Is this thump normal?

[pyonc]

With a dummy load, (aka 8 ohm resistor) attached to the output, you also attach the leads of the Fluke to the output. Depending on polarity of the DC offset voltage, you may need to reverse the leads on the Fluke.
Baron030

Thanks a lot for your kind explanation.
As I'm not that familiar on this tech thing, can you identify the output with something like an arrow on the circuit board?

By the way, in the manual on D.C. at output section, I see:

1. +_ 0.1 volt normal
2. + _3 volts adjust R9 for 0 volt D.C.

So, I guess you're right. R9 is the DC offset voltage adjuster.
Attached is a brighter pic of SE408S where once can see the R9 more clearly. I see sort of bolt-like cross in the center, which I think is intended for adjustment, right?

[Baron030]

Hi Pyonc
Well, maybe I am not very clear in my explanations. Fundamentally, we are testing to see just how much DC voltage would be going to the speakers when no input signal is present. So, the “outputs” are on the back panel and are called “Channel A Transducer Terminals” and “Channel B Transducer Terminals”. And rather than risk some irreplaceable JBL voice coils, an 8 ohm resistor will fill in as a dummy load for this test. And the Fluke is measuring the DC voltage across this 8 ohm dummy load resistor. If the DC voltage measured across the “Transducer Terminals” (aka dummy load) is under 0.1 volt DC than the amplifier is good working order. But, if it is higher than 0.1 volt than the R9 trimmer pot needs to be turned a little bit to lower the DC voltage helpfully to zero. The bolt-like shift with the cross in the center is what will be needed to be turned and you will find that a Phillips head screwdriver will fit into that cross nicely. While monitoring the DC voltage across the load resistor, if you turn the R9 shaft the one direction and the DC voltage may go up. And if you turn it the other direction than the DC voltage may go down to zero and if you continue turning in that direction the DC voltage will then reverse polarity and then go back up again. So, you will need to turn the R9 shaft back and forth, until you find that sweet spot where the DC voltage will be very close to or at zero volts. In your picture, you can see that someone in the factory marked R9 trimmer pot with a felt tip marker, so that one of the cross points is pointing to that mark, which was the sweet spot when you amp was manufactured. Over time, if a component has changed in some way, then you can see where R9 will need to re-adjusted. And then you repeat the whole process again for R59, which is for Channel B.
Baron030

[pyonc]

Hi Pyonc
Well, maybe I am not very clear in my explanations. Fundamentally, we are testing to see just how much DC voltage would be going to the speakers when no input signal is present. So, the “outputs” are on the back panel and are called “Channel A Transducer Terminals” and “Channel B Transducer Terminals”. And rather than risk some irreplaceable JBL voice coils, an 8 ohm resistor will fill in as a dummy load for this test. And the Fluke is measuring the DC voltage across this 8 ohm dummy load resistor. If the DC voltage measured across the “Transducer Terminals” (aka dummy load) is under 0.1 volt DC than the amplifier is good working order. But, if it is higher than 0.1 volt than the R9 trimmer pot needs to be turned a little bit to lower the DC voltage helpfully to zero. The bolt-like shift with the cross in the center is what will be needed to be turned and you will find that a Phillips head screwdriver will fit into that cross nicely. While monitoring the DC voltage across the load resistor, if you turn the R9 shaft the one direction and the DC voltage may go up. And if you turn it the other direction than the DC voltage may go down to zero and if you continue turning in that direction the DC voltage will then reverse polarity and then go back up again. So, you will need to turn the R9 shaft back and forth, until you find that sweet spot where the DC voltage will be very close to or at zero volts. In your picture, you can see that someone in the factory marked R9 trimmer pot with a felt tip marker, so that one of the cross points is pointing to that mark, which was the sweet spot when you amp was manufactured. Over time, if a component has changed in some way, then you can see where R9 will need to re-adjusted. And then you repeat the whole process again for R59, which is for Channel B.
Baron030

Thanks a lot, Baron030. Okay, this is the back panel of SE408S. As I've never done this kind of check before, I'd like to double check:




Re output, you're referring to the transducer black and red terminals of Channel A and B, right?
Now, you said "With a dummy load, (aka 8 ohm resistor) attached to the output, you also attach the leads of the Fluke to the output," I still don't get it clearly. For example, if you want to check the DC volt on Channel A, do you mean hooking the 8 ohm resistor leads and Fluke leads into its transducer terminals on Channel A at the same time? Or do you mean hooking the resistor leads first into the terminals, and then Fluke leads also into the resistor leads? How? I see this 8-ohm resistor has leads on its both ends.

[grumpy]

Pyonc,

Please take this with the best of intentions: you should not be doing even this simple procedure if you value your equipment. Take it to someone who is willing to -show- you what needs to be done, then
-perhaps- try it yourself later if you are comfortable with it. There is 120v AC inside that case. It could harm or even kill you. I would tell the same thing to my brother, who has a similar level of electronics understanding. I certainly encourage learning more about electronics and testing, but please get someone local to help you out.

best regards.

[Wagner]

Pyonc,

Please take this with the best of intentions: you should not be doing even this simple procedure if you value your equipment. Take it to someone who is willing to -show- you what needs to be done, then
-perhaps- try it yourself later if you are comfortable with it. There is 120v AC inside that case. It could harm or even kill you. I would tell the same thing to my brother, who has a similar level of electronics understanding. I certainly encourage learning more about electronics and testing, but please get someone local to help you out.

best regards.

Good advice, as well as the fact that DC offset (DC on the taps) adjust procedure is not he same for all solid state amps
Most commonly, it is checked and measured with no load present
As for specific adjustment procedures, the manual should be followed
I am NOT a "tech", but on every solid state amp I have ever performed this adjustment on (those which allowed for it obviously), it was done (measured) by simply placing the DVOM across the output taps with no signal present
Some amps (like KENWOOD) used matched pairs of transistors and provide for no manual adjustment (differential pairs)

[pyonc]

Hummmm..... I get a bit confused, grumpy and Wagner.
Am I supposed to do this test with the unit's power on? Then, you're right.
I just assumed I would do it with no power. No?

[Wagner]

Hummmm..... I get a bit confused, grumpy and Wagner.
I just thought it's a simple procedure as I would be dong this on this amp with no input signals, namely with the power cord pulled out.
With no power on the unit, is this handling of the unit still a dangerous thing?

After reading the post above, I can say with a clear conscience that you need to consult with a known competent technician (or someone that understands how amplifiers work and possesses the practical skills with which to effect safe and competent repairs and adjustments)

You need (based on your posts) someone to help you with this that knows what they are doing

The adjustment being discussed (DC off set) is done with the amplifier powered up, as are most if not all

For what it's worth, MOST no signal cone excursions/"thumps" coming from speakers upon amplifier power "on" or "off", in my experience, have been capacitor or power supply related (needed fresh caps)..............note, I said MOST (I am certain someone will jump in with an exception)

ESPECIALLY when dealing with aging and ancient solid state equipment..........."thump" on power down is a very common capacitor issue

You could also conceivably have a switch concern, but I doubt it based on your video (these will generally manifest as the nasty transient "pop" upon power "on"), but without actually having the amp on a bench in front of me, all of this is nothing but worthless speculation, a losing game

Excessive DC on the taps is like high blood pressure, a silent but deadly killer (of tweeters especially)

[pyonc]

After reading the post above, I can say with a clear conscience that you need to consult with a known competent technician (or someone that understands how amplifiers work and possesses the practical skills with which to effect safe and competent repairs and adjustments)

You need (based on your posts) someone to help you with this that knows what they are doing

The adjustment being discussed (DC off set) is done with the amplifier powered up, as are most if not all

For what it's worth, MOST cone excursions/"thumps" coming from speakers upon amplifier power "on" or "off", in my experience, have been capacitor or power supply related (needed fresh caps)..............note, I said MOST (I am certain someone will jump in with an exception)

ESPECIALLY when dealing with aging and ancient solid state equipment..........."thump" on power down is a very common capacitor issue

Excessive DC on the taps is like high blood pressure, a silent but deadly killer (of tweeters especially)

I see... That's good to know.
As you suggested, I had better stay away from it this time.
Let me take it to a vintage technician nearby.

[Wagner]

I see... That's good to know.
As you suggested, I had better stay away from it this time.
Let me take it to a vintage technician nearby.

Wise move

Basic adjustments and measurements for your personal equipment are something you could certainly learn and become comfortable with if you were so inclined, but are still not something that you want to approach with the random, anecdotal internet advice ("well, when it happened to me it was................(fill in the blank)") sort of poking around when you are trying to solve a specific problem or answer a specific question (you're in the I want it diagnosed/answered mode of thinking) and starting from scratch (zero)

Seen too many guys go down that rabbit hole and in more than a few instances the outcome wasn't a happy one

[grumpy]

I suppose I should add that I am fond of my brother and on good terms

[Doc Mark]

Good Evening, Grumpy, and All,

Many years ago, I build a very large subwoofer for our band's keyboard player, with two JBL 2235H transducers inside it. This large and very heavy unit was then placed into a special recess that he had built into his wall to handle it. The grill frame was flush to the wall surface. My buddy was using 4 smaller JBL Control speakers, one in each corner, for his satellites, and we crossed the sub/sats at around 80-100 HZ, using a Crown VFX-2A electronic crossover. This fellow had bought a huge Carver amp, and a nice Carver pre-amp/tuner to round out the system, with some sort of CD/DVD player, the brand of which I have forgotten.

Upon turn-off, the Carver amp would discharge a huge POP back through the subwoofers, and those two 2235's would literally fly forwards so hard that I thought the cones would rip right off the frames and slam against the far wall!! If you stood in front of the sub when the amp discharged, your the material in your pants legs would move as if blown by the wind!! We swapped out the Carver amp for an old Crown DC 300 that he had, and sent the Carver in for service. My friend got his amp back in super quick time, with a note that everything had been checked, and no problem had been found. By the way, the Crown did NOT discharge like the Carver had done. When we put the Carver back into his system, nothing had changed, and the amp still caused the woofers to fly forward with a very loud POP!! My friend called Carver, and talked to Bob Carver, himself, who asked him to return the amp once more.

To cut to the chase, in the end, there was never a problem found with that Carver amp, and even Bob could not figure out why it was discharging like that, and fix it. I don't recall what electronics my Bandmate ended up using to replace the Carver, but whatever it was, it did not pop like that.

I wonder if DC offset was the problem, and if so, why couldn't Bob Carver have found and adjusted that to cure the problem? Any ideas? I have nothing but respect for the Carver name, and for Bob Carver, personally, whom I believe to be an honest man, and very talented engineer. But, to this day, I still wonder why that specific combination of amp/pre/speakers created a horrible sound like that, which most certainly could have damaged the JBL's had it been allowed to continue. Any ideas? Anyone else have something similar take place with Carver, or other amps? Thanks for your comments and ideas! Take care, and God Bless!

Every Good Wish,
Doc

[grumpy]

If one voltage (of a bipolar supply) discharged faster than the other, for whatever reason,
there are several scenarios where it could cause transient/unintended signals to show up on the output...

There are so many variables in setup/environment it's tough to know why a problem wasn't found
or reproducible elsewhere.

[Doc Mark]

Hey, Grumpy,

Thanks, Dave, for your thoughts. That's pretty much what I thought, especially when Bob Carver couldn't figure it out, either. For what it's worth, I thought my friend's system sounded OK, but that he could have done much better in his choice of satellite speakers. He wanted "small", so that's what he got. But, they always reminded me of Radio Shack Optimus 7 mini speakers, on steroids!! The subwoofer sounded tight, ballsy, and IMHO, fantastic! Too bad the top couldn't have been as solid as the sub. Oh, well....

In the end, my friend sold that house, and let the pair of 2235H's just set on a shelf until their surrounds rotted. He sold them to me for a good price, and I took them to OCS and had them refoamed with JBL parts, and they currently reside in those old 4333 cabinets that you sent me way. I'm kind of keeping them out of harm's way by having them mounted, even if they are not being used right now.

Thanks, again, for the shout, and he, I've got a "BBQ Jones" going. We should try to arrange another lunch, or dinner meeting, when Sweet Bride gets back from her current gigs! Take care, and God Bless!

Every Good Wish,
Doc

[Baron030]

Wow, I am having second thoughts. And I hate to have to say this. But, I really don’t think you have enough knowledge and experience working with electronics to safely test and possibly service your amplifier. So please don’t take this as personal offense. I just don’t think you realize the dangers and the risks you would be taking in attempting to service your amplifier. Adjusting the zero balance requires the amplifier to be running, which means there is 110 volts AC going into the power supply. And since this is a power amplifier, there would also be + and – 46.8 Volts DC on the power rails. And while 46.8 volts does not sound all that dangerous, actually it is dangerous, because across the two power rails the voltages add up to 93.6 volts DC. So, there is a real danger of electrocution. And even when an amplifier is turned off and un-plugged, there is still an enormous amount of power stored up in the capacitors. I have seen screw drivers get arc welded to a chassis when a power cap accidently gets discharged. And dead shorts are really hard on capacitors as it tends to lead to catastrophic failures.

And there is one other risk with adjusting R9. R9 is a wire wound trim potentiometer and its wiper has not moved since the amplifier has left the factory. Moving the shaft even slightly could cause the wiper to move from an un-oxidized winding to an oxidized winding. I think we have all experienced changing an L-Pad setting and hearing a tweeter cut in and out as the wiper is going across good and oxidized windings. Well, R9 is not all that different from an L-Pad. It is still prone to oxidation. But, in this case, it is carefully balancing currents in the differential input stage of the amplifier. A wiper cut out of the R9 trim pot could lead to a gross imbalance in the differential input stage and this gross imbalance would then be amplified by the “T-Circuit”. The result could be an excessive amount of current flowing through one of the output transistors. Unfortunately, this could lead to a very loud bang and a puff of smoke and one very dead amplifier. And I suspect it is going to be very hard to find exact replace transistors for an amplifier this old. So, do yourself a favor and have a professional look at your amp.

Baron030

[Wagner]

Wow, I am having second thoughts. And I hate to have to say this. But, I really don’t think you have enough knowledge and experience working with electronics to safely test and possibly service your amplifier.
Baron030

Do not know why you would "hate" to say it? It is honest, helpful and thoughtful advice (not to mention the correct advice) You and others (not attacking your post specifically, just the now all too common phenomena) make it sound as if it were some sort of personal attack or insult to make an honest (and responsible) statement or assessment based on the facts at hand

It is also good advice for someone you do not know personally, as many individuals just do not know when to STOP; that their knowledge of a specific field or trade is lacking (and that said same lacking could pose a potential hazard)

Unless of course the OP is interested in doing some self teaching from on-line resources, the library or textbooks; that possibility was mentioned (and plausible) except from a time perspective if he were hot for a quick resolution

The OP came to that realization and expressed it in post# 23

It is not as if you, or anyone, were insulting the guy

We all know what we know and do what we do

It is nothing to apologize for

What has this World come to, in which one has to quantify and qualify every single objective (and honest) remark made for fear of "hurting" someone's "feelings"? I am happy to report that I do not feel this is the case with this OP

The internet's vast resources of information does not qualify us all in all trades and professions although it falsely makes many think that it does (auto repair forums are some of the most entertaining)

I, for example, do not perform my own eye exams, teeth cleanings, minor surgeries or FM alignments

"Political correctness" and "sensitivity" is going to be the death of us all