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Author Topic: Fender guitar amp started humming, can't figure out why  (Read 14370 times)

brodie1600

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Fender guitar amp started humming, can't figure out why
« on: April 27, 2015, 04:57:55 PM »
My buddy has a Fender Frontman 212R solid-state amp that he bought used from Guitar Center, and it's been performing fine for almost a year now. He plugged it in a couple days ago and it produces this 120 Hz buzzing, even without an instrument or cable plugged into the amp, and even with all knobs set to 1. My first thought was that it was a grounding issue, but we checked out the inside and everything's grounded properly, and nothing's wrong with the IEC cable. I did a scan over the board to check for any damaged components and made sure everything was plugged in properly. After looking around online, some people are saying that there's either DC on the output of the amp due to a bad output transistor, or it could be caused by a bad filter capacitor. I haven't taken the cover off the front of the enclosure to see whether or not the speakers were moving, but I can feel them vibrate when I touch the backs of the speaker cones, so I don't think there's DC signal getting sent to the speakers. I'm also not the best when it comes to electronics, especially amps, so this one's really got me stumped.
I should also add that the buzzing doesn't change when any of the pots on the amp are turned and regardless of whether or not a cable or instrument is plugged into the amp.
If anyone could provide some insight as to what might be the cause of the hum and/or any possible way to fix the amp, that would be appreciated.
Here's a link to the schematic of the amp:
http://www.amparchives.com/Amp%20Archives/Fender/Schematics/Fender%20Frontman%20212R%20Schematic.pdf

Loudthud

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Re: Fender guitar amp started humming, can't figure out why
« Reply #1 on: April 27, 2015, 05:03:55 PM »
Check for a broken solder joint on one of the filter capacitors.

Enzo

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Re: Fender guitar amp started humming, can't figure out why
« Reply #2 on: April 27, 2015, 08:14:03 PM »
Solid state amps that hum REAL loud are usually either missing filter on one power supply, or they are pushing DC out to the speaker.  Both of those makes a loud hum.  The reason you hear a loud hum is that the speaker cone is vibrating.  That is how it makes sound regardless of why.  So you will feel the cone vibrating either way.

Can you not see the speaker cones from the back, since you can access them with your fingers I assume so.  Use a flashlight if necessary, but WATCH as you power on.  The speakers either move one direction and stay there (even though vibrating) or they do not. 

You can also just connect your volt meter to the speaker leads and see if it says 30-40v of DC or not.

I agree with LT, a very likely problem is cracked solder on one or both of the main filter caps.

brodie1600

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Re: Fender guitar amp started humming, can't figure out why
« Reply #3 on: April 28, 2015, 12:32:53 AM »
I took a close look at the speaker and watched it kick back and stay there when I powered the amp on, so it's definitely DC at the output. Someone told me it could be caps 71/72 or 75/76 that need to be replaced, or D53-56 might be shorted. Since I know that the DC is the issue, is there any other component in the schematic besides those that might be the culprit? I'll have to get my hands on a multimeter to check the offset voltage and to check for shorts in the diodes, but would it be safe to say that replacing all four caps would solve the issue?

DrGonz78

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Re: Fender guitar amp started humming, can't figure out why
« Reply #4 on: April 28, 2015, 04:27:07 AM »
At this point you need to disconnect the speakers from the amp as you do not want DC voltages on the speakers. The DC voltage will burn up the speakers and then you will need new speakers too. So do not power up with speakers and then confirm the DC voltage by measuring with volt meter across the speaker terminals. It is not safe to say that replacing the filter caps will fix anything at this point. In fact a failed transistor(s), more than likely, could be the problem too. Foremost it would be best to know for sure what DC voltage is on the output of the amp first. Solid state amps are not like tube amps and do not require a load (speaker) when turned on. Also research light bulb limiter on this page too, especially if the amp starts blowing fuses.

http://www.ssguitar.com/index.php?topic=2093.0

“A person who never made a mistake never tried anything new.” -Albert Einstein

Roly

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Re: Fender guitar amp started humming, can't figure out why
« Reply #5 on: April 28, 2015, 11:38:29 AM »
Quote from: Loudthud
Check for a broken solder joint on one of the filter capacitors.

Quote from: Enzo
I agree with LT, a very likely problem is cracked solder on one or both of the main filter caps.

Agreed - a simple test for a common problem.

- bright lamp and a lens
- identify the solder connections for C71 and C72 (look okay, or is one cracked?)
- gently push on each cap as you look at the joints with the lens
- any movement means it's bad and needs to be re-soldered.

If so that's likely to be the end of the story.

If not;
Get your meter out and measure the DC voltage across the output and post it here.


Quote from: brodie1600
Someone told me it could be caps 71/72 or 75/76 that need to be replaced, or D53-56 might be shorted. Since I know that the DC is the issue, is there any other component in the schematic besides those that might be the culprit?

"Someone"?  Caps C71 and C72 are where we are telling you to check for cracked solder joints first.  That is more likely than one of them being duff ('tho that's still possible, but we check the simple main chances first).

C75 and C76 are the bypass caps for the preamp, but your someone has overlooked, or doesn't understand, the role of these caps or zeners D56 and D58.  The most likely result of either of these caps going open wouldn't be hum, it would be preamp instability, oscillations, putt-putt motorboating.

D53-D56 are the main bridge rectifier diodes.  Typically it's one goes and they all go, puts a dead short circuit across the power transformer which then either blows fuses or suffers a rapid smoking death.  If a rectifier is shorted it doesn't rectify any more so you don't get DC.  Again someone who has never actually repaired an amp with a shorted rectifier and is having wild shots in the dark.

Other components?  Sure, where would you like to start?  The output transistors, the driver transistors, the pre-driver transistors ... but they could all be innocent and it's just a cracked joint on C71 or C72.  This is a particularly complicated power amp circuit with quite a lot of protection circuitry, excessive in my view, and will take some care and accuracy to service, but it's possible because I did a curly fault in one of these by e-mail earlier in the year.

We will get to the transistors if that's where the symptoms lead, but first we verify the power supply, the first step in all repairs - "have we got good power?".

Quote from: brodie1600
would it be safe to say that replacing all four caps would solve the issue?

Quote from: DrGonz78
It is not safe to say that replacing the filter caps will fix anything at this point.

No it would not be safe to say.

This is called "scattergun" servicing, picking components at random and replacing them, a favorite of the clueless forums but it doesn't work.  Professional techs (who you are talking to here) first diagnose the fault, then replace the faulty components, promptly and accurately repair the amp, and stay in business.


Investigate the main cap solder joints and report findings.   :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.

Enzo

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Re: Fender guitar amp started humming, can't figure out why
« Reply #6 on: April 28, 2015, 02:50:21 PM »
The filter caps would have been a good suspect, but you found DC on the output.  That DC comes from the power supply, so we no longer suspect the power supply.  You have a problem in the power amp output section.

As Roly said, replacing those caps will do nothing, and your rectifier diodes are also not at fault.


It could be as simple as a shorted output transistor, but that often blows fuses.  Simple enough to check with an ohm meter.  But it could also be a smaller component directing the outputs over to DC.

Oh, while unlikely, it is possible you are missing one of the main supply voltages.

Roly

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Re: Fender guitar amp started humming, can't figure out why
« Reply #7 on: April 28, 2015, 06:34:17 PM »
Quote from: Enzo
but you found DC on the output

I'll wait for that, and the supply voltages, to be confirmed by actual measurement with a voltmeter - it's only implied at this stage.
If you say theory and practice don't agree you haven't applied enough theory.

brodie1600

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Re: Fender guitar amp started humming, can't figure out why
« Reply #8 on: April 28, 2015, 11:50:06 PM »
Thanks for the help so far.
Here's a link to some photos I took: http://imgur.com/a/WFPFq#0

There's an offset voltage on the output of -1.19 V. The diodes pass current just fine. I checked the solder joints for each capacitor, and they seemed to be making contact just fine, but I noticed a possible short on C75 - you'll see it in the photos. I also measured AC/DC voltage through each capacitor:

42 VDC / 92 VAC across both C71 and C72
15.6 VDC / 32 VAC across C75
16.3 VDC / 35.7 VAC across C76

I hope this information is useful. What would be my next course of action here?

Edit: I was also unaware that there should only be DC flowing through the caps. I provided the AC readings in case they proved useful, but they're most likely not.
« Last Edit: April 29, 2015, 12:56:50 AM by brodie1600 »

galaxiex

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Re: Fender guitar amp started humming, can't figure out why
« Reply #9 on: April 29, 2015, 12:09:03 AM »
Thanks for the help so far.
Here's a link to some photos I took: http://imgur.com/a/WFPFq#0

There's an offset voltage on the output of -1.19 V. The diodes pass current just fine. I checked the solder joints for each capacitor, and they seemed to be making contact just fine, but I noticed a possible short on C75 - you'll see it in the photos. I also measured AC/DC voltage through each capacitor:

42 VDC / 92 VAC across both C71 and C72
15.6 VDC / 32 VAC across C75
16.3 VDC / 35.7 VAC across C76

I hope this information is useful. What would be my next course of action here?

WOW! I see lots of very dodgy looking solder joints on that board!
Never mind the ones of interest that you have marked in the pictures!

*My* first course of action would be to re-flow/solder almost every joint on that board!
If it ain’t broke I’ll fix it until it is.

Enzo

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Re: Fender guitar amp started humming, can't figure out why
« Reply #10 on: April 29, 2015, 04:05:02 AM »
What kind of meter are you using, those AC readings on top of the DC readings seem unlikely to me.  I think your meter does not like to measure AC with DC present.

As to your short, it just looks like two joints close together on the same trace to me.. If you are not sure, simply clean the solder off the two joints and see if there is a trace between the two points.

Roly was right, if you only have 2v DC offset at the output, your output stage is not likely blown.

And I agree, get out your iron and resolder all those iffy looking connections.  Looks like some of those caps would fall out if they got a chance.

Roly

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Re: Fender guitar amp started humming, can't figure out why
« Reply #11 on: April 29, 2015, 06:57:14 AM »
Wot dem gentlemines said.   :dbtu:

I'm not real thrilled by that soldering either.  Dirty wave solder bath I'd guess.



The two joints arrowed are typical of the sort of cracks that can occur with larger components (or external sockets) where the joins can get strained and then crack;


The joints on the main filter caps don't appear to be cracked, but they don't look too darn hot either.

The scarring around C75/76 look like a bit of rough rework to me, perhaps somebody already replaced these at some point (but I suspect they were on the wrong tram), or maybe just a rough operator post-solder wave fit.

No, I don't think that's a short either, but check as Enzo advises.


So what's going on with your voltage readings?

The DC readings seem okay but the AC readings on the same points are way off.

The reason is that most less expensive meters cannot read AC accurately when there is also DC present, they see the DC as a much higher AC voltage.

The way to measure AC ripple on a DC rail with such a meter is to use a small cap, say 0.1uF, in series with the meter probe.  This blocks the DC but passes just the AC to the meter.


However, given;
Quote from: brodie1600
42 VDC / 92 VAC across both C71 and C72
... I don't think there is a crack on either cap, this is what I'd expect if the PSU was okay, and that the problem lies elsewhere.



A snippit of theory;
Quote from: brodie1600
I was also unaware that there should only be DC flowing through the caps.

Ah, no, caps are insulators so DC doesn't flow through them.

These main filter caps also used to be called "reservoir" caps because they act as storage pools for electrons.  Every half-cycle of the mains the rectifier dumps a bucketload of electrons into the cap and its voltage rises a bit (Q = CV).  The cap provides a steady outflow of electrons to the output stage (Q = It).  Therefore there will always be some voltage ripple during this charge/discharge process, depending on the size of the "reservoir", bigger cap = less ripple (but it never gets to zero ripple no matter how big the caps are).


(this is either half of your power supply.  The dotted line shows the output if the cap is disconnected, pretty serious hum)

These charge/discharge cycles do produce an AC current through the cap we call ripple current which works out to be the same as the average DC output current.  If you want to investigate the physics of how an AC current can flow through an insulator look for capacitor displacement current.



Very loud hum doesn't automatically mean that there is something wrong with the power supply.  This can also be caused by the output stage drawing excessive current due to a fault such as a blown transistor.  This is why we all said "no" when you asked;
Quote from: brodie1600
would it be safe to say that replacing all four caps would solve the issue?

Unless it turns out to be something simple such as a dud solder joint (which are fairly common) the output stage has many more components and has to be a serious suspect.


{Meanwhile, in a parallel Universe; if you had followed the advice of "someone" you would now be down two large caps, two small caps, four heavy diodes, and the time to fit them, none of which you can ethically charge to the client because there was nothing wrong with them - dead loss.  And you've still got the fault!}


(Movin' right along...) So we've got -1.19V on the output now as our main (and only) symptom.

{did I say I wasn't mad about this output stage?  :( }

This circuit contains a number of fusible resistors marked "Fu<value>".  In the pre-driver R87, R88, R89, R90.  in the driver R102, R104, R106.  These would be the next obvious and easy thing to check.

You can try measuring the resistance of each in situ and should get reasonable results, but it would be more useful to us if you could make up a list of the voltages at either end of each of these resistors.  That will give us a scatter of voltages across the output stage which may help to paint the picture.

Test conditions; all controls minimum, speaker disconnected.
If you say theory and practice don't agree you haven't applied enough theory.

brodie1600

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Re: Fender guitar amp started humming, can't figure out why
« Reply #12 on: April 30, 2015, 02:46:33 PM »
For R89, 90, 102, 104, and 106, the values changed when I flipped the leads. The values did not change when I did this for R87 and 88. Here's a link to some pictures I took, as well as the values I recorded from my multimeter, measuring both resistance and voltage: http://imgur.com/a/iN7Zz#0
"+ top(left) / - bottom(right)" means the positive lead of the multimeter made contact with the top(left) lead of the resistor, with the negative lead making contact with the opposite end as determined by the pictures of the top face of the board. The same goes for the opposite column.

Roly

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Re: Fender guitar amp started humming, can't figure out why
« Reply #13 on: April 30, 2015, 05:32:35 PM »

R87  0V
R88  0V
R89  1.5mV
R90  14mV

R102 45mV
R104 47mV
R106 1.03V

1.5mV across R89, but 0V across R87.

If the current through R89 isn't coming through R87 where is it coming from?   ???

On the strength of these voltages I'd test Q11 out of circuit ... but I'm dubious because it seems like a rather odd failure mode - an open emitter with a leaky collector.  Not impossible, but ...


{thinking out loud;
In this amp the natural state of the upper and lower output "cells" is on (possibly the reason for the excessive protection circuitry, Q12-17).

The upper cell is turned on by R91 and R91 and this current injection is shunted through D28.

Simlarly the lower cell is turned on by R99 and R100 and this current injection is shunted through D29.

These voltages suggest that there is nothing wrong with the output transistors or drivers, but things get strange when we get back to the pre-driver Q11.}


I must confess that I'm at a bit of a loss at the moment.

BRAINS TRUST!  HALP!  Gitin'ere, we need more smarts on this one.


brodie1600 please double check your voltage measurement across R87.

Also please measure and post the voltage to ground at the collectors of Q9, 10, and 11.

If you say theory and practice don't agree you haven't applied enough theory.

brodie1600

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Re: Fender guitar amp started humming, can't figure out why
« Reply #14 on: May 08, 2015, 06:07:31 PM »
Voltage across:

Q9: 40.3 V
Q10: 41.4 V
Q11: 82.4 V

I tried measuring the voltage across R87 for about 15 minutes. Every now and then I get a reading on my multimeter, but I don't know if it's reading the voltage across the resistor. It'll read around 12 mV intermittently. 90% of the time, the multimeter will read zero. I attached some pictures here: http://imgur.com/a/DmXuw#0
The leads are definitely making contact. I don't understand it either.