Home » VintageKustom.com » Repairing Kustom Amps » 745 PA amp (crackling sound)
745 PA amp [message #21561] Sun, 28 September 2014 23:49 Go to next message
Iowa Boy is currently offline  Iowa Boy
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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 messageGo to next message
stevem is currently offline  stevem
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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]

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Re: 745 PA amp [message #21563 is a reply to message #21561] Mon, 29 September 2014 10:21 Go to previous messageGo to next message
Iowa Boy is currently offline  Iowa Boy
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The cables from tank were unhooked from the board and not touching anything.....I have now totally removed the tank....problem is still there.

Checked everything else you said and all is tight. Light flickers when there is crackling sound.....very loud.

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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 messageGo to next message
Iowa Boy is currently offline  Iowa Boy
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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]

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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 messageGo to next message
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 messageGo to next message
Iowa Boy is currently offline  Iowa Boy
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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]

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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 messageGo to next message
Iowa Boy is currently offline  Iowa Boy
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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]

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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 messageGo to next message
stevem is currently offline  stevem
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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]

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Re: 745 PA amp [message #21570 is a reply to message #21561] Mon, 29 September 2014 15:13 Go to previous messageGo to next message
Iowa Boy is currently offline  Iowa Boy
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This is too weird........when I laid the cabinet face down, the crackling stopped. Tapped on the circuit board that the reverb tank was plugged into and heard a crackle or two and then it stopped. Amp is acting ok now. What's up with this?

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Re: 745 PA amp [message #21571 is a reply to message #21561] Mon, 29 September 2014 15:22 Go to previous messageGo to next message
Iowa Boy is currently offline  Iowa Boy
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Since I have this all open now......the reverb tank when connected chattered all the time and never noticed a true reverb from the get go. The top of the tank is open, springs are all intact, no loose wires, looks to be ok. Is there suppose to be a foam cover over the top? There is a hissing sound when the foot switch is turned off and then is quiet when the switch is on.

Also, I have wiped everything down with a q tip that I can get at. I did buy Deoxit D5 and F5. What do I need to do to use this stuff and clean this amp properly?

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Re: 745 PA amp [message #21572 is a reply to message #21571] Mon, 29 September 2014 16:54 Go to previous messageGo to next message
fieldflower is currently offline  fieldflower
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Could it be the speaker cord?
Since it changes when You laid the speaker down (and I saw the problem persisted with another speaker).

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Re: 745 PA amp [message #21573 is a reply to message #21561] Mon, 29 September 2014 16:57 Go to previous messageGo to next message
stevem is currently offline  stevem
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First off what amp do you have, is the reverb tank screwed to the inside wood top of the cabinet , or is it suspended by two aluminum bars across the top of the amp and does the face of the amp have fuses and or a circuit breaker on each side of the on / off switch?

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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 messageGo to next message
Iowa Boy is currently offline  Iowa Boy
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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]

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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 messageGo to next message
stevem is currently offline  stevem
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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]

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Re: 745 PA amp [message #21576 is a reply to message #21561] Mon, 29 September 2014 18:37 Go to previous messageGo to next message
Iowa Boy is currently offline  Iowa Boy
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Nope.....nothing was there and I even wiped it clean under the board last night with alcohol and a q tip but it still was crackling this morning. Laying the amp on its face did something but I sure the hell don't know what it did.

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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 messageGo to next message
Iowa Boy is currently offline  Iowa Boy
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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]

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Re: 745 PA amp [message #21581 is a reply to message #21577] Tue, 30 September 2014 10:33 Go to previous messageGo to next message
fieldflower is currently offline  fieldflower
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Really interesting that it stopped when You put the amp on it's face.
It smells like a short or bad solder or something hanging loose...

What if You lightly push and bend components and boards (without electrocuting yourself) and see if You can make it start/stop that way?

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Re: 745 PA amp [message #21583 is a reply to message #21581] Tue, 30 September 2014 11:00 Go to previous messageGo to next message
Jerrybass1955 is currently offline  Jerrybass1955
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I would take a look at the jacks themselves. Perhaps they should be tightened to get a good ground grip or the springs that make contact to the plugs need to be bent in a little to assure you always have good contact. I would also be inclined to not rely on the connection of the jack to the chassis for ground. A separate wire will assure that you get a good ground if the jack works loose. Since the problem disappeared for a while when you moved the speaker, perhaps the problem is in the speaker jack or connections to the speaker itself.
Good luck.
Jerry

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Re: 745 PA amp [message #21584 is a reply to message #21561] Tue, 30 September 2014 11:16 Go to previous messageGo to next message
Iowa Boy is currently offline  Iowa Boy
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Well speaker jacks are very tight, the positive prong does make contact on both jacks as well. This is not the problem because when the amp was making noise, it didn't matter which speaker input I used, popping and crackling was still present.

This amp had been dropped at one time or other since the chassis was bent where the transformer sits and I straightened it the best I could. Also the support bars for the tank were also bent.

I inspected the board the tank plugs into and didn't see any hair line cracks but there might be some on the underside of the board. All the component on that board are tight but yesterday I did push on it when the amp was making noise and it popped a couple time and that was the last of it. So I'm guessing that is where the problem is at. For now its working fine in fact I play my bass thru it for about 2 hours last night without any problems. I'm Leary about messing with the board since its working right now and afraid I might really end up with a lot of headaches.

Back to earlier questions I asked about using Deoxit D5 and F5.......what and which should be used on what? Also the reverb tank......does it have to have a cover on it to work properly?

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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 messageGo to next message
C4ster
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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 #21586 is a reply to message #21561] Tue, 30 September 2014 12:12 Go to previous messageGo to next message
stevem is currently offline  stevem
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I use Deoxit F5 for cleaning and lubing pots and I use Doxit D100 for metal to metal connection points, as on RCA ,input jack , speaker jack, Molex and tube pin to socket points.

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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 messageGo to next message
Iowa Boy is currently offline  Iowa Boy
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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]

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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 messageGo to next message
C4ster
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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 #21589 is a reply to message #21588] Tue, 30 September 2014 14:40 Go to previous messageGo to next message
pleat is currently offline  pleat
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Not a tech, but from all the reading, the amp 795PA is a K200A-5 4 channel PA.

All the A series used a lot of white Mallory Caps that Kustom had a ton of problems with. They look like a long white pencel eraser. Kustom eventually traced the problem with all the failures. The girls soldering the components would wiggle the caps right after they were still hot from soldering and were pulling the legs loose inside the cap and eventually they failed out in the field. I'm betting the amp still has a boatload of those caps and may need to be replaced.

pleat

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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 messageGo to next message
stevem is currently offline  stevem
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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]

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Re: 745 PA amp [message #21591 is a reply to message #21561] Tue, 30 September 2014 16:08 Go to previous messageGo to next message
Iowa Boy is currently offline  Iowa Boy
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Pleat is right, this thing is loaded with the white Mallory caps. I cleaned every one of them with alcohol and a swab they were so yellow and dirty. All seemed to be tight but maybe like Pleat said there is one or two that are broken inside the cap. Guess I could power the amp up again and go around to each cap and jiggle it a little to see if the noise comes back or not. Checked the board with a mirror and everything underneath looks to be ok...again no arcing indicated or rub marks. And that board has a ton of those caps on it.

I'll do some more checking later tonight and see if anything shows up. Wonder if in Kustom's tech journals anything was ever mentioned about turning the amps face down.......seemed to work in this case. Go figure. Thanks everyone for your help.

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Re: 745 PA amp [message #21592 is a reply to message #21561] Tue, 30 September 2014 19:01 Go to previous messageGo to next message
stevem is currently offline  stevem
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The good thing about the PA heads is that they do not stack the boards like on the other A series models with effects so you can work on the boards without having it be a pain in the ass !

I would make plans to change out all of those caps one board at a time, if not and you only play thru one or two of the channels you can disconnect the bad boards blue output wire and at least not have the noise issue crop up until you get to fixing those bad boards!

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Re: 745 PA amp [message #21593 is a reply to message #21561] Tue, 30 September 2014 20:33 Go to previous messageGo to next message
Iowa Boy is currently offline  Iowa Boy
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I still think the board that the reverb pan connects to is the problem.......that is what I pushed on and the noise stopped. I did notice when I first got the amp that once in a while when playing, that the sound level would drop and then come back again. Then I'd change to another channel and after awhile it might do it also. But now that I play the amp everyday and usually for an hour or two, haven't noticed the problem anymore. Was hoping it just needed to loosen up a little since I figured it probably hadn't had much play time in a while. But your right about doing a change out. I would like to get this thing back to what it was 40 some years ago.

Right now, I am working on another deal with the same amp head as this one and a 4-10's cabinet and then an additional 4-10's cabinet to go with it also.......all in blue. Both cabinets are Jensen's so I know they're made for guitar so am thinking if I can find a guitar amp head cheap with all the effects that still works, I can switch out the amp cabinets and have a complete setup all in blue. Since I can't play guitar any longer, figured I could resell it and make a modest profit from the sale. Think it would make a very impressive sight......almost as big as the 3-15's cabinets. And you know I like the big speaker look.

Still hoping to get the reverb to work on this amp.........I've never yet heard what a Kustom reverb even sounds like since they don't work on any of the amps I have and almost everyone selling an amp says the reverb doesn't work either. Guess that will be another problem to work on later when its too damned cold to be outside working. Thanks Stevem you've been a great help, same with everyone else that has jumped in. This really is a great website for us Kustom Kats!

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Re: 745 PA amp [message #21594 is a reply to message #21593] Tue, 30 September 2014 20:58 Go to previous messageGo to next message
pleat is currently offline  pleat
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One thing to be careful of. A K200A Chassis is different from a K200B chassis. The A series head case is not as deep as the B series. The B Series will stick out the back of the A series head case and you'd have to remove the 1/4" wooden support blocks from the A series.

The A series chassis have 5 large hex spacers mounted the bottom of the chassis that accepts the bolts to hold the chassic to the amp head case.

pleat

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Re: 745 PA amp [message #21595 is a reply to message #21561] Wed, 01 October 2014 00:07 Go to previous messageGo to next message
Iowa Boy is currently offline  Iowa Boy
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Well that justs sucks! And here I thought I was being clever. Obviously the engineers at Kustom never went to the Henry Ford School of Mass Production. Would have been a whole lot smarter and more cost effective to have made all the similar amp heads with the same cabinet and same chassis and just added or deleted those effects that weren't needed.

Guess I will have to hope for a blue T&R guitar head or have it recovered. Thanks Pleat for the info.....would have been pissed to have found out the hard way that the two heads aren't the same.

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Re: 745 PA amp [message #21596 is a reply to message #21561] Wed, 01 October 2014 06:11 Go to previous messageGo to next message
stevem is currently offline  stevem
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Back up here now!
I asked you if the reverb cables where plugged in and you posted back no, and now you are saying that plugging them back in cured the issue?!

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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 messageGo to next message
Iowa Boy is currently offline  Iowa Boy
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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]

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Re: 745 PA amp [message #21598 is a reply to message #21561] Wed, 01 October 2014 15:35 Go to previous messageGo to next message
stevem is currently offline  stevem
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It's a good sign that you get that buzz when you tap the end of that cable as that means the recovery amp is working as it should in the reverb circuit !
Now to test the reverb pan do you have volt ohm meter as we need that to test the input side of the pan which I would say is bad.
Due to the fact that you have heard the reverb clang when the pan Is plugged in that means that we know the output side of the pan is good.
I'am racking up more beer here am I not?

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Re: 745 PA amp [message #21599 is a reply to message #21561] Wed, 01 October 2014 17:27 Go to previous messageGo to next message
Iowa Boy is currently offline  Iowa Boy
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Between you and the other rocket scientists on this site....I'll have to buy a brewery! Well of course I own meters......5 in fact, I am and HVACR technician. Went to tech school for TV's/stereo equipment after high school but couldn't take sitting in one classroom with 30 other guys and no women for 7 hours a day. That lasted about 2 months!

So where do you want me to take the reading from.......off the input connections on the tank itself or from the RCA plug coming out of the board?

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Re: 745 PA amp [message #21600 is a reply to message #21561] Wed, 01 October 2014 17:59 Go to previous messageGo to next message
Iowa Boy is currently offline  Iowa Boy
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Found the problem. After looking at how the RCA jack made contact with the plug, noticed that nothing was making contact with the positive end of the RCA jack. Prong was bent up and not making contact. It now works......not very impressive for reverb........my old 67 Camero stereo would have put this reverb to shame. Anyway, problem has been solved. Thanks for pointing me in the right direction.

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Re: 745 PA amp [message #21601 is a reply to message #21561] Wed, 01 October 2014 21:15 Go to previous messageGo to next message
stevem is currently offline  stevem
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Yup, they do not have verb even equal to a twin reverb until you get into the 71 and latter metal face amps with the tone control in the reverb section and then the verb sounds good!

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Re: 745 PA amp [message #21602 is a reply to message #21561] Wed, 01 October 2014 21:53 Go to previous messageGo to next message
Iowa Boy is currently offline  Iowa Boy
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Solved one problem and now I have another one.

Made a cover for the reverb tank out of laminated cardboard and the sound is so much better now. Really does sound like true reverb.

Now for the new problems. I spray Deoxit D5 on all the speaker jacks and instrument jacks. Also sprayed all the pots with Deoxit F5 where Stevem said (slot where wire terminals are). Several hours had gone by since I had done this. When I went to plug in my bass......no sound other than a high pitch feedback hum. Thought my guitar cord was bad so used one of my new speaker cords......still no sound and hum. Took alcohol and cleaned all the jacks. Now have sound but still have a very high pitch feedback hum. Especially noticeable on the treble controls when turned up. I even unplugged reverb tank thinking maybe this had something to do with it. Nope. Used to have this hum before getting my footswitch. When I did get it, by turning on the button (light come on but reverb is apparently off) the hum went away. Now it doesn't matter if button is on or off, hum is always there. Also just noticed that the tone control on my bass also increases or decreases the feedback hum. Now what has happened

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Re: 745 PA amp [message #21603 is a reply to message #21602] Wed, 01 October 2014 22:19 Go to previous messageGo to next message
pleat is currently offline  pleat
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Speaker cords won't work for guitar cords. Guitar and Mic cables are sheilded with the center wire. Speaker cables are not sheilded and the wires run parallel to one another, same as a lamp cord. Audio cables must be of the sheilded type which would explain the loud hum you have now.

I would not use sheilded cables for speaker wires. looking at the mass of the braided sheild and compare it to the thin single conductor center wire there is not equal transfer of voltage and with a high power amp, the center wire can melt through its insulator and short to ground and may cause a power amp to go down.

pleat

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Re: 745 PA amp [message #21604 is a reply to message #21603] Wed, 01 October 2014 22:22 Go to previous messageGo to next message
pleat is currently offline  pleat
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Correction to my last post. Audio cables must be of the sheilded type. Using a speaker cable to connect your guitar to the amp would explain the loud hum you have now. Is what I meant to say.

pleat

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Re: 745 PA amp [message #21605 is a reply to message #21604] Wed, 01 October 2014 23:27 Go to previous messageGo to previous message
Iowa Boy is currently offline  Iowa Boy
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Registered: June 2014
Senior Member
Yeah....I figured that out also. Had grabbed up that cord by mistake forgetting about it not being shielded. Apparently my usual cord has a short in it.

Any idea what's up with using the Deoxit D5 on the input jacks and nothing working until I re-cleaned them with alcohol? Considering what I paid for this stuff wondering if its any good at all.

The good thing is that the reverb does finally work now and does sound pretty good. And the crackling sound is also gone for the time being. Life is good......for the moment.

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