Crown DC150A amp overheating

[scott2000]

Here's a piece of the schematic with Q113 checked.......

JR mentioned a bias issue earlier......????

 

[Gene Pink]

Anyway, I need to be able to disassemble those round grey parts to get to the circuit board on the other side.  ...don't know what they are and I want to make sure I don't damage them, by unscrewing those.  They have the reddish washers on the top.

Those are chokes, one end connects to a speaker binding post. Just a sintered iron toroid core painted grey, and 5 turns of copper wire. Not particularly delicate, the red fiber insulating washer is there to keep steel hardware from cutting into the copper wire insulation and shorting it to ground. There should be another fiber washer underneath, to insulate it from the heatsink.

Just unscrew it while noting how tight the nut is for reassembly, and make sure the underside washer doesn't slip out and get lost, and gets put back in.

Gene

 

[musika]

Thanks Gene,

The way this is assembled, those bolted chokes hold the ventilated cage to the heat sinked frame that the power transistors are mounted on with long stand offs.

Will make special note of the parts for putting this beast back together...

Does anyone know if these power transistors are still available?  A quick search yesterday didn't provide much.  Motorola 2n3990

 

[musika]

Here is the parts list page from the original instruction manual.

 

[musika]

Original performance report just for fun.

 

[JohnRoberts]

Here's a piece of the schematic with Q113 checked.......

JR mentioned a bias issue earlier......????

Q113 is the driver for Q114, if Q114 is not working Q113 will try to do the work (and get hot)...  I have seen power transistors fail open base, but it isn't as common as shorted collector-emitter, but in those cases the amp no longer passes audio, because the output is shorted to one rail or the other .

JR

 

[musika]

Thanks  JR for the explanation on this... it is great to have a suspect right from the start!

 

[JohnRoberts]

Thanks  JR for the explanation on this... it is great to have a suspect right from the start!

With power amp failures it is often good practice to replace all the devices connected to the one that released it's smoke... a subtle partial failure could lead to the new parts releasing their new smoke.

JR

 

[musika]

Does anyone know... are these power transistors readily available?  Motorola (or others) 3295 and 3990.

 

[scott2000]

Does anyone know... are these power transistors readily available?  Motorola (or others) 3295 and 3990.

They tested bad??????

I was reading somewhere that the print on the transistor is the actual Crown part number which doesn't make sense. So, the parts list should have the actual Motorola number.....

like this is the 3990

https://www.mouser.com/ProductDetail/ON-Semiconductor/2N5631/?qs=vLkC5FC1VN8pO%252bXU5lSZyQ%3D%3D

the 3295

https://www.mouser.com/ProductDetail/Central-Semiconductor/2N3585/?qs=Ypxpq5eNvNUgBE9FuccXhQ%3D%3D

weird....

I'll look into it a bit later if you don't get it sorted..... Maybe there are other replacements available somewhere as well.....maybe even different part numbers....

I actually have some original Motorola 15025 and 24s I pulled from a working Altec amp..... wonder how close those are....

 

[JohnRoberts]

back in the day motorola would put house numbers on power devices for big customers.

at Peavey we had several sets of to-3 devices for different power points.  (before motorola sold their factory in mexico and got out of the to-3 business. 

JR

 

[musika]

I was reading somewhere that the print on the transistor is the actual Crown part number which doesn't make sense. So, the parts list should have the actual Motorola number..... 

I think you and JR are right - the Motorola part numbers are the correct replacement transistor parts -2N5631 and 2N3585.  This really can be confusing.

They tested bad??????

Haven't tested yet...

 

[scott2000]

I think you and JR are right - the Motorola part numbers are the correct replacement transistor parts -2N5631 and 2N3585.  This really can be confusing.

Haven't tested yet...

. 

Make sure to read section on troubleshooting.... It mentions some of the suspect areas to check as well when overheating is an issue.  Oscillation can be caused by faulty resistors.

 

[musika]

Make sure to read section on troubleshooting.... It mentions some of the suspect areas to check as well when overheating is an issue.  Oscillation can be caused by faulty resistors.

Thanks for the suggestion.

I have tested power transistors before, but I am certainly no expert... and have been collecting some materials and doing research on making sure I understand all this before I get started. 

Can you point me to what you are talking about here? 

 

[scott2000]

Section 7 is the trouble shooting from what I can tell. It mentions suspect components to check . Overheating caused by bias,offset,oscillation.... leaky transistors will pass signal......

Shouldn't take too long to run through everything I'd imagine. I appreciate it when they include the pcb/foil layout with components too as it makes it a lot easier to see what goes where.

 

[musika]

Is this what you are referring to ?

https://www.crownaudio.com/forums/index.php?/forum/22-amplifier-troubleshooting/

 

[scott2000]

Is this what you are referring to ?

https://www.crownaudio.com/forums/index.php?/forum/22-amplifier-troubleshooting/

Sorry....I was talking about the service manual.......

But that forum may be a good place to check around too????

 

[musika]

Thanks.  This Service Manual is what I should have been looking at all along. 

It does provide the transistor voltages along with a good description of the circuitry, the voltages of which I was planning on documenting when I tested them, but must admit I may be out of my league on this repair after a quick read. 

Thanks again.

 

[scott2000]

Thanks.  This Service Manual is what I should have been looking at all along. 

It does provide the transistor voltages along with a good description of the circuitry, the voltages of which I was planning on documenting when I tested them, but must admit I may be out of my league on this repair after a quick read. 

Thanks again.

You should be able to get pretty far with just your multimeter set to diode.....Check the transistors for obvious out of spec readings, remove and double check if you get a bad reading, ...

Resistors are good to check too.... if you can....

I'm not experienced enough to give any real direction but have been more or less successful with any repairs I've had to make using pretty limited tools...Just takes more time( and parts :-[ sometimes).... It's as much an organic experience as it is technical for me sometimes....Kind of feeling my way around how things interact and what affects what.....

You'll definitely get it figured out and, there are two channels so you can always compare readings of the one that seems fine with the one with the questionable component to get a better idea of what may be going on and where..........

Good Luck!

 

[scott2000]

Thanks.  This Service Manual is what I should have been looking at all along. 

The hard version of the 150 Service manual...... Very cool...not sure how different it is from the A though....

https://www.ebay.com/itm/Crown-Service-Manual-D-150-Dual-Channel-Power-Amplifier/372092291535?hash=item56a26dc9cf:g:Oo4AAOSwRMtZa6zd