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Author Topic: AMP only plays when I strum really hard.  (Read 16088 times)

J M Fahey

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Re: AMP only plays when I strum really hard.
« Reply #30 on: February 14, 2011, 06:42:19 PM »
Quote
I assumed you meant check the ends of the 38 and 39 caps. to ground.  If that is NOT the case, that would explain the sparks.  Am I doing this correctly???
We are talking measuring voltage here, with a multimeter.
Sparks? ... what sparks?  :o

Quote
R-74 and 75 I would say are probably warmer than intended.  Not burning but pretty warm. NO voltage on either end but on one end they both sparked pretty well  D 19 and 21 are NOT HOT. NO voltage and on D 21 when I touched the probe on one end the speaker popped VERY LOUDLY.  Scared the ***** out of me.
Sparks? ... what sparks?  :o [2]

"No voltage" and "sparks" contradict each other.

dan92y

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Re: AMP only plays when I strum really hard.
« Reply #31 on: February 14, 2011, 06:59:31 PM »
Quote
I assumed you meant check the ends of the 38 and 39 caps. to ground.  If that is NOT the case, that would explain the sparks.  Am I doing this correctly???
We are talking measuring voltage here, with a multimeter.
Sparks? ... what sparks?  :o

Quote
R-74 and 75 I would say are probably warmer than intended.  Not burning but pretty warm. NO voltage on either end but on one end they both sparked pretty well  D 19 and 21 are NOT HOT. NO voltage and on D 21 when I touched the probe on one end the speaker popped VERY LOUDLY.  Scared the ***** out of me.
Sparks? ... what sparks?  :o [2]

"No voltage" and "sparks" contradict each other.

OK I rechecked the readings.  D 19 and 20 ZERO volts DC and when I touch the probe to one end nothing.  The other end sparks when the probe touched it like a little static shock.  R-74 and 75  ZERO volts and sparked a little when the probe touched.  The caps same thing, but BIG sparks.  Maybe I should just send this to you...

joecool85

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Re: AMP only plays when I strum really hard.
« Reply #32 on: February 15, 2011, 08:32:37 AM »
OK I rechecked the readings.  D 19 and 20 ZERO volts DC and when I touch the probe to one end nothing.  The other end sparks when the probe touched it like a little static shock.  R-74 and 75  ZERO volts and sparked a little when the probe touched.  The caps same thing, but BIG sparks.  Maybe I should just send this to you...

What are you using to check the voltage?  Is it a multimeter?  If so, is it possible you have it set to check resistance instead of voltage?  Or maybe it is set to check too high or low of a voltage?

There should be no sparks measuring voltage on a multimeter, regardless of component type of voltage.
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J M Fahey

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Re: AMP only plays when I strum really hard.
« Reply #33 on: February 15, 2011, 09:18:26 AM »
Or it might be set to Current.
Even worse, the Red probe could be plugged into the  2 or 10 Amp jack, which internally has a thick piece of copper wire (a shunt resistor) which shorts it internally to the black jack.
*That* will happily spark all day long.

dan92y

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Re: AMP only plays when I strum really hard.
« Reply #34 on: February 15, 2011, 03:01:58 PM »
Or it might be set to Current.
Even worse, the Red probe could be plugged into the  2 or 10 Amp jack, which internally has a thick piece of copper wire (a shunt resistor) which shorts it internally to the black jack.
*That* will happily spark all day long.
BREAKTHROUGH- I plugged the meter lead into the CORRECT spot and got the following readings.
C38  25.1v - 0v
C39  25v - 0v
R74 - 25.2v - 15.1v
R75 - 25.1v - 14.2v
D19 - 0v - 15v
D21 - 0v - 14v
I apologize for the meter being plugged in wrong.  I am going back through the previous posts and re-measuring the ones you told me to do before as the meter has been this way the entire time. More to follow.

J M Fahey

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Re: AMP only plays when I strum really hard.
« Reply #35 on: February 15, 2011, 03:05:42 PM »
Well, it seems you *do* have +/-15V, great news.
Maybe we can get again on the right track.
Good luck.
PS: I had thought I was the only one who did not read instruction manuals.  :'( :-\
« Last Edit: February 15, 2011, 03:07:01 PM by J M Fahey »

dan92y

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Re: AMP only plays when I strum really hard.
« Reply #36 on: February 15, 2011, 03:27:11 PM »
Well, it seems you *do* have +/-15V, great news.
Maybe we can get again on the right track.
Good luck.
PS: I had thought I was the only one who did not read instruction manuals.  :'( :-\
No, I try to shoot from the hip until I hit a wall.  I apologize for wasting the time before.  Also, I rechecked IC3 and have the following.
pin 1-0v, 2-0v, 3-0v, 4-14.1v, 8-15v.  Are those capacitors only to have voltage on one end?

dan92y

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Re: AMP only plays when I strum really hard.
« Reply #37 on: February 18, 2011, 03:06:02 AM »
Well, it seems you *do* have +/-15V, great news.
Maybe we can get again on the right track.
Good luck.
PS: I had thought I was the only one who did not read instruction manuals.  :'( :-\
No, I try to shoot from the hip until I hit a wall.  I apologize for wasting the time before.  Also, I rechecked IC3 and have the following.
pin 1-0v, 2-0v, 3-0v, 4-14.1v, 8-15v.  Are those capacitors only to have voltage on one end?
I am starting to think that the pots may in fact be the culprit.  Any credence to this thought???

phatt

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Re: AMP only plays when I strum really hard.
« Reply #38 on: February 18, 2011, 07:06:59 AM »
Hi Dan,
I'd be looking at R6 (50k Volume) and possibly R22 (50k Master)
If R6 wiper breaks from ground then the Amp would have very little gain but still pass a small signal. Which I believe is the symptom you described at the start.

Make damn sure you did not cold solder the new pots.
You need to inspect these things in very good light and even with a magnifiying glass.

Some of these modern Amps use very fine tracks and reinstalling commponents often lifts the track off the board.
A blob of solder may look perfect but cover a fine track that is broken.

I once built a circuit and inadvertantly bridged a very fine strand of wire as I soldered.
You can't see these things without magnification.

One tiny bit of hair wire caused me a week of frustration. :grr

As and old wise teck once said to me 90% of all electrical problems are in the connections,,,, I tend to agree 8)

no matter,,, at least now you have learnt how to setup a DMM and use it.
Phil.

dan92y

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Re: AMP only plays when I strum really hard.
« Reply #39 on: February 18, 2011, 12:11:40 PM »
Hi Dan,
I'd be looking at R6 (50k Volume) and possibly R22 (50k Master)
If R6 wiper breaks from ground then the Amp would have very little gain but still pass a small signal. Which I believe is the symptom you described at the start.

Make damn sure you did not cold solder the new pots.
You need to inspect these things in very good light and even with a magnifiying glass.

Some of these modern Amps use very fine tracks and reinstalling commponents often lifts the track off the board.
A blob of solder may look perfect but cover a fine track that is broken.

I once built a circuit and inadvertantly bridged a very fine strand of wire as I soldered.
You can't see these things without magnification.

One tiny bit of hair wire caused me a week of frustration. :grr

As and old wise teck once said to me 90% of all electrical problems are in the connections,,,, I tend to agree 8)

no matter,,, at least now you have learnt how to setup a DMM and use it.
Phil.
OK, good advice.  If there is a broken wire or a strip that has lifted from the pcb, is there a way to fix that or not???  Thanks

joecool85

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Re: AMP only plays when I strum really hard.
« Reply #40 on: February 18, 2011, 01:31:52 PM »
OK, good advice.  If there is a broken wire or a strip that has lifted from the pcb, is there a way to fix that or not???  Thanks

You can repair them.  I've done it with a liquid in tube that I got from RadioShack a few years ago.  It has metal in it so once it sets up it works just like a regular trace.  I can't remember the name, but if you are interested I will check when I get home.
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dan92y

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Re: AMP only plays when I strum really hard.
« Reply #41 on: March 10, 2011, 01:56:38 AM »
OK, good advice.  If there is a broken wire or a strip that has lifted from the pcb, is there a way to fix that or not???  Thanks

You can repair them.  I've done it with a liquid in tube that I got from RadioShack a few years ago.  It has metal in it so once it sets up it works just like a regular trace.  I can't remember the name, but if you are interested I will check when I get home.

OK.  Found nothing as far as broken tracks or shorted solder joints that I can find.  Does someone know what I should look for (and where) to replace the pots that still may be the problem??  Thanks

Jack1962

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Re: AMP only plays when I strum really hard.
« Reply #42 on: June 04, 2011, 07:38:20 AM »
if you have a line out(or effects loop send) go from there to a known good amp , basiclly if you have a good sound when you play your preamp isn't the problem , if you don't the preamp is the problem. check all your solder joints not only on the parts you replaced but on the entire board.

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LJN

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Re: AMP only plays when I strum really hard.
« Reply #43 on: June 04, 2011, 06:11:39 PM »
Try cleaning the pots. even if they're new, they could still have some corrosion. I always try the simple things first. Things seem to reveal themselves if you try the simple stuff first, and work up to the more complex things.
I hardly ever read manuals, myself.
Hope that helps.
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