| Repair advise [message #19064] |
Fri, 09 August 2013 15:44  |
Jonomega3
Messages: 81
Registered: November 2012
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I had this on my other "Kustom Repairman" post but thought I should start a new one since I've decided to try to fix it myself, with a lit'l help from my friends! lol.
OK, I have this A-4 out of its cab and I'm checking around being very careful not to get electrocuted and not to mess anything else up. But with it plugged into a speaker cab and turned up I was tapping lightly on some of the caps and there is a static pop when I tap 4 of the white Malory caps at the top of PC boards 103 on both sides of the amp 2 are 25MFD 35VDC the other 2 longer ones are 100MFD 15VDC. I have another A-4 that I can compare with and works properly (just doesn't sound as good, lol)but it doesn't make the pop noise when I tap the same caps on that amp. So, I guess I'm wondering if this is familiar to anyone? I would really appreciate any input. I've been on CL and all over the internet doesn't appear to be any tech close to me, I'm pretty handy and I'd love to keep this amp working so.....thnx!
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| Re: Repair advise [message #19065 is a reply to message #19064] |
Fri, 09 August 2013 16:28  |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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If the amp was affected by a lightning strike, I would think that the main power supply would have been damaged, not just one channel board. More likely the board just decided to act up.
What you are describing sounds like a bad solder joint. Either that or a bad cap. I think Pleat has a story about the white Mallory caps and the factory assemblers.
If you are handy with a soldering iron you can fix it up with no problems. You will need a low wattage iron (25 watts) and some rosin core solder.
Turn off and unplug the amp from the wall outlet. Then unbolt the PC103 board from the chassis. Be sure not to pull too hard on any of the wires that connect to it. Carefully inspect the bottom of board where the copper traces are and see if you can see any broken solder joints. Even though the noise happens when you touch the caps, check the entire board as it may be something else that is loose.
If you find something that is loose, reheat the joint with your iron, adding a small amount of new solder to help flow the solder on the joint. Reinstall the board and test to see if the noise has gone away and more importantly if the channel works again.
If the noise continues, a lead that goes into the cap may have come loose and while securely soldered to the board is still not making a good connection on the inside of the cap.
Let us know what you find.
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| Re: Repair advise [message #19070 is a reply to message #19064] |
Fri, 09 August 2013 21:36 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Do you have a multimeter with a diode test function? I think that you need to check the transistors on the dead channel.
I'd go back and double check whatever you did to the working channel board. If it was working well before you pulled it, then something happened while you had it out.
If the caps need to be replaced, equivalents are available from places like Mouser Electronics or Jameco or Allied or Newark, etc. Until you prove that they are bad don't worry about replacing them.
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| Re: Repair advise [message #19078 is a reply to message #19064] |
Sat, 10 August 2013 12:55 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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The diode test function will usually be marked with the schematic symbol for a diode, which is an arrow with it's tip touching a short straight line.
If your meter doesn't have that function, then you can just use the resistance function.
Yes, as Pleat points out, the boards are grounded by the mounting points. If you do power up the amp without the boards being mounted, things may not work right if at all. If you have any jumper wires, you could ground the board with a jumper for testing purposes.
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| Re: Repair advise [message #19089 is a reply to message #19064] |
Sun, 11 August 2013 14:34 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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The part that you are referring to is a resistor. I doubt that it is leaking. What you are seeing is more likely residue from the original one that burned up. That board also shows that two transistors have been changed. The original blue resistors are metal film types that are rated at 1%. They were chosen for their low noise characteristics. The one that is in there is a standard carbon comp resistor.
The tooth lock washers are there to keep the mounting nuts from coming loose and to insure that the board is grounded to the chassis. You do not want the board insulated from the chassis metal.
There is some stupid repair or mod done to the power amp board that shows the quality of the earlier work done on the amp. There are two resistors and a blue cap that are floating off of the board. If the power amp is working and you are happy with the sound, leave it alone and just fix the preamp.
Now that we've seen the amp, tell us which channel is dead, the straignt or the fx channel?
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| Re: Repair advise [message #19090 is a reply to message #19089] |
Sun, 11 August 2013 14:46 |
Jonomega3
Messages: 81
Registered: November 2012
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Member |
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It looks burnt in the pic but it just looks that way, there is some sort of dark goo that is partially on the resistor and dripping to the board. I tried to take a closeup pic but it was too out of focus. The fx channel is dead the straight one works but is kinda intermittent. I think the solder points on the board are touching the chassis cause there are only two tooth lock washers on it, I'll try it with alligator clips to see if the straight channel cleans up like mentioned before.
[Updated on: Sun, 11 August 2013 15:09] Report message to a moderator |
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| Re: Repair advise [message #19092 is a reply to message #19064] |
Sun, 11 August 2013 22:20 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Your eyesight is probably better than mine, it's just that I've seen it before so many times that I can smell it from here.
If the FX channel is dead, then it will take a little doing to fix it, because the signal runs in series through all of the FX boards. The first step will be to try and find out where the signal stops. Which channel board has the changed parts, straight or FX?
Do you have the schematics for your amp? My K200A-4 is the newer version than yours, as you have the circuit breaker version, probably with the aluminum front panel. I'm not sure, but I think that electronically your version is similar if not exactly the same as the later version that I have.
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| Re: Repair advise [message #19094 is a reply to message #19064] |
Sun, 11 August 2013 22:53 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Go to the technical section of this site and look at the K200A schematics. They are mixed in with the schematics for the Frankenstein heads, but there is enough information there to cover your amp.
I will take a look and try and figure out a plan of attack for you to follow.
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| Re: Repair advise [message #19101 is a reply to message #19064] |
Mon, 12 August 2013 12:57 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Yes, the PC102 circuits are the same for both amps.
Do you have the footswitch for your amp? If you do, turn on and off each of the FX and see if the signal on the dead channel comes through. If you don't have the footswitch, then use a jumper wire to short each of the 4 pins to the chassis.
On the K200A series amps, all of the FX are normally turned off until you turn them on with the footswitch.
[Updated on: Mon, 12 August 2013 15:29] Report message to a moderator |
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| Re: Repair advise [message #19106 is a reply to message #19101] |
Mon, 12 August 2013 22:43 |
Jonomega3
Messages: 81
Registered: November 2012
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Member |
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Yes, I do have the footpedal and I just went and tried the fx and you can hear the effect, rev (if you shake the can you hear the splash) and you can hear the vibrato (hiss and wavey sound) as well as selective boost (you can hear the eq change with the boost switch but the sound of the guitar doesn't come through just the effect itself.
Almost as if the input jack isn't making connection. I did try both input setting High and Low inputs
[Updated on: Mon, 12 August 2013 22:44] Report message to a moderator |
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| Re: Repair advise [message #19107 is a reply to message #19064] |
Tue, 13 August 2013 00:26 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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That's a good sign, I'll guess that the FX boards are all working. You can try and bypass the signal around the preamp board by jumpering from the input jacks to the output line from the preamp. If you hear the guitar signal, then you'll know that the problem is on the preamp board.
If you look at the bottom edge of the board, there will be four wires two red in the center and two single blue ones at the outer sides. One blue wire comes from the input jacks and the other one is the output that goes to the boost/clipper board. If you can jumper between the two blue wires the inputs will directly feed the the boost/clipper circuit. Then if you can hear the guitar signal you'll know for sure.
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| Re: Repair advise [message #19111 is a reply to message #19064] |
Tue, 13 August 2013 11:35 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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When you did that, were the fx still turned on?
Following that same idea, if you can jumper the input jacks to the input of the reverb/trem board and see what happens.
The circuit flow is from the preamp to the selective boost, to the clipper, to the vib/trem, to the reverb, to the power amp. You need to try and find out where the signal stops.
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| Re: Repair advise [message #19117 is a reply to message #19116] |
Tue, 13 August 2013 13:48 |
Jonomega3
Messages: 81
Registered: November 2012
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Member |
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Its very strange though because I can hear eq change with the switch on the Selective Boost as well as the reverb pan feeding back as well as Trem noise, you shouldnt hear any of that if there wasn't a through signal, correct? could there be a ground issue maybe causing this? there is a resistance of 1.303 reading on my meter when I touch both positive and ground on the input jack. Maybe that's normal?
I got a little zap when I touched the ground on the reverb pan, lol.
OK got it, I was looking in the model section, thanks!
[Updated on: Tue, 13 August 2013 14:52] Report message to a moderator |
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| Re: Repair advise [message #19121 is a reply to message #19117] |
Tue, 13 August 2013 15:08 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Without knowing what the 1.303 value refers to, I can't tell you if the jacks are okay or not. 1.303 megohms, kilohms, volts?
Yes, if you can hear sound through the output from the clipper then the signal should be passing the other circuits.
Set you meter up to read dc voltage. Attach the black lead to the chassis. Touch the red lead to the input of the boost/clipper circuit. There shouldn't be much voltage there if any, but touching the red lead to the audio input should cause a hum through the speaker. Just like touching the end of a guitar cord that is plugged into an amp.
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| Re: Repair advise [message #19125 is a reply to message #19123] |
Tue, 13 August 2013 16:16 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Can you try the same test at the input to the trem/rev board?
If the meter was set to continuity, then that would probably be 1.303 ohms or nearly a dead short. Did you have a cable plugged into the jack when you tested it? There is a shorting contact on the jacks that grounds the inputs when not in use to kill any hum.
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| Re: Repair advise [message #19131 is a reply to message #19128] |
Tue, 13 August 2013 17:16 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Okay, it's time to test the preamp board. Do you have an old RCA cable around? Something from an old stereo or video unit would work. If not a regular jumper wire will do. Either plug in the RCA cable into the jack on the back of the amp, the monitor jack or hook a jumper wire to the back of the jack. This is a direct line into the power amp.
Connect the other end of the cable to the output wire of the preamp. If you disconnect the output wire from the rest of the amp circuit you can disregard the fx units for now. If the rest of the circuits are still connected, then turn down all of the fx controls and turn on the fx.
If the preamp is working, you should hear the signal through the speaker. If not, then we will start testing parts on the preamp board.
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| Re: Repair advise [message #19133 is a reply to message #19064] |
Tue, 13 August 2013 18:16 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Start by turning off and unplugging the amp.
Transistors are three element devices. They have a base, a collector and an emitter. On the schematic they are drawn like a letter Y on it's side. The single element side is the base and the side with the two lines are the collector (plain) and the emitter (arrow).
To test the transistor you set the meter to diode test and then connect the red lead to the base and then touch the black lead to the collector. You will either get a reading or it will test the same as having the meter not connected to anything. Then you will reverse the two connections. If the junction is working properly there will be a low reading maybe .6-.7 volts in one direction and a wide open reading in the other direction. Then you need to repeat the same test, but this time touching the leads to the base and the emitter. And again touching the collector and the emitter.
There are two types of transistors in the preamp, NPN and PNP. The NPN will read low when the let's say the red lead is connected to the base and high when the black lead is connected to the base. The opposite will be true of the PNP transistors they will read low when the black lead is connected to the base. This will actually be dependent upon the voltage polarity of your meter, but I hope you get the difference.
Now test all of the transistors on the board. They all should have high and low readings from base to collector and base to emitter. And they should all have open readings from collector to emitters.
There may be a few cases where the reading will be skewed by resistances in the circuit that parallel the transistor elements. If you can't get a good reading, you may need to remove the transistor from the circuit to get a final answer.
I don't know if you have a known good transistor available to test out of circuit. If you do, it may help you to understand the process better.
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| Re: Repair advise [message #19139 is a reply to message #19138] |
Wed, 14 August 2013 12:33 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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It's always handy to have a few parts on hand to fix these old gems. A few different transistors and maybe a couple of caps would be helpful. I'd guess that there is a Radio Shack closer to you than 40 mins., so you could go there to pick up a few parts. They don't really carry the selection that they used to, but in a pinch they will sometimes have what you need.
Unlike these Kustom amps, a lot of modern electronics are designed to be manufactured as inexpensively as possible, without any thought given to repair. Every major manufacturer has what they call a do not repair list. These are items that are cheaper for them to replace than to pay to have fixed.
Every vintage Kustom amp was designed and manufactured with the highest quality parts. They were using high tech space age grade components like G-10 epoxy pc boards, metal film resistors and tantalum caps long before the rest of the industry. The worst of the damage that I see in these is badly done repairs. So I like working on them, and whenever I can, I like getting another one fixed and back into service.
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