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oscillations


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8/20/1999 8:02 PM
paul gambon
oscillations
I'm rewiring a PC board Vox AC-30 to an eyelet board. I'm getting some severe oscillations/motorboating. I've read where black phoenalic boards can carry eddy currents. I thought this sounded strange as phoenalic is considered to have high die-electric strength. Itouched parts of the board with the DVM probe and found .15 volts DC in certian areas. this seemed strange, so I measured the fiber board in my tweed twin. I found up to .50 volts DC. Very strange! Anyone have any input?
 
8/20/1999 8:15 PM
Gil Ayan

quote:
"I'm rewiring a PC board Vox AC-30 to an eyelet board. I'm getting some severe oscillations/motorboating. I've read where black phoenalic boards can carry eddy currents. I thought this sounded strange as phoenalic is considered to have high die-electric strength. Itouched parts of the board with the DVM probe and found .15 volts DC in certian areas. this seemed strange, so I measured the fiber board in my tweed twin. I found up to .50 volts DC. Very strange! Anyone have any input?"
 
 
 
 
Well, I guess somehow you go have something riding on that board... How are your ground connections? Lead dress in general? What type of phenolic board are you using, the Fender 1/16" piece of shoe sole stuff, or the nice .18" Whestinghouse board? I use the latter and it seems to work fine, the Fender stuff, on the other hand, warps and bends and looks pretty awful after a few years. Additionally, it is capable -- alledgedly -- of absorbing moisture.  
 
 
 
But, maybe even in spite of the "floating voltage board" you have something else wrong in your amp, in terms of stray capacitances coupling parts of the circuitry. Of course, that would be more little to generate HF oscillations, not the motorboating you describe...  
 
 
 
Does the motorboating go away if you turn the bass down, or the volume down?  
 
 
 
Gil
 
8/20/1999 9:36 PM
SpeedRacer
Gil: FWIW I firmly beleive that the Fender boards get water in them, and I've fixed a fair number of amps with a hair dryer! (or running them for a day or two in a dry room). Most often I've seen moisture manifest itself most often as odd squeaks and chirps ("mice from space" said one client). A friend at Fender who's worked in restoration as well as another long time tech friend both swear by soaking the old boards in MEK (or sim) to blow out any moisture. I'm not that zealous
 
8/20/1999 9:45 PM
R.G.

Pure grain alcohol is a great one for sucking water out of things as well.  
 
 
 
If one uses FR4 from their friendly neighborhood plastics dealer, one doesn't have water soakage problems.
 
8/21/1999 1:25 PM
Graywater

R.G.,  
 
 
 
I reserve my ppure grain alcohol for more noble pursuits :). Now, automotive fuel line dryer is a) unpalatable, b) cheaper, and c) more "dry" (closer to true "anhydrous" alcohol). I do tend to use it outdoors for safety sake but I'd also use ethanol outdoors 'cuz the fumes make me dizzy.  
 
 
 
GW
 
8/21/1999 7:34 AM
pgambon

A little history on this project: I got this amp at a blow out sale. It didn't work,someone had tried t repair the pc board. I couldn't make heads or tails of the pc board. I deceided to make it a hard wired job. I used Hoffmans board layout (great web site). However the tubes are laid out in reverse of his diagram. Lead dress looks good, short wires, plate leads away from the rest. I've narrowed the problem to the phase inverter. The power tube don't motorboat until I install the PI tube. The board material was purchased from McMaster Carr (great source,will sell to anyone,hardware, raw materials,etc). I don't believe moisture could get into the phenolic. I think I read in Weber's book that the black boards have carbon in them which conducts current. I'm still not buying this. I've tried jump-ering over the P supply caps. Still I get violent motorboating. I haven't given up, and am open to suggestions.
 
8/22/1999 10:51 PM
Tony M.
Re: curiosity
Do you use the gpo-3 or the grade xx Garolite for your boards,from McMasters?
 

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