Fixing a 30-year-old Crate

[scissortail]

Anyone want to help me fix my 30-year-old Crate KX-160 amp ? It's nothing fancy but it has sentimental value to me. And it's good workhorse if you want something clean and loud. It's got four input channels, an fx loop, and a 7 frequency EQ. The user manual is online here:

link to manual

and I found a schematic here:

link to kx160 schematic

It started cutting out intermittently, and there was noise when I adjusted gain, volume, and EQ. So I thought the potentiometers had aged out. Replaced them all (both rotary and slide) and I cleaned all the plug contacts. That got rid of the noise, but the amp still cuts out.

Although cutting out is intermittent I can generally make it happen when I turn the gain and the 7-frequendy EQ down to low levels. After a minute or two of noodling on a guitar, the audio cuts out, and I can then restore it by turning the master volume down (just for safety) and then turning the gain and EQ up near or at maximum. Then I can slowly bring the volume up and the amp is working again. It tends to remain stable when the gain and EQ remain at high levels. It's the same story on all four input channels. So I'm guessing there's something like a bad transistor that is not responding properly to low input signals from the preamp. A top candidate in my mind is the op amp that links the preamp to the amplifier circuit (IC4 on schematic; page 3 far right). But I'm just guessing.

Another thing I noticed, is that the amp is not producing much heat. My memory tells me that the whole outer casing used to get pretty warm. Now the only things that get noticeably warm are two 330 ohm resistors situated side by side on the amplifier circuit board. They are large 10W resistors, so it does not seem strange to me that they should get warm. But there are also 4 other 10W resistor (at 0.33 ohms), and they stay cool. Also the parts attached to the heat sink don't heat up noticeably. But then, maybe I'm just not pushing it hard enough or long enough in my testing.?.?

I've got a hobbiest electronics background, and I'm good at fixing circuit boards. But I'm more familiar with digital electronics than analog. I removed and examined the circuit boards. The solder joints are all clean and shiny and there are no obviously burnt out parts. I applied contact cleaner to all the wire connection terminals, and they were clean and free of corrosion. I'm hoping someone recognizes the symptoms I've described and can suggest some testing procedures or part replacements. I've got a digital oscilloscope I can bring to bear, but I don't have a signal generator.

Thanks for any help

[DrGonz78]

What happens in these Crate amps is that the heat dissipates across those 10w wire wound resistors. They are used to drop B+ to a 15vdc or something lower voltage. This voltage controls the preamp. Check for cracking and cold soldering on those dropping resistors. My hunch is that you are losing the supply and it's cutting out. The heat even wears out the soldering pad so be patient resoldering as it damages things even more sometimes.

I noticed that you said the joints looked good but test the voltage. When it cuts out do you lose any preamp voltage?

[g1]

Connect the FX send to FX return jack with a patch cord, see if you still get the cutting out issue.
Aside from that, the schematic link does not allow me access to both pages, please upload a copy here.

[DrGonz78]

Connect the FX send to FX return jack with a patch cord, see if you still get the cutting out issue.
Aside from that, the schematic link does not allow me access to both pages, please upload a copy here.

Oh yeah 👍 It's always this!!

[scissortail]

Connect the FX send to FX return jack with a patch cord

Oh yeah 👍 It's always this!!

Not this time. I saw that trick in several other posts and gave it a go. Bridging the FX loop with a patch cable does not help.   

[scissortail]

Check for cracking and cold soldering on those dropping resistors

Took another look at the amp circuit board. The solder joints on the big resisters looked perfect. I reflowed them just to be sure. Didn't change anything. Pretty sure it is something else.

I noticed that you said the joints looked good but test the voltage. When it cuts out do you lose any preamp voltage?

I'm not sure what voltage you are talking about here, but I measure +16 and -16 going to the preamp. It is steady when dropping out occurs.

[scissortail]

the schematic link does not allow me access to both pages, please upload a copy here.

Attached. You cannot view this attachment.

[DrGonz78]

You need to determine if the issue involves components of the preamp or the power amp. Can you take a line out signal to another amp to hear the preamp isolated? Inject a signal into the line in on the troubled amp. It won't be as gain sensitive bypassing the preamp but this is a quick way to isolate the problem. Good work troubleshooting so far 👍

[Tassieviking]

Next time it cuts out try giving the amp a thump on top and see if you can hear the reverb sound coming out of the speaker, naturally have the reverb turned on and up loud.
Also try the guitar directly into the effects return when it cuts out, it wont be very loud but you can usually hear some signal come through if the power amp is ok.

If these are no good then measure the voltage on IC6 on pin 4 and 8, there should be 30 volts across there, then we can go from there.

Is J1 connector a plug or soldered in ? It connects the pre-amp to the power-amp and if its a plug/socket it might have a dirty contact.
Cheers
Mick
 

[g1]

Bridging the FX loop with a patch cable does not help. 

It seems there are 2 FX loops in this unit, did you try them both? (one is called 'pre-master loop')

[scissortail]

It seems there are 2 FX loops in this unit, did you try them both? (one is called 'pre-master loop')

I'm not 100% sure, but this may have nailed it. The amp worked with the patch cable installed. They after I took the cable out it CONTINUED working!!. So maybe a contact was bad on one of the plugs. I'm going to reassemble everything, continue testing, and report back. I never thought about the pre-master loop.

@DrGonz78: I tried the line out thing, and it did not cut out on the other amp. I'm not sure what to make of that given the solution with the pre-master loop. Seems like both amps should have cut. I didn't try the line in trick, since there was an apparent fix with the pre-master loop.

@Mick: Thanks also for your suggestions. If the problem reemerges I will carry out the proceedures you described. I should note that the reverb doesn't seem to work at all. It never sounded very good, so I don't miss it. I hope it is not a symptom of some other problem. . .

Thanks folks. More soon.

[DrGonz78]

Yup just spray some contact cleaner in there and exercise it with a male 1/4 plug to clean it out. Sometimes I will pull the chassis and use burnishing tool to clean deep into the switching contact.

[g1]

To be more specific, it is the 'line in' jack of the pre-master loop that is using it's switch contact.  It is shown as J8 on the schematic, lower left corner of the 3rd page.
Likely that when you exercised it, some oxidization was cleared, but it should get a shot of cleaner like Dr.Gonz suggested.

[scissortail]

Yep I sprayed it straightaway. It held steady for an hour+ long jam last night.

[DrGonz78]

It's strange that earlier you put patch cables between FX In/Out and it didn't restore the connection. I assume you are relying on the switch jack to do its job but perhaps the mechanical function of the switch is worn out? Did you try to restore the signal through amp with patch cables when it failed?