Crown dc300a mk2 - loud pops after shutdown

[smilan]

Hi,
Lately my old amp started to make loud pops after shutdown.
When the volume pots are on "0" position and the power button is turned off there's a light on the over load LEDs and the speakers making loud pops before it's completely shutdown .
I've checked the two 13000/70 caps (C2 and C3) and both tested good with low ESR.I've replaced C4 (10/150) since it was faulty.
What other components can cause this problem?
Here's a link to the schematic:
https://adn.harmanpro.com/site_elements/resources/973_1425479950/DC-300AII-Service-Manual-DC300AIIServiceManual_original.pdf

 

[JohnRoberts]

The DC300 is notorious for being DC coupled and killing speakers when faults in it or upstream drive DC into loudspeakers.

It can be difficult to trouble shoot turn on/off transients but with an amplifier that old you are probably looking at the likely suspects (tired old electrolytic caps).

JR

 

[smilan]

Thanks,
I'll order new caps and replace the old electrolytic caps.
Does anyone knows where can I find replacement caps for the two large 13000/70 with the same size of the originals?

 

[mjrippe]

You might find 15000/100v and they still may be smaller in physical size.  Higher values are ok, lower are not.

 

[JohnRoberts]

Perhaps scope or measure the AC ripple on those large reservoir caps. If ripple voltage is high, or different on both, that suggests possible tired capacitor.

Unfortunately at the end of all that work and expense you still end up with a DC300. Small caps are cheap, but we don't know for a fact that replacing them all will correct the pop.

JR

 

[guy_4]

Hello smilan,

This amp is suffering of a bad design from the beginning.

As there is not a DC protection and turn-on / turn-off delay circuit ( driving a relay)  between the amp output and the Loudspeakers connectors. 
Such a shame !

I would highly recommend you to  buy one ASAP (  Velleman has a kit - ref K4700 )  and install it inside your amp.

It will also save your Loudspeakers, opening the relay, when a transistor of the power stage will short, one day or another....
... 80V DC to the coils means smoke, a bad smell in the house, and some tears too...    :'( :'( :'(

Best,
Guy

 

[smilan]

Hello smilan,

This amp is suffering of a bad design from the beginning.

As there is not a DC protection and turn-on / turn-off delay circuit ( driving a relay)  between the amp output and the Loudspeakers connectors. 
Such a shame !

I would highly recommend you to  buy one ASAP (  Velleman has a kit - ref K4700 )  and install it inside your amp.

It will also save your Loudspeakers, opening the relay, when a transistor of the power stage will short, one day or another....
... 80V DC to the coils means smoke, a bad smell in the house, and some tears too...    :'( :'( :'(

Best,
Guy

Thanks Guy,
I've just ordered  this kit

 

[smilan]

One more thing,
I would like to test the output transistors (MJ15150) but I can't find any datasheet.
Does someone have it / can tell me how I can identify the emitter, collector and the base for testing it with a diode tester?

 

[JohnRoberts]

One more thing,
I would like to test the output transistors (MJ15150) but I can't find any datasheet.
Does someone have it / can tell me how I can identify the emitter, collector and the base for testing it with a diode tester?

IIRC that design is quasi-complementary so all outputs are NPN... TO3 transistors always have Collector connected to metal case. With a vom in diode mode you will find a diode drop between base (one of the two pins) and collector (case). You should likewise measure another diode drop between base and emitter pins.

The power devices may be a case of "don't fix em if they ain't broke".... unlikely to cause the symptom you are chasing.

JR

 

[smilan]

IIRC that design is quasi-complementary so all outputs are NPN... TO3 transistors always have Collector connected to metal case. With a vom in diode mode you will find a diode drop between base (one of the two pins) and collector (case). You should likewise measure another diode drop between base and emitter pins.

The power devices may be a case of "don't fix em if they ain't broke".... unlikely to cause the symptom you are chasing.

JR

I want to check all the transistors before I'm ordering parts from Mouser.
Since I'm living in Israel I can't get quality components on local electronic stores so it's better to check every thing before placing  an order.
I've already checked one half of the output transistors, all seems to test good with voltage drop between base and collector and another drop between base and emitter pins.

 

[Dan Kennedy]

It's been more than 20 years since I've worked on these things, but I remember the problem as bad or dying electrolytics around the opamp.

I kind of remember it being a timing issue on shutdown on how fast each low voltage supply decayed and therefore maintained stability until it didn't matter.

I would recommend replacing all electrolytics with similar value/voltage types, they're ancient now anyway.

 

[CJ]

here is a link to audio caps by Nichicon, they have diameters listed, or use new clamps to fit an undersized cap to the hole,

find right cap then order  from digikey,

https://www.nichiconcapacitors.com/product-category/nichicon-kg-series/

 

[smilan]

It's been more than 20 years since I've worked on these things, but I remember the problem as bad or dying electrolytics around the opamp.

I kind of remember it being a timing issue on shutdown on how fast each low voltage supply decayed and therefore maintained stability until it didn't matter.

I would recommend replacing all electrolytics with similar value/voltage types, they're ancient now anyway.

here is a link to audio caps by Nichicon, they have diameters listed, or use new clamps to fit an undersized cap to the hole,

find right cap then order  from digikey,

https://www.nichiconcapacitors.com/product-category/nichicon-kg-series/

Thank you Dan and CJ, I'll order new electrolytics today

 

[smilan]

I would highly recommend you to  buy one ASAP (  Velleman has a kit - ref K4700 )  and install it inside your amp.

I got the k4700 kit,
The manual of the  k4700 says "if the amplifier to which the module is to be connected has a simple power supply (asymmetrical supply) i.e an amplifier with output-elcos, the following resistance R23 toR26 has to be mounted.
ATTENTION: if the involved amplifier has a symmetrical power supply, those resistances may NOT be mounted."

What is the case with the DC300A?
And if someone can explain the theory behind symmetrical / asymmetrical power supply I'll be great full.

 

[JohnRoberts]

I got the k4700 kit,
The manual of the  k4700 says "if the amplifier to which the module is to be connected has a simple power supply (asymmetrical supply) i.e an amplifier with output-elcos, the following resistance R23 toR26 has to be mounted.
ATTENTION: if the involved amplifier has a symmetrical power supply, those resistances may NOT be mounted."

What is the case with the DC300A?
And if someone can explain the theory behind symmetrical / asymmetrical power supply I'll be great full.

Perhaps an awkward translation....  The DC300 from the link you provided looks like +60 and -60V power supplies, so clearly symmetrical.

Perhaps they meant "single" supply with capacitor coupled speaker output?

JR

 

[smilan]

Perhaps an awkward translation.... 
Perhaps they meant "single" supply with capacitor coupled speaker output?

JR

I've translated this section from the Spanish manual using Google translate and it seems like the manual says: "For asymmetric power amplifiers (where there is an electrolytic capacitor)"
*in Spanish "para los amplificadores de alimentación asimétrica (donde existe un condensador electroliticode)"
So I guess you're right.
So in my case there's a 0.1uF capacitor across the outputs, so I need to install the four resistors?