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Old 26th March 2012, 04:49 AM   #1
puppet is offline puppet  United States
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Default Amplifier pwr down "thump" issue

Rane MA6S .. all six channels "thump" when amp is switched off.
Thump is pretty large ... moves woofer full Xmas.

http://www.rane.com/pdf/old/ma6sdat.pdf
http://www.rane.com/pdf/old/ma6ssch.pdf

According to the data sheet and schematic, there are relays to shunt the speaker(s) output either by power down or large DC offset.
Finding it a little unusual that all (6) relays are/have become faulty all at once.

Any techs here able to direct my attention toward a component responsible for activating that circuit, or any other cause (pwr. switch?) for something like this?

There is also a very low level buzz (sounds like 60hz) if that is even related here. <---- this isn't a huge deal, unless it is related.

I posted this issue on Rane's forum and the pwr switch was first suggested ... the unit still does this even is unplugged directly (no switch toggled). Next was the relays ... but all six?

Any help is appreciated.
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Old 26th March 2012, 05:57 AM   #2
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by puppet View Post
Finding it a little unusual that all (6) relays are/have become faulty all at once.
IMHO, It's most likely a problem with the relay control circuitry.
Now to wade through the schematic and try figure out what's going on....
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Old 26th March 2012, 06:01 AM   #3
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when an amplifier starts up or shuts down it will require a few seconds for all divisions of the circuit to balance or simplified load or unload in a proper operating voltage

in between and at both procedures thumps and DC at the outputs is expected that is why circuits exist to delay connect the speakers at start up and quick disconnect at shut down ...

the time line for both procedures is determined by small capacitors in the relay circuit ..still if other like power supply capacitors are too old or leaky may the original designed time frame doesn't work properly and amplifier fails to start up in time or shut down quick enough

Combine this with the buzz which if not a ground issue then obviously is a capacitor issue...

of course the amp might have other problems such is soldering or someone in the past was messing with

Kind regards
sakis
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Old 26th March 2012, 02:35 PM   #4
puppet is offline puppet  United States
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The amp doesn't display any issues on pwr ON. That delay works as advertized.

I took a look inside the amp yesterday .. before this thread was posted. I see no indication of "repair attempts" or any leaking components. The amp looks to have been used in a environmentally "friendly" area it's whole life. Very clean. No idea of how many hours on the piece though.

I have not taken any "technical" measurements ... other than to plug it in and use it. Aside from the issue(s) posted here, it works great. (overload, clip, LED indicators work ... no cracking/popping, no overheating, no surging ... everything else really seems to be A-OK).

As a possible indicator here .... the buzz seems to me to be related to the DC chassis fan. It sounds like it is around the same frequency level. ... and is the relay circuit also DC driven?

Could the nature of the problem (being all the relays effected) be confined to DC level component(s) on the power module?

Anything I can check with a multimeter to help narrow this down?

Thanks again.
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Old 26th March 2012, 04:25 PM   #5
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Hi,
I was looking at you schematic and all 6 amplifiers are independent of each other. The only common is the power supply. Like you said each amplifier have a relay. I do not think all relays are bad. It is possible that you may have a problem with one of the power supplies. I see a common signal to all of the amplifiers and it is through the zener diode D1. If you follow the signal it's goes to the relay and I think here is were you let the others amplifiers that one amplifier have a problem and need to shutdown. The power +15 volt supply voltage to the zener diode D1 and then goes out to all the amplifiers through pin "B". Here is were you may have a problem in the +15 volts or in the -15 volts. I would check all the power supplies first for DC volts and them AC. Read the +/-60 +24 +/- 15 using the voltmeter in AC. This will tell if you have ripples in the power supplies. I maybe be wrong but if one of the capacitors in one of the power supplies are bad it will discharge faster than the others and that maybe the thumps that you are experiencing. Also the test that you did pulling the plug and you have the same symptom mean that is not the switch.
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Old 26th March 2012, 10:14 PM   #6
puppet is offline puppet  United States
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tauro0221 .. I see what you are talking about. There is only one PS though. (one PS module bussed to the 6 amp cards)

Can I check the outputs right off the buss connection?
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Old 26th March 2012, 10:21 PM   #7
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Hi,
You can check it any places that easy accessible to check. If you get a high AC voltages in one of the supplies that is the one that the capacitors are going bad.
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Old 27th March 2012, 04:52 AM   #8
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Quote:
Originally Posted by puppet View Post
Rane MA6S .. all six channels "thump" when amp is switched off.
Thump is pretty large ... moves woofer full Xmas.
The output of Z2 (5534) has a bad habit of swinging wildly when powered off. This is caused by the +/- 15V rails collapsing at different rates. Since the amp's supply rails take longer to collapse, the amp's output will swing very high, possibly to amp's supply rails because the base of Q10 (MPSA93) is being injected with DC up to +/-15V.

There are a few ways to remedy this.
1) Shorten the output relay Cut-off time so that the relay opens before the swing. This will obviously also shorten the power-up delay as well.
2) Short the output of Z2 to 0V before it starts to swing.
3) Cut the +/-15V supply instantly.
4) Change Z2 to an opamp that doesn't swing when powered off.

All the above require modifications.

The easiest way is to leave everything as it is.
5) Install another output relay but one that runs off 120Vac. Wire this relay to the power switch of the amp. This will give you instant open/close.

Quote:
There is also a very low level buzz (sounds like 60hz)
If it's coming from the transformer, it's probably some DC on your Mains.
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Old 27th March 2012, 07:08 AM   #9
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Right now the question to ask is if this is a day one or it is a recently problem. In the schematic also I see a 4.7K resistor installed in the -60 volts and not in the +60 volts. In the +60 volts it is missing. This will make the discharge of the capacitors uneven. In other word the -60 volts will discharge faster than the +60 volts. Second the speaker output contact it is connected with two diode to -60 volts and +60 volts and the speaker. The relay when powered down it will short the speaker output and the two diodes to ground. Now If the 24 volts take too long close the ground then the speaker will make the "thump" noise. Like Michael Chua already described the problem that will cause if the relay stay longer. I will try to fix the problem by doing these two modification. Add a resistor in parallel to the relay coil to discharge the +24 volts faster and add a 4.7K 2 watt resistor same as the -60 volts to the +60 volts. This will allow the -/+ 60 power supplies to discharge both evenly when the amplifier it is powered down. I will do this to just one amplifier and see if it is fix the problem.
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Old 27th March 2012, 01:46 PM   #10
puppet is offline puppet  United States
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Quote:
Originally Posted by Michael Chua View Post
The output of Z2 (5534) has a bad habit of swinging wildly when powered off. This is caused by the +/- 15V rails collapsing at different rates. Since the amp's supply rails take longer to collapse, the amp's output will swing very high, possibly to amp's supply rails because the base of Q10 (MPSA93) is being injected with DC up to +/-15V.

There are a few ways to remedy this.
1) Shorten the output relay Cut-off time so that the relay opens before the swing. This will obviously also shorten the power-up delay as well.
2) Short the output of Z2 to 0V before it starts to swing.
3) Cut the +/-15V supply instantly.
4) Change Z2 to an opamp that doesn't swing when powered off.

All the above require modifications.

The easiest way is to leave everything as it is.
5) Install another output relay but one that runs off 120Vac. Wire this relay to the power switch of the amp. This will give you instant open/close.



If it's coming from the transformer, it's probably some DC on your Mains.
Thanks for your input Mike. I saw this circuit used by somebody and posted on the web. I was curious about it ... as is did seem to be a simple solution.

On the TF, I'd have to check lead length first but was wondering if rotating it might help w/the low level buzz?

@tauro0221 .. I honestly don't know if this is/was a day one condition. (I acquired this amp second hand)
I do know that people @Rane say this should not "thump" like it does now. This is through conversation on their troubleshooting forum. It is also no longer serviced by them (limited parts, etc. I suppose). On the upside, I guess, a workable solution can be implemented without worrying about warranties.

Both you and Mike have my thanks. I'll check the voltage levels as you suggested and go from there.
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