Re: ampeg ss150 supersonic/parasitic oscillation

[ilyaa]

alright:

if i short out the loop contacts altogether with a known good lead - still thumps
if i short out the preamp - no thumping

the thumping is unpredictable, though, so its hard to say 100%, but i did not get a single thump with the preamp shorted out. which points to that part of the amp as the culprit, probably, correct?

whats the next step - start checking voltages on the preamp?

because of the weird negative ripple coming off the power supply, and the two high negative voltage, would it make sense to start with checking any V- hookups on the preamp? and the preamp power supply, knocks the 39v down to somewhere near 16?

there are a lot of ICs around - are they likely suspects?

[ilyaa]

Alright!!

Another victory (let's hope)!!

With the preamp shorted out via the mains in, I could still hear the amp making kind of a weird sound with no signal or anything going into it - perhaps a quieter version of the THUMP??

i scoped the output and could see tiny little hiccups when the sound happened. i scoped the preamp out and saw HUGE spikes - 10+ volts - that coincided exactly with the sound. briefly unplugged the shorting lead and sure enough it was my THUMP.

so the problem was (yes, i said was!) definitely in the preamp.

i had been suspecting the weird too high negative voltage for some time, so i thought id start with the power rails going into the preamp. sure enough, right away i saw something funny on the negative rails. if i checked one side of the 330 ohm, 10 w power resistor (the first thing the power coming from the supply sees), id get one voltage, then id check the other side and get something way lower, but not consistent and not around -16 V (what i should be getting). checking across the resistor id get zero volts....i figured thered have to be at least SOME drop across the resistor...i checked and it didnt seem shorted....what the hay?

THEN all of a sudden i realized, while prodding it, that it was loose!!! and as i prodded it, i heard the amp making a little mini-thump. i disconnected the shorting lead from the f/x loop and found i could reproduce the thump exactly by wiggling that little chalky mofo.

i remembered how roly had mentioned that the wrong voltage seemed to suggest there was a loading issue somewhere and here it was! the loose resistor was cutting out V-. it wasnt so loose as to be predictably disconnected, but once i wiggled it a bit it became clear.

just a dash of solder later and i think the problem is fixed!!!

im now getting +38-39 V and -39-41 V on the main rails, which seems much better, right?

the power resistor feeding the preamp DO seem to get kind of hot kind of quick, though...think that's a bad sign? they are big 10 watters but still, they are hot to the touch....could this issue have fried the zobel resistor? should i just roll with the fix for now and see what happens??

either way, im stoked!! thanks for your help, everyone!

[Roly]

With the preamp shorted out via the mains in

It's not "shorted" ghoddamit!  :grr  The contacts are normally closed (and thereby hangs the potential problem), so when you plug in to Main you open the preamp output and take over the Main amp Input.  (but the contact you are going in on can be equally "iffy").

THEN all of a sudden i realized, while prodding it, that it was loose!!! and as i prodded it, i heard the amp making a little mini-thump. i disconnected the shorting lead from the f/x loop and found i could reproduce the thump exactly by wiggling that little chalky mofo.

That, my friend, is a 100-pointer, take +1 Internet sir;



Your eyes are your best service instrument.  Paying attention helps.





Well, that just about wraps it up for tonight folks, we hope you enjoyed watching.

the power resistor feeding the preamp DO seem to get kind of hot kind of quick, though...think that's a bad sign? they are big 10 watters but still, they are hot to the touch....could this issue have fried the zobel resistor? should i just roll with the fix for now and see what happens??

This is a dubious "feature" of a few preamps, excessive zener current cooking stuff - like the zeners ... then their resistors when they shorted...

Can't see a direct connection to the Zoble failure, no.

either way, im stoked!! thanks for your help, everyone!

:dbtu:

[ilyaa]

moving forward with the amp working, should i be considered that the negative voltage out of the main power supply is a couple volts off? by the time it hits the preamp the + and - Vs are leveled out, but i can still see a bigger ripple and larger negative voltage on the main rails. not huge, just a volt or two -

should i just use the amp and see if things stay normal or try and see if the power supply needs some fine tuning?

[Roly]

Some minor difference in given voltages are quite normal, as is a small amount of ripple on the supplies.

should i just use the amp and see if things stay normal

Yes.  There isn't much in the average power supply you can "tune" anyway, and the differences on the supply rails will almost certainly be due to slight difference in the load, not the supply.  Again, this is normal.  If the supplies are withing 10% of expectation, and the output is within half a volt of zero, you shouldn't have too much to worry about.   :dbtu:

[J M Fahey]

As Roly says, lower voltage and higher ripple in one of the rails means that , everything else being equal, it has higher current consumption.

That said, it **might** be that that rail main filter cap is somewhat dry, which means it both loses capacitance and raises ESR, its internal resistance, both cause poorerv filtering.
Easy to check, tack solder a similar cap in parallelo (no need to pull the original one).
If voltage and ripple change little, the original one is fine; if big change, it was quite dry.
In that case replace both main fiters, since they are from the same era and probably came out of the same box.