tubmyk2
Active Member
Here's a longshot; I have a feeling there aren't too many people familiar with these amps but:
I picked it up for pennies on CL. I'm not sure why I wanted or needed it but it didn't quite serve my purpose. It's not that old of an amp, and these are still sold new today. But what I was noticing was that if I connected a speaker to the output and ground, even with no audio signal connected to the unit, there was a loud hiss. It would get better or worse with the different ohm outputs and also was driven by the treble/bass controls. I spoke with the service dept at Bogen and they told me the amp would be in need of service. I don't want to send it to them, I'd rather fix it myself if it's a simple replacement part.
So I did some troubleshooting and found that if I completely removed the RC4558 op amp, the amp was sort of "bypassed" and the external audio flowed directly through to the speaker outputs with no problem, but no amplification. So I swapped out the chip which made no difference. I then proceeded to re-cap the entire audio control board and still made no difference.
Am I looking at filter caps on the power supply possibly? If not specific to this amp, what would normally cause a nasty hiss like that on the output with nothing even connected to it?
I'm having a hell of a time finding anything on these amps and I'm not surprised because they're not a popular item amongst collectors. If anyone can help me though, it would be awesome...
I picked it up for pennies on CL. I'm not sure why I wanted or needed it but it didn't quite serve my purpose. It's not that old of an amp, and these are still sold new today. But what I was noticing was that if I connected a speaker to the output and ground, even with no audio signal connected to the unit, there was a loud hiss. It would get better or worse with the different ohm outputs and also was driven by the treble/bass controls. I spoke with the service dept at Bogen and they told me the amp would be in need of service. I don't want to send it to them, I'd rather fix it myself if it's a simple replacement part.
So I did some troubleshooting and found that if I completely removed the RC4558 op amp, the amp was sort of "bypassed" and the external audio flowed directly through to the speaker outputs with no problem, but no amplification. So I swapped out the chip which made no difference. I then proceeded to re-cap the entire audio control board and still made no difference.
Am I looking at filter caps on the power supply possibly? If not specific to this amp, what would normally cause a nasty hiss like that on the output with nothing even connected to it?
I'm having a hell of a time finding anything on these amps and I'm not surprised because they're not a popular item amongst collectors. If anyone can help me though, it would be awesome...