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Messages - Enzo

#2086
Schematics and Layouts / Re: Marshall G10 Mk-II
October 11, 2008, 04:32:12 PM
The MG10- Mk2 is listd as a "replace only" model on my dealer site.  No schematic is offered.

But as slideman suggested, they are generally very simple amps, and most likely failure is the powr amp chip.
#2087
Amplifier Discussion / Re: Amp speaker
October 11, 2008, 04:26:20 PM
Are you sure the old speaker was only rated at 40 watts?  That would be most un-Peavey-like to mount a speaker barely rated for the amp.
#2088
It is your basic Crate solid state Bass combo.

Isolate the problem - preamp or power amp.  Plug something into the Line In jack.  You have a pair of jacks Line In and Line Out, your effects loop basically.  They are next to the headphones jack.  The Line In is the power amp input.  Plug the guitar into it and turn it to zero volume. Still hum?  Or did the hum diminish or leave?

Likewise send the signal from the Line Out jack to the input of another amp for a listen.  Is there the hum on that signal?

The power amp is all transistor, no op amps, so the whole thing runs on the +/-40v rails alone.  There is also a +/-15v set of rails that serve only the preamp.

If the hum is in the power amp, then check the two 40v rails.  Are they both up to about the same voltage - like within a volt of each other?  And is the ripple level about the same in each?  Best is to scope them for ripple, but lacking a scope, set your meter to AC volts and measure the 40VDC rails that way - the meter will ignore the DC and just measure ripple.  Well unless it is a $5 meter.

The low voltage rails for the preamp - the +/-15VDC - derive from the 40v rails through dropping resistors and zeners.  They are actually 16v zeners, but check right on one of the op amps in the preamp.  Are both 15v rails up to the same voltage and free of ripple?  If one is several volts off, that is a problem.

Does the heatsink get pretty warm just idling?  If the amp is baised wrong - on the too hot side - it can hum.

Scope the output for RF oscillation.  That will usually make it run hot, but it will also add hum, and usually distortion as well.

If turning the controls on the amp all to zero doesn't kill the hum, then I'd agree it is in the power amp.  Check the solder on the main filter caps - reflow the solder anyway - and make sure the screws are tight.  Note the mounting screws for the circuit board.  At least the one in the rear corner behind the input jacks has a ring terminal for the screw to go through.  And I think maybe also the one behind the power switch.  These ground the circuit to the chassis.  A loose screw there could hum you.

A thought: C29 is immediately behind the ribbon cable to the graphic controls.  Check for rippple on it - there should be none at all there.  To the left or it are a resistor and then a diode.  The end of the resistor towards the front is connected directly to the + on That cap, a handy place for a reading.  For that matter, right behind the High Boost button is one bare wire jumper aiming front/rear.  That is the ground for that cap.  It would be a better spot for the meter ground for this than the chassis.

And are the output transistors Moto or Texas Inst?  It matters to the bias.
#2089
Because while it might be annoying, it is not a threat to the amp, so they don't go to the extra expense.  The pop is limited to the power rail voltage, just like the signal is.  SO the pop will never be any louder than the peak signal can be.

Isolate the problem.  If you want to see if it is the power amp settling, plug something into the power amp in jack to disconnect the preamp signal, then cycle the power.  If it no longer pops, the preamp was the popper.  If it still does, then it is power amp.

Likewise, you can send the preamp out signal to some other amp to monitor it and see if the pop comes out of the preamp.

SOme amps pop because the power switch arcs as it opens.  The power supply charge lasts long enough to amplify the resulting spike.  To test for this, leave the power switch on and just pull the power cord from the wall outlet instead.  If it doesn't pop that way, then the power switch is sparking.

In my experience, when I see any of the Fender solid state amps with that wedge shaped chassis - front panel short, rear panel tall with sloping chassis bottom - come into my shop, I expect to have to resolder the two main filter caps sticking up from the main board.  They never seem to be bad, but they love to crack free of their solder.  If yuo still have the amp, do your customer a favor - glob a bunch of silicone sealer or hot melt glue between those caps to glue them together,  They'll vibrate kess.  Then run a bead of glue around the base of each.
#2090
My first thought is the footswitch jack itself.  If you are NOT using a footswitch, there is a cutout contact in the jack that completes a circuit through the panel switches.  WITH a footswitch, it cuts those switches out and ignores them.  If that cutout contact gets iffy or the solder cracks under the jack, you can get similar symptoms.

If you ARE using the FS, the jack still can cause these problems.

Without the FS, check the panel channel switch with an ohm meter.

The Fender one wire FS system puts an AC voltage on the FS jack, then various diodes and zeners switch in and out and then op amps decode the result to control the amp.

Isolate the problem.  Random channel changes could mean the control circuit is directing the signal circuits to do it, OR the control circuit is OK, but the signal switching is screwed up.  The question to answer is this:  when the channel switches itself, do the indicator lights on the panel shift or stay steady?  If they shift, that means it is in the control circuits - like my FS jack notion - and the signal switching is just following orders from it.  If the lights stay where they should be, then you have a signal switching element at fault.

And another question:  is it really actually changing channels?  Or is it more a matter of the channel stays put but the gain makes major changes?
#2091
The Newcomer's Forum / Re: pot problem Torque bass amp
September 27, 2008, 05:11:04 PM
Let me understand, you are putting 3-leg pots in where 5-leg pots belong?  And it doesn't work?

COuld be a defective pot, could be it is not appropriate for that pc board layout, could be a solder issue, could be damaged copper traces from installation.  If nothing else you can stick the old scratchy pot back in to make sure the rest of the amp is OK.
#2092
Amplifier Discussion / Re: Laney GC 60A Troubleshooting
September 27, 2008, 05:03:56 PM
CLose, it is the RETURN jack that is iffy.  The return is the one with the cutout contacts.  they get dirty and this happens.
#2093
Amplifier Discussion / Re: Proper use of shielded cable
September 27, 2008, 05:02:19 PM
And this:  if you are running a wire and just want it shielded, then ground one end somewhere.  But if the shield is also a ground connedction then both ends are connected.  An example of that is the guitar cord.  the shield is ALSO the ground return.  SO if you have a remote pot such as your example of a pot wired down to a preamp board, and there are the hot and wiper wires, if the shield serves as the ground for the pot too, then both ends are connected.  If the pot is just grounded to the panel, then one end is enough.
#2094
Amplifier Discussion / Re: Power Transformer replacement
September 19, 2008, 07:52:46 PM
Well, if you are looking for 29.36v transformers, you won't find them, but 30v center tapped transformers should be easier to find.

Remember that if a transformer is rated at some output voltage - 29.36 or 30 or whatever - it will ONLY produce that voltage when the primary voltage is ALSO at the specified level.  In other words if the part is specified as 30v out with 120 on the primary, and your local mains are sitting at 124VAC, then your output will be 31v.  SO don't sweat those partial volts.

If you are confused by VA ratings, you can think of VAs as Watts, it is close enough for this discussion.
#2095
Schematics and Layouts / Re: Peavey's steel amps
September 13, 2008, 11:25:35 AM
Here is the factory drawing for the Nashville 400 for comparison for you.


Oh poop, the file is roo large to attach here, so here is a link to it elsewhere.

http://www.ampix.org/albums/userpics/10003/PV_Nashville_400.pdf
#2096
That's a great list.

Keep in mind too that while online files are handy at 4:30AM - the current time for me - there are other resources.  Many companies will send you the schematics if you ask for them.  Peavey is a good example, they will send you any you ask for.

This is not only useful when there is not an online library, it is also useful when an online collection is missing one.  They may have it in their offline files.
#2097
Schematics and Layouts / Re: Peavey's steel amps
September 12, 2008, 04:13:07 AM
Without me looking them up and doing a side by side, are these circuits different from the PV published ones?
#2098
Amplifier Discussion / Re: Short before Diode?
June 14, 2008, 12:19:59 PM
Try this.  Plug a spare cord into the effects send jack, and then the other end right back into the effects return jack.  if htis restores the sound, then the return jack needs service or cleaning.

If the amp has a pair of jacks marked preamp out and power amp in, try the same test there.