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Frontman 15G Suddenly Died

Started by Bakeacake08, October 02, 2016, 12:35:25 PM

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Bakeacake08

I have a Fromtman 15G I've been using as a practice amp (and live a couple times) for about a year and never had a problem with it. Last night I was playing on it with my phone plugged into the auxiliary input. I played for a few minutes and heard a kind of popping/scratchy noise, like you'd hear if you moved a bum cable the wrong way. It happened once for about a half second and ten went back to normal. Then give or ten minutes later it made the noise again and the amp *s!!t* down--no noise, LED off, like it was unplugged. I opened it up and checked the fuse and continuity on the switch. Both were good, so now I'm out of ideas.

In typing this out I realized I should check the transformer secondary and make sure the IEC plug is working. Assuming those are okay, where should I look next?

Schematic here:
http://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Frontman-15G-Schematic.pdf

DrGonz78

Check some voltages on the amp to confirm what is missing. Do you have -/+23v rails? -/+15v rails missing? CR11 & CR12 are they good? I would not think that the mains power rectifier diodes are bad, since no fuse blew, but still check them. Best way for us to tell what is going on is some voltage checks. Also if the PT was responsible I would guess the fuse would have gone too.
"A person who never made a mistake never tried anything new." -Albert Einstein

Enzo

Unplug from the wall and check the power transformer PRIMARY for opens.

Bakeacake08

Well, one of my worst fears happened: I opened up the box and pulled the chassis and when I turned it on everything appears to be working normally again.  :) I haven't tried sound yet, but the LED is on and all voltages seem to be appropriate.

22.7
-22.7
15.7
-15.0

I'm not sure how else to test a zener, but CR11 and CR12 both acted like a normal diode on the DMM (as I [think] I should expect).

So where do I go from here? Count it as a fluke and put everything back together? Leave the chassis out and olug in my gear to try replicating the problem? I know I had the volume quite low (night time, so barely on), if that means anything to anyone. Awaiting further instructions.

Enzo

The entire purpose of the unit is to make sound, so you can't ignore that test.  Does it work or not?  if it works, then all your readings will probably be normal.  That doesn;t mean it won't drop out again, but you usually only get funny readings when it is DOING the bad thing.

The purpose of a zener is to establish a voltage.  So if you have a pair of 15v zeners, and your 15v power supplies are OK, then the zeners must be working.   Whatever rating the zener has, if it is in the power supply, it ought to have that rated voltage across it.

There are zeners in other uses where they protect against over voltage, and under normal conditions there may be no voltage across them.


Ball up your fist and whack the top of it, does that make it cut out?

DrGonz78

I have had a few power cables where the neutral or hot wire had worn out from bending over the years. So sometimes the unit would power up fine and then it wouldn't. In that case you have to run continuity test on each wire of the power cable while bending it around to uncover the faulty wire. First thing first give it a good ole Enzo Whacking!!
"A person who never made a mistake never tried anything new." -Albert Einstein

Bakeacake08

The power cord leads tested fine. I partially reassembled it and both the guitar and Aux inputs played normally. The "Enzo Whacking" procedure did nothing to recreate the fault. I just thought to test the outlet I was using, but everything there looks normal as well. Where to next?

Enzo

If you have both 15v rails and both 25v (or whatever they are) rails, then clearly the mains side is working.

So now I try signal tracing or injecting.  Use your meter probe.  Don;t ground the black probe, just set it aside.  Now probe circuit points with the red probe.  It will act like a hum antenna, and inject hum into the circuit.  Try the input pin of the power amp IC, and try both input pins of the op amps.  Do you get hum/noise out th speaker when you touch them?

Alternatively, we play a signal into the regular input jack, and follow it along the signal path.  Is the signal present on the output pins of the op amps (1 and 7)?  Does it make it to the amp IC?  For that matter, what if the amp IC is working, but something is not allowing it to reach the speaker wires.  Like headphone jacks.

Does it work on head phones?  You might just have a bad cutout in the headphones jack.

Bakeacake08

I got him and occasionally some random radio station came it very clearly. That was fun.

I got him on the following pins:

U1: 2, 3, 6
U2: 2
U3: 3, 6
U4: 1

Guitar input and Aux inputs still functioning correctly.