Trying to repair a "hissy" Bogen C60 Classic Series amplifier...

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...
 
filter caps would tend to make it hum, not hiss.


The schematic I'm seeing from Bogen shows an LM387A op-amp used for the two mic inputs. This is not used for the aux input. Have you tried potting down the mic1 input all the way, and using the aux input only to see if it is noisy? The aux/mic2 switch would need to be set to the aux position. The mic is a very sensitive input, and if the mic amp circuit is noisy it will produce hiss. Turning it down and switching to aux should isolate that circuit entirely. If its quiet then, honestly I'd run it like that unless you specifically need microphone inputs. The aux input is pretty sensitive too.
 
Yeah, the problem is that there is nothing plugged in and all the inputs are turned all the way down, and I just get a hiss through the speaker. If I tweak the treble, bass controls, the hiss follows the tone. All the jacks work and I do get amplified audio. I'm just trying to see if this amp is just inherently "hissy" and I'm just being a pain, but it's pretty horrible by my standards...
 
not sure how old such a beast is. If it has any carbon composition resistors... they can get noisy.
Hiss in solid state amplifiers is usually just too much gain -- my guess is a ss Bogen PA amp wasn't built with high s/n being a key design parameter.
 
It shows 150mv as the sensitivity on the input jack. Standard line level is 1v, so its possible its just noisy by nature. Consider what these guys were designed for too. You'd never notice a bit of residual hiss in a typical PA installation.
 
With all that said, I have gone through a number of SS Bogens, and they do tend to have some noise floor more than home audio would have, but you are on the right track. The LM387 and 4558 series are pin for pin replacements as I recall from the days I was building pre amps around the LM387's, there is some noise that can be inherent in the Op Amps, but if you want to hear that kind of noise magnitudes greater just drop in 1458 series (ouch!)

If it were one on my work bench, I would first look for bad solder joints, especially around the inputs- reflowing the connections and the solder traces on the board, or turret board connections or terminal strip connections and reseating the controls to make sure they ground to the chassis is the first order of business. A loud hiss means something in the audio chain is floating.

If all of that has been done, check for external sources of noise- CFL's Lighting dimmers, "Dimmable" LED lights, flourescent displays, etc. Then lastly go through the active devices on the amp, Op Amps, discrete transistors(often the source for noise)- sometimes tapping the device will reveal the issue, sometimes not. Even though a transistor may pass as good on a simple in circuit transistor tester, theck with a high impedance Ohm meter. There will be a "Diode" effect across the junctions of a transistor, when these break down they become more like a resistor (they measure nearly the same forward biased versus reverse bias) and become noisy in the process. And lastly, any clamp diodes, check those or just replace them.
 
I will add, a poor solder connection may have oxidized making that connection into roughly a point contact oxide rectifier, which makes it hiss instead of hum. But I suspect a failing transistor if not a bad connection.
 
discrete transistors(often the source for noise)- sometimes tapping the device will reveal the issue, sometimes not.

Well, I usually use the freeze spray to isolate faulty components if something isn't obvious just by looking, and when I sprayed at the transistors it did affect it. I also sprayed at the op amp and got a similar reaction but the op amp was easier to test because I had sockets and chips around. I think I will swap out the trannies next and see what it does, then I'll probably give up because I'm guessing it is unnoticeable to most people in the environment these are used.
 
I remember most of the Bogen's in my area market (long ago) were decent enough for low vol. as an informal amp. mostly used for intercoms etc.

I think some transformer matching is required for a mic perhaps. You could spend money on a decent Class D for the time and $.

I did like their cases, totally vented box on the bogen's tube jobs and SS wasn't bad for the buck. They didn't make many as most p.a. stuff goes anyway.
 
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