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SWR Studio 220 Capacitor Question

Started by Hawk, February 19, 2015, 08:41:01 PM

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Hawk

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!

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.

Hawk

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:


'.

J M Fahey

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 :)

Hawk

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!

Roly

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

J M Fahey

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 :)

Hawk

 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.