Mesa Boogie???

[bsco]

I thank everybody for helping me in the problems I have had and have posted here concerning Fender Amps. Of course the owner has talked to someone who talked to someone...etc,etc... now I have the privledge or trying to get a couple of Mesa Boogie amps fixed up for a couple of fellow musicians. I think that in all due respect and consideration I should only post one request at a time. Somebody dropped off a Mesa Boogie Lonestar that was picking up radio stations in his area. This particular unit has the switching capability of 5/15 and 30 watts. The rf problem was being caused by the chassis not making a good connection to the foil lining which is glued to the inside of the top cabinet. There are 4 screws, one in each corner of the chassis. I removed them and installed a washer under the head of each and reinstalled. Put the chassis back and no rf. I still have the other problem which is a hum that can be heard on the clean channel with all associated controls turned down except the input gain and master volume controls. Turn the master volume control up, and slowly increase the gain and about 1/3 of the way you can hear hum and as you advance the control further the hum gets louder until you reach about 8 to 9/10 of rotation when the amp will be quiet and this is pretty much at max volume with no input connected. I checked for some bad grounds couldn't find any. I am in the process of trying to hunt down a schematic for this sucker...no luck as of yet...As usual I am open to any and all suggestions.

[soundguruman]

I am thinking that this is a high frequency oscillation and the hum is a symptom of it.
or a sympthum.
You should look at that output with a scope while the hum is occurring, I think you are going to see some high frequency noise.
The oscillation turns on the output tubes. But it usually comes from the preamp, I am thinking.
The frequency of the oscillation is too high to hear. So you need to look.

[bsco]

Sorry about the delay in getting back to you....Apparently, the problem turned out to be ME!! In an attempt to find out why the amp would hum when my hand was bought near the input area( and rf problem), even when the chassis was installed in the amp, I had played around with the controls and apparently when I had them set to full off and had the master and gain controls turned up, I set up some sort of feedback or ground loop and this is what was causing the hum to occur....When I set the controls for a normal playing level, there was no hum....but I thought I had a problem until a tech from Mesa had en-lightened me..... So there you have it....in an attempt to find the source of this un-explainable hum, I had actually caused it....how stupid of me....I will now know better for the next time...Thank you for your reply.
I appreciate your input even if I sent you on a wild goose chase so to speak.....

[Jazz P Bass]

TIF.
Tech induced failure

[soundguruman]

TIF.
Tech induced failure

Tech Induced Failure Syndrome is treatable, ask you doctor.

[Chuck H]

The hum issue sounds like phase relationships and ground scheme are working together. Probably not much to be done since it's a PCB so changing any layout or grounds is impractical. But, have you measured the hum to see if it's 60hz or 120hz? If it's 60hz then you may be able to get a reduction by modding the filament supply with a DC elevation or even changing to a DC circuit if the amp doesn't already have these in place.

[trem]

The hum issue sounds like phase relationships and ground scheme are working together. Probably not much to be done since it's a PCB so changing any layout or grounds is impractical. But, have you measured the hum to see if it's 60hz or 120hz? If it's 60hz then you may be able to get a reduction by modding the filament supply with a DC elevation or even changing to a DC circuit if the amp doesn't already have these in place.

Pardon me for butting in.....
I have seen this asked before. Do you make this measurement at the speaker jack.? Will the "Frequency" setting on a DMM do the job.?
Thank You

[Enzo]

For an experienced tech, it is usually just done by ear. 120Hz and 60Hz are the same "note", but 120Hz is one octave higher.

But with distorted waves it can be confusing, so in the shop for a quick reference I use my finger. I touch my scope probe tip with a finger and see how many humps are across the waveform, then I can touch the probe to the circuit with the signal and see if I get the same number or twice as many.

Speaker jack is as good a place as any, after all, it is where you are hearing the sound - out the speaker. But one could easily scope mid-circuit somewhere.

A freq counter function on your DMM? Probably work, try it. COnsult the meter owner manual for voltage limits when in freq mode.

[trem]

Thanks Enzo -
Good tip about checking how much voltage my meter can take in that mode.
Thanks Again