Guitar Amp w Cement Resistors Loud Pops & Noise

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
So I got my hands on another guitar amplifier for cheap and without the knowledge of how to
use an oscilloscope or variac, etc. (I own neither), I replaced all the electrolytic
capacitors since the amp is over 20 years old and also replaced a few of the power amp chips.

While I do have sound now when I play a guitar through it; I also have a loud hiss. Sounds
like an open mic or radio that is not quite tuned in. This happens without having anything
plugged into the amp and no matter the volume turned up or down, along with bass, mid and
treble set at normal levels.

On closer inspection, I see 2 cement resistors (Pair 1) have small cracks in their cement tops.
They also look like the wire inside them also has raised up, they look a bit deformed compared
to Pair 2 in the middle. Maybe this is normal due to heat? They do have me wondering and that
is why I am asking you pros, "could these cause all this noise even when the amp has been
running for just a few seconds?"

Another investigation I tried was plugging in a ¼ jack cord into each input (one at a time)
and a couple of times this caused the noise to decrease a little bit. A couple of other times
I got a loud pop by plugging in. Seems like this is a grounding issue but how can I check?
I also thought it's interesting that pair 2 and 3 are .33 ohm while this pair 1 with the most
cracking and deformity is 330 ohm. It does match what is on the schematic. What are your
thoughts? Thanks in advance.1.JPG 2.JPG 3.JPG 4.JPG F1.JPG F2.JPG F3.JPG
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
Since it is snowing in Denver today (Saturday April 29th), this is a good day to warm my hands on this Pair #1 resistors. I turned the amp on for less than a minuet and then disconnected power and put my hand over them they are scorching hot. While new ones may act the same, I am going to change them out and see if they have anything to do with the noise.
 

MrChips

Joined Oct 2, 2009
25,932
I would not suspect the high wattage ceramic resistors.
Disconnect all inputs, outputs and speakers.
Turn on the amp. The resistors should not be hot. If they are, then something is wrong with the DC balance of the output stage.
The 330Ω resistors could be from a different part of the circuit.

Make and model number of unit?
What is the part number of the power amp chip?
Do you have a circuit schematic?
 

dl324

Joined Mar 30, 2015
14,339
Another investigation I tried was plugging in a ¼ jack cord into each input (one at a time)
and a couple of times this caused the noise to decrease a little bit. A couple of other times
I got a loud pop by plugging in. Seems like this is a grounding issue but how can I check?
A schematic showing what you're plugging in to and what your're plugging in would be helpful.

My guess is that you're shorting whatever you're plugging in to ground (tip to ring).
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
* At first I thought you were right concerning the hotness. They were cool not like when I had the speaker plugged in but after leaving it on for about 10-15min they are super hot now, while the other 2 pairs (2 & 3) are not. I do have the schematic for it but right now I forget what chip/part I replaced to get it going. When I bought this there was no sound and I believed because of age maybe just a re-cap would bring it back to life. But that did not work so then I started replacing chips since I have had good luck doing that in the past with other amps.

Make and model number of unit?
What is the part number of the power amp chip?
Do you have a circuit schematic?
 

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Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
Do any of you think that these super hot (to me failing) resistors, could cause noise or are they just a result of something ahead of them? I uploaded the schematic btw. Thanks for your inquires.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
On the parts replaced question = I replaced 1 Tip32C, 1 Tip31C, 2 50v 4700uf caps, 2 50v 47uf caps, 2 50v 220uf caps, 2 35v 22uf caps and 1 slider pot that had been busted off in the slider area on front (equalizer).
 

MrChips

Joined Oct 2, 2009
25,932
Why are you replacing parts willy nilly?

Do you have a voltmeter?
Disconnect all inputs, outputs and speakers. Turn VOLUME control to lowest setting.

With the voltmeter set to 100 or 200V range, connect black lead to GND or chassis.
Measure the voltage (using the red lead) on both ends of the two 330Ω resistors (four readinds).

Measure the voltage on both ends of the four 0.33Ω resistors (eight readings).

Alternatively, if you can locate TP6, TP7, TP8, measure the voltages at these test points.
Measure the voltage at the two terminals where you would connect the loud speaker.

All voltages are measured with the BLACK voltmeter lead connect to GND.
 

MrChips

Joined Oct 2, 2009
25,932
Am I going to be reading DC or AC at the 300 resistor?
Sorry.
All measurements are DC Volts.

Here are your expected values:
on 330Ω resistors: TP9 +16.5V, TP10 -16.5V
on 0.33Ω resistors: 0V

TP1 0V
TP2 15.6V
TP3 0V
TP4 +0.75V
TP5 -0.8V
TP6 0V
TP7 +35.8V
TP8 -35.6V
TP9 +16.5V
TP10 -16.5V
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
I only have an analog meter and if I am reading it right on the 300 resistors in one direction it reads about 15vdc on the other end it is about 40vdc.
When I check the other one the red goes to ground and it reads about the same one end 15vdc and the other end 40vdc. That's all I've tried so far the others are a little close to other things and I do not want to get zapped.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
I guess I finally need to break down an get a digital multimeter because this analog even though it shows you can put it on a .25 vdc setting the scale readout I guess switches to decibels (db). On a chart off to the side it says 0 db = 1mW. Pfft.... I don't know what I need to see for .33 ohms. I think it's time. I guess Harbor Freight or maybe Sears?
 

MrChips

Joined Oct 2, 2009
25,932
I only have an analog meter and if I am reading it right on the 300 resistors in one direction it reads about 15vdc on the other end it is about 40vdc.
When I check the other one the red goes to ground and it reads about the same one end 15vdc and the other end 40vdc. That's all I've tried so far the others are a little close to other things and I do not want to get zapped.
We can work with what you have for now.

15VDC and 40VDC appear to be correct.

Measure the voltage at the speaker terminals with respect to GND (with no speaker connected).
Expected reading should be 0VDC.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
The speaker output jack is enclosed so can I plug a cord into it and check the other end of the cord. Should I turn the volume up and to what?
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
This circuit board is not printed but I will look at the schematic and try to find it from topside. To bad they don't give you a picture of the actual layout. Fender had that and it was a great help. If numbers were on the board I could maybe find which 22k resistor they are talking about by TP6 but I don't even know that from looking at it. I did the tip of the cord and there was nothing on the scale of 2.5vdc but I did get a little movement on the scale that I do not know how to read. The .25dvc scale. I know I need a digital who's is okay = Harbor, Sears, Radio Shack a cheap one? BTW my wife says that is about it for today, so I had better scoot for now.
 

MrChips

Joined Oct 2, 2009
25,932
This circuit board is not printed but I will look at the schematic and try to find it from topside. To bad they don't give you a picture of the actual layout. Fender had that and it was a great help. If numbers were on the board I could maybe find which 22k resistor they are talking about by TP6 but I don't even know that from looking at it. I did the tip of the cord and there was nothing on the scale of 2.5vdc but I did get a little movement on the scale that I do not know how to read. The .25dvc scale. I know I need a digital who's is okay = Harbor, Sears, Radio Shack a cheap one? BTW my wife says that is about it for today, so I had better scoot for now.
Funny how the lady of the house does that when things just start to get busy.:D

No movement on the meter is good news on the output. All .33Ω resistors should be cool at this point.
The 330Ω resistors are dissipating 2W which should be a tad warm.
Do you have any music source such as an mp3 player that you can plug into the LINE IN on a 1/4" jack?
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
The meter does pick up movement on the output when I have the analog set at it's lowest scale. I will buy a digital meter this week and look back at this. For the next few weeks though, the weekends will be tied up so I will not be able to get back to the amp for awhile. I hope you are around in a couple of weeks (or more), so I can get a little more guidance. Thanks for sticking it out with me. I will look on here once in awhile in the meantime. I'm still wrestling with what cheap meter to buy will they all pick up .33 or less?
 
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