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Old 26th July 2013, 08:49 PM   #1
roc4fun is offline roc4fun  United States
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Default Fender Frontman 212

Hi All, During my morning run I found a Frontman 212 at the curb. I was hoping someone's ex kicked a perfectly good amp to the curb, but it was not the case...

Plugged in it makes a very loud popping and crackleing sound. It makes it with the volume and tone controls all the way down, and either channel selected. Inserting a patch cable into the power amp in causes the sound to stop.

I currently have the amp disassembled. There are no loose components, and all the solder joints look good.

I'm not a tech, but I have a DVM, I know what most electronic components look like, and can solder/unsolder.

I'd appreciate any help troubleshooting you might give.

Thanks in advance,

Dave
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Old 27th July 2013, 12:44 AM   #2
Enzo is offline Enzo  United States
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You have isolated the problem to the preamp section.

Do you have a scope?

Aside from the noise, does the amp work, play music, sound OK?
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Old 27th July 2013, 04:48 AM   #3
roc4fun is offline roc4fun  United States
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Hi Enzo, Honestly, the noises it was making were frighteningly loud. I didn't try and play it. Would that be a valuable diagnostic?

I don't have an occillloscope.
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Old 27th July 2013, 06:32 AM   #4
Enzo is offline Enzo  United States
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Plugging into the loop return kills the noise. OK, plug the guitar into the FX return, does that come out the speaker strong and clean? I bet it does. That means the power amp works. If you plug your guitar into the input, and strum it, THEN turn on the power, you may hear the noise, but with the guitar strings still going, it takes only a second to hear if it comes through the preamp or not. If you can stick some sort of volume control in the FX loop, you can turn down the volume of whatever noise is leaving the preamp.


So you have a voltmeter? You got noise on the preamp out, let's see if we can measure it. Plug a cord into the FX SEND, at the far end of the cord, measure AC volts from tip to shaft of the plug. If your meter has ranges, try one that will measure like half a volt to a volt. Get any readings that seem to represent the noise? If so, we can look back through the circuit for similar AC volts.

Got any extra amps? You can make a simple probe for the input of one, and use it too "listen" to stages in the amp.
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Old 27th July 2013, 04:43 PM   #5
roc4fun is offline roc4fun  United States
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I think I've got some good data...

Strong, clean sound from the FX return.
Both inputs give good sound from both channels. Tone controls work as expected as does gain and drive.

Some minor random popping was present with the guitar plugged in, but no crackling. Unplugging the guitar returned to loud popping and crackling.

My DVM autoranges on AV volts. I got no ACvolts at the FX send. I did get 14-17 DC volts using the 200m DCV range.

Could this amp have some kind of "protection" circuit when nothing is plugged in? This seems like good diagnostics.
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Old 28th July 2013, 07:01 AM   #6
JMFahey is offline JMFahey  Argentina
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Quote:
I did get 14-17 DC volts using the 200m DCV range.
That's like saying I put 15 horses inside a shoe box

I *guess* you read 14/17 milli Volts, using the 200 milli Volt range/scale or 14/17 full Volts, using the 200 full Volt range/scale but please confirm.

*If* a guitar plugged at the input sounds reasonably well, but unplugging it brings major havoc, I suspect a bad jack, cracked pads/tracks/splder around the input jack or all at the same time.

Please check it under good light, preferrably using a loupe or strong reading glasses (+4).
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Old 28th July 2013, 03:00 PM   #7
roc4fun is offline roc4fun  United States
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Hi JmFahey, Sorry I wasn't clear about the volts measurement. My meter shows 200m on the scale I was using. My inclination would be that it is 14-17 milli volts.

I rechecked very carefully, bright flashlight and strong glasses, I detect no solder faults at either of the input jacks.

When plugging in to either of the input jacks, or the FX return the same behavior is manifested... The amp is on and making loud popping/crackling sounds. Plugging into any of those 3 jacks there is a distinct time, .5 to 1 sec, where the sound continues, then it becomes quiet. It was this pause that made me consider it was a circuit thing. You would know better.
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Old 30th July 2013, 09:57 PM   #8
roc4fun is offline roc4fun  United States
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Are there some other ways to check the jacks besides visually? It seems like a stretch that both input jacks have a problem. I hate to throw parts at it.
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Old 30th July 2013, 10:55 PM   #9
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Make sure all of the control fixing nuts, behind the control knobs, are tight and the jack socket nuts are tight. The main chassis connection is via the front panel and the control metalwork with the input jack outer connection. If they are loose, there will be odd things happening, noise wise.
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