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Old 9th November 2006, 08:14 AM   #1
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Default DC300A II tweaks

Hello to all from the Windy City,

First post at diy, I'm looking for the straight dope on tweaking my series II DC300A's (bridged mono drivin' vintage HPM-150's)

Freshly recapped and rezenered, I'm ready to replace:

Motorola LM339M input op amp IC
NSD128 voltage amp (Motorola 713 151)
D40P3 predriver (Motorola MPS U10 726)
2N4929/SS7304 predriver (Motorola D2923 K8724).

This are late production series II amps w/MJ15150 output transistors.

Those of you with tested and true improvements on these devices please lay em' on me. Like to keep with OE configurations where pin assignment is concerned.

Steve "Chicago Blues"

In the words of the late, great John Belushi, "May I suggest you buy as many blues albums as possible".
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Old 9th November 2006, 06:48 PM   #2
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Default DC300 thread

If you have not yet seen this, it may interest you much? I will post the transistor selection when I get back home tonight.


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Old 10th November 2006, 04:01 PM   #3
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Please note that there are big differences between the DC300 and the Series II

I've got two Series II my self, and just love them
Free Schematic and Service Manual downloads www.audio-circuit.dk, Spare time company (just for fun): www.dupont-audio.com
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Old 12th November 2006, 12:08 PM   #4
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hi guys,

yes the dc300a mk2 is a cracker, it has a totally redisigned main board. its amazing how many mk 1.5's there are about, (new style black front panel , upsidedown chassis and mk1 main boards) just confuses the whole issue, as if we needed too!
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Old 14th November 2006, 12:29 AM   #5
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Hey Guys,

Thanks for the posts. Thought I would update everyone on what I found out from Crown (right next door in Indiana).

BOTH Q104 (NSD128) and Q106 (D40P3) are now replaced by D40V2.

Q107 is now 2N3637.

No change to LM339N op amp.

So far, I have not found any current day drop in to replace the LM339N.

LF357H is now LF357N.

I have not yet found spec sheets for these new parts, but I thought I would let everyone know what Crown is shipping.

Anyone with improvements or thoughts on these parts, I would love to hear from you.

I post again after installation and bench test.


Steve "Chicago Blues"

In the words of the Motor City Madman Ted Nugent - "This here guitar is guaranteed to blow the balls of a charging rhino at 60 paces"
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Old 13th August 2008, 04:32 AM   #6
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Location: MN
I have one of these on my bench now that i need to troubleshoot. It oscillates at a low level no load and no input. The positive wave form clips sooner then the Neg. not sure if thats normal or not. there are some large wire wound resistors on the main board that have gotten quite hot but they don't look discolored. But there is soot around them on the PCB and on every component above them!

Probably needs new caps all around...

Any common issues to look for??

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Old 13th August 2008, 06:40 AM   #7
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Default dc300a mk2

I have a similar problem with the two resistors, im going to replace them with allen bradley 3 watt 2.7k carbon, and i will certainly lift them off the board a bit, damn silly putting huge resistors righ on the board. i like the ceramic stand offs, they keep the resistors steady.

The newer make of resistors really get hot, (they look like metal film?) watch the tracks, they come loose fairly easily, especially where they have baked with the heat.

I have an amp similar to yours..... they look ok apart from that, i usually just change the smaller elec caps and check zeners are ok.....

all the best

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Old 14th August 2008, 03:13 AM   #8
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I fixed my Dc300II, C4 the 10uf 160V capacitor that filters the supply for the opamps and front end was bad. while in there i replaced R1 the 1K 1 watt to a 1K 5 watt and moved it so it had some room to breath. fixed the oscillation problem.

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