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Old 10th July 2011, 07:47 PM   #1
mitchba is offline mitchba  Canada
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Default a40 oscillation

20 years ago I purchased a40 circuit boards and partial parts from Old Colony. I got as far as stuffing the boards, then life happened. Just recently, I was afforded the opportunity to finish the amp Ė see a40 pic attached sans perforated chassis covers.
Sounds great, but there is oscillation in the amp and comes across as a high frequency buzz. See attached pics.
Scope1 jpeg. The top trace is the +32v rail. Scope settings: 2v per div vertical and 5ms per div horizontal. The bottom trace is the output of the amp into an 8 ohm speaker load at 1v per div.
Scope2.jpg, same settings as above except I cranked the time per div out to .5us per div.
What I have done so far:
Got feedback from Mr. Pass (thanks so much!) pointing out I could use more capacitance (hence replacing the 20 year old caps) and noted the RF noise. Replacing the caps greatly reduced the hum.
Found Erics great a40 site (thanks Eric!) and rewired the amp plus moved the caps/bridge rectifiers around to minimize noise and same for the wires going to the bias on Q7 and Q8.
Tried testing the FET 2N5248 as per Ericís site, but did not get reliable readings with the ohm meter. So canít be 100% sure these are installed right.
What should I do?
Somewhere I read that the Old Colony boards needed a trace modification, but I canít find what it is exactly.
Looks like a snubber RC across the bridge rectifiers would really help (as per Erics site), but I donít think I have the gear or know how to correctly measure and calculate the RC values. Any suggestions for the RC values?
Looks like putting a 4.7uf film caps like http://www.partsconnexion.com/capacitor_film_axon.html across the electrolytic caps would help?
Does this make sense? Any other suggestions?
Best regards,
Mitch
Attached Images
File Type: jpg a40 front.jpg (461.8 KB, 376 views)
File Type: jpg Scope1.jpg (491.5 KB, 366 views)
File Type: jpg Scope2.jpg (491.4 KB, 329 views)
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Old 11th July 2011, 01:18 AM   #2
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I see that your channels are at some distance from the supply. Do you
have local supply capacitors on the channels themselves?

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Old 11th July 2011, 02:43 AM   #3
mitchba is offline mitchba  Canada
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Wow Mr. Pass, thanks so much for responding!

Yes, each channel is ~4ft from the supply. See attached pic.

No, I do not have have local supply capacitors on the channels. What would you recommend?

Many thanks, Mitch
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File Type: jpg Rig.jpg (809.7 KB, 315 views)
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Old 11th July 2011, 02:53 AM   #4
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ANYTHING!

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Old 11th July 2011, 05:33 AM   #5
mitchba is offline mitchba  Canada
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:-) Ok, I put my old 10,000uf caps right at the amp. It helped, but still buzz. See scope1a.jpg and scope2a.jpg - same scope settings as my orginal post.

The 3rd trace scope, scope3.jpg is at 5us per div, almost looks like a square wave while fiddling with the trigger on the scope.

When I play music through the amp, at low volume, it sounds a bit distorted with no bass, then as I turn up the volume, the bass kicks in and can go quite loud, but still sounds a bit distorted, almost motorboat kind of sound riding on top of the audio...

Thanks again for your help!
Attached Images
File Type: jpg scope1a.JPG (507.2 KB, 294 views)
File Type: jpg scope2a.JPG (486.5 KB, 22 views)
File Type: jpg scope3.JPG (459.2 KB, 31 views)
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Old 11th July 2011, 09:34 AM   #6
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It looks like you have to use very long wires from the driver board to the output transistors. Usually you should try to keep connections to transistors as short as possible (parasitic capacitance and whatnot). This could be causing your oscillations, maybe. Solution: Short wires or maybe ferrite beads/cores.
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Old 11th July 2011, 05:03 PM   #7
mitchba is offline mitchba  Canada
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Thanks Rodeodave. I did rewire the amp since the photos above. See attached. I moved the bias wires to top of the amp - the red wires. On the bottom of the amp, I kept the supply wires away (green) from the signal wires (black).

I find the bias and collector wires going to Q7 to be the most sensitive. If I move them around a bit, it does affect the buzz, but never goes away. Moving any of the other wires around has little to no effect.

The kit did not come with TO3 sockets, so I soldered the wires directly to the output transistors - could be a cold solder joint... (I have sockets on order).

Attached is a scope trace - top trace is +32v rail (same scope settings as above) and the bottom trace is the output of the amp (with input shorted). Scope setting is 100mv per div.

I could move the driver board to underneath the amp and rewire - this would provide the shortest possible path. But I am wondering with a scope trace like that if I have broken something...?
Attached Images
File Type: jpg Amp Top.JPG (475.0 KB, 107 views)
File Type: jpg Amp Bottom.JPG (471.0 KB, 127 views)
File Type: jpg Scope4.JPG (492.8 KB, 96 views)
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Old 11th July 2011, 11:20 PM   #8
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I hate to say that, but your wiring looks like it's contributing to the problem. Making all the connections with parallel pairs of wires may seem like a good idea at first, but in reality it's adding capacitances everywhere. I don't know how sensitive the A40 circuitry is to such parasitic capacitances, but that's usually one way to get an amp to oscillate.
Keep the wires coming from the driver board as short as possible, really. Use single wires to exclude the parasitic capacitances. Maybe moving the Zobel network (R20 and C5 at the output) closer to the output transistors helps. And keep the feedback path (R5) as short as possible. If necessary, remove the heatsinks from the case and get the output transistors as close to the driver as possible. Every inch counts. That way you can eliminate all parasitic influences.

How do your voltages check out? Nelson mentions a couple test points in the A40 article, under Final Test.
Where does the oscillation first appear? Short the input to ground, and prod around with the oscilloscope probe. Work your way from the input to the output transistors. Check the bases of all transistors, what do you see? Any idea how fast the oscillation is?

Edit: The envelope of the top waveform in the last picture looks like a typical charging/discharging capacitor, what's the time resolution of the scope in this shot? Where did you put the oscilloscope's ground clip for that measurement?
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Last edited by Rodeodave; 11th July 2011 at 11:24 PM.
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Old 12th July 2011, 04:26 AM   #9
mitchba is offline mitchba  Canada
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Rodeodave, many thanks you are absolutely right! I mounted the circuit board under the amp and rewired with single wires as short as possible. See pic. The alligator clips are to a couple of local supply capacitors as suggested by Mr. Pass.

I can almost get the buzz to be inaudible by fiddling with the wires.

It is really sensitive, particularly the wires going to the bias adn collectors on Q7 and and Q8. I suspect one of my issues is that I don't get a good connection on the collectors of the output transistors as I can only screw down the lug with a nylon nut that are easily stripped. Waiting on real TO3 sockets.

I will check the voltages again, but I do remember them being what was specd under Final Test when I tested them eariler.

See scope trace 4a and scope 4b. Input shorted measuring the output across the speakers terminals, probe on red, ground on black.

4a is at 10ms per div and 100 mv per div. Noise seems to be 500mv peak to peak. 4b is the same except for .2us per div. That oscillation is in the Mhz range. Do you think getting some ferrite beads would help get rid of that?

What's the trick with the wiring? I am using regular 18 awg hook up wire. Should I be using different wire? Should I move the R16 thru R19 closer to the emitters of the output transistors?

Again many thanks for your assistance!
Attached Images
File Type: jpg UnderAmp.JPG (478.7 KB, 64 views)
File Type: jpg scope4a.JPG (492.6 KB, 54 views)
File Type: jpg scope4b.JPG (460.1 KB, 28 views)
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Old 14th July 2011, 03:56 AM   #10
Eric is offline Eric  United States
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Hey Mitch,

What wire gauge did you use to connect your power supply to the amp boards? I would recommend a minimum of 14ga, perhaps 12ga. Also, how did you connect your output transistors to the heat sink without the use of TO-3 mounting sockets? If the transistors are not coupled to the sinks, there can be all kinds of trouble as they heat up.

I'm sorry that I don't have the original papers that detailed the error on the circuit board (you need to cut one trace - its very easy to do with a razor knife), but if you compare the traces on your PCB with Fig 9/10 in the PDF for the a40 project, you'll find it. My boards are buried so deeply in the amp chassis that I can't easily get them out to photograph them.

Eric
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