|Kekke466||7/29/2018 7:36 AM|
|Hartke A100 bass amplifier hum and crackle|
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||7/29/2018 8:35 AM|
|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||7/30/2018 3:57 AM|
|J M Fahey||7/30/2018 10:03 AM|
To be more precise, "it receives no external influence".
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.
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).
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