745 PA amp [message #21561] |
Sun, 28 September 2014 23:49 ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
Iowa Boy
Messages: 767 Registered: June 2014
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HELP! My amp took a crap last night. Loud crackling sound that sounded like the speakers were going to explode. Even with nothing plugged into the amp head and all volume control shut down, crackling sound is still present. Tried different speakers and still there. Something must be fried. Reverb tank is unhooked so know that's not the problem. Where do I start? Checked speaker cords and they work fine on other amps. Power cord has been replaced with grounded plug and resistor was removed. Up until now, this has been a great amp. Have a very good Fluke meter so point me in the right direction please.
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Re: 745 PA amp [message #21562 is a reply to message #21561] |
Mon, 29 September 2014 06:19 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
stevem
Messages: 4772 Registered: June 2004 Location: NY
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Since you unplugged the tank did you tape off both rca ends and restrain them from moving and shorting out on something?
Slide the amp out and move the wires around and tap on the boards, does this action make the amp act up?
Are the connections on top of the power supply filters tight?
Are any of the input jacks on the front of the amp loose from the chassis?
Are any of the circuit board mounting frame to chassis hold down screws loose?
[Updated on: Mon, 29 September 2014 07:42] Report message to a moderator
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Re: 745 PA amp [message #21564 is a reply to message #21561] |
Mon, 29 September 2014 10:27 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
Iowa Boy
Messages: 767 Registered: June 2014
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Is it possible I have a short in the on/off switch.....doesn't seem to matter which way I turn it there is still the load crackling/popping sound. The second I turn the power on it starts making noise. I had been playing the amp for about an hour last night when it started acting up.....just all of a sudden. Nothing inside smells like burnt electronics and see no signs of arcing anywhere.
[Updated on: Mon, 29 September 2014 10:42] Report message to a moderator
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Re: 745 PA amp [message #21565 is a reply to message #21561] |
Mon, 29 September 2014 11:58 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Is this amp the early K200A head?
I can't stress enough the fact that making a mistake while doing these tests can do damage to you and or your amp, so please be careful and know your own limitations.
Steve asked if tapping on the boards caused the noise to react in any way. Does it?
Visually inspect the wiring in the amp. See anything loose or odd? Carefully check the wires going to the output transistors. Carefully check the two wires that connect the bias diode that is mounted in the clip on the chassis to the circuit board. Have any of these wires come loose from the pc board?
Disconnect the speaker from the amp and connect your voltmeter to the speaker jack, black to ground and red to the tip connection. Set the meter to read dc voltage, 100 volts or so scale. Now turn on the amp and see if there is any voltage spiking on the output.
The amp may not act up if the speaker is disconnected, but watch the meter and if you say that the pilot light reacts to the noise, watch that as well to see if the amp is acting up.
Next read the dc voltage across each of the main filter caps. Are both stable at around 40 volts?
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Re: 745 PA amp [message #21566 is a reply to message #21561] |
Mon, 29 September 2014 12:37 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
Iowa Boy
Messages: 767 Registered: June 2014
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I check voltage at speaker jacks.....nothing.
Voltage at filters.....41-42.
Light does not flicker with speakers unhooked.
No loose wires anywhere.
I can't tap boards since it cracks and pops so loud I'm afraid to leave running for fear of blowing speakers.
I believe it is a K200 head.
[Updated on: Mon, 29 September 2014 12:38] Report message to a moderator
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Re: 745 PA amp [message #21567 is a reply to message #21561] |
Mon, 29 September 2014 12:45 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
Iowa Boy
Messages: 767 Registered: June 2014
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I noticed that the speaker jacks appear to be wired funny. Top jack is not wired to negative but looks like it was soldered and wire had been cut. Wire runs from circuit board to positive on top speaker jack to positive on lower speaker jack and then from negative to one of the filters. Is this wired correctly?
[Updated on: Mon, 29 September 2014 12:48] Report message to a moderator
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Re: 745 PA amp [message #21569 is a reply to message #21561] |
Mon, 29 September 2014 14:13 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
stevem
Messages: 4772 Registered: June 2004 Location: NY
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First off if your reverb cables un plug from the board then you have a 200A head, not the Frank type 200!
The speaker jack can get its ground from the chassis and does not need a what should be a black wire going back to the common of both main filters, who's wires at that point should be black also.
Since you checked the 40 volts neg and positive and did not report that those voltages went on and off with the noise then your amps power supply section is likely good.
Into the Amps output driver board you will see two blue wires, un solder and or clip off each one, one at a time and see if the noise stops , if it does not than the problem is in the driver/ regulator or output stage .
If like Bill asked you to check and you gave no dc voltage on the speaker output when the amp I snaking the noise than you are safe from blowing the speakers so just lie the cabinet on its face when making checks so the noise will not be so loud!
[Updated on: Mon, 29 September 2014 14:18] Report message to a moderator
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Re: 745 PA amp [message #21574 is a reply to message #21561] |
Mon, 29 September 2014 17:55 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
Iowa Boy
Messages: 767 Registered: June 2014
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Amp head was laid on front like Stevem said, not the speaker......cables are fine.
The 795 head has only a fuse soldered into the main circuit power board. Tank is suspended by aluminum tubes. Amp has run now for about 2 hours and haven't heard any noises coming out of the speaker. Nothing appears to be loose on that board the tank plugs into. Tried plugging the tank in while it sitting on the desk......when foot switch is off, if you move the tank you hear a clanking sound but not an echo. The volume on the amp is definitely louder when the foot switch in on though. If this was Kustom's idea of reverberation, its pretty lame.
[Updated on: Mon, 29 September 2014 18:26] Report message to a moderator
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Re: 745 PA amp [message #21575 is a reply to message #21561] |
Mon, 29 September 2014 18:29 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
stevem
Messages: 4772 Registered: June 2004 Location: NY
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Ok, let's take some big steps back
I wanted you to lay the speaker cabinet on its face on the floor so the noise would not be so loud for you when it did take place, but I guess flopping the head over cured the problem which would lead me to think that something metal that should not be in the amp was making for your noise issue!
Has anything fallen out if the amp since you slid it out of the cabinet?
[Updated on: Mon, 29 September 2014 18:33] Report message to a moderator
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Re: 745 PA amp [message #21577 is a reply to message #21561] |
Mon, 29 September 2014 18:39 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
Iowa Boy
Messages: 767 Registered: June 2014
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Sure hope this isn't like an RV refrigerator where you have to turn it upside down every now and then to make it work right! lol Sorry about the confusion about laying the cabinet down on its face. I thought you meant the amp face and not the speaker cabinet. Being stupid this time paid off.
[Updated on: Mon, 29 September 2014 19:01] Report message to a moderator
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Re: 745 PA amp [message #21585 is a reply to message #21584] |
Tue, 30 September 2014 12:10 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
C4ster
Messages: 686 Registered: June 2001 Location: Mukwonago, WI (Milwaukee...
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You have a cracked circuit board or at the very least a cold solder joint. Look VERY carefully at the entire board for cracks especially around larger components that could have broken loose during the fall. Take the entire board out to look for the break. Isolate the problem by disconnecting the signals coming from the preamps. Like Steve mentioned, its the blue wires coming from the preamps to the power amp. You mentioned that the chassis was bent around the transformer. That is a good indication that the unit was dropped. Look for other bent or damaged areas but realize that the board could have been twisted and the crack is nowhere near the damaged chassis. Circuits boards don't heal themselves so that problem WILL return, probably while you are playing a gig. Not a good idea.
Conrad
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Re: 745 PA amp [message #21587 is a reply to message #21561] |
Tue, 30 September 2014 13:02 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
Iowa Boy
Messages: 767 Registered: June 2014
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Senior Member |
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Thanks for the advice on the board......yeah I figured I'd have to eventually take it out and look it over. Hope it is just a cold solder point and not an actual crack in the board circuitry. Fingers crossed on that one.
Hey Stevem, when spraying the pots......do you just spray it in the center of the back? What about the reverb tank.....is there suppose to be a cover on it to make it work right? What about the plug ins? Tank is marked input and output and the output is suppose to be away from transformer but does it matter which RCA jack on the board the tank is plugged into? There are no markings on the board saying input or output.
[Updated on: Tue, 30 September 2014 13:03] Report message to a moderator
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Re: 745 PA amp [message #21588 is a reply to message #21587] |
Tue, 30 September 2014 13:42 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
C4ster
Messages: 686 Registered: June 2001 Location: Mukwonago, WI (Milwaukee...
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If the board is cracked, its not a hard fix. First of all, it is a single side board, very old school. The copper trace is covered with solder so all you need to do is clean the area and use a short piece of copper wire, any gauge will do, and bridge over the crack. That will reinforce the board if it is cracked all the way through. Just bridging with solder will work but is can still crack again so the wire will help greatly.
Conrad
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Re: 745 PA amp [message #21590 is a reply to message #21561] |
Tue, 30 September 2014 14:54 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
stevem
Messages: 4772 Registered: June 2004 Location: NY
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You spray the pots thru the slot below where the solder tabs come out from.
In stock form there is a card board cover on top the tank to keep out dust and other unwanted items that my slip thru the top of the chassis.
The output side of the tank is the side to keep away from the power transformer .
The reverb cable jack that gives a buzz noise when you touch the RCA end of a cable plugged into with the reverb control up us the one that gets plugged into the output jack on the pan.
Kustom circuit boards are very thick and unless the chassis where the board is mounted got bent in the fall it took then I do not think you have a cracked board, I think it may be a failed solder connection!, or like pleat posted a bad Mallory cap!
[Updated on: Tue, 30 September 2014 19:02] Report message to a moderator
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Re: 745 PA amp [message #21597 is a reply to message #21561] |
Wed, 01 October 2014 13:15 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
Iowa Boy
Messages: 767 Registered: June 2014
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Senior Member |
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No Stevem, the cables were not plugged in when the amp was making all the noise. They were unplugged from the board but still attached to the tank and were rolled up and not touching anything metal in the amp head. Board was isolated from the tank until last night when I tried hooking it back up like you said to see if the reverb worked or not, thought maybe someone had put the wires in the wrong RCA jacks......it still doesn't work. Pulled the tank out of amp....cables and all; played amp again last night to the wee hours of the morning and still working fine.
Could you explain how the reverb works in Kustom equipment? I do get the hum from the output cable when you touch the tip; springs in the tank are connected and suspended like they are suppose to; wires are connected to both input and output RCA jacks and not broken; unit does "clang" when moved. On all channels its still the same......if you have the footswitch off, there is a buzzing sound coming from the speakers and turning the reverb pot does not do anything. With footswitch on, buzz is gone, volume gets louder when turning pots counter clockwise.
[Updated on: Wed, 01 October 2014 13:23] Report message to a moderator
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