Randall Commader cutting out

[ChewyNasalPrize]

Thanks Roly!

This one doesn't have effects send and return. Just the high/low inputs for each channel and the footswitch, speaker out (only one jack) and preamp out.

[ChewyNasalPrize]

Just to make doubly extra-sure, I played it a good while at bedroom levels to see if it would fart out using it's on-board power amp. Nothing. Then I turned it up a little over "1" and after playing for about 10 more minutes it started to crap out intermittently (sounded fuzzy/buzzy/muddy with a drop in volume but not much). It came in and out for several minutes then after about 5 minutes, it stayed farted-out. I switched it off then back on and it was OK for about 20 seconds then it did it again. Switch it off and back on and it stayed farty.

Also, there was a low-level hum that was present at the same intensity no matter where the volume settings were placed. This happened before but I thought it was because of the disconnected reverb (which is now connected) but the hum was back along with the farty sound. Swiched off the amp and back on quickly and the hum was still there. Switched it off and left it off for a several seconds then turned it back on and the hum was gone.

[DrGonz78]

Still we have a hum and a crackling fart to worry about lol. I am attaching that same schematic from previous post but this one shows the power amp side of the page better. There are plenty of test points to analyze. One idea is to test the output of the amp with your multimeter set on DC Volts. Clip a lead on both end of the output speaker jack, black on jack sleeve and red on the jack tip. Easy to clip on the solder lugs if you have the amp out on a bench. Have the speaker plugged in and when it starts farting, lets see if there is any DC at all on the output. I don't think there will be but if there is then we will need to stop connecting a speaker to the amp. At least analyze the speaker output to make sure what we are putting out of the amp onto the speaker. More ideas will come as we all proceed from this point further and there are some great troubleshooters here at SSguitar.

Note: Mainly just wanted to update a post here with this other copy of the schematic.

[ChewyNasalPrize]

Thanks DrGonz!

Waiting on a new multimeter. Will perform suggested test as soon as it arrives!

[ChewyNasalPrize]

I got a set of clamp adapters for the analog multimeter I already had. Want to see if this works first before I get a digital one.

It has three ranges for DCV: 10, 250 and 500. Which range should I use?

I also got a can of that spray-cold stuff to check transistors if it comes to that. I've never used it but it might come in handy.

Now I just have to wait for the little woman to leave the house so I can CRANK it! 

[DrGonz78]

Well the main thing we are wanting to know is when that hum begins if it is a sign of DC voltage onto the speaker. For one we don't know what is at fault and for another we are not sure if the fault occurs without a speaker load on the amp. Right now we know with a load it fails at higher volumes. I don't expect to see DC on the output but then again better safe than sorry.

So connect black probe to sleeve and red probe to tip on the inside jack. Run signal through the amp and wait for it to fail. At that point check the reading on the voltmeter, then turn off audio signal. Keep the amp running with speaker connected and check the reading on the voltmeter. Disconnect the speaker from the amp and check reading on the meter. Every time you check the meter do it on the 10 setting then the 250 setting and take a quick note. That way we will check if there is voltage above 10vdc and better accuracy if there is any readings below 10 vdc. Better accuracy would be a digital meter, but really we are looking to see if there is more than 1/2volt on the output once the failure begins. Let us know what you find out.

Note: I am not sure the accuracy of the meter you have now if it is not digital, probably okay. We might be seeing only millivolts of DC on the output and not so sure how well that meter will display such small readings. A lot of times the output will have a range of 0-500mv DC on the output of many solid state amps. Sure we ideally want to have no DC voltage on the output, but that is rarely the case.

[ChewyNasalPrize]

OK- I think I found something... :dbtu:

I connected the multimeter as described then plugged in and turned on and the needle didn't move at all.

I played at volume set at 2 (highest so far lol) and it cut out right away. I hadn't played the amp for several days. Noticed the meter needle still did not move on either 10 or 250 settings.

This time it stayed cut out so I had a constant farty signal. While trilling on the guitar with my left hand to maintain the signal, I poked around again with the wooden kabbob skewer with my right hand. When I pressed on (what I believe is) the negative connection of one of the filter caps, it stopped cutting out right away. It went to a clean signal and I could not get it to fart out again after playing for several more minutes and while pressing and prodding the connections of this filter cap.

Solders look OK to me but maybe something needs cleaning or resoldering or possibly replacing?

Picture of the location marked by kabbob skewer of what I think is a bad connection and at this point the most likely cause of farting is attached. 

[ChewyNasalPrize]

Also- I think the "hum" was associated with my guitar being too close to the amp while I was testing it. I was wearing it while testing so it was only inches from the transformer at times.

At least I hope that's all it was.

[ChewyNasalPrize]

Just played it some more and for a good while at high volumes and it NEVER cut out. It also sounded "punchier" it seemed. Maybe my imagination.

[Roly]

When I pressed on (what I believe is) the negative connection of one of the filter caps, it stopped cutting out right away.

Yep, that is normally a pretty strong clue you are hot on the trail.

I think the "hum" was associated with my guitar being too close to the amp

That will do it.

When testing you are generally better of using an MP3 player, cassette deck, or radio as a signal source because you don't have to try and do two things at once, and you aren't clinging on to grounded guitar strings with one hand while poking around in the innards with the other {yes the "chopstick" is insulated, but it's still not a great idea}.

Try giving the cap lug a bit of a wiggle and see if you can determine if it's the solder joint, or perhaps the lug is losing connection inside the cap.  Try re-soldering the whole joint, but if that doesn't clear it up then it looks like the cap might have a faulty internal connection.  You certainly seem to have "touched" the fault, which is progress.

Maybe my imagination.

Sorry, but yeah, that is a fairly well-known subjective effect.

[DrGonz78]

Well looks like we are now back to the earlier tests proving that something is loose/intermittent. Much better than something starting to fail and putting DC or something on the speaker. Still I am confused by one thing this whole time... The preamp test showed that we could not get the amp signal to fail running it into the Peavey hybrid amp. So apparently this little intermittent connection is very elusive.

Does your meter have a an ohms setting? With amp off, touch black probe to that negative terminal and red to the other end where that black wire goes. We are looking for very low resistance here 1 ohm or less on the meter. Does the meter have a continuity setting? A continuity setting on a meter will give you a beeping noise to let you know there is low resistance. It is good to have a that beeping sound going on and then you might wiggle the wire to check it for shorts. The ohms meter will do the same thing but is harder read on an analog meter.

So it could be bad solder joint, bad black wire, and the filter cap might have an internal shorting developing. I guess it could be more than those 3 ideas there but lets start here.

You can also place your probe over the connections of that cap and check the DC readings. Black probe connect to the chassis ground and red probe connects to each side of the filter cap. Meaning we will be measuring each leg of the cap one at a time. Try the black one and you should see negative DC voltage, set you meter to read 250.

[DrGonz78]

When grounding your meter connect it to the green wire to clip it to where it bolts down to the inside of the chassis. The black spray might prevent a good conductive ground surface. Also, there are two black wires that connect to the pcb board and perhaps thats why preamp is not failing earlier. Perhaps not... Either way make sure to wiggle those wires around, especially the thick black one.

Yeah and Roly is dead on there about using a signal other than you playing your guitar at the same time as prodding with the stick. Good luck

[ChewyNasalPrize]

OK- There are three terminating points from that joint. Two to the PCB board and one to the other filter cap. Readings at all points read below 0 ohms.

My meter does not have a continuity setting.

I checked the filter cap connections and while the amp was off, the needle did go slightly into negative territory when I touched the connection for the red wire. It didn't seem to do anything when touched to the black.

I also checked the other filter cap and same for the black but it went slightly into positive territory when I touched to the connection for the red wire. I guess this is normal?

I wiggled the lug but didn't cut out.

I'll resolder that connection and report back.

Thanks guys!

[DrGonz78]

I checked the filter cap connections and while the amp was off, the needle did go slightly into negative territory when I touched the connection for the red wire. It didn't seem to do anything when touched to the black.

I also checked the other filter cap and same for the black but it went slightly into positive territory when I touched to the connection for the red wire. I guess this is normal?

Yes very normal indeed. You now see the residual voltage of the cap is positive on one of the red terminal lugs and negative on it's corresponding filter cap. Those are your amps B+ rails that provide the higher voltage to your output section of the amp. They will also use that B+ voltage to drop it down to a lower voltage for the preamp section. Now you see how one cap is the positive side and the other negative side of this equation.

[ChewyNasalPrize]

Ok-

I clipped the three wires at the solder joint, removed existing solder and cleaned the tip of the connector. Stripped the wires and reconnected to tip of the filter cap connector and resoldered. It's about as clean a connection as I know how to make.

Played it for a while at high volumes and no trace of cutting out. I'll leave it open and test from cold to hot several times over the next few days. It may still yet be the lug connection inside the filter cap and my fooling with it made it have a good temporary connection so I'm not totally ready to say it's fixed but I am pretty sure at least the problem lies here with this filter cap and/or connections to it.

It doesn't look like the lug comes out of the filter cap- should it?. If it is the lug, I'm guessing new filter cap? Or maybe both filter caps for good measure?

Thanks guys!