Crate G120CXL Rebuild

[russell072009]

Hello.

So in my infinite wisdom i decided i wanted a project to play with. I found a cheap crate amp and decided to rebuild it. I have spared no expense, upgraded every component and generally spent a whole lot of time soldering. I finally got the thing back together yesterday and to my surprise it didn't burn up when i powered it up. Everything works but the chorus. So i have spent a bunch of time trying to hunt down that problem. I can't figure it out. It's been a very long time since electronics class. If anyone can take a peek at the schematics and give me an idea of where and how to figure out what's wrong i would really appreciate it.

All i have for test equipment is a DMM. So far i have checked that i have proper voltage with the switch on and off at the designated test point. I have checked for power from IC7 through IC12. Other than that i don't have a scope to check for wave forms. My biggest issue is the schematic confuses me. I would have thought the chorus effect would be isolated more than it is. i can't tell where the effect circuit starts or ends.

Anybody want to help?

[Enzo]

Hi, welcome. I hope you had fun with your project, though I suspect it was gilding the lily.

The amp is stereo, each speaker has its own power amp. Do they both work? You can check each by plugging your guitar or for that matter most any signal into the left and the right line in jacks. Actually it should also work if you plug into the line out jack. If both power amps work, but one speaker stays silent otherwise fed from the preamp, then first jump across the send/return jacks for that side. You can also test the preamps by connecting each line out to some other amp. Does that make sound or not over there?

Do you know how it works? IN dry mode, the same mono signal feeds both power amps, thus both speakers. In chorus mode , ONE amp/speaker stays dry, the other amp gets the chorused signal. Two different signal then for the two speakers. SO, if only one amp works, then we either always get one speaker of dry signal, or the one speaker signal goes from dry to total effect. Depends which channel is dead. If both amps work, then we can have both dry, but when it goes to chorus, the chorus speaker goes dead. SO does turning the chorus on and off make one speaker turn off and on? A lack of chorus could mean the chorus side amp is dead, or it could mean the internal signal routing is bad, or it could mean the actual chorusing circuit is bad.

There is also an FX loop, so we can input a signal into the FX return and bypass most of the preamp, that might reduce any confusion.

[russell072009]

Hi Enzo,

Thanks for helping out. So thankfully the amp uses plugs for the speakers instead of being wired directly. I can verify that both channels operate in clean and overdrive. However once i switch the chorus on i lose the right channel. I'm assuming the chorus circuit is not right. It's all been replaced with new components but i'm guessing either i put the wrong part somewhere or one of them is bad. I ran through the layout and schematic to verify but it's hard for me to isolate the chorus circuit in the schematic. It's not as isolated as i would think it should be so i don't know what is chorus and what is preamp.

[Jazz P Bass]

Hello.

So in my infinite wisdom i decided i wanted a project to play with. I found a cheap crate amp and decided to rebuild it. I have spared no expense, upgraded every component and generally spent a whole lot of time soldering. I finally got the thing back together yesterday and to my surprise it didn't burn up when i powered it up. Everything works but the chorus. So i have spent a bunch of time trying to hunt down that problem. I can't figure it out. It's been a very long time since electronics class. If anyone can take a peek at the schematics and give me an idea of where and how to figure out what's wrong i would really appreciate it.

All i have for test equipment is a DMM. So far i have checked that i have proper voltage with the switch on and off at the designated test point. I have checked for power from IC7 through IC12. Other than that i don't have a scope to check for wave forms. My biggest issue is the schematic confuses me. I would have thought the chorus effect would be isolated more than it is. i can't tell where the effect circuit starts or ends.

Anybody want to help?

That schematic link is dead.

Try 'zipping' the file & reloading it.

[Enzo]

nickb has it here: Guitar Amplifier Schematics and more

Under Crate G120C

You can replace a million parts, but what if the issue was a cracked copper trace? Or some connector?

The chorus circuit is switched in and out by a bunch of JFETs with control voltages to their gates. The chorus circuit could be bad, or the JFETs or the switching control circuit. That is why we troubleshoot rather than wholesale parts replacement. Are the JFET gates getting control voltages or not as you toggle the effect on and off?

[russell072009]

nickb has it here: Guitar Amplifier Schematics and more

Under Crate G120C

You can replace a million parts, but what if the issue was a cracked copper trace? Or some connector?

The chorus circuit is switched in and out by a bunch of JFETs with control voltages to their gates. The chorus circuit could be bad, or the JFETs or the switching control circuit. That is why we troubleshoot rather than wholesale parts replacement. Are the JFET gates getting control voltages or not as you toggle the effect on and off?

I did the wholesale parts replacement because i wanted to rebuild it with overall better components. Like i said it was a project for me to play with. The fact that it didn't burn up on initial power up was awesome. Now i just need to figure out the chorus. While it is possible there is a bad trace i did look over the board very well while i was replacing the parts. All the traces look fine. I assume the chorus circuit receives the signal at all times and there is a switching circuit that changes the input of the right channel from direct from the preamp to the chorus output. I just don't know what components are part of the switching circuit.

[Enzo]

You can see gross problems visually, but trust me, you can't tell by looking. A typical copper trace is a thin stripe of copper, that widens into a "pad" at the end for the solder around a component lead hole. What can happen is the solder makes its little volcano around a resistor wire or whatever, but the trace cracks right at the edge of the pad. You'll never see it. I don't know that you have that, but it is certainly a possibility.

Center right on the preamp page of the schematic are a bunch of JFETs, those are teh switching elements.

[Jazz P Bass]

I did the wholesale parts replacement because i wanted to rebuild it with overall better components. Like i said it was a project for me to play with. The fact that it didn't burn up on initial power up was awesome. Now i just need to figure out the chorus. While it is possible there is a bad trace i did look over the board very well while i was replacing the parts. All the traces look fine. I assume the chorus circuit receives the signal at all times and there is a switching circuit that changes the input of the right channel from direct from the preamp to the chorus output. I just don't know what components are part of the switching circuit.

The chorus circuit is on Page #1.
The point on the lower left marked 'C' all the way across the right side of the page.

The switching is over on the middle right where the Chorus foot switch is labeled.
The voltage markings for the footswitch are labeled.
This will test the function of the switch circuit. (Q11).

The Jfet marked Q10 is the active element.
It is Normally closed Drain to Source. (Gate at -0.5Vdc) Chorus will be On.
It Opens (ie: chorus Off) when the switching circuit provides the Gate with a much higher negative voltage.

You will notice that 'C' again.
It physically connects to the other 'C'.

[russell072009]

So keep in mind i'm measuring with a DMM. I get about 1vac on the output of IC8 pin 7. I also have the same on pin 3 of IC10. i do not however have output from pin 7 or 8 from IC10. I've already changed the MN3007 thinking that might be it but it did not seem to help.

The gates, Q9 and Q10 seem to be working. chorus on, gate on Q10 has power and the gate for Q9 does not. Chorus off the gate on Q9 has power and Q10 does not.

I also noticed that my -16V supply is more like -25V.

[Enzo]

So fix that, nothing works right with bad power supply. Possible open zener?

[russell072009]

does not look like it. I've got about 30 VAC from both sides of the secondary. I've got about 40vdc after the rectifier though. So my 29.5 supply to the amp is about 40. any ideas on that one?

I'm assuming that is why my + and - 16V are high. Input for the circuit is supposed to be 29v not 40.

Ok let me edit this, i just checked again, if i measure across one of the secondary windings i get 25vac on power up. from there it slowly creeps up to about 35vac.

same goes for the DC side if i measure from one of the 6800uf caps it slowly creeps up to over 40vdc as well.

bad transformer? i can't think of anything else that would cause that.

[Enzo]

30vAC rectifies and filters to about 30 x 1.414 = 42v more or less, so that looks normal, 29 does not...to me. Are both pos and neg sitting at 40v?

Look upper left on page 1, below input jacks, see the two zener circuits? Those are the 16v supplies. Your 40/29v drops to the 16v zeners through resistors. The zeners determine the voltage, not the input from the supply. If I see a 16v zener with 25v across it, I assume it is bad. I stock a drawer full of 1N5353, it is not an unusual thing to replace.

If your 16v rails are 25v each, then your op amps must have about 50vDC between pins 4 and 8. Is that the case?

So Q9,10 are the switches, (Q7,8 are power up mutes on the preamp outs) if their gates toggle, that is good, not test their function. Your dry works, right? And when you turn on the chorus, the dry goes away in that channel, so the dry side JFET appears to work. The FX side JFET is in question. Looks like Q9. SO you know the gate pin, the other two are source and drain, doesn;t matter which is which. A JFET is ON until turned off by a voltage at the gate. Power off, Measure resistance source to drain of the good one - should be a lowish resistance. Now do the other, measure similar?

With power on, you can measure resistance source to drain and see if toggling the effect on/off makes the resistance change substantially. My offhand thinking is that JFET might be bad, it seems stuck in the OFF mode even without a gate drive. Tack a wire from source to drain on Q9, and see if the FX is restored. You won't be able to turn it off that way, but it would demonstrate the JFET is the issue. In any case, wwhatever method you use, Q9 and Q10 ought to measure the same.

If your meter on AC volts can detect the signal through the amp, see if it is present at AP2 there left of Q9. If not, then Q9 has nothing to switch.

I often used my meter for that, when my scope was off and it was just one or two readings. But a useful tool you can make is a signal tracer. Essentially it is a small amplifier (well any size would work, but seems pointless to use an SVT) with a probe p[lugged into the input. A probe is a shielded wire. We add a series cap to block any DC. Ground the tracer to the circuit under test, and then probe points in the s=circuit with the input cable. You can listen to what is ther at any point.

[russell072009]

Well i popped a 10ohm resistor on the 29V rail so half my amp is dead. i'll have to buy some more resistors.

I did replace both 1N5353 diodes when i upgraded the components. What do you think the odds of both being bad are? Any brand recommendation for replacements?

Also the voltage on the zener is slowly climbing with the 29v rail as well. So measuring from the + side of D5 where it meets R133 shouldn't go much past 16v? having it climb to over 20 is not normal?

[russell072009]

On another note i have a G212XL amp that was 100% working before i pulled it out of the cabinet to put the G120 in to it. I checked the power supply on that amp and i have 41VAC on the secondary and 25VDC on both the 16V rails. I am totally confused now. What are the odds that the voltages i am getting are normal for crate?

[Enzo]

What are you using for the meter ground point? Did you try the voltage from pin to pin on the IC power pins? The dual op amps cannot stand 50v, more like 35 tops. If you find 30v across the ICs, then we have a measuring issue, rather than a voltage problem per se.