Robert, if your getting better gain when injecting into the return, that is promising. With all the other gains at zero, or with a dummy plug into the return, can you turn the master volume all the way up with virtually no sound from the speakers? then when you inject a signal into that Return, you can get the rated power out of the amplifier under test by turning up the master volume?

What is the model of your multimeter? That way I can get one of their manuals and see what your capable of measuring.

I'm sure you are using good speakers.

If you replaced the electrolytics, can I assume you installed each of them properly ... polarity wise.

 

Incredible. The other day when I tried a guitar in the return I got nothing. This morning I tried the guitar
and all of a sudden I have sound (normal sounding). I have no idea why other than maybe I did not have it plugged
all the way in or something. Strange. This is with all the gains set at zero and the only things turned in are the bass and treble.

I quickly went to the regular input (around front) but there is still nothing, even with the gain turned up. I just get the garbel-ed sound
that I had before. * To address your other questions = I just bought the DMM = Klein MM300 through Home Depot. The speakers are good
and I am pretty sure the caps are correct.

 

This morning I tried the guitar
and all of a sudden I have sound (normal sounding).

That's big. Eliminates a lot of possibilities and allows focus on the preamp.

 

I concur with wayneh. You now know the problem is BEFORE the return jack.

Did you put a battery in the pickup to give your guitar a higher level to drive the amp?

To test the preamp, take the SEND jack signal and send it to another amplifier's RETURN jack, or you can hook up some headphones to the jack, high impedance headphones.

 

I have a cheap Squier guitar and it is passive pickups, no battery. I tried your test like this = output jack of the amp into the speakers - guitar into the return jack on the back of the amp - plugged a set of headphones into the send jack. No sound whatsoever through the headphones but normal sound from the speakers. I did not turn the amp up very loud maybe noise would have come through the headphones, but I was a bit afraid to try that because I do not want to damage my headphones. The volume, bass & treble were the only things I adjusted, everything else was at zero. Also I started to check the test points 1, 2, 3 come in about right but 4 is only showing 11m, 5 is showing 12mv . Unless I am reading these wrong TP4 should be 50m and TP5 should be 99m. Any ideas if this is indicating the failure?

 

When you put the headphones into the send jack, the guitar needs to be in it's normal input .... and the gain of the preamp adjusted.

50 mV AC and 99 mV AC. The meter you chose has two options for AC ... and neither of them can read those two figures.

Plug your guitar into the input, headset in the tune, adjust the preamp gain and hear what you hear. See if you have distortion. The master volume can be at zero. You can manipulate any control you want to see if you can force it to distort.

I don't know how much you paid for that meter, but it's not going to help you troubleshooting your amplifiers, other than the DC voltages.

 

The meter was recommended to me by the Moderator and he may have not noticed that there wasn't mv on the AC. * In regards to the AC/DC, I asked that question early on. While the schematic can tell a person to check AC, what they don't explain is where to place the common. For example = on the schematic page 1 area 3A, it says you can check AC on the chassis. If this is the case, what does it matter if the meter is set to AC or DC, since it seems to me that this is more of a DC situation? The meter does have mv on DC and that is what I was using because of my theory. I still don't understand the difference or where I should place the common if my meter 'could' check AC. What meter would you recommend because maybe Home Depot would take this one back?

 

Doing the test you suggested = there is some very low modulation noise/hum that I can hear with everything set at zero, without strumming the guitar (maybe normal?). When I push in the active passive button the volume decreases slightly with little to no distortion whatsoever. Adjusting everything across the front of the amp with many variations I heard no changes in the sound. Only when I strum the guitar and bring the Gain off of zero, there might be slight distortion at about gain-2 and the yellow light comes on. The distortion though is very light/slight and increases as you turn it up (slightly hear it). Around 7 the red light comes on and the distortion is very noticeable then and gets worse at full gain (maybe normal?).

 

All measurements are to common. You can try to measure the 50 mV AC signal. I just didn't see a frequency response specification for the meter or a range function. Measuring the AC voltages you would usually inject a known frequency at a specified level. A signal generator or app can do that. A mp3 player could do it, I have a few mp3s with different frequencies at a neg 10 dB level for that purpose.

Hum, to me is either 60 Hz or 120 Hz. Distortion is not faithfully reproducing the input signal. If you have an Android phone or tablet, you can download a frequency counter and signal generator. If you hat two Android devices, one could provide a signal and the other could measure it.

Since we know it's present at the line out, your wife will be happy you won't be turning up the master volume until you solve the problem.

Let me know if you have an Android device and I'll recommend the app you would need.

 

She does have an Android phone but I think the battery is on it's way out. Not sure she will be willing to put more stuff on it or be willing to take the time. Is there any other way to check this amp? Should I go back to doing the test points? I see AC scale starts at 200v I wonder if it will show anything on those mv's.

 

AC scale starts at 200v

You can try, but if it's not auto ranging, what you read will be within the error of the count.

Check and see if you have a function generator at the app store for the phone you use. You need to inject a sine wave into the input at the appropriate level (50 mV rms) so you can do the AC measurements. Do you have some form of a sine wave generator? Do you have a mp3 player?

You can divide the output voltage down to 50 mV using a resistor divider circuit.

 

At best you can read the ac voltages above 1 volt.

 

Okay Joe, so if I am not beyond the point of no return with Home Depot
what multimeter would you suggest for a budding novice?

 

... did you get a chance to check test point 17? That test point is associated with R143, a 10 ohm resistor ... maybe the resistor that was cracked. ... Should expect a reading of 445 mV AC, at the output of op amp U12B.

 

This is where I am a bit stuck drc_567. The multimeters available to most of the consuming public do not have a setting for AC millivolts. So checking the test points 1-18 you are forced to buy this big lunchbox of a machine for around 100 dollars which defeats the purpose, since in Denver I will be lucky to get 150 for the amp working. It's kind of goofy since in some places on this schematic it gives what you can read at the test point in both AC and DC. If they did that at every tp I could look at it.

 

So Joe, we found a signal generator at the app store but won't I still have the same problem, since my meter won't go that low on the AC? Not sure I follow how I can use this. Here is a picture of it =

 

Ok ... Let me look at amazon to find an inexpensive multimeter that can help you out. This will give you a break from this problem.

Do you have a device that can play mp3s ?

 

Ok ... Let me look at amazon to find an inexpensive multimeter that can help you out. This will give you a break from this problem.

Do you have a device that can play mp3s ?

 

We don't have an mp3 player but we did download a signal generator. Of course I do not know how to set or use it.... but 1 step at a time. I wonder if AAC has a post on those? This could come in handy in the future if I can learn how to use it to determine stuff.

 

Found another generator, maybe this one is easier to use but I do not know how to apply it to the amp
and then use the signal as the fault finder; especially when I do not have a meter that can read AC millivolts.