Symptom is no audio, power supply is fine, +/-15 is good, and output stage is oscillating correctly. I was able to figure out that there is audio present to the input of the preamp card but nothing coming out so I’m guessing an OPAMP went bad, but everything on the board is so tiny I can barely make anything out. If anyone has repaired these before, is the audio path through the JRC4560’s near the phase switch? If so I can likely follow the signal till it disappears then replace the offending IC, but I’ve never worked with this brand before.
I saw that for sale, almost grabbed that one 😉..
I would work your way through the preamp with o-scope. Could be an issue with a switch or maybe bad coupling capacitor?
I would work your way through the preamp with o-scope. Could be an issue with a switch or maybe bad coupling capacitor?
Switches are all good, audio disappears between the crossover pot and the gain pot somewhere so still searching but I’m straining to see anything even with a magnifying glass. The parts are sooo tiny...
Pffff, try working with 0201 parts all day. Then you can complain hahaha.
In all seriousness, a boom microscope is almost a necessity if you are serious about repairs. I got one for home use years ago.
Anyway, see if you can find a coupling cap that has signal on one side and not on the other. The one audiocontrol amp I've worked on did have a burned up opamp fwiw, could actually see a burn hole in it.
In all seriousness, a boom microscope is almost a necessity if you are serious about repairs. I got one for home use years ago.
Anyway, see if you can find a coupling cap that has signal on one side and not on the other. The one audiocontrol amp I've worked on did have a burned up opamp fwiw, could actually see a burn hole in it.
+3 dioptre or two pairs of +2 dioptre cheaters (stacked) and good lighting help greatly for working with small parts.
The Dr Mom otoscope has great light (shows colors perfectly) and magnification for times when you don't have to be hands-free.
Doctor Mom LED Otoscope Ear Tube - Walmart.com - Walmart.com
The Dr Mom otoscope has great light (shows colors perfectly) and magnification for times when you don't have to be hands-free.
Doctor Mom LED Otoscope Ear Tube - Walmart.com - Walmart.com
I definitely need some optics in the future, I’ll add it to my want list.
Meantime the high side of the gain pot seems shorted to ground, that can’t be right...
Meantime the high side of the gain pot seems shorted to ground, that can’t be right...
Pot high side is indeed tied to ground and it’s supposed to be, perhaps it’s in the local feedback of its respective OPAMP instead of simply varying the signal coming out of it, Ive seen that before.
The OPAMP the gain pot is tied to isn’t seeing any input either, so I’ll have to work my way back farther. Think I’m going to wind up removing this card to get a better look at everything, so off to buy some chipquick for now. On the plus side if I inject audio to the wiper terminal of the gain pot I do get clean output, so once I find the loss of signal it will likely be a solid repair.
The OPAMP the gain pot is tied to isn’t seeing any input either, so I’ll have to work my way back farther. Think I’m going to wind up removing this card to get a better look at everything, so off to buy some chipquick for now. On the plus side if I inject audio to the wiper terminal of the gain pot I do get clean output, so once I find the loss of signal it will likely be a solid repair.