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Messages - Enzo

#1966
The Newcomer's Forum / Re: Crate G80XL fading out
January 29, 2010, 04:31:22 AM
Also squirt some cleaner into the footswitch jack and probe a plug in and out of it a few times.  That jack could be involved in your channel switching problems.

Your problem is probably not a bad partt, more like a bad connection.

Next time this happens, ball up your fist and whack the top of hte amp.  If the amp reacts, something is loose.
#1967
Look at your second photo, at the circuit board on the right.  ALong the bottom are two white stripes showing.  Right above each stripe appears to be a pattern of about 15 pins.  On the other side of the board, are there two 15-legged ICs mounted to the heat sink?  the white stripe being heat sink grease showing underneath?

Possible number on the things might be TDA7293 or LM3886 maybe?

If so, it would be one or more of those that I suspect burnt up.  usually the smoke comes out of them around the legs.

Your transformer is the last thing I'd suspect, though like anything else, it is not immune to failures.
#1968
Or bolt one of these things over the cone.  They come in all sizes.

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=260-373

And if the grille projects beyond the front edge a little, what harm?
#1969
Actually, in the USA, a "real" hardware store should have T-nuts.  And stores like Lowes and Home Depot should as well.  They come in stardard imperial threads, so 3/16 would work, or 12-32 or 10-32.

You are just bolting a handle to a box, this is not critical as to the size screw you use.

Of your speakers are mounted using T-nuts, again use whatever makes sense.  Typical speaker screws are #8 or #10.  I wouldn;t use smaller.  And if you want to go larger, make sure the screws fit the holes through the speaker.
#1970
Amplifier Discussion / Re: Help repairing crate xt65r
January 26, 2010, 03:45:32 AM
The insulating piece is under each transistor?  Pull the mounting screws.  Are the tabs still shorted to the heatsink with the screws out?  Each screw MUST have a plastic shoulder washer on it.  Otherwise the screw shorts the transistor to the heatsink.

The shoulder washer - or T-washer if you prefer - os a plastic washer with a short tube projecting out one side.  This bit of tube sheaths the screw shaft as it passes through the transistor hole.  It keeps the screw cenetered in the hole and insulates it.

The heatsink is brounded to the chassis.  The tabs of the power transistors are connected to the power rails.  If the tabs are griounded to the heatsink, then you have a dead short across your power supplies.  Fuses blow.
#1971
Isolate the problem.  There are two speakers.  When the chorus quits, does one speaker go silent?  Only one of the two speakers makes the chorused so9und, the other remains dry.  SO if the chorused speaker goes silent, you lose the effect.

That could be an initermittent speaker, a failing cord from the amp down to the speaker, a loose connection to the speaker wiring at the amp.

Spray cleaner in each jack and probe a plug in and out a few times.  Cutout contacts on FX loop jacks and power amp in jacks can cut out your signal.
#1972
Um, did you try a different guitar cord?
#1973
Amplifier Discussion / Re: Test Signal Help
January 23, 2010, 12:28:28 AM
Look at your test point, it is the input jack.   Showing the 2mv signal at the input jack is the same thjing as specifying a 2mv input.

Be aware all those signal voltages are appoximate.  DOn;t waste time trying to "correct" a 34mv signal point that is supposed to be 38mv.
#1974
Amplifier Discussion / Re: Help repairing crate xt65r
January 23, 2010, 12:24:46 AM
OK, first, the transistor tabs are NOT ground.  That is why there is an insulating washer under each one and a plastic step washer on the mounting screw.  The metal tab is not grounded.  Use chassis as your ground point.

You didn't have to remove the power transistors to test them for shorts, but since they are out...   Each one has three legs.  That is three combinations of two legs.  Are any pair of legs shorted together?


You will find that the center leg of the transistor is connected to the mounting tab.  In each case that is the collector of the transistor.

When installed, the tab of one ought to have +40v on it, and the tab of the other ought to have -40 on it.

I am not sure what you mean the two red wires are at 56v.  56v from one to the other?  That sounds right.  But they are not 56v to ground.  I bet you have about 28v to ground from each one.  Look at the schematic, there is a center tap on that transformer and it is grounded.

If you rectify and filter 28VAC, you get just about 40VDC on the nose.

Even though you assumed wrong that the transistor tab was ground, you still got readings of about 80v betwen two of their legs
, so I bet the +/-40 are OK.



















































#1975
Amplifier Discussion / Re: Help repairing crate xt65r
January 21, 2010, 10:23:02 PM
Yes of course you can connect a transformer to the mains with the secondaries not connected.  But the power transformer is not your problem.  It won;t place DC on the speaker leads.

You have the oldest problem in the solid state world, a blown output stage.  Probably one of those TIP142 or TIP147 transistors is shorted.   Check them with your meter.  REplace the pair even if only one is bad.  If they both seem OK, then continue.

Since it doesn't blow fuses, disconnect all speaker loads - the amp doesn't need them, and you don;t need to burn them up.   Once the amp no longer makes DC, you can connect one.

First verify power supply.  The metal tabs on the output transistors is a handy test point.  Looks like 40v rails, so you got +40 on one and -40 on the other?   More or less?
#1976
Amplifier Discussion / Re: Power Chorus with no chorus
January 20, 2010, 11:12:06 PM
First, are both speakers making sound?
#1977
Whatever this is, it is going to require soldering on the circuit board inside the amp.  Are you up to that?

Probably not, which means a technician is needed.   And that gets expensive for a small amp.  You need to find a friendly local electronics guy willing to help you in exchange for a bottle of beer or something.  Figuring out what is wrong should not be difficult for anyone who is familiar with circuitry.

If you are near Lansing Michigan USA, I would be happy to look at it for you.
#1978
This is a simple amp.  If the master vol comtrol turns the noise down, then the noise is before it.  You say it is the only control affecting the volume.  So that means the noise is still there even with both channels having ALL controls at zero?

I would be surprised if it were not IC1.  A TL072, very common part.  In any case it sounds like a noisy semiconductor to me.  And before the MAster vol control there really are not many, other than the two ICs, there is the JFET TR1, and really less likely are the three bipolar muting transistors.
#1979
Amplifier Discussion / Re: Gorilla GG-110
January 07, 2010, 09:28:56 PM
Pignose distributes them, but I don;t know if they make them or not.  Seems to me that Pignose acquired the brand a while back
#1980
The Newcomer's Forum / Re: Newcomer with questions
December 22, 2009, 04:59:06 AM
So you have some bad part of failed connection in that preamp channel.

Do a search on this forum for Lab Series, and also specifically the L7.  You may find some insights, and I think we have posted the schematics of these.

As to why there is no signal, apply a steady good signal to the input and trace it through the amp.