Crown DC300A Series II OP Amp Circuit Voltage

[stratcat442]

Hello all, 

DC300A Series II early model has low output on both channels.  I am running in Dual mode.  Have cleaned/checked Dual/Mono switch and it's good.  Have pulled power transformer and checked it and it is good as well.  When checking main board voltages I find less than .5 volts at both ends of D100 and D200 (see pg. 34 in Service Manual).  This is near the center of schematic at outer ends of op amp.  It's supposed to have 2V and 1.35V at these locations.  The curious thing is when I turn off the power switch with my meter leads still attached, I get the required voltage momentarily until the caps start bleeding off.  I'm thinking a soft short in a transistor but am new to this game and was hoping for some experienced input.  Maybe even the op amp?

Link to Service Manual:  http://www.crownaudio.com/media/pdf/legacy/DC300AIIServiceManual.pdf

Thanks for your consideration,
Steven

[smackoj]

senor Strat; I suggest searching the web for a forum that deals mostly with Crown amps. Probably one or more out there. Also give a look at
ssguitar.com site. I believe the forum originates in Australia and these chaps are very adept at diagnosing SS amps. They might even have a couple threads dedicated to the DC300A.

good luck with your search,  jack   

[PRR]

1) You are in the wrong place. This is a Tube Amp community

2) In a past life, I did several years fixing the solid-state stuff. The main thing to know is that direct-coupled amps fail DOMINO style. One blow-up causes another and another. Any slip or part-repair causes another near-total blow-up, with loss of time/labor and not-cheap parts.

Low bias will not kill a transistor amp, just make it "hoarse" at bedroom level.

The observation about bias coming up as rails drop is interesting, but I don't see what it proves. It could be totally-short output stage, or Crown's bootstrap (ah, not used on the Series II which is after my time), or the self-protection system (which is good but not fail-proof).

The DC300 has a LOT of NFB and is very high-strung. The speed of every device has to be factored-in or it will oscillate and kill itself (faster than the protection can act). I know on the 1967 series you just can not get power devices with the right spec today. Even 1986 is a very long time ago in semiconductor history.

At this amp's age, we must also consider stuff that "didn't fail" when these amps were young, and stuff that has failed on the 100,000 miles of bad road this amp may have seen. (Though Crowns are notorious for surviving extended abuse, there are limits.)

Thinking about the time and parts to set-up a test rig, remove and test devices, probe the half-naked amp, find and order parts, smoke-test, and likely do it again and again.... I'd really rather go to the pawn shop and buy 300W of modern amplifier.

And I reall DO like the DC300. When it is healthy.

[6G6]

Crown has their own forum.
There are a few die hard D series fans there.
Mostly it's kind of like trying to find information about you great grandfather's brother's rumored bastard child.
However, when you do find anything, there is usually a Crown engineer on the forum that will chime in to verify that
what you are seeing is valid.