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 (http://www.amparchives.com/Amp%20Archives/Fender/Schematics/Fender%20Frontman%20212R%20Schematic.pdf)
Check for a broken solder joint on one of the filter capacitors.
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.
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?
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
Quote from: LoudthudCheck for a broken solder joint on one of the filter capacitors.
Quote from: EnzoI 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: brodie1600Someone 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: brodie1600would it be safe to say that replacing all four caps would solve the issue?
Quote from: DrGonz78It 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:
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.
Quote from: Enzobut 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.
Thanks for the help so far.
Here's a link to some photos I took: http://imgur.com/a/WFPFq#0 (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.
Quote from: brodie1600 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 (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!
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.
Wot dem gentlemines said. :dbtu:
I'm not real thrilled by that soldering either. Dirty wave solder bath I'd guess.
(https://learn.adafruit.com/system/assets/assets/000/001/978/medium800/tools_Header_Joints.jpg?1396777967)
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;
(http://www.agthompsonfamily.com/vw/DoorLocks9.jpg)
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: brodie160042 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: brodie1600I 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).
(http://www.ratopx.com/imagens/diode23.gif)
(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: brodie1600would 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.
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 (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.
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.
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 (http://imgur.com/a/DmXuw#0)
The leads are definitely making contact. I don't understand it either.
When you say "Voltage across:" what does that exactly mean? The transistors have 3 legs so is it base to emitter, base to collector or emitter to collector? Roly asked for voltages in relation to a chassis ground reference. How are you measuring those transistors?
Also if we are not measuring any voltage across the R87 then that really implies that the resistor is open. UPDATED: Oh I see how those are low resistance fusible resistors. So yeah not much of a voltage drop to measure there... Try grounding your black probe and measure either side for the voltage with the red probe.
Foremost please let us know exactly how you are testing voltage with your meter using a ground reference. Just got to clarify for safety reasons IMHO.
look at your schematic. The power rails are 42v each, adding up to 84v. You report you have 84v across Q11. The only way that possibly makes sense is if you mean emitter to collector, written E-C. 84v ther tells me the transistor is not conducting AT ALL. And that means zero current through it. Since R87 current can only come through the resistor, that mean there is zero current through it as well. Ohm's Law tells us that zero current through any resistor will mean there is zero voltage drop across the resistor. So with 84v across the transistor E-C I would expect to see zero volts across R87.
Why would ther be zero conduction in Q11? The Q11 could be open. Or it could be not turned on at its base. That base has to be more negative than the emitter by half a volt or so for the thing to conduct, so is it? If that base is indeed more negative, then Q11 is likely bad. If the base is not, then we would look to Q10 for answers.
An open R87 or R89 could also shut off current through Q11.
Yes, either of those would remove current from Q11, however, if one of those were open, you would not get 84v E-C on Q11.
Sorry, misunderstood when Roly asked me to measure the transistors.
Q9 from collector to chassis ground: 440 mV
Q10 from collector to chassis ground: sits at 7 mV for the most part, but fluctuates intermittently to 0 mV and 24 mV
Q11 from collector to chassis ground: 42 V
Also measured R87 as DrGonz78 suggested. Multimeter read 42 V, measuring both sides of the resistor to chassis ground.
In general, when we are measuring DC voltages in a power amp, a few millivolts don't matter.
I'd be trying a new Q11 at this point.
Quote from: brodie1600Q11 from collector to chassis ground: 42 V
Quote from: EnzoI'd be trying a new Q11 at this point.
Me too.
In the same way that the collector of Q9 is sitting about one diode drop
above the half rail;
(400mV + V
D28 - V
BEQ18 - V
BEQ20)
so the collector of Q11 should be sitting about one diode drop
below the half rail;
(-400mV - V
D29 + V
BEQ18 + V
BEQ20)
The fact that it isn't suggests that it has failed shorted C-E.
I'd guess open, as there is 84v across the C-E. Shorted would imply zero volts C-E. Either way - bad.
Quote from: brodie1600Q11 from collector to chassis ground: 42 V
Quote from: Enzoas there is 84v across the C-E
These are in contradiction. He can't have the 42V +ve rail on the collector of the VAS
and have 84V across the VAS C-E. I'm uncertain how the "84V" measurement was made, there was some confusion about "across", but I suspect it was VAS collector to -ve rail, which would be consistent with 42V to ground, and a shorted VAS.
Whatever, Q11 looks SNAFU. The question now is what do we replace a 2SA1016K with? (150V/50mA/400mW) An MJE350 should do. {In Oz most retailers don't carry 2Sx series, you have to go to the equipment manufacturer for a "genuine spare part" which costs $40 for a $4 part, or substitute.}
I guess the mists will clear.
apologies to all, I had Q11 upside down in my mind, forgetting a moment it was PNP. I pictured +42 on the top lead and -42 on the bottm. I now see the E and C are reversed of my thinking.
I believe we earlier clarified the 84v he was reading was E-C. But if V+ is on the collector, well now it gets confusing...
If no 2SA series are available, suitable subs might be 2N5401 or MPSA92 o r93. Those are common enough in the USA, Just be aware the legs are in different order.
Suburban electronics has 2SA1016 in stock.
New 2SA1016 came in the mail and I soldered it to the board and reconnected the speakers. Still getting buzzing. Still measuring 42 V from collector of the new Q11 to ground.
Please measure EBC voltages to ground on Q11, all three points. One thing that bothers me is that the collectors of Q9 and Q10 should be around 39v if I am understanding this correctly. It would be beneficial to do voltage checks on Q9 and Q10 as well. What voltage drop is occuring across each D21-24.
Also note that testing a transistor after pulling it out for replacement is easy. You could have tested the old one compared to the new one to see if there was any gross differences. Perhaps that is not necessary but it is something I have done as way to learn bettter.
Don't need to hook speaker up until you measure only millivolts on the output.
Transistor readings are each point to chassis ground:
Q9
E: 1.47 V; C: 42 V; B: 3 mV (essentially nothing)
Q10
E: 1.9 V; C: 42 V; B: 5 mV (essentially nothing)
Q11
E: 42 V; C: -43 V; B: 42 V
Diode measurements:
D21: 1.12 V
D22: 1.16 V
D23: .29 V
D24: 1.47 V
Okay now I think I can see, possibly where we are making mistakes. I think you might be confusing the transistor base legs for the collector legs. The -43v on the base of Q11 makes no sense to me and perhaps that is really the collector. Now you have written 42v on the emitter and collector of Q11 and -43v on the base of Q11 would also mean there is -43v on the collector of Q10. That is wrong.
What I think is going on is that Q11 is really E: 42V B: 42V C: -43V. Same with Q9 and Q10 that you have the bases and collectors flipped flopped. Lets start there and find the datasheets for those transistors. Once you figure this out the whole thing should be making way more sense.
Also that reading across D23 might be suspect.
EDIT: I attached the pinout for those transistors. Please check to make sure you are reading them correctly.
Just as I thought look at my edited post for the correct pinout of those transistors. Here is the actual data sheet. We can explain how pin outs are different in the US, Europe and Asia later.
Edit: Now type up the correct voltage chart on your next post and lets start there.
Alright, I understand now. I edited my last post, so the measurements should be correct this time.
Okay that makes sense finally. For the life of me and everyone here, we were brain farting on how that collector of Q11 could have 42v and not negative voltage. So I would first unplug the amp and test D23 to make sure is not shorted. Do you know how to test a diode with your meter?
I don't have a diode test setting, so I measured resistance across it. It was 13.5 kilohms, no measurement the other way.
Try measuring the resistance of the other diodes D21-24 to see a difference. Also try reversing the probes as well to see if there are any differences.
These were measured with the negative lead to the cathode, and positive lead to the anode:
D21: 13.7 kilohms
D22: 13.7 kilohms
D23: 13.7 kilohms
D24: 12.85 kilohms
Couldn't measure anything with the leads flipped.
Well that looks fine actually and just had to make sure. I have been reading up a bit on this amp and there is many problems with this amp. First off the mute circuit causes many problems. So again measure the DC voltage on the speaker output, but this time insert a guitar cable in the input jack. We want to see if this makes any changes to the DC voltage on the output of the amp.
Measured the voltage on the output again and it read -400 mV. This is strange because right after I installed the new transistor, it was still delivering the same hum to the speakers. Output voltage measures 10 mV with a guitar plugged into the amp. I plugged the speakers in and played the guitar a bit and got sound from the amp.
Safe to say that it's fixed? Or should I check anything else before putting everything back together?
So the hum is gone? Now unplug guitar cable with the amp just at idle, is the hum there now? Even better hook up the voltmeter to observe voltage on the speaker while you unplug the guitar cable from the jack.
Yeah, no more hum. Just normal amp hum. Had the leads on the speaker outputs and watched the voltage drop from 10 mV with the guitar plugged in, to -400 mV when it got unplugged.
Well that is as good as it gets and it seems to be back to normal. Still there is a concern that Q11 had nothing to do with the problem and as you said it did not change the hum earlier. Something triggered this problem and something triggered it to go away. Thus meaning the problem could return. I would put the amp back in the cab and play it for a while. Also, get a rubber mallet or use your fist to hit the top of the amp to see if the hum comes back. In other words this could still be an intermittent problem down the road.
That's true. All I remember is that I still heard hum at one point after I installed Q11, but not anymore - the amp seems to be working fine. I'm betting it was Q11 that was causing the issue.
Everything's put back together and we're testing all the knobs and buttons.
If the issue comes back, I'll know it's an intermittent issue caused by something else, and I'll post another thread on the forums.
You've all been a great help, and you saved my friend and I loads of money. And I learned a thing or two in the process. We're both real grateful - thanks again everyone!