Roland AC100 Buzz

[drot]

I am a new member, so greetings to everyone in the forum.
There is an extra amount of buzzing coming from my newly purchased, 20 year old, AC100. Most notably, when the chorus is engaged. The pots are good, I cleaned it and it looks properly grounded.
If anyone has any ideas, I would be most thankful.

[Chuck H]

Does it buzz without an instrument plugged in?

Please forgive if this seems like a pedantic question. You're a new member and these things often need to be handled like "technical support" calls... "Is the unit plugged in"...

[drot]

Thanks for the response. Unfortunately, tech support is non existent in these parts.
With nothing connected, except the power (you made me laugh), it makes a hissing noise which is increased when the chorus is engaged. Otherwise this amp has amazing tone. Master volume also increases the problem. Most amps make a bit of noise, but this is audable at 5 meters.

[drot]

With no instrument plugged in, the hissing is present and increased when chorus is engaged. It is audable at 6 meters.

[Chuck H]

Hiss is a byproduct of gain or, more rarely, carbon composition resistors at high voltages. We can rule out CC resistors at high voltages for that amp. You may have one or more amplifiers in the circuit that are noisy. I've fixed hiss in a couple of amps by just replacing transistors or op amps. Sometimes you can locate the offending part if hitting it with a blast of freeze spray quiets it down. There may be amplifiers in that amp that are made of unobtanium. It's usually possible to get a suitable alternative or alternately wire a part that is the same but with a different pinout.

Buzz above 120Hz is usually EM interference, a bad amplifier or a bad ground. It's possible then that you could solve for this when fixing the hiss.

Buzz, more like hum, at 100Hz could be old power supply filters that have stopped eliminating ripple from the amps voltages. Those filters are a capacitor type that goes bad with age whether the amp is used or not. So it's good to know if what you hear is 100Hz.

You should look around for a schematic to post. The guys here appreciate that and it puts everyone on the same page if the discussion gets deeper. It may also help to know your level of electronics experience and what tools you have access to (multi meter, signal generator oscilloscope, etc.).

[Jazz P Bass]

With no instrument plugged in, the hissing is present and increased when chorus is engaged. It is audable at 6 meters.

Try turning Off the Chorus using the Chorus Ft sw jack.

If the buzz stops, then you have possibly narrowed it down to the Chorus circuit.

Roland_ac-100ut.zip


As always, verify that you have a healthy power supply.
Minimum Vac ripple on the high voltage rails.

Zero Vac on the opamp suppliers.

[drot]

I will try that. Thanks also for the link.

[drot]

Chuck thanks for the detailed information.I have basic electrical skills, so your advice will be put to use.
I will let you know when the problem is solved. I am leaning towards the 100Hz since the amp is 20 years old. This amp is worth the trouble, to my ears anyway.