Another Guitar Amp Need Help w Fix

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
Because of it's age, I replaced 5 of the largest standing capacitors because they were electrolytic and they say after 10-15 years you should. I also found a Motorola IC chip that had broken in half and a fired 10 ohm resistor. But even after replacing these the amp will not work right. While I am a novice I have learned a few things. I've gone across the board and checked most of the IC's (leg 8 but a few that were different according to the schematic) and they read 15.5vdc consistently. So how do you narrow down the problem area? I thought that would do it but it revealed nothing that I understand. The amp just makes an awful noise when turned up and has heavy distortion and will not get loud like it should. I did unplug the pre-amp side from the power board and the modulation sound was very quiet (very low hiss). I'm looking for suggestions at this point and attached the schematic. Thanks in advance.
 

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wayneh

Joined Sep 9, 2010
16,276
See all those "TP"s on the schematic? Those are places you can test to see if the proper voltage appears there. I'd start at the power supply rectifiers and start moving forward.

A broken IC indicates some pretty serious damage, probably from mechanical impact. I'd look carefully at the PCB in that area for cracks or damage to the traces. Parts supporting that IC may have been shorted or overloaded as well.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
Thanks for the comment wayneh. So what concerns me is the voltage at some of those locations. TP1 for example = do they mean somehow check it with the wire connector on it or the wire connector removed from it and then just touch the male sticking up off the board ?DSC02388.JPG DSC02389.JPG
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
Looking closer at the schematic (page 3 A6 area) so are they saying check the AC here neg on the orange wire at the bridge rectifier from P5 and then the pos on the orange coming off the white red P7 ? Again, do you disconnect the wires from the rectifier or leave them on it and try to go under them?
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
Although I skipped a few AC tp's I have gotten up to tp11 and at that point on the diode it read 14 not 10vdc, I wonder if this is anything? Also wondering how you check those 3 legged bjt's. I see the pdf for the 2N4403 (which is near this diode), do you need to remove it from circuit or can you check it in circuit on diode mode or something?
 
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JoeJester

Joined Apr 26, 2005
4,194
So how do you narrow down the problem area?
You follow a set procedure when you troubleshoot. Accomplished troubleshooters do a few of these steps without the aid of notes. Others, not so much.

6step.png

Your toolbox needs to be filled appropriately and you take copious notes in your "engineering notebook".
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
Thank you Joe I will need a few months to digest all that and it looks like a cool resource to learn and get a better understanding of things.
 

JoeJester

Joined Apr 26, 2005
4,194
Start with block diagrams. Circuits are built into functional blocks. You know you have a pre-amp, buffer amps, equalizers, power amplifiers and power supplies.

Your KX-160 manual has a decent block diagram. You can see what is part of each of the blocks of that amplifier.

You can use associative learning to construct a basic functional block diagram of this newer amplifier from the schematics. Then your basic functional test is to check each block to see if you get the expected output from a standard input.

Right now you see that amplifier as one block. You put an input in ... you don't get the desired out.

KX-160 Block.png

You have four input channels. You can turn down the master volume and see if your getting those noises at the line out. That would be my initial functional test.

If there is no error there, then the problem is more than likely in the power amplifier side.
If there is an error, turn down each input channel individually. The one that reduces the error is the channel at fault.

One quick functional check. Take copious notes.

What test equipment do you have?

What is the model number of the amplifier?
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
The Crate amp that you are writing about above Joe, is on another thread that I started. On that amp I was helped by Mr.Chips and he came up with a way to try an filter the noise out of the system. I am waiting on parts to try an complete that amp. * On this thread/this amp, a Fender Bassman 400. I am up to TP21 (the fan circuit). At this point the voltages are not coming in right. The fan runs okay and seems to work right but all the test points in this block, do not test at the right voltages (21,22 & 23). So I jumped ahead.... but found more voltage trouble with 24 & 25 also. I think for now I need to take a break and maybe revisit it this weekend.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
I suppose because the experts could not easily draw the fan circuit on the schematic (connected to all the rest), they separated it out and put it at the bottom of page 3 area A5 (from the schematic for this amp - post 1). That makes checking the directly related parts of that circuit easy to locate. I wish they would do that with the entire amp. Even the block diagram you posted Joe, for me is not easily separated out on the schematic. When you have something so complex with so many parts in it; it would be nice to have them separated with an overlay drawing showing the boundaries on the actual schematic. Alas, it is what it is.
 

JoeJester

Joined Apr 26, 2005
4,194
Is your amplifier one of the user manuals I've posted?

Yes, they don't make block diagrams any more. In your KX-160 schematic, the power supply, is on two pages. One has the flow from left to right and the other from right to left. The other problem is the schematics are more then likely drawn in the "block" method. It would have been nice if they offset it from the others by actually enclosing the section in a block.

When you said you turned down the gain, did you mean the master volume?

The second step in the process is you elaborating on the problem. You manipulate the associated controls to see which section is causing the problem. With these amplifiers, you have one distinct advantage, you can turn the main volume down and monitor the line output signal. Then you can manipulate the controls while observing a low level signal. That test could be done before you OPEN the enclosure to look inside.

I know it's more "FUN" to jump right into the attempted repairs.

The method you are currently using is checking every test point, which is time consuming. A lot of technicians know the systems well enough they have a mental block diagram. They will split the problem the best they can before opening the enclosure.

Did you have a reason to "suspect" the power supplies? Confirming the voltages at the power supply itself is normally a good test, but you didn't "have to" test them at each and every chip.

What is your background in electronics?

Do you have an oscilloscope?
What test equipment is available for you to use?


Post a picture of the front and rear panels of this amplifier.
 
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Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
Q. Did you have a reason to "suspect" the power supplies? A. I was just following the suggestion of a person who left a comment in post 2. * Q. What is your background in electronics? A. I was made an offer I could not refuse a couple of years ago on a broken guitar amp.... and that got me started. Btw, I've repaired a few just by replacing their caps, pots and chips as-needed; maybe about 5 under my belt is all. * Q. Do you have an oscilloscope? A. No * Q. What test equipment is available for you to use? A. I just have a DVM, AVM and a cheap combo cap, transistor tester, that I bought off ebay for 10 bucks. BTW.... I probably won't be able to dedicate much time to the amp until next week but maybe a little bit here an there in between. I must fix it or sell it for scrap because my wife wants it out of our front room.
 

JoeJester

Joined Apr 26, 2005
4,194
Which model of the BASSMAN 400 do you have? Is it one of the three pictured in the user manuals? If so, which one?


The amp just makes an awful noise when turned up and has heavy distortion and will not get loud like it should. I did unplug the pre-amp side from the power board and the modulation sound was very quiet (very low hiss). I'm looking for suggestions at this point and attached the schematic
As I stated in post 8, you can monitor the line level output to see that annoyance is there, and you have the master volume turned down. That can eliminate a lot of circuitry by half splitting the problem.

You can set the potentiometers, other than preamp gain and master volume, to mid range.

In the Bassman models, there is a switch near the input that is for pickup sensitivity. What are you using for an input source?
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
I have the Bassman 400 (no H, just 400). I attached the schematic for it in post 1. This is how I did your test. Let me know if it's right or not. I turned only the bass and treble up midway (not the eq section), everything else was at zero and then I tested the output jack, to see if there was any voltage reading = .008. There was some fluctuation at turn off. It climbed to .929 (2 volt scale dc) and then dropped back down. I suppose that happens because of capacitors. When I try the amp I use the passive guitar setting (button out) input with a guitar.DSC02390.JPG DSC02391.JPG
 

JoeJester

Joined Apr 26, 2005
4,194
The pdf is just the english pages and the front cover of an earlier manual.
The 400-front and 400-rear are the front and rear pictures.

When I said you can monitor the line out, that would be at the xlr connector on the rear. That point is just before the master volume potentiometer and you can switch it to pre and post equalization. Turn the line gain all the way up. That point splits the amplifier in two sections. What we want to know is if the distortion is present at that point. The setup is all equalizers are set to mid point. That means the gains are on zero, not attenuating nor amplifying the specific bands. The equalizer in / out switch should be in the out. The master volume could be set on low, as we will be turning up the preamp gain.

On the left side, the gain should be about mid range, the room balance, bass, treble is also mid range. the enhancement is off.

Turn the left gain all the way up and see if you can produce the distortion. If you have no distortion, increase the master volume, to the point you have the distortion. Remember the master volume gain setting.

If you have access to another amplifier, you can connect a cord to the SEND jack of the good amplifier and the other end to the RETURN jack of the amplifier under test. If you can not hear the distortion with the amplifier under test's master volume at the same point as the previous sentence, the problem in the amplifier under test is BEFORE the power amplifier. If it distorts, the problem is in the power amplifier.

The problem we are having, is that you must HEAR the distortion and without an oscilloscope or distortion analyzer, you can't measure it. I hope you would have some high power dummy loads to substitute for your speakers, your ears would appreciate it, not to mention your family.

With an oscilloscope, you can "see" when the waveform distort.

The tune jack in the rear is before the mute switch. You could use some headphones to see if the distortion occurs at this point. The master volume would be at zero or the mute button would be pressed during this test.

The distortion is just distortion and not oscillation (squealing) ... correct?

What country are you in as you have not filled out your location?
 

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Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
I need to explain the distortion sound better = there was very little sound when I had everything turned up and a guitar plugged in. The sound was very heavily distorted and even made like a reverberation distorted noise. It was not loud at all. I will do the test you have laid out but it's only 6:30am in the condo we live at and my wife would not be happy with any noise. BTW I do have the pdf of the owners manual and it is like the one you posted. Thanks for your help.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
So no matter how I had the line out in the back (up down in or out), the scratching was the same with the non-recognizable chord coming from the guitar when I strummed it. Sounds more like scratching. You don't need to turn the left gain up very much before you get the yellow clip-light to jump-flicker, every time you strum either. The master on the far right was turned up just a little bit.
 

JoeJester

Joined Apr 26, 2005
4,194
So the distortion occurs only when your playing the instrument, the amp is "silent" when no chord is played? From the user manual:

preampled.png

Now, is your instrument a passive or active pickup? Make sure the input is properly selected.

Bypass the equalizer, do the same test again, only manipulate the controls on the left, one by one, to see if you can change the error.

I'd like for you to determine if the error is before the power amplifier (master gain set to zero).

Did you place a jack in the "RETURN" rear panel jack and turn up the master volume to see what sounds emit from the speaker? Just a jack, no instruments, no sound. Turn the input gain to zero.

Is it a particular chord that "distorts" or will it distort with all chords?
 
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Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
The amp is silent when no chords are being played. The guitar is passive (it does not have a battery) and I always plug-in
at the input with the button out. I have tried with the button in just for testing every conceivable test but it made no difference.
I need to emphasize that the amp must be turned up to it's max for you to get any sound from the speakers.

I have done the test you are suggesting in almost every conceivable way and it makes no change to the unrecognizable sound
coming from the speakers. It does not make any difference what note is played on the guitar; the sound is same.

Testing the return was the most interesting, it sounds like I have a hot microphone ready to be spoken in to. This seems like the volume
that the amp would produce. It was the most promising test, I've done so far.
 
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