More amplifier Problems

[Ian Mackenzie]

The Phase Linear 700B used to power my brothers 2235H's in 4343's has died.

Luckily he is astute enough to always put the line speaker fuses in AFTER turning on the amps.

A few days ago he turned on the 700B and the meter needles went up into the red and smacked into the end stops and stayed there.

On the bench today and both rail fuses (something like 10 amps) had vapourised and 2 output transistors are short collector-emitter.

The thing is why? We think the problem might be a dry joint somewhere as it did this to the other channel about 15 years ago.

I think it's going to be nasty.

Ian

[Ken Pachkowsky]

Luckily he is astute enough to always put the line speaker fuses in AFTER turning on the amps.
Ian

I had one do the same thing back in the late seventies. It fried the 2231's in my 4350's left channel. Like the Crown 300A, these don't die in a gracefull way.

The output transistors can be difficult to find these days.

Ken

[Ian Mackenzie]

They stopped production of the original power transisters in 1980 according to the PL web page. However there are known modern alternatives that are probably superior and more reliable.

[Hamilton]

I had a Flame Linear 400 kill all my speakers twice, I guess I was a slow learner.

[Ian Mackenzie]

Twice, now that is very unfortunate.

[LE15-Thumper]

Bipolar transistors can just get plain old tired out and develop a bit of leakage which can lead to thermal runaway.


Or they can just throw up their arms and say: "To hell with this ! There is only so much current I can deliver ! "


The connections to the die inside can fail with Heat/Time.

I my experience........Some of those Phase Linears can turn into revolving doors.

Although the hardcore PL fans out there will argue to the death about the virtue of these. The term "Flame Linear" Or "Fuse Linear" does have some basis in fact.

[Ian Mackenzie]

Agreed.

What we can't quite figure yet is why transisters on both halfs of the output stage blew in one channel and blew both rail fuses with the loudspeaker not connected and no signal going through it.

He (Mark) is going to set it up on the bench with a CRO and monitor it before putting it back into service. No point not finding the real cause.

[Zilch]

Source and sink arrays conducted simultaneously, and the fuses did not react fast enough, looks like....

[Ian Mackenzie]

Thats right but its A/B , on..off..not both on full fighting each other.

[Zilch]

Yup, but whatever's supposed to keep them mutually exclusive failed, or a HF oscillation occurred at zero crossing.

With no speaker connected, there's no other high current path between them except each other, it doesn't appear....

[Hofmannhp]

The Phase Linear 700B used to power my brothers 2235H's in 4343's has died.

Luckily he is astute enough to always put the line speaker fuses in AFTER turning on the amps.

A few days ago he turned on the 700B and the meter needles went up into the red and smacked into the end stops and stayed there.
......
Ian

Hi Ian,

during my early amplifier years I had similar problems with some amps....
Try to find out (Oscilloscope if available) if there's the posibilty of high frequency oscillation of the output stage, specialy when not loaded. This is posible to 100s of KHz.
Maybe the Zobel circuit in the output is not well functioning (bad caps), which should fight against this as a low impedance feedback circuit.
This may also be the reason for the "red zone" level meters.

my 0.2 cents

HP

uups.....saw Zilch is on the same way

[doodlebug]

after looking at the schematic are:

1) check all the connections on all the output devices for cleanliness.
2) I'd also check all the physical ground and power connections for corrosions and other crud that could cause oscillations or small ground loops. At this amp's age, any of these should be suspect. Had this probelm with a Hafler DH-220 some years back.
3) Check *all* the emitter resistors for proper value
4) Lastly, I'd pull all but a single set of those outputs, since they're in parallel and bring the amp up on a variac to at least get the 2 channels working together even with one channel at lower power.
5) I'd also be monitoring the voltages around those driver circuits where there are electrolytic caps in some sort of bypass function. If they're leaking DC - watch out. Of course, this wouldn't explain why only some outputs blew in the way described.

My old engineering prof always pressed us plebes to look for physical problems before suspecting failures in a circuit, assuming it was properly designed on paper, of course! This seems to apply in this case.

Cheers,

David

[Audiobeer]

Sounds like some of the problems we experienced in the Navy with communication equipment. The answer then was put a bigger fuse in it!

I love the old Carver and phase linear stuff but frankly I can't afford the historic malfunctions that they can be prone to. With the older phase linear and Marantz stuff I've owned, I always sent them off to the techs who had the experience and the knowledge to modify them so the problems wouldn't happen again. But I will say it always changed the characteristics of the sound they originaly had. I was never one to think that I would be able to tell the difference but in the case of the Marantz I could easily tell the difference in sound. I think the 700 would definitely be worth bringing back.

[LE15-Thumper]

I had an amp drive me nuts. Would blow outputs at random. Turned out the bias pot had an intermittent OPEN causing sky high bias to the output stage. Didn't take long before the outputs went into thermal runaway.

HTH

[jbl]

Check the diode drop at D6, 7 & 8 along with R720 along with Q6. Also check for bad solder joints. I had that problem with my Marantz 300 DC.