Old crate G40C buzzing after warm

96lt4c4 · 2nd February 2011, 01:41 AM

I have had this amp for 10 years and it has always done this. After about 5 to 10 minutes of the amp being turned on it starts to buzz, humm and it
oscillates. I found the schem. for it on this site and have it tore down. I Found one cap that looks like it had came apart and someone wrapped black tape around it. It was a 1 mF 5 volt. I replaced it with a 1 mF 10 volt and still have the problem. I am going to check the voltage coming out the secondary side of the transformer next. Just wondering if anyone has any ideas. I have no idea how old this thing is.

llwhtt · 2nd February 2011, 02:57 AM

Almost all Crate amps of that vintage use zener regulation for the op amps. The 5 Watt dropping resistors and the zeners themselves run very hot and the solder joints go bad. Also check the solder joints on the main filter caps, they tend to break. Another problem you'll see when you take it apart is that all of the nuts behind the knobs will be loose, certainly not tight, anyway check all of the solder joints on the controls and anything on the PCB that goes to the outside the chassis, jacks, controls, switches, etc. In my experience that should take care of 90% of Crate problems that are sorta still working, ex-warranty shop. Get a magnifying glass to inspect the solder joints, look for "rings" around the leads, those are cracks.

Craig

Enzo · 2nd February 2011, 08:03 AM

And don't bother the transformer, the likelihood of it being wrong is very tiny.

As Craig points out, check the +15v and -15v power supplies.

96lt4c4 · 2nd February 2011, 01:10 PM

I knew the likelihood of the transformer being bad was slim. Thanks guys i will check all the solder joints.

96lt4c4 · 2nd February 2011, 07:44 PM

I found a bad diode, it looked like it had been getting pretty hot and I wasn’t getting any voltage across it so i pulled it out. It was shorted. It is on the power supply side next to a very large 120 ohm ceramic resistor. The diode was pretty good size as well. All I had were some bridge rectifiers so I used one side of the rectifier to replace it and cut the other 2 legs off. Now I am getting 15 vdc on one side of the diode. I have some pictures that I can post later. I also pulled the 4 diodes off that made the bridge and replaced them with an integrated bridge rectifier.

96lt4c4 · 2nd February 2011, 09:11 PM

New bridge

New diode

llwhtt · 2nd February 2011, 09:21 PM

The 120 Ohm 10 Watt resistor (R115) is the dropping resistor for the + supply, D17 (1N5357A) is the zener. Did you replace it with a rectifier diode?

Craig

96lt4c4 · 2nd February 2011, 09:51 PM

Quote:

Originally Posted by llwhtt

The 120 Ohm 10 Watt resistor (R115) is the dropping resistor for the + supply, D17 (1N5357A) is the zener. Did you replace it with a rectifier diode?

Craig

Yes using one diode in the rectifier, 2 legs cut off. I didnt have one as big or the same as the one that came out. Will this not work?

96lt4c4 · 2nd February 2011, 09:55 PM

Its been a long time since I was in tech school. I just looked up a Zener, and it looks like thats what needs to go back in there.

llwhtt · 2nd February 2011, 10:04 PM

It needs to be a zener, the rectifier you have there now is not doing anything. The voltage will be too high, do not run the amp until the zener is replaced. D17 is a 1N5357A and D18 is a 1N4744A.

Craig

2nd February 2011, 01:41 AM	#1
96lt4c4 diyAudio Member Join Date: Feb 2011 Location: Bardstown, KY	Old crate G40C buzzing after warm I have had this amp for 10 years and it has always done this. After about 5 to 10 minutes of the amp being turned on it starts to buzz, humm and it oscillates. I found the schem. for it on this site and have it tore down. I Found one cap that looks like it had came apart and someone wrapped black tape around it. It was a 1 mF 5 volt. I replaced it with a 1 mF 10 volt and still have the problem. I am going to check the voltage coming out the secondary side of the transformer next. Just wondering if anyone has any ideas. I have no idea how old this thing is.

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2nd February 2011, 02:57 AM	#2
llwhtt diyAudio Member Join Date: Jun 2008	Almost all Crate amps of that vintage use zener regulation for the op amps. The 5 Watt dropping resistors and the zeners themselves run very hot and the solder joints go bad. Also check the solder joints on the main filter caps, they tend to break. Another problem you'll see when you take it apart is that all of the nuts behind the knobs will be loose, certainly not tight, anyway check all of the solder joints on the controls and anything on the PCB that goes to the outside the chassis, jacks, controls, switches, etc. In my experience that should take care of 90% of Crate problems that are sorta still working, ex-warranty shop. Get a magnifying glass to inspect the solder joints, look for "rings" around the leads, those are cracks. Craig

2nd February 2011, 08:03 AM	#3
Enzo diyAudio Member Join Date: Sep 2004 Location: Lansing, Michigan	And don't bother the transformer, the likelihood of it being wrong is very tiny. As Craig points out, check the +15v and -15v power supplies.

2nd February 2011, 01:10 PM	#4
96lt4c4 diyAudio Member Join Date: Feb 2011 Location: Bardstown, KY	I knew the likelihood of the transformer being bad was slim. Thanks guys i will check all the solder joints.

2nd February 2011, 07:44 PM	#5
96lt4c4 diyAudio Member Join Date: Feb 2011 Location: Bardstown, KY	I found a bad diode, it looked like it had been getting pretty hot and I wasn’t getting any voltage across it so i pulled it out. It was shorted. It is on the power supply side next to a very large 120 ohm ceramic resistor. The diode was pretty good size as well. All I had were some bridge rectifiers so I used one side of the rectifier to replace it and cut the other 2 legs off. Now I am getting 15 vdc on one side of the diode. I have some pictures that I can post later. I also pulled the 4 diodes off that made the bridge and replaced them with an integrated bridge rectifier.

2nd February 2011, 09:11 PM	#6
96lt4c4 diyAudio Member Join Date: Feb 2011 Location: Bardstown, KY	New bridge New diode

2nd February 2011, 09:21 PM	#7
llwhtt diyAudio Member Join Date: Jun 2008	The 120 Ohm 10 Watt resistor (R115) is the dropping resistor for the + supply, D17 (1N5357A) is the zener. Did you replace it with a rectifier diode? Craig

2nd February 2011, 09:55 PM	#9
96lt4c4 diyAudio Member Join Date: Feb 2011 Location: Bardstown, KY	Its been a long time since I was in tech school. I just looked up a Zener, and it looks like thats what needs to go back in there.

2nd February 2011, 10:04 PM	#10
llwhtt diyAudio Member Join Date: Jun 2008	It needs to be a zener, the rectifier you have there now is not doing anything. The voltage will be too high, do not run the amp until the zener is replaced. D17 is a 1N5357A and D18 is a 1N4744A. Craig

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