Crown DC-300 Schematic / Service Manual

[Brian Roth]

I may be "tripping", but I am almost certain that Crown used a (hard?) anodized aluminum insulator under the outputs, vs mica or sil-pads. 

Before I moved, I had a small pile of "burnt" D-150 and DC-300A amps in the garage and I never recall seeing mica or sil's...

Bri

 

[JohnRoberts]

If the heat-sink aluminum is anodized that is not conductive... You can check for that with a  VOM... on ohms scale...

Be careful not to scratch the anodize layer because it is not very thick.

I've never messed around inside a DC 300 so that is possible.

JR

 

[gridcurrent]

I'm not finding any mica insulators, I'll order them up an install them. 

note the mention of the insulators on .pdf page 25 of the service manual.

 

[Biasrocks]

Yes, the service manual (pg 25) says that some units have an insulated coating and earlier units used insulators.

This guy appears to have an insulated coating, but it has been breached somehow (to be determined).

I'm planning on installing the mica insulators as a precaution and to hopefully solve the problem.

note the mention of the insulators on .pdf page 25 of the service manual.

 

[JohnRoberts]

Anodized aluminum is insulated and if it was reliable it would be superior to mica washers that exhibit some thermal insulation  (maybe0 .1'C/W) , but the anodized layer is very thin so can be pierced a little too easily by steel TO-3s tightened down too tight (just for one example).

Note: with or w/o mica, thermal grease is important to fill in all the tiny air gaps that are poor thermal conductors.

JR

 

[Brian Roth]

I've been "intrigued" by the sil-pads I see used from time to time in amps, PSU's, etc.  No sticky heat sink grease!  But, I have no idea how they compare to Olde School mica + grease.

Bri

 

[JohnRoberts]

I've been "intrigued" by the sil-pads I see used from time to time in amps, PSU's, etc.  No sticky heat sink grease!  But, I have no idea how they compare to Olde School mica + grease.

Bri

They are competitive and deliver similar thermal resistance.

Loose heat sink grease is messy.

I killed a lot of brain cells worrying about this stuff when the industry transitioned from 2 screw steel or kovar base power transistors to one screw or less (metal spring clip) plastic power devices.

Modern plastic device amps had less thermal headroom for multiple reasons, but the modern class D technology, and very low on resistance devices make this pretty much moot.

JR

 

[gridcurrent]

TO-3 insulating "plates" from early production unit.

 

[Andy Peters]

I've been "intrigued" by the sil-pads I see used from time to time in amps, PSU's, etc.  No sticky heat sink grease!  But, I have no idea how they compare to Olde School mica + grease.

They're superior to mica and grease in every way.

 

[David Kulka]

I worked on loads of 300's and 300A's back in the 80's and 90's. Replaced plenty of output devices but another very common failure was the little electrolytics and zeners on the bias rail.

I remember one unit with weird HF osc. and buzz on the outputs that I spent many, many hours on. In the end I found that moving a piece of hookup wire an inch or so away of some other wires made everything right.

In failure mode they sometimes latched to the rail, an instant speaker killer. And like John Roberts, I was never really sold on the idea of a direct coupled speaker amp.