Guitar Amp w Cement Resistors Loud Pops & Noise

MrChips

Joined Oct 2, 2009
25,932
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?
Not to worry. AAC will be here at your service.

There is no need to sell the barn in order to acquire a decent DMM. They often go on sale at your local hardware store for under $20, sometimes as low as $10.

Check out your local Home Depot. There is a Klein Tools model MM300 for under $30.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
So I make the mistake of getting an "auto-ranging" meter. Did you know that these pick up static in the air and the digital numbers jump around? I called the maker of the meter and they told me, "it will settle down when you measure DC, but just standing it will jump around." While they claim it will settle down when you check DC, I am returning it because it did not settle down enough. Mailing it in on Monday and went ahead and bought the Klein mentioned above. Next, I am having trouble figuring out where TP1 is and so on because this board is not printed. Where is TP1 and 2?InputAreaTP1.JPG
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
I am a beginning novice. Can you show me where TP1 and TP2 is according to this schematic and then show where on a picture of the actual board? I have successfully repaired a Fender guitar amplifier before but this Crate guitar amp does not have the numbers printed on the board, so I am trying to figure out where exactly is it telling me to test given the actual board.
 

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ColoradoRobert

Joined Jan 22, 2016
155
Hmm sorry, just trying to get the answer and assumed my most recent comment on this old post was difficult to find or would not be answered. Solution: push the question to the front by introducing it as a new post.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
Anyways, Thank you Mr. Chips. I am going to study this for awhile. I have not got the Klein DMM
yet. So while I can't test these today with my analog meter and tell much, since reading that
is difficult by the scales shown on the front of it; I thought the first thing I need to learn
is where are these test points, how do they translate to the actual un-printed board. I will
study it.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
Just to see if I am identifying parts right: Is CP1 and CP2 capacitors = are they the mustard colored components or is one of them the blue one with the arrow on it also? What is that blue one with the arrow? TP2 = I can easily see the 3 diodes (clear-glassy looking components orange-ish with a bold black line on one end, showing the direction; like a plumbing check valve). So the mustard colored one must be a capacitor C6?
 

WBahn

Joined Mar 31, 2012
26,398
So I make the mistake of getting an "auto-ranging" meter.
Most auto-ranging DMMs provide a means of manually setting the range and thereby disabling the autoranging functionality. I've never seen one that didn't, but that doesn't mean they don't exist. But since there will always be measurements that wander back and forth cross the range selection thresholds, even with hysteresis, I suspect that very few meters wouldn't provide this option.
 

MrChips

Joined Oct 2, 2009
25,932
Just to see if I am identifying parts right: Is CP1 and CP2 capacitors = are they the mustard colored components or is one of them the blue one with the arrow on it also? What is that blue one with the arrow? TP2 = I can easily see the 3 diodes (clear-glassy looking components orange-ish with a bold black line on one end, showing the direction; like a plumbing check valve). So the mustard colored one must be a capacitor C6?
Let's start with the easy component, IC1A and IC1B.
IC1 is a RC4558 dual opamp. That is why it is labeled as IC1A and IC1B in the schematic.
This is in an 8-pin DIP (dual in-line plastic) package.


The pins are numbered starting with 1 at the top left corner, 4 on the button left, then 5 to 8 going up on the right hand side.



Once we can identify the pins on the IC, we can follow the PCB traces and match up with the external components, resistors, capacitors and diodes as indicated in the schematic drawing.

Small signal diodes such as 1N914 are encapsulated in glass. Note that the black band on the glass marks the cathode (N) of the PN junction diode.

Resistors are the tan (4-band) or blue-green (5-band) components with color bands for the resistance code.
It would be worth the while to learn the resistor color code. This will help you confirm which resistor you are looking at.



The mustard colored components are ceramic disc capacitors. These are non-polar capacitors and hence orientation does not matter.

The blue cylindrical components are also capacitors. These tend to be higher value aluminum electrolytic capacitors. If these are polarized, orientation is important. You want electrolytic capacitors to be biased with the more negative voltage on the -ve lead of the capacitor.
 

WBahn

Joined Mar 31, 2012
26,398
So I make the mistake of getting an "auto-ranging" meter.
Most auto-ranging DMMs provide a means of manually setting the range and thereby disabling the autoranging functionality. I've never seen one that didn't, but that doesn't mean they don't exist. But since there will always be measurements that wander back and forth cross the range selection thresholds, even with hysteresis, I suspect that very few meters wouldn't provide this option.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
You identified the TP1 area and right by the mustard cap (C1 poly) is a 4 band resistor. Using the chart b1 = 2, b2 = 2, b3 = 10k so what does that mean the resistance is 22,000 (22k)?
 

MrChips

Joined Oct 2, 2009
25,932
You identified the TP1 area and right by the mustard cap (C1 poly) is a 4 band resistor. Using the chart b1 = 2, b2 = 2, b3 = 10k so what does that mean the resistance is 22,000 (22k)?
No.
I see RED-RED-YELLOW-GOLD which is 22 x 10k 5% = 220kΩ ±5%
We are looking at R2 in the schematic, in parallel with C2 to GND.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
So the blue arrowed component is C1 poly and C2 is the mustard colored one? On the resistor it's 22 x 10k =220k.... okay. In regards to the numbers on the IC (integrated circuit) are they numbered like this?IC leg numbers.JPG
 

MrChips

Joined Oct 2, 2009
25,932
I cannot see the writing on the blue capacitor. I don't think that is a "poly", short for polyethylene or polypropylene plastic capacitor.
The blue cap looks like an aluminum electrolytic to me.

Trace the PCB from the input jack to find R1 and C1.

Also trace Channel 2 input jack to find R21 and C15.

Some components might have been substituted. If the blue caps are indeed C1 and C15, I would replace them with non-polarized caps as suggested in the schematics.

Yes, you have identified the pins of the IC correctly. The indented dot on the top of the package locates pin 1.
 
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