Autotek Street Machine SX5750

[Dr Zeus]

I've got this amp in for repair which seems to be suffering from opamp errors on Channel #4. This being a 5 channel amp there are a lot of controls and what not for all the channels.



The nice thing about fixing this amp is, CH1/2 opamps are all on the main PCB, while CH3/4 are on the following riser board which makes troubleshooting CH4 a bit easier.




When I first hooked up the amp, I tested each channel with the scope and a test speaker. All but channel 4 played anything. Upon activating CH3/4's crossover switch, CH4 pulsed rail DC voltage on the speaker terminals. I began testing all the opAmps and found good +-15vDC on power pins. All the opAmps (Marked BA4558N) were measuring ~0vDC on the output pins except the one at ICF5. It was measuring either 10 or 12vDC depending on the crossover switch selection.

I replaced ICF5 with a spare NJM4558L and created an improved condition. Now, CH4 has major static on it and no discernable audio. The output pin which previously measured ~10-12vDC was fluctuating between 2~5vDC for a few minutes.

After a few minutes, I believe the newly replaced opAmp failed and now back supplying ~10-13vDC on its output pin. There is no longer lethargic static on the speaker output terminals, and I smell a very faint scent of a burned opAmp. I have more spares but need to figure out what the other issue is.

[Dr Zeus]

With ICF5 isntalled:

Pin 1: 12.01
Pin 2: 3.812
Pin 3: 0.213
Pin 4: -15.79
Pin 5: 0.003
Pin 6: 0.003
Pin 7: 0.004
Pin 8: 15.28

With ICF5 removed:
(These readings are the same on all opAmps)
Pad 1: 0.002
Pad 2: 0.002
Pad 3: 0.002
Pad 4: -15.85
Pad 5: 0.003
Pad 6: 0.002
Pad 7: 0.002
Pad 8: 15.44

[Dr Zeus]

I started tracing through from RCA to opAmp and noticed what I think is a very bad gain pot. Its almost disconnecting the RCA from the rest of the amp. Its a B-5-0-3 which others on the board are measuring ~50k. I think I have a spare or two

I pulled the bad pot off the PCB and its measurment is dead on all leads. I bet it popped like a fuse internally.

[ComputerJLT]

might check the crossover frequency pot...

i had a pg amp that used the rmnet chips and one had a broken pin on it. it did weird things and would cause that channel to spit out rail voltage when you tapped the xover switch.

sounds like a similar issue maybe; or maybe not just thought i'd mention it lol

/edit: it didn't cause the opamp to fail but it did cause one of the small driver transistors in that channel to fail causing that channel to intermittently pop

[Perry Babin]

If the inverting input is greater than the non-inverting input and the output is near the positive rail, the op-amp is defective.

[Dr Zeus]

Perry,

The fix for this amp was to replace the gain control for the defective channels. The opAmp which measured the faulty voltages became corrected, without replacement, when a new gain pot was installed.

I almost want to say that since the opAmp had noting connected to one of the inputs that it was creating some kind of loop-feedback which caused a pegged output pin to ~12v. It stopped doing that after the gain pot was replaced. The non-inverted input was essentially disconnected from most/all circuits.

I took apart the defective gain pot and its internal contacts had hairline cracks and lack of continuity.

[Perry Babin]

I don't think it's possible for an op-amp to have those voltages without something driving it's output terminal, unless it's defective.

[Dr Zeus]

Hmm Im not too sure what was driving the output to 12v. With the opamp removed the voltages on the output pad was ~0. Whatever the cause or case, after the gain pot was replaced that opamp's output corrected. My guess is something of an OpAmp mystery.

I test ran this amp at high output until it got warm with test speakers and everything seemed ok. I returned this amp to the owner yesterday. Of course I told him if there are any problems to please bring it back. I think its fixed though.

[Perry Babin]

Nothing seemed to be driving the output. There was likely an error in the readings or one input was completely open and the meter caused the voltage to change when the probe was touched to the terminals. It's not a big deal either way but the voltage readings definitely indicated that the op-amp was defective.