Crate MX65R . . . vague problems . . .

[poplars]

ok, well this is my first post but I'm not sure where to start as far as this amp goes . . . I guess I'll put the most detail yet into this thread since there are people here capable of helping me.

so, it all started one day when we took it out of this trailer from the middle of nowhere . . . plugged it in, plugged the guitar in . . . the input jack barely worked. the red LED for the gain channel didn't turn on, and you could hardly hear anything the guitar was putting out.

so, we plugged in the pedals . . . they didn't light up, or show any signs of making any difference whatsoever. we tried adjusting all the adjusters . . . no audible difference (but it could have potentially made one . . . )

THEN we plugged the guitar into the Insert jack, and we got perfect clean sound (but again, nothing would change on the amp, pedals don't do anything, etc.)

so after much delay, my friend and I finally opened it up and checked it out. NO apparent damage to any of the capacitors . . . so we take out the dmm. tested the 2 largest capacitors on the board . . . two 4700µF 50V capacitors . . . one of them tests bad. we replace it, luckily I bought two because we put the first one in wrong and it blew up . . .

so after replacing the capacitor, we're back to square one. everything works exactly the same as before. . . except the vague just barely audible noise the guitar would make in the input jack is almost completely inaudible. the insert jack still works fine.


the past two weeks I've been testing everything with my friend and I'm not sure where we should go next . . . the transformer is fine, but this board is really complex . . . lots of components.

things you should know in general: the red LED for gain/overdrive channel doesn't turn on at all. the green LED for clean channel is always on and doesn't change despite the switches being turned on or off. and the red LED for the power indicator turns on . . .


thankfully, crate supplied me with the schematics for this amp. I will add more details and possibly pictures later if necessary.

http://d.turboupload.com/d/1946094/schematics.rar.html

thanks.

[oc disorder]

Hi.. well firstly it appears that the power amp section is ok ie the section after the insert jack so the problem must be in the pre-amp section.
Good place to start is the power supply - firstly make sure all live connections
(where the 110/230 volts come in depending where you live) are safely
covered.. ie mains switch , connections to fuse ,power selector (if any)
so you live to report back to the forum, I often use proper insulating tape
as a temp measure just in case I get distracted and put my hand where I shouldn't !. Looking at the circuit the first place I would look is at the two zener diodes ?25 and ?26 (cant read it very well) and the two "largeish"
270 ohm 5 watt resistors and check for dry joints .
By measuring with a meter across c43 and c44 they both should have aprox 15 or 16 volts across them.If its hard to get the probes there put one probe on the chassis (ground or earth) and measure from the junction of the zeners
and the 270 resistors (ie two seperate readings) one should be +15/16volts
and the other - 15/16 volts. I would suspect the 270 ohm resistor in the negative supply first ... as the incoming supply is aprox 40volts and the zeners are approx 15volts - the 270 ohm resistors should have aprox 25volts
across them.See how you go with that and post your findings hopefully someone has a clearer copy of the schematic as I am finding it hard to read
with foxit reader.

[poplars]

about the schematic, if you read it with adobe reader but you to go view and click lineweights, you will find it is MUCH easier to read.


I thought I was referring to the power supply when I said 'transformer'. we tested all the power outputs to the board, they all tested up to spec according to the schematic. I haven't tried any of the test points yet, I'm waiting on that due to the electrical danger here.

btw. this reply isn't addressing your entire post, just what I can say at the moment.

[Enzo]

Well eventually you will have to stick your voltmeter probes in there and check the voltages. D25,26 are those 16 volt zeners. They have to have about 16 volts across them. DOn't worry if it is off a volt or something. We are looking for a complete absence of +16 or -16.

I would suspect the insert jack itself if it were just low output, but with the loss of LEDs and lack of channel switching, I'd bet on power supply.

The transformer is part of the power supply, but it is not THE power supply. Power supply includes the transformer, the rectifiers, the filter caps, the dropping resistors, the zeners or voltage regulators, and anything alse that supplies power to the various circuits.

[poplars]

well, we tested all the power cables comming from the power supply that plug into the board and they all tested up to the correct voltages.

not sure about everything else.

[poplars]

so I found something interesting here. D15 has a huge (for its size) bubble-like thing sticking out of it. my friend and I looked over the schematic, and apparently if the D15, 16 and Q17 were all damaged, that would explain the symtoms for this entire deal.

seem possible to anyone?

[poplars]

well, it seems that we're back to square one. I really have no idea what to do now.

I don't have a voltague generator, I don't have any special tools for this job. just a DMM (possibly an unreliable one) and a soldering iron :P.

after some extra testing, I found that D15, D16, and Q17 are fine. Q17 looked damaged at first, but apparently it isn't.

so honestly I have no idea what to try next . . . I would appreciate some idea's, you guys have the schematics if you need them (first post.)

[poplars]

hey all, well I believe P1 (a 10k potentiometer) is busted . . . I have absolutly no idea where I can buy a replacement and I'm also not sure if P1 alone can account for all of these issues, but it seems entirely possible to me.

here's the writing on the back:
10k
70-103-61
0122

any idea's anyone?

[Enzo]

Start with the closest Crate dealer. 70-103-61 is the factory part number. 0122 is the date code - part was made 22nd week of 2001.

Most any 10k pot would work, but the factory part will fit the panel hole, and the printed circuit footprint as well as fit the knob.

[poplars]

after much testing, it seems that there is only 9ohms of resistance between v+ and ground. . . . v+ is scoring 1v when it should be scoring +16v. the test points A B C scored the following:
-------------------A---|----B--|---C
CHAN A GAIN1 -.07v | +.8v | +.9v

CHAN A GAIN2 -.10v | +.5v | +.9v

CHAN B CLEAN -.04v | +.7v | +.9v


if you refer to the schematic, under SWITCHING LOGIC; you will see tht all of these results are either + or - 10v off.

any idea's on what to try testing next (short of everything on this board) would be greatly appreciated.

thanks

[Enzo]

V+ the +15v power supply? The most likely problem is a shorted zener diode. I can barely read it, but it looks like D25, a 1N5353. Or the filter cap is supplies - C43. REmove those and see which one measures 9 ohms.

[poplars]

ok, thanks. I don't really understand much of this stuff, I take the readings and my friend interprets the schematic.

also, if you want to make it easier to read, and you're using acrobat reader, go to View, and click Line Weights (makes a world of difference)

[poplars]

I think D25 is to blame, it scores around 8-10 ohms on each side (averaged at 9ohms)

I'll test C43 when I find it

edit: C43 tested a massive amount of resistance on both sides. 1.4mohms on one side, .8mohms on the other :P.

so is it D25?

[oc disorder]

Yep 1N 5353 16 volt 5 amp Zener Diode I'de be surprised if it cost more than
$1.... while you are there might as well buy 2 and replace the other one
cause they are constantly under minor stress...
Lets hope there is no short elsewhere that caused it to break down, but I guess as the cap had such a high reading its unlikely.

"Looking at the circuit the first place I would look is at the two zener diodes ?25 and ?26"

"D25,26 are those 16 volt zeners. They have to have about 16 volts across them."

[poplars]

ok I ordered 3 of them. 84 cents, too bad the shipping will probably jack it up to like 4 bucks :P. but it's all good, still less than paying 200 bucks to have someone else fix this thing .

anyways, I'll post back after I get the parts, thanks everyone.

http://www.alliedelec.com/Search/Pro...42D003041617F#

those are the ones I ordered, I'm pretty sure they're the right ones, they look exactly like the one I took out . they're 1N5353B instead of 1N5353, I doubt it makes any difference, they're 16v 5w .