Yet another SL2442FX-PRO repair thread

[Enzo]

Unless there is some creeping corrosion or something awful, I would not worry just where the break was. If a jumper wire fixes it, the circuit is restored. Move on to the next thing.


I am just in love with those little metal cylinder flashlight that Harbor Freight gives away or sells for a dollar or two. Nice bright light for shining on or through circuit boards. That and my magnefying glass.

[slobb11]

I'm not sure whether I should be starting a new thread, or continuing this one...
I'm working on another SL2442FX-PRO now...
It had more issues than I have fingers and toes, but I've managed to fix them all by sacrificing one channel and raiding all the components I needed from that strip to fix up the others.
It all went well, but upon reassembling, channel 2 had it's signal light on solid, and produced no output. Ch 2 had no issues originally so I hadn't gone near it. The closest I got to it was when i replaced the mid parametric pot on channel 1.

So...I 've tried to be as systematic as I can in finding out what went wrong with channel 2, and eventually discovered -12.8VDC between the ch2 output of IC1 (the 4580 in the pre-amp for ch1&ch2) and ground. So I replaced that op-amp as I had 20 spare op-amps from the last repair job on one of these units.
And here's where I got so confused and frustrated that I've come here to beg for help again....
The 12.8vdc is still there on the ch2 output, but the signal light now doesn't work at all. Channel 1 is now in the exact same state as channel 2, with a 12.8vdc charge on it's output from the pre-amp 4580.
Both channels work now, but at a greatly reduced level. They sound as if there's a 40db pad on them.

The DC voltages on the 4580s pins are....

1. -12.8vDC
2. -9.5vDC
3. -9.2vDC
4. -14.5vDC
5. -9.2vDC
6. -9.5vDC
7. -12.8vDC
8. -+14.5vDC

And here's the readings from IC2 (the 4580 from the pre-amp for channels 3&4 which are working perfectly)...

1. AC voltage relating to signal strength
2. -8.3vDC
3. -8.3vDC
4. -14.5vDC
5. -8.3vDC
6. -8.3vDC
7. AC voltage relating to signal strength
8. +14.5vDC


Any ideas what I've managed to do to channels 1&2?

Cheers,
Mike

[Enzo]

MY first thought was you either clicked on the mute or maybe the PFL by accident. But you probably have a bad transistor. See the four transistors and a couple diodes before the IC?

[slobb11]

Thanks Enzo!
I tested the transistors and as far as I could tell they were alright.
Given that the symptoms changed slightly when I replaced the 4580, I decided to switch the old one back in. Boom! The symptoms went back to how they originally were.
So, I then swapped the ch1&2 4580 with the ch3&4 4580, and the problem moved to channel 4.
So....yeah.
How unlucky is that?!
The brand new IC that I used as a replacement, was also faulty.
I've now put yet another new 4580 in, and she's all good