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Old 1st August 2009, 09:32 PM   #11
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Quote:
Originally posted by nigelwright7557
If the lamp is continuous lighting then there is a serious short circuit somewhere.
Not necessarily. As I said above, the odd design of the 400-II using an op-amp for the input stage makes it behave differently than most amps that use a discrete input stage at low line voltages. If the op-amp is slammed against one of its rails because it's not getting enough voltage, the entire amp is thrown off balance and may well try to draw more power from the AC line (lighting the bulb).
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Old 1st August 2009, 09:40 PM   #12
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Quote:
Originally posted by RocketScientist


Not necessarily. As I said above, the odd design of the 400-II using an op-amp for the input stage makes it behave differently than most amps that use a discrete input stage at low line voltages. If the op-amp is slammed against one of its rails because it's not getting enough voltage, the entire amp is thrown off balance and may well try to draw more power from the AC line (lighting the bulb).

There is no problem with the amp slammed against one rail so long as the speaker isnt connected.
At this stage a speaker shouldnt be anywhere near the amp until it is proved to be working.
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Old 1st August 2009, 09:48 PM   #13
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Quote:
Originally posted by nigelwright7557



There is no problem with the amp slammed against one rail so long as the speaker isnt connected.
At this stage a speaker shouldnt be anywhere near the amp until it is proved to be working.
I've seen several amps that work perfectly fine that won't power up with a light bulb (or similar current limiting) even with no load connected. It depends on the topology of the amp. The DC feedback loop and/or VAS stage in some amps needs to be established at something near the correct operating point before the output stage will behave and bias correctly.

A lower than normal voltage and current limited power supply can set up a feedback loop where what would normally be just a brief current blip during power up creates oscillation or otherwise prevents the amp from powering up normally. Your advice for turning down the bias is good as really high bias could be causing the problem. Of course one needs to figure out which direction that is on the trimpot and it's also possible the bias circuit has a problem.

But yes, I agree you certainly shouldn't have a load connected while testing. Blue lander has explained he's already gone through much of what you describe regarding not having the output devices connected and the amp powers up OK. And they're brand new output devices. So it's not likely a short or a bad output transistor as you suggest.
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Old 1st August 2009, 10:01 PM   #14
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Quote:
Originally posted by RocketScientist

And they're brand new output devices. So it's not likely a short or a bad output transistor as you suggest.

But I have been there before.
Fitted new output transistors and there is still a short because there was a fault with the bias stage causing both output transistors to switch on and short out the power supply.

He needs to check the output transistors again, just in case.
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Old 1st August 2009, 10:15 PM   #15
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That is good advice. Even with most cheap DMMs you can check all the transistors for obvious shorts. If they seem to be shorted while in circuit desolder at least 2 of the leads and test them again. If you need help on how to test a transistor with a DMM, just ask

And, as mentioned, look really hard at the bias circuit (everything between the bases of the driver transistors). Make sure everything is wired/installed correctly, are the correct parts, nothing is open or shorted, etc. And it could be something as simple as the bias pot is cranked to the wrong end of its range.

But, seriously, depending on your DIY skills, the LME49810 solution may be worth seriously considering. You could end up with a very nice amp that way. Also, another tip is to look on eBay for speaker protection modules from vendors like diy-gene. Assuming there's room in the rather odd 400-II chassis to mount such a thing, it might be cheap insurance. Fuses often do a lousy job of protecting speakers (or for that matter the rest of the amp) against a failure.
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Old 1st August 2009, 10:33 PM   #16
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I don´t know for sure, but Michael of Amps Lab seems to have a special liking for old amps like the Phase Linear ones. The link is for a 200 series II and his site is of course commercial.

http://ampslab.com/phaselinear2_4.htm
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Old 1st August 2009, 11:02 PM   #17
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A quote from AmpsLab on the page linked above:

"The bane of Phase Linears, instability, has been resolved with this Model 200 Series2. After much analysis and redesigning, this Phase Linear remains stable in operation. "

Instability can also be a serious problem with the 400-II. I'm not trying to be unfair, as I've met and hung out with Bob Carver on a few occasions and he's a smart fun guy. But times have changed since the late 70's when Phase Linear had a strong following. There really isn't much to like about any of the Phase Linear designs by modern standards.

They were generally under heatsinked, had undersized power supplies, tended toward instability, mostly used inferior quasi-complimentary output stages, lacked a lot of modern day distortion reduction techniques, used drivers and output devices that are vastly inferior to those available today, lacked modern protection circuits and suffered from other assorted issues like some really marginal engineering (i.e. the red hot resistors in the 400-II). And some, like the 400-II, used poorly conceived topologies.

They were popular because, in the 70's, cheap high power amps were rare. And they tended to be marginally more reliable than some of their competition (like the infamous Ampzilla) but they still had the well earned nickname "Flame Linear"--often from instability issues and/or the lack of sufficient heat sinking. Carver did a great job of turning out Phase Linear amps with an impressive dollar per watt ratio, and some of them even looked cool, but they're otherwise vastly inferior to lots of later designs.
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Old 1st August 2009, 11:10 PM   #18
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Quote:
Originally posted by RocketScientist
A quote from AmpsLab on the page linked above:

"The bane of Phase Linears, instability, has been resolved with this Model 200 Series2. After much analysis and redesigning, this Phase Linear remains stable in operation. "

Instability can also be a serious problem with the 400-II. I'm not trying to be unfair, as I've met and hung out with Bob Carver on a few occasions and he's a smart fun guy. But times have changed since the late 70's when Phase Linear had a strong following. There really isn't much to like about any of the Phase Linear designs by modern standards.

They were generally under heatsinked, had undersized power supplies, tended toward instability, mostly used inferior quasi-complimentary output stages, lacked a lot of modern day distortion reduction techniques, used drivers and output devices that are vastly inferior to those available today, lacked modern protection circuits and suffered from other assorted issues like some really marginal engineering (i.e. the red hot resistors in the 400-II). And some, like the 400-II, used poorly conceived topologies.

They were popular because, in the 70's, cheap high power amps were rare. And they tended to be marginally more reliable than some of their competition (like the infamous Ampzilla) but they still had the well earned nickname "Flame Linear"--often from instability issues and/or the lack of sufficient heat sinking. Carver did a great job of turning out Phase Linear amps with an impressive dollar per watt ratio, and some of them even looked cool, but they're otherwise vastly inferior to lots of later designs.

Its called making things down to a price.

I was in computer system design and quite often on high number runs we were asked to cut every penny we could.
If your churning out 10,000 units each penny saved gives you £100 back.
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Old 1st August 2009, 11:47 PM   #19
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Quote:
Originally posted by nigelwright7557


Its called making things down to a price.

Agreed. And, in that regard, Carver did a brilliant job with the Phase Linear amps. I just wanted to clear that, while they may have nostalgic value to some, and be a decent source of cheap watts to others (like perhaps Blue Lander), they're otherwise not generally worth investing a lot of time or money in except perhaps to use the chassis, heatsinks, power supply and (if applicable) metering as a starting point for a DIY project based on a more modern, stable, reliable and better performing design.

I should add many ended up in semi-pro use for live sound, DJ duty, etc. with huge fans behind them to keep them cool enough. While nowhere near as robust as say a similar vintage Crown, they were much cheaper per watt. So that may also explain why there are so many Frankenstein-like examples around today.
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Old 2nd August 2009, 12:40 AM   #20
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I'm going to check all my new transistors to make sure they're good. I bought them from Digi-Key and just assumed they were good. Do I need to put each and every output transistor in for testing? I only ask because they're a gigantic pain in the butt to screw in and out.

Assuming the output transistors check out, I'll remove all the transistors, check them, and check the values on the resistors and caps. I'll also set the the bias to zero and double check every component connected to it. There's no circuitboard scorching around the large sandbox resistors. They're actually sitting quite high above the circuit board. I assume the previous owner did that to keep them from cooking the surrounding components. It looks like s/he also touched up the traces on the board with solder, but I'll double check for breaks there too. I'll reheat all the solder joints while I'm at it too in case any have frozen.

The reason I'm bothering with such an ancient and fussy amplifier is that I'm trying to use it to drive my set of 4 Large Advent speakers. I wanted something "authentic" from that era to run them off of. I'm beginning to think I should have gone with a Crown DC300a, though! Although the LEDs are really cool...
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