Guitar Amp w Cement Resistors Loud Pops & Noise

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
Hey Joe, Regarding channel 3 being the same as 4.... a huge problem Ifor me is knowing what component actually goes to 3 since they are all unnumbered. When I hold the board up to the light an try an trace things out, I find things connecting to each other all the way across the board. So while your snapshot shows an outline of just-3 with a few components; the truth is when I look at the board, things are not sectioned off clearly at all.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
The smallest cap I have is a 22uf 35v. BTW, I am trying to be as careful as possible to make sure the meter is set to read the right amount. But in some cases it has not been clear to me which they want (acdc). I have encountered going back and forth from AC to DC. If you give me instruction can you tell me each time either AC or DC; that would help me a great deal, so I give back the right thing.
 

JoeJester

Joined Apr 26, 2005
4,390
Robert, that capacitor will do. As far as finding components tied to the specific chip, you can measure the resistance between the ice pin and the component. Zero ohms is what your looking for. But first ..

Place the capacitor in series with the red probe. Using the lead on the capacitor as a probe, measure the three test points again.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
First I am not for certian what is C15 and R21 but I drew a circle around the ones I think they are by IC 2,
by the back of input jack for channel 3. So here is what I did:

#1. Inject tone into the channel 3 input jack (1khz @ 100mv).
#2. Turn amp on and set channel 3 gain to 9 oclock, turn master to 9 o'clock.

#3. Clamp the 22uf cap on my red meter probe (positive side of the cap) and use the neg leg of the cap to touch pin 1 on IC 1.
Then I'll touch C15 both ends and R21 both ends. Results: Pin 1 = 0.00acmv R21 0.00 the other end 0.80acmv and makes amp buzz.
C15 read and did the same.

Next, when I did the tests without the cap on my probe: Pin 1 = 00.40's to 00.50acmv (reading jumped around).
R21 read 00.16acmv - on the opposite end it makes the amp buzz through the speaker. Meter read about 00.80acmv.
C15 read and did the same thing. * I'm including 2 pics showing where I think IC 1 and IC 2 are.
 

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JoeJester

Joined Apr 26, 2005
4,390
IC1A is channel 3.

Ok. On the female plug, measure the point where the tip raises the leaf to see if you have the 100 mV signal there. You do not need to have the master volume up, in fact, since your overdriving the amplifier (100 mV rms vice 35.35 mV rms) you will just get distortion.

I'll assume you measured the tip and ring to set the voltage before using it.

0.80 mV is a far cry from 100 mV. Remove the power from the set and unplug it. Using the ohm meter, measure between the jack 3's tip and R21. On side you should read zero and the other side you should read R21's value. You could put a dummy plug into Jack3 and do the same test easier, measuring the plug's tip connection to R21. If you read an open, you will find the problem to be:
- bad soldering connections on the jack (j3) or R21
- hairline crack on the run between the jack (j3) and R21



In summary:

We half split the problem

The first test, when you did the three IC1 pins, 1, 2, and 3 we found out there was no AC voltage there.
The second test when you went to the point where R21 and C15 connected to each other, we half split the problem between the Plug in J3 (good) and the Pin 3 of IC1. You went further and measured the other end of R21 and found 0.80 mV (essentially zero).

Before you dig into this further, do you have a 10k resistor hanging around? If you do, connect it to the tip and sleeve of the signal generator (where you measured it) and measure it again under load. We want to make sure the signal doesn't drop causing the problem.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
Okay, so I inserted the male into channel 3 input jack and measured the lifted leaf spring and
it does measure the 100mv acmv. At R21 I measure 0.00 and the other end 00.10acmv.

Second test I removed the channel 3 plug and touched the tip leaf spring and then R21. One end
I get sound on the ohm scale and it reads 000.1 ohm scale. Other end reads OL but then saw 199.4k. Test 3: I
inserted a dummy in channel 3 jack touched leaf spring and R21 reads OL on both ends (ohm scale).

On the resistors I only have a few left overs: 66pf - 10ohm - 46.5ohm - 82.5ohm - 220k ohm
 
Last edited:

JoeJester

Joined Apr 26, 2005
4,390
Second test I removed the channel 3 plug and touched the tip leaf spring and then R21. One end I get sound on the ohm scale and it reads 000.1 ohm scale. Other end reads OL but then saw 199.4k.

R21 is a 22k resistor. That resistor is bad.

This is where we stand after your troubleshooting:

Channel3-bracket1.png

After you replace that resistor, when you adjust the audio to inject into that channel, adjust it for 35.35 mV.
 
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JoeJester

Joined Apr 26, 2005
4,390
Here is the two states that the Channel 3 Jack has:

TSNotes.png

So, a quick test would be to see if the junction of C15 and R21 measures the R21 resistance to ground when there is no plug installed. In fact if it were open, it could become an unwanted signal pickup source.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
Since many of our Radio Shacks are closing, moving and who knows what else; they don't have it so I placed an order = 1 to 5 days. I hope wattage doesn't matter because I ordered a 2 watt because the 1/4 watts were on back order. I sure hope this fix does it. I'll post when I find out one way or the other.
 

JoeJester

Joined Apr 26, 2005
4,390
Who did you order it from? That resistor is no where near 2W. In fact, you might have a hard time inserting the leads into the PCB with a 2W.

There are 50,000 plus 22k 250 mW (1/4 W) resistors at mouser for a dime each.

I sure hope this fix does it.
Did you check all the highlighted components prior to placing an order?

Notes.png

R21 measured open. C15, C16, R22 complete the input circuitry, a low pass and high pass filter, forming a bandpass filter.
 
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Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
Arrow has worked better for me than Mouser and I have had Mouser take the same amount of time. It's usually quicker than the total 5 days. I think I did check all the diodes and some of the small caps around the area. I have though cleaned up the mess out of our front room and put everything back in storage, including the Fender amp that you were helping me fix. I'll let you know what happens after I get it installed.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
Well the good news is, Arrow was very, very fast on delivery but after I installed R21 the channel is still dead.
One thing I did notice when I was under there is the little yellow mustard drop cap right next to it (C15 )
has a solder bridge on both ends to R21. Do you think this is right?
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
Well what do you know, it was a solder bridge; but one that was missing on IC 2.

I saw this area before but did not want to mess with it because the rest of the amp worked
and I didn't think it played into the circuit on channel 3. So this time when I got to looking at it again
one pin coming off of IC 2 connected to a close by diode and now channel 3 works.

Back to Craigslist as soon as I put it all together. Thanks again Joe.
 

JoeJester

Joined Apr 26, 2005
4,390
Robert,

It's good you found the solder bridge. I don't understand how it was on both ends of R21 as it is suppose to be in series with that resistor. I hate to ask this, but, were you soldering in that area and could have possibly created the problem?

I never asked if you tested the 22k ohn resistor out of the circuit. Typically on problems like this we continue to troubleshoot the immediate area to see if another component caused R21 to fail.

I know I'm an asshole when it comes to troubleshooting by the six step troubleshooting procedure. You can reap the benefits by doing the full functional testing before you crack open the equipment to get to the innards. You can see the sections on these amplifiers with the basic block diagram of this KX-160. We knew that Channel 3 and only Channel 3 was at fault as Channel 4 worked and was in parallel with Channel 3 after the first amplifier.

I take it we will be back to the Fender sometime soon. Take a break, review what you learned and any mistakes you feel you have made, and any questions you might have for me. If necessary, we can discuss that in a conversation vice a thread as you would have total control over who is invited to that conversation. That being said, the main thrust of the troubleshooting still should be in open forum. Ancillary information can be handled in a conversation.

Good job on locating the solder bridge.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
Thanks for the advice and help Joe. I had not messed with the area around R21 except I replaced all the IC's across the board at one time.
Thinking about this further I guess seeing parts bridge together on both ends is not uncommon but very normal, they just have underlying traces
doing it, not solder. On the IC 2, that one leg always drew my attention because you could see the area right next to the diode leg had been chewed up by a razor blade making sure there was no connection between the IC and the diode. But at this point since I had drug everything out again hoping it was that resistor, I decided if it doesn't make a difference to make a bridge, I am not out anything but on the other hand this could be the culprit and it was. Not sure when I'll be back on the Fender but one thing I did do was test all the larger caps and all the diodes. I am suspecting the IC's now but I need to make sure they all do the same things and their legs are the same before I start swapping them out. The squelching clipping sound when a play a chord seems like it would be more of an opamp than resistors, but that's just my theory.
 
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