Guitar Amp w Cement Resistors Loud Pops & Noise

MrChips

Joined Oct 2, 2009
25,928
Yes, I can see the chicklet which looks like C1. But where is the other one (C15) for Channel 2?

Trace the PCB to confirm.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
This amp board is a bit strange C15 goes to channel 3 (the upper board), not channel 2. The number on channel 1 chicklet (C1) is 473k 100v (I don't see a dot). One leg of that chicklet looks like it goes to leg 4 of IC1B the other leg goes to a 22k resistor (R1 ?). The other end of that resistor goes to the input jack. The blue directional caps are 10uf 16v.Chicklet.JPG
 

MrChips

Joined Oct 2, 2009
25,928
Sorry, my mistake. I misread the schematic. I can see CHANNEL 3 INPUT now.

KX-160 Channel 3.jpg


So the chicklets are C1 and C15. The marking of 473 is as for resistors, i.e. 47000pF = 47nF = 0.047μF
We're good with that.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
I wonder why they have them designated with C's instead of R's. Regardless I need to get that meter. They say it is en-route. Thanks again for all your time, I appreciate it.
 

MrChips

Joined Oct 2, 2009
25,928
Capacitors are used for different functions in electronic circuits.

In this example, C1 and R2, and C15 and R22, 0.047μF and 220kΩ combination form a high pass filter with a cut off frequency of 15.4Hz.

The purpose of this is to remove any DC voltages from the input source.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
When the amp is noisy with nothing plugged in and all the volumes at zero, would a person normally start testing from the output to the input, or does it matter?
 

MrChips

Joined Oct 2, 2009
25,928
When the amp is noisy with nothing plugged in and all the volumes at zero, would a person normally start testing from the output to the input, or does it matter?
At this point, it doesn't matter. It is really hit and miss as long as you have a consistent plan and procedure.

The plan at this point is to short the inputs to ground.
You can do this one of two ways.

1. Get yourself a short piece of insulated hook-up wire. You can short the signal to ground if you are certain that you are not going to disturb any DC bias.

2. Get a spare capacitor, 1μF to 10μF, non-polarized, but an electrolytic capacitor will do for now.
Ground one lead of the capacitor. Use the other lead to short the AC signal to GND.

I would start with the signal input to the power amp.
Short pins-2 to pin-3 on J1 connector.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
Let me make sure I'm understanding what to do = with the power on and with the speaker hooked up
(but all volumes at zero), touch a leg of this input jack to ground (the frame). I don't know for certain which leg
maybe the middle one, but does it matter? As far as disturbing the DC bias, how would I know if I did that?
Also, my wife just brought home my new meter. DSC02356.JPG
 

MrChips

Joined Oct 2, 2009
25,928
If you don't have a steady hand it is best to make alterations with the POWER OFF and AC unplugged.

Locate J1 connector. This is a 5-pin connector on the power amp board. Jumper pin-2 to pin-3 or resistor R2 to GND.

Plug in and turn on the amp. There should be no sound from the speakers.

KX-160 Power Amp Input.jpg
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
Two tests I've done: 1. On the cement resistors that looked cracked to me and maybe failing. Here were their readings = 330ohm A-Pair 1 17.1v and 46.0v * B-Pair 1 = 46.0 and 17.1v. On Pair 2 and Pair 3 zero volts. ~ Another test I went back and did = plugging in a guitar cord
to where the amp would output to the speaker, I checked for any voltage at the tip of the plug and it measured 0.27v on the 2volt scale. This is the noise that is heard when the amp is switched on with all levels at zero.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
Maybe also the 2 big caps that I had thought tested good are releasing and that is why there is also a pop thump when I turn it off and it is hooked up normal to the speaker. As I wrote down they read 4726uf and 4709uf (4700uf 50V). Thoughts?
 

MrChips

Joined Oct 2, 2009
25,928
Two tests I've done: 1. On the cement resistors that looked cracked to me and maybe failing. Here were their readings = 330ohm A-Pair 1 17.1v and 46.0v * B-Pair 1 = 46.0 and 17.1v. On Pair 2 and Pair 3 zero volts. ~ Another test I went back and did = plugging in a guitar cord
to where the amp would output to the speaker, I checked for any voltage at the tip of the plug and it measured 0.27v on the 2volt scale. This is the noise that is heard when the amp is switched on with all levels at zero.
What do you mean by:

A-Pair 1 17.1v and 46.0v
B-Pair 1 = 46.0 and 17.1v
 

MrChips

Joined Oct 2, 2009
25,928
Maybe also the 2 big caps that I had thought tested good are releasing and that is why there is also a pop thump when I turn it off and it is hooked up normal to the speaker. As I wrote down they read 4726uf and 4709uf (4700uf 50V). Thoughts?
The thump you hear on turn ON and turn OFF is normal.

Unless I am testing wrong TP2 has zero volts on both the little mustard cap and the diodes around it. The 8th leg on the IC 1B checks 15.1v
Nothing unusual you have reported so far.

Have you done the test in post #50?
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
No, I disconnected the 5 pin connector and then I made a jumper with 2 crimp fittings and pushed them over those two pins. The 5 pin was just hanging off to the side. I can make a jumper I guess across those 2 and then connect it back, should I try that ?
 
Last edited:

MrChips

Joined Oct 2, 2009
25,928
No, I disconnected the 5 pin connector and then I made a jumper with 2 crimp fittings and pushed them over those two pins. The 5 pin was just hanging off to the side.
I will assume that you jumpered the "wrong" end of the connector. But that's ok. It still gives me some useful information.

Let's move on from here.

I need some clear, well focused photographs of the top pre-amp board, the top board that does not have the big ceramic resistors.

I need to see where that 5-pin connector with the red wires enter that top board and the components around that portion of the board.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
DSC02357.JPG DSC02358.JPG DSC02359.JPG If you need closeups just let me know. I could also post them full size and then go back and delete them after you've taken a look
 
Last edited:
Top