digitaldrummer
Well-known member
So I've been looking at some of the metas and posts re: oscillation but haven't gotten any closer to a fix yet...
I have a Yamaha PM-1000 Echo channel that I am "racking" and I'm getting a weird oscillation - it's audible -- around 2.1KHz. unfortunately I don't have a scope so I only figured that out using a freq. analyzer plug-in. the oscillation occurs even when there is nothing connected.
I've recapped it, replaced the transistors (Fairchild MPSA18's), etc. I have had similar problems (but higher frequencies) in the past and most of the time it has been related to grounding. I think I have a good ground though... there is a single gain pot and when I move it, the oscillation changes -- at lower gain (higher resistance on the pot) it is more audible and when the gain is all the way up, it goes away but then I get some "white noise" or what appears to be much higher frequency noise.
In troubleshooting I have disconnected the first gain stage (the "boost amp") - basically removed the wire leading from the volume pot into the second gain stage ("line amp"). so at this point I have no input load and only the "line amp" gain stage and I still get the oscillation. for output, this one has unbalanced, 1/4". the echo board has two identical channels and I'm getting the same oscillation on both - although the volme/gain pots give slightly different results when changed.
the power supply is an inexpensive external, brick (switching) - and yeah I know that is not ideal -- but I have used them many times and not had this problem. I have several different types and have tried them all with the same oscillation - so it's not a particular noisy supply.
I did increase the input cap (replaced the tant with a 2.2uF film), I am using some Beyer input transformers and I did the direct-inject mod, but I've tried disconnecting the input transformer, then the entire first gain stage, so I don't think this is the problem. it's disconnected right? I also increased the output cap (was 100uF electro, changed to a 220uF).
I'm just banging my head and getting a headache now...
If anyone has any suggestions to get me on track again I would appreciate it.
oh - and the schematics for this are on the Yamaha web site, but I noticed they are actually wrong -- the boost amp circuit has changed at some point. the "line amp" circuit is the same though and the boost amp circuit is the same as what is shown for the master module.
thanks, Mike
I have a Yamaha PM-1000 Echo channel that I am "racking" and I'm getting a weird oscillation - it's audible -- around 2.1KHz. unfortunately I don't have a scope so I only figured that out using a freq. analyzer plug-in. the oscillation occurs even when there is nothing connected.
I've recapped it, replaced the transistors (Fairchild MPSA18's), etc. I have had similar problems (but higher frequencies) in the past and most of the time it has been related to grounding. I think I have a good ground though... there is a single gain pot and when I move it, the oscillation changes -- at lower gain (higher resistance on the pot) it is more audible and when the gain is all the way up, it goes away but then I get some "white noise" or what appears to be much higher frequency noise.
In troubleshooting I have disconnected the first gain stage (the "boost amp") - basically removed the wire leading from the volume pot into the second gain stage ("line amp"). so at this point I have no input load and only the "line amp" gain stage and I still get the oscillation. for output, this one has unbalanced, 1/4". the echo board has two identical channels and I'm getting the same oscillation on both - although the volme/gain pots give slightly different results when changed.
the power supply is an inexpensive external, brick (switching) - and yeah I know that is not ideal -- but I have used them many times and not had this problem. I have several different types and have tried them all with the same oscillation - so it's not a particular noisy supply.
I did increase the input cap (replaced the tant with a 2.2uF film), I am using some Beyer input transformers and I did the direct-inject mod, but I've tried disconnecting the input transformer, then the entire first gain stage, so I don't think this is the problem. it's disconnected right? I also increased the output cap (was 100uF electro, changed to a 220uF).
I'm just banging my head and getting a headache now...
If anyone has any suggestions to get me on track again I would appreciate it.oh - and the schematics for this are on the Yamaha web site, but I noticed they are actually wrong -- the boost amp circuit has changed at some point. the "line amp" circuit is the same though and the boost amp circuit is the same as what is shown for the master module.
thanks, Mike
