I've included two images. Amp belongs to a friend. There are three leads from the two caps on the board and only two are being used. To me this doesn't look like a dual cap, so I don't understand why there is an extra lead. It's not used in the amp and the amp works just fine. ??? Thanks!
Well, first it looks like a part someone cobbled in there at some point.
That is a physically large part. If soldered to the board standing up as intended, it is tall and heavy. Larger caps like that often include an extra dummy pin or two for stability. it usually has a solder pad on the board for the extra leg, but it doesn't connect to anything. It is soldered to provide extra strength. A lot of makers also run glue around the bottom of the cap to glue it to the board for additional strength.
There ain't no way that cap is original, and I'm none too impressed by the workmanship - or lack of it. Caps of this size must be securely mounted or they will just rip themselves free with normal transport vibration and impacts.
The third leg is an isolated support pin, and in some caps it is more physically robust than the electrical connections.
Thanks Roly and Enzo. Good info moving forward. Caps were definitely bent to fit into chassis--they can't be stood up as they would stick about 1.5 " above chassis. Not sure what the orginal caps were but I will find out to satisfy my curiosity.
Questions: The graphic equalizer faders on this amp are in rough shape, full of static, seeming to lose their smoothness and "sticking" ability once pushed downward to their lowest setting (they suddenly speed up as if freed). There are four and one of the far left cuts out the signal when pushed down to the bottom. There are de-greasers, de-oxit products on the market, but they are not cheap! What are the best products in your opinions when it comes to a good FaderLube and contact cleaner. And what would you do, in terms of a fix, for these tired old faders (aside from trying to buy new ones which SWR no longer provides. Didn't have much luck so far trying to locate any).
I live in Canada.
Thanks again!
Does anyone know where I can find a schematic for the power amp section of an SWR studio 220 bass amplifier?
Also looking for a decent pre-amp schematic of for the SWR220--the ones I've uncovered so far are blurry when enlarged and hard to see otherwise. You think SWR would have a decent archive of their schematics!
There are a few power amp versions, what are the output transistor types?
Preamp schematic attached.
Deoxit makes a fader lube, as you say it is expensive. Other types of cleaners tend to change the feel of the faders. In Canada you can find the deoxit sold by Hosa accessories at music stores. Alternately you can take the faders apart, use regular deoxit or equivalent, then lube the moving parts with white lithium grease or similar.
I'll just put both power amp schematics up anyway. Might come in handy for someone in the future :cheesy:
http://www.eserviceinfo.com/downloadsm/58493/SWR_basic%20black.html (http://www.eserviceinfo.com/downloadsm/58493/SWR_basic%20black.html)
Thanks g1 and Roly! That preamp schem. is much clearer than the one I have!
Question about eservice. I was nervous about downloading from there as it looks like they will somehow be part of my chrome browser and it would be tough to get rid of it if I didn't want it anymorel Is this a safe site?
Question about the white lithium grease. Is this conductive? Wouldn't it mess with resistive tracks of the faders?
It's non-conductive in it's normal state. But you don't want it on the tracks anyway. Just where you have mechanical contact of moving parts (aside from the tracks and their contacts). The tracks you would clean with a regular contact cleaner that contains a lubricant, whatever you are using for regular pots.
Quote from: HawkIs this a safe site?
As far as I know, but then I'm using Firefox. I didn't see any obscure "I'm happy for you to hijack my machine" tick boxes to untick. It downloaded directly for me, but I am cautious about sites that send you a "downloader" first because they often try and spin other (unwanted) software on you in the process if you don't pay attention and untick the default extra downloads.
Try Googling "SWR basic black" for alternatives.
Talking of safe, it is good practice to treat
all of these compounds and chemicals you use, lubricants, thermal pastes, contact cleaners, etc, as if they are toxic, don't get them on your skin and don't eat or smoke until you have given your hands a good washing. They are not particularly dangerous, but they won't do you a lot of good either.
:)
Speaking of caps: I tested both 6800uf caps with my blue esr meter and found one to have a high reading. There is a fair bit of hum from the amp. I will probably go ahead and replace both. However, I do not have a power supply schematic for this amp (don't think one exists). Any suggestions for a "correct value" to use as replacements? There is probably a formula to figure out how to determine proper cap size in the power supply but I'm not sure how to do that....thanks.
Best to stick with the value that is in there. If they are running at a voltage too close to the rated voltage, then you should go with a bit higher voltage rating.
And higher temperature rated caps (105C instead of 85C) should last longer.
Quote from: Hawk on February 25, 2015, 10:14:56 AMAny suggestions for a "correct value" to use as replacements? There is probably a formula to figure out how to determine proper cap size in the power supply but I'm not sure how to do that....thanks.
mmmm ... yeah.
You can take the rail voltages (+/-55V), or nominal output power rating if known, and the output impedance, and work out the maximum RMS output current, then select your filter caps to have
ripple current rating higher than this ('tho you might find this an elusive figure for most caps).
For example the ST-220 is specified as 220 watts RMS @ 4 ohms.
P = I
2R
(P/R)^0.5 = I
(220/4)^0.5 = 7.4 amps rms
http://au.mouser.com/Search/Refine.aspx?N=12013589 (http://au.mouser.com/Search/Refine.aspx?N=12013589)
Crap. I stopped by my shop recently and meant to take a thumb drive of all my schematics. Unfortunately I took it back out of my pocket and put it back to look something up, then left it there. I am sure there is a schematic for it. Perhaps using the correct value will allow the caps to fit on th board instead of keeled over like that.
Before just throwing new caps at it, you might check to see if the cap is really the source of the hum. If only one of the caps measured funny, then presumably the two supplies will have differing amounts of ripple. Do they?
Oops! Sorry, when I said go with the values in there, I forgot these were not the stock parts. :-[
The supply schematic attached says they were 4700uF for this model.
I will respond in more detail to these posts soon as I've been very busy but wanted to say thanks a million for the power supply schematic!! Talk about helpful! I was struggling with that. :dbtu:
I do have some time:
Roly, thanks for the mouser link but what temps/load life would you put in? Without all filters I can't "apply filters".
Enzo, specifically, to test for ripple current, using a voltmeter set on AC, where would you put your test leads? (Black to ground, red to?). I'm thinking at the positive end of the furthest supply cap. Is that right? I've read that you don't want more than 2vpp ripple? I have a scope, so wondering if I would do the same thing--probe on far side of scope, scope ground to chassis ground?
Having said that I realize I should also test for dc voltage on coupling caps...Thanks!
Quote from: Hawkwhat temps/load life would you put in? Without all filters I can't "apply filters".
I think you can select "any", but I don't use the company myself, just included it to show an example of ripple current rating.
Quote from: Hawkto test for ripple current
We need to be clear that
Enzo is talking about ripple
voltage which is
across the capacitor sitting on the supply voltage; I was talking about the ripple
current which is
through the capacitor and a function of the load current.
Using your CRO you will need to select AC input coupling, CRO ground to amp ground, then probe as you like.
We can't just say "2V max" because the ripple voltage will also depend on the loading on the supply. When there is no load there won't be any ripple at all, at idle a little, and moving on up to full output the ripple voltage will get larger as the caps are discharged more by the amp between being refreshed via the rectifier. This ripple voltage will be a sawtooth, so the most sensible way to measure/quote it is peak-to-peak.
Thanks Roly. So when you say: "When there is no load there won't be any ripple at all, at idle a little, and moving on up to full output the ripple voltage will get larger as the caps are discharged more by the amp between being refreshed via the rectifier"
Do you mean the speaker load and actively playing the bass as I watch my scope? I suppose I could inject a signal into the amp with speaker attached and probe from there...definitely easier hands free...make sense?
At the top of the power supply stage schematic, on the +/- 15 power supply, we have two IN4004's, one on the neg and one on the pos. rail. If we already have a neg and positive voltage coming off the rectifier why do we need these diodes? Voltage dropping? And then what is the purpose of the zener diodes at the far end? Thanks for your patience!
That simple supply creates +/- 15V out of main rails, which are much higher, between +/-6- to 75V .
Voltage is dropped through power resistors R1/R2 and voltage is stabilized at +/- 15V using Zeners 1N4737 , which must be 7V5 each .
I guess that the purpose of the 1N4004 diodes is the following:
in many amps, there's a harmless but mildly annoying noise when amp is turned off, a few seconds later you can hear a "squeeeeeiiiiikkkk" type sound, clearly an oscillation.
I guess that when amp is turned off, main rail voltage drops quickly, preamp rails do not because Zeners keep them stiffly at 15V, in a way "n isolating" them from main rails ... until main rails drop below 15V (they can still produce quite a few watts at that level, over 15W :o ) and isolation disappears, R1/R2 are very low value and 15V filter caps are puny, only 100uF ; at that point preamp is directly coupled to main rails which are still feeding a loaded power amp ... the conditions are set for some oscillation or motorboating.
For a couple seconds only, of course, until fully discharged.
The added diodes become reverse biased once main rails drop below 15V and prevent that.
Just an educated guesss, of course.
Thank you. On the pos. leg of both 6800 uf cap I see 40mv peak to peak sawtooth waveform,. So far I don't see the sawtooth anywhere else.
With bass unplugged, Gain at 6, Master at 6, the hum increases into a loud buzz when I turn the Treble to 10. When I crank the Bass to 10 it gives me a very a low "bassy" hum. Any thoughts? Oscillations?
Quote from: Hawk on February 28, 2015, 09:02:07 PM
Thank you. On the pos. leg of both 6800 uf cap I see 40mv peak to peak sawtooth waveform,. So far I don't see the sawtooth anywhere else.
With bass unplugged, Gain at 6, Master at 6, the hum increases into a loud buzz when I turn the Treble to 10. When I crank the Bass to 10 it gives me a very a low "bassy" hum. Any thoughts? Oscillations?
I do hope you mean the +ve end of the +ve supply cap and the -ve end of the -ve supply cap, otherwise one of them is in the wrong way around! (not impossible with a somewhat bodgy retro-fit)
All hum (and noise) tests must be done with the input shorted (to sort any internal hum from external pickup), and with the tonestack set to neutral, 5/10, 12 o'clock.
Are they?
Same here.
With bass unplugged, and nothing else I imagine all sorts of horrid things, you need a shorted plug in the input or nothing plugged in the amp input while your description hints at a cable plugged in the amp input, nothing at the other end ("no bass")
Thanks.
A few things: by shorted you mean attaching a 1/4" jack but with out cable attached? (You're right about the noise being worse with the guitar plugged in. Without anything plugged in the amp, the hum is much better I just want to know that I can reduce the hum as much as possible. Again, increasing either the bass/treble increases the hum considerably. I don't have a new amp to compare it to so I'm not sure how much is too much).
I took the ground lead of the scope and attached it to chassis ground and the scope lead to the positive lead of the cap, then did the same with the other cap. However, now that I look at the schem. I see that the pos of one cap goes to the +ve rail and the neg side of the other cap goes to -ve, so my inclination is to check the -ve as well which I didn't do before. Is 40mv ripple acceptable? Also shouldn't I really be checking at R1/R2 as the caps are supposed to reduce ripple and the result would show at R1/R2? Thanks everyone!
(I have assumed that the caps are wired in correctly but I have will check that out later when I'm home.)
Quote from: HawkWithout anything plugged in the amp, the hum is much better
In most amps the input sockets are arranged to short the input when nothing is plugged in. Attempting to deal with internal amp hum with an open circuit input picking up all the external noise in the world is a bit like running an air conditioner or heater with all the windows and doors open.
Quote from: HawkIs 40mv ripple acceptable?
Under what conditions? With the amp at idle that would be quite acceptable.
Looking at <SWR Studio 220 ps.pdf> there are a couple of filter caps for the main +ve, C1 & C3, and -ve, C2 & C4, supplies. R1 and R2 come off the same points, (A) and (B). Generally speaking we assume wire or PCB traces have zero resistance, so what you measure at the caps should be identical to what you measure at R1 and R2.
These are the supplies for the main power amp which is fairly insensitive to hum/ripple. With the main or master volume turned down whatever hum and noise you can hear will be due to the power amp alone.
The other supplies to check are the +/-15V for the preamps (across the 1N4737A zener pairs) the hum here should be quite low, but similarly the split supplies imply op-amps in the preamp and these are also highly resistant to hum and noise on their supplies; so if you have excessive hum and noise our first guess is that it's getting into the signal path somehow, say due to poor grounding or layout.
So we are not trying to hit a moving target (and bamboozle ourselves) we conduct all tests with all EQ set to flat, 5/10, 12 o'clock, and any Fx off/minimum.
Representative gain and master volume settings would also be 5/10, 12 o'clock.
With the master at 5/10 but the gain at 0/10 you will add the hum/noise contribution of the preamp back end, and with the gain at 5/10 you will add and get and idea of the hum/noise contribution of the preamp front end.
Not too may preamps are totally silent, particularly if they are high gain, but you must remember that hum and noise performance is referenced to
full output power level. This is a high power amp intended to fill an arena and that under those conditions the nearest audience member may be some tens of metres away from the backline and the residual hum and noise quite inaudible. With such an amp I would expect that its hum/noise up close in a quiet bedroom or workshop would seem quite high.
But you can get an absolute handle on it by actually measuring the hum and noise on the output with the gain, master, and EQ controls all set for 12 o'clock. This will give you a representative peak-to-peak value which you can compare to the full output peak-to-peak (the +/-ve supply rails, in this case I think +/-60V, and work out in dB just how far down your hum and noise is on full output. You may discover that it's within a reasonable spec.
dB = 20 * log
10(V1/V2)
Great, thanks. Excellent info.
I'm going to try and drill down here so I understand everything that is being said.
"The other supplies to check are the +/-15V for the preamps (across the 1N4737A zener pairs) the hum here should be quite low" Here, using my scope I found some ripple but very low, approx 20mv. Even with a 50 ma 1khz signal injected the ripple wasn't much more. Acceptable?
"With the master at 5/10 but the gain at 0/10 you will add the hum/noise contribution of the preamp back end, and with the gain at 5/10 you will add and get and idea of the hum/noise contribution of the preamp front end." Tried this but no audible difference. A good thing, right?
"But you can get an absolute handle on it by actually measuring the hum and noise on the output with the gain, master, and EQ controls all set for 12 o'clock. This will give you a representative peak-to-peak value which you can compare to the full output peak-to-peak (the +/-ve supply rails, in this case I think +/-60V, and work out in dB just how far down your hum and noise is on full output. You may discover that it's within a reasonable spec. dB = 20 * log10(V1/V2)"
Okay, this really interests me but not quite sure how to proceed. Do you mean V1 is the 12 o'clock voltage and V2 is the voltage with the gain and master full-on? With the settings at 12 o'clock, and the speaker output attached to a 4 ohm dummy load, and nothing plugged into the amp, I get a value of 13 mv (using my voltmeter). With the gain and master at full volume I read a voltage of 70 mv. Am I on the right track before I start play with the log calculation? Thanks for all your help.
Having listened to and tried to apply everything so far I'd say it's fair to assume that this amp doesn't have a lot of hum, and, at first, a lot of it was coming from my bass plugged into the amp. The controls are dirty so I'll apply some deoxit. Otherwise, at this point, I'd say this amp is doing well.
P.S. How do I insert quotes on this forum? :) Also, how can I save all these posts as they are great?
Enzo said: "Before just throwing new caps at it, you might check to see if the cap is really the source of the hum. If only one of the caps measured funny, then presumably the two supplies will have differing amounts of ripple. Do they?"
On the 250 volt supply I probed at points from the rectifier to the tube and there was no AC ripple. But when I checked the 15 V supply I found ripple on the neg side of the 6800uf cap and on the pos. side of the other, approx. 40 mv. But when I probed R1, R2, and the diodes/zener diodes there was no ripple...all tests done with a 50mv/1khz signal injected. Thanks Enzo!
Look just above the posting box while you write. Just below the FONT SIZE box is a small symbol that looks like a thought balloon like in a newspaper cartoon. THAT is the quote button. If you click on it first, you find the starting and closing things for your quote. Just copy and paste the quote between them.
I prefer to do it the other way. Copy and paste the quote into the box, then go back and highlight the quote and THEn click the quote button. it puts the quote things around whatever is highlighted.
Here, I copied the first sentence, and paste it here:
Look just above the posting box while you write.
Now I will do it again, then highlight it with my mouse, then hit the quote:
QuoteLook just above the posting box while you write.
QuoteLook just above the posting box while you write. Just below the FONT SIZE box is a small symbol that looks like a thought balloon like in a newspaper cartoon. THAT is the quote button. If you click on it first, you find the starting and closing things for your quote. Just copy and paste the quote between them.
Thanks Enzo! (Still hoping for a response to the first part of my post re:hum/noise calculation :cheesy:
'.
Quote from: Hawk on March 02, 2015, 10:25:24 AM
But when I checked the 15 V supply I found ripple on the neg side of the 6800uf cap and on the pos. side of the other, approx. 40 mv. But when I probed R1, R2, and the diodes/zener diodes there was no ripple.
40mV ripple on a main rail is nothing.
You have even less (as in zero) at Zener diodes ends because that's what they do, keep a voltage value
fixed (ripple is an up/down
variation)
Somewhat in a hurry now, later will re-read all the thread to see whether I can offer something :)
So from the pic you can see the heat sink and two power transistors at the front of the pcb, but there are also two at the back, plus others. From the circuit board there are holes so that you can use a screwdriver to remove the transistors. Guess the only way to remove a defective transistor here is to remove the heat sink from the back of the chassis then unsolder all the power transistors/transistors so that the pcb can be removed so that I could change the defective transistor (there isn't one but just wondering). Am I missing something here? Is there an easier way? Should I consider long thin pliers to snip the transistor leads and unsolder the leads? Or would it be best to unscrew the pcb from the heat sink and unscrew all the transistors and re-apply thermal paste to all of them and screw them, including the new one down? Lot of work to change one output transistor.
What works best for you techs out there? What would be the most efficient way?
Also, the red caps. To get a true reading in both the small pre-amp caps and the small power amp caps it looks like I'd have to remove the boards so that I could unsolder the leads and check them with a cap meter? Is there an easier way?
Thank-you!
Holy himm ... WTF?
Oh that is slightly nasty. However, somebody put it together so it can be taken it apart, repaired, and put back together.
I'd investigate the ease of demounting the entire heatsink assembly from the chassis.
I'd do some in situ resistance test, but I wouldn't expect too much of them.
Then I'd solder-suck the major device leads and the thermal comp transistor, and with care the sub-PCB should lift off. The you have the FET's (or whatever they are) stark naked for a good testing.
That's how I'd do it - but why? Why would you want to attack the power amp module while trying to trace a source of hum/noise injection?
(I've lost track...)
Do we have a baseline measurement of hum+noise at the output with the input of the power amp shorted (Main 0/10) ?
Referring strictly to power amp disassembly and not saying that that is the problem to be solved, just stating this for the record and
possible need to repair the power amp in the future:
QuoteSo from the pic you can see the heat sink and two power transistors at the front of the pcb, but there are also two at the back, plus others.
I only see
power transistors in the back of the PCB, which are bolted to the heat sink and with legs bent forward towards the PCB.
This are the only parts which complicate replacement because they are physically joined to 2 different bodies, the heatsink and the PCB.
Those much smaller (probably TO220) in the front, are just
drivers, mounted conventionally on the PCB and with small,
individual, floating mini heatsinks, so no big deal.
Your picture only shows the top, and only useful data we can see is that the PCB is solidly mounted to the heatsink with spacers.
That makes me think that the whole block can be pulled out, as a unit, for further testing and repair.
You'll have much easier access to everything and if needed, a fine tip screwdriver through the PCB dismounts power transistors, then reverse procedure.
An experienced Tech (the kind SWR will authorize to repair it) will repair that amp on the first try, so no big deal cleaning, replacing mica and regreasing; now a less experienced one who might need to disassemble and reassemble the amp 5 times because every time he finds it
still needs something else, will be driven up the wall.
FWIW that kind of construction has a justification: it allows for a far more compact module.
Rodd Elliott's big power amps also are designed that way, the power transistors or MosFets are sandwichwd between the PCB and the heat sink.
I am designing a couple thin "sandwich" power modules myself, in 100/150 and 200/300W size, because the power amp block will be some 4" by 2" by 1/2" thick ... relatively light, cheap and easy to send by mail, either with plug-in or screw terminals, the idea being that if a customer in a neighbouring Country has a problem and is within warranty, I plain send him (one way) a replacement module and even himself, if he's handy enough, or a friend, (not necessarily an Electronics Tech) can replace it in, say, 30 minutes top.
And we avoid moving heavy stuff back and forth, crossing Customs, or my having to setup a Service network (impossible for somebody my size, and even more involving foreign countries) .
So this apparently klunky construction technique, sometimes solves certain problems.
Check how the heatsinks are mounted to the chassis and you have 30% problem solved.
QuoteFrom the circuit board there are holes so that you can use a screwdriver to remove the transistors.
Yes.
QuoteGuess the only way to remove a defective transistor here is to remove the heat sink from the back of the chassis then unsolder all the power transistors/transistors so that the pcb can be removed so that I could change the defective transistor (there isn't one but just wondering).
No.
It is somewhat complex but not that much, see above.
QuoteAm I missing something here? Is there an easier way? Should I consider long thin pliers to snip the transistor leads and unsolder the leads?
See above.
QuoteOr would it be best to unscrew the pcb from the heat sink and unscrew all the transistors and re-apply thermal paste to all of them and screw them, including the new one down? Lot of work to change one output transistor.
That's roughly the way it is.
All in this case is a grand total of
two, not that big deal.
QuoteWhat works best for you techs out there? What would be the most efficient way?
See above :)
Thanks for the detailed answer JM!
QuoteThat's how I'd do it - but why? Why would you want to attack the power amp module while trying to trace a source of hum/noise injection? (I've lost track...)
Sorry Roly, I'd gone on to another question as I was hoping for a reply to my reply regarding my novice attempts to apply Enzo's hum/noise injection statement: " "But you can get an absolute handle on it by actually measuring the hum and noise on the output with the gain, master, and EQ controls all set for 12 o'clock. This will give you a representative peak-to-peak value which you can compare to the full output peak-to-peak (the +/-ve supply rails, in this case I think +/-60V, and work out in dB just how far down your hum and noise is on full output. You may discover that it's within a reasonable spec. dB = 20 * log10(V1/V2)"
Okay, this really interests me but not quite sure how to proceed. Do you mean V1 is the 12 o'clock voltage and V2 is the voltage with the gain and master full-on? With the settings at 12 o'clock, and the speaker output attached to a 4 ohm dummy load, and nothing plugged into the amp, I get a value of 13 mv (using my voltmeter) across the dummy load. With the gain and master at full volume I read a voltage of 70 mv. Am I on the right track before I start play with the log calculation? Thanks for all your help.QuoteDo we have a baseline measurement of hum+noise at the output with the input of the power amp shorted (Main 0/10) ?
Please see my attempts in the above pgh.