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Author Topic: Intermittent loud crackle from Fender Deluxe 112 Plus  (Read 541 times)

traxcontrols

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Intermittent loud crackle from Fender Deluxe 112 Plus
« on: November 04, 2019, 08:56:34 AM »
Hi all, I have an interesting question that someone on here may be able to answer - I have a Fender Deluxe 112 Plus combo on the bench which is causing a few sleepless nights, as it has a fault of the type I have not ever come across before...

Randomly it will suddenly make a loud, short buzzing sound, no longer than a second in duration, which resembles the sound of a bee in a bean can. It usually occurs about a minute or so after switch on, and can happen at any time after that. All earthing checks out OK.

The fault is there whether the volume control is up or down, suggesting a power amp problem, but having monitored dc conditions in that area, nothing changes when the annoying sound occurs.

This is the baffling part - most of the electroytics have been replaced, including the two large smoothing caps, as have all of the FETs, and U1 (an MC1436 opamp).

The output transistors have not been replaced, as they appear to be in good order. Intermittent breakdown ocurring in one of those maybe?

As you can appreciate, a fault of this type is very difficult to trace - would anybody here have any experience of this fault, or some suggestions as to what to check?

g1

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Re: Intermittent loud crackle from Fender Deluxe 112 Plus
« Reply #1 on: November 04, 2019, 11:59:47 AM »
A cracked trace or pad can also do that.
And at the main filters, they can be very difficult to find if the caps are glued down.

Enzo

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Re: Intermittent loud crackle from Fender Deluxe 112 Plus
« Reply #2 on: November 04, 2019, 01:27:43 PM »
Something external to the amp can do this.  Does the amp act this way if you take it to a different place, like a friend's house?

By the way, not only power amp, but power supply is included in your narrowing down.

Watch with a scope. 

traxcontrols

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Re: Intermittent loud crackle from Fender Deluxe 112 Plus
« Reply #3 on: November 04, 2019, 01:45:01 PM »
I have checked the pcb traces, but no problems there.

The crackle does indeed sound like interference on a radio - so maybe location DOES have something to do with it - but would Fender allow such interference on one of their amps? Still, it's worth a try...

Enzo

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Re: Intermittent loud crackle from Fender Deluxe 112 Plus
« Reply #4 on: November 05, 2019, 12:23:27 AM »
Fender has little to say about it.  Fender may not RADIATE interference, but how sensitive it  might be is a mater of how well do they shield it.

The whole essence of troubleshooting is to isolate the problem, so the first thing to answer is whether the problem is even inside the unit or out.

fender or anyone else takes standard steps to reduce sensitivity to external noise, but there is a limit.  Common sources are cell phones.  Many have found that if they sit their phone on top of the amp, the amp picks up ticking sounds.  Ever been in or lived in a home where every time the furnace kicked on or the sump pump kicked on, you heard a quick pop or noise on your TV or AM radio? 

traxcontrols

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Re: Intermittent loud crackle from Fender Deluxe 112 Plus
« Reply #5 on: November 05, 2019, 05:50:25 AM »
Fender has little to say about it.  Fender may not RADIATE interference, but how sensitive it  might be is a mater of how well do they shield it.

The whole essence of troubleshooting is to isolate the problem, so the first thing to answer is whether the problem is even inside the unit or out.

fender or anyone else takes standard steps to reduce sensitivity to external noise, but there is a limit.  Common sources are cell phones.  Many have found that if they sit their phone on top of the amp, the amp picks up ticking sounds.  Ever been in or lived in a home where every time the furnace kicked on or the sump pump kicked on, you heard a quick pop or noise on your TV or AM radio?

I live in a terraced house which is a fair way away from any sources of interference - it's not my phone that is causing it, and the neighbours are out most of the time so it's not theirs either. The heating bolier is not the cause, having checked it (there's no interference on the radio), so it all seems to point to an internal fault.

I have ordered some replacement output transistors (they are relatively cheap), so we will see what happens once they are fitted, as everything else seems to check out.

traxcontrols

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Re: Intermittent loud crackle from Fender Deluxe 112 Plus
« Reply #6 on: November 05, 2019, 06:01:44 AM »
A small clue that makes me suspect the output transistors - when the owner brought it in for repair, the terminal board on the speaker had been ripped off, and the terminals may have shorted against the speaker frame, possibly damaging the the transistors.

phatt

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Re: Intermittent loud crackle from Fender Deluxe 112 Plus
« Reply #7 on: November 05, 2019, 06:12:07 AM »
If the Amp works (makes music as intended) then unlikely replacing transistors will fix it.
hint, Use Enzo's logic,, bang your fist on the amp and see if you can get it to pop.
If so it would point to a mechanical problem, i.e. a cracked solder or similar issue.
If something has been ripped out then look for hairline cracks in that area.
Phil.

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Re: Intermittent loud crackle from Fender Deluxe 112 Plus
« Reply #8 on: November 05, 2019, 08:53:29 AM »
If the Amp works (makes music as intended) then unlikely replacing transistors will fix it.
hint, Use Enzo's logic,, bang your fist on the amp and see if you can get it to pop.
If so it would point to a mechanical problem, i.e. a cracked solder or similar issue.
If something has been ripped out then look for hairline cracks in that area.
Phil.

I have tried this approach, but no change, the crackle cannot be made to happen by physical shock.

Enzo

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Re: Intermittent loud crackle from Fender Deluxe 112 Plus
« Reply #9 on: November 05, 2019, 11:22:20 AM »
Sump pumps and furnaces were just examples, we had a drop forge a half mile from our shop, and the heavy motors and such in that place put noise on the power mains.  At another location, we had a traffic light with a noisy triac a half block away, and so whenever ONE direction had a yellow light, we could hear power line noise, so every minute or so, we got 10 seconds of noise.  Point being, we need to eliminate the possibility of external sources.  We cannot just reason them away.   One trick is to set an AM portable radio between stations so all you hear is hiss, set it near the amp.  DO you ever hear noises from the radio at the same time as the amp?

I realize this could be something totally unrelated, but it is just a question to answer:  inside the amp or out.

traxcontrols

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Re: Intermittent loud crackle from Fender Deluxe 112 Plus
« Reply #10 on: November 05, 2019, 03:31:46 PM »
Sump pumps and furnaces were just examples, we had a drop forge a half mile from our shop, and the heavy motors and such in that place put noise on the power mains.  At another location, we had a traffic light with a noisy triac a half block away, and so whenever ONE direction had a yellow light, we could hear power line noise, so every minute or so, we got 10 seconds of noise.  Point being, we need to eliminate the possibility of external sources.  We cannot just reason them away.   One trick is to set an AM portable radio between stations so all you hear is hiss, set it near the amp.  DO you ever hear noises from the radio at the same time as the amp?

I realize this could be something totally unrelated, but it is just a question to answer:  inside the amp or out.

I'll try this tomorrow, it's been an exhausting day!

traxcontrols

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Re: Intermittent loud crackle from Fender Deluxe 112 Plus
« Reply #11 on: November 09, 2019, 01:23:37 PM »
Well, the amp itself confirmed that the fault was internal a day or so ago - by blowing the output transistors and mains fuse!

The customer has since taken it back, as the repair bill would now be pretty high (he can get a good used one on eBay fairly inexpensively), so we may never know what the actual cause was.

My money is on one of the many diodes in the power amp section breaking down.

The mystery remains unsolved!


 

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