on IC 4:
Without opamp:

Pin1: 0V
Pin2: 0V
Pin3: 0V
Pin4: -15,24V

Pin5: 0V
Pin6: 0,240V
Pin7: 0,240V
Pin8: 14,96V

With opamp:

Pin1: -13,90V
Pin2: 0,103V
Pin3: 0V
Pin4: -15,25V

Pin5: -13,92V
Pin6: -13,89V
Pin7: -13,89V
Pin8: 14,96V

[J M Fahey]

My DC readings on IC 4:
Without opamp:

Pin1: 0V
Pin2: 0V
Pin3: 0V
Pin4: -15,24V

Ok, left half of IC4 , plain tone control, looks fine.
To be more precise, "it receives no external influence".

Pin5: 0V
Pin6: 0,240V
Pin7: 0,240V
Pin8: 14,96V

You should have zero Volts on highlighted pins, what makes me suspect they come through ribbon connected to slider sub-board.
Please unplug Con4 (female on ribbon?) from Con3 (male on PCB?) and recheck whether you still have 240mV on IC4 pins 6 and 7 or not.
If now 0V we must trace back along ribbon, sliders and gyrators (IC5-6-7-8) to check whoīs misbehaving and sending unwanted DC; if we still have significant DC without connector plugged in, check around IC4 solder pads for a bridge or very contaminated flux allowing unwanted DC there.

With opamp:

Pin1: -13,90V
Pin2: 0,103V
Pin3: 0V
Pin4: -15,25V

Pins 2 and 3 should be same voltage, 0 or close to 0, thereīs 100% DC NFB there, through R14, VR2 and R15 ... and their associated tracks ... and your added socket and related soldering operations ....
The path is broken somewhere, and even slight (~100mV) voltage on pin2 (- IN) slams OUT (Pin1) against negative rail.
Why negative? ... because itīs the *inverting (-)* input.
So check for DC continuity between IC4 pin1 and pin2, with amp Off and NO IC in the socket.
Correct what you find (broken track or poor soldering).

Pin5: -13,92V
Pin6: -13,89V
Pin7: -13,89V
Pin8: 14,96V

Ouch!!!!
First correct possible open DC paths as suggested above.
After doing so, plug in a new Op Amp there, the old one might have died.

Hope itīs a DIP8 package and not some cheesy Asian SIL package; if so try a regular TL072 there, same pinout and functions and you probably have quite a few available (and if not, you *should*).

As you see, these problems look mechanical/connector/soldering type, you might throw 1000 new parts at that board and never solve the problem.

Shotgunning is only good for bringing ducks, geese or pheasant home and nothing else

[Kekke466]

Thanks for the input there, I was able to trace this to a hairline crack in the carbon board of the VR2 pot (bass control). Almost impossible to see! After fixing this I still had DC, I socketed almost every opamp to properly isolate it. There were traces of liquid around IC8 that have caused oxidation and a serious short circuit. It was only visible by de-soldering IC8. One hell of a repair on this one! This one had DIP8 package opamps, I would have given up if they were SIL.

Many thanks for your input here Juan Manuel!