Phase Linear 400, SERIOUS AC!

[Planerguy]

Anyone have any idea's on this - A Phase Linear 400, Series 2.
with pure AC for output?
I tried replacing the power supply caps, the bridge rectifier replaced, and has all new matched outputs, per stock.
Brought up on a variac, ac output begins around 15volts, and by 50 volts or so, is shaking any speaker hooked up to it pretty close to distruction.

It doesn't make sense to me, as the AC component should be pretty well filtered out, if nothing else. It actually seems like you're hooking a speaker directly to an AC wall outlet.

Thoughts?

[sakis]

it could be a couple of things

for once sothing is seriouslly wrong with the amplifier and of course you dont mains ac out ..... you get audio signal .....

it could be just a ground missing from your plugs so of course you get 50hz out noise that could easilly foul you for ac....

to determine what exactly this is you need a scope ....

if your so called AC is controled by the volume knob then is not ac ...its audio ....

then might be also something wrong with the amp and is ocilating modulated to 50 hz !!!!.....

the chance that any cable melted in you have mains AC in the out for me is one in a billion

best regards sakis

[bear]

No volume control on a Phase Linear 400, maybe on the Series II, dunno.

Probably you have a shorted output device or a few.

Remove the fuses that are on the back, and see if one or more is blown.

No blown fuses, and not drawing too much power from the wall (variac and ammeter on the line are a good idea), then short the inputs and try again. Also with the amp off check the continuity between the ground of the speaker outs and the ground of each input RCA. Should be a dead short reading.

Then check between the center pin of the each RCA (power off) and ground, both should read something in the kohm region, both the same.

A shorted output device and or driver will draw a ton of current and pull the PS into big ripple, which = AC hum...

_-_-bear

[Planerguy]

No input connections, no signal applied. AC being 50/60 cycle output at the speaker terminals. Not a missing ground.
50 Htz. oscillation? Is that possible?

I have signal generators, oscilliscopes, etc., and know how to use them, but my background is in RF, and I'm used to seeing oscillations in mhz., not 2 digits. Clearly I'll be learning something new here.

Thanks for having responded!

[eyoung]

Hello all,

Low freq oscillation is more common to tube amps but can be in solid state. It is known as motor boating. I'm thinking the old Phase Linear 400 had some kind of weird darlington affair in the outputs and dont like being run into no load . Maybe someone ha s a schematic and some experience troubleshooting these, however the most likely cause is a leaky cap being as how these were early 70's amps and are aged like me.


Regards, Elwood

[wg_ski]

Originally posted by Planerguy
Anyone have any idea's on this - A Phase Linear 400, Series 2.
with pure AC for output?
I tried replacing the power supply caps, the bridge rectifier replaced, and has all new matched outputs, per stock.
Brought up on a variac, ac output begins around 15volts, and by 50 volts or so, is shaking any speaker hooked up to it pretty close to distruction.

It doesn't make sense to me, as the AC component should be pretty well filtered out, if nothing else. It actually seems like you're hooking a speaker directly to an AC wall outlet.

Thoughts?

Bring it up with a light bulb limiter, first with NO load. The op-amp front end is unbalanced, and sometimes doesn't like low operating voltage. Those amps "squeal" upon shutdown occasionally for the same reason. If it comes up ok, connect a speaker and play it at low volume - if it's ok proceed to set the bias and you're done. It may come up with DC to the output, in which case the first thing to check is the +/-15v shunt regulator to the op-amp front end. If something is off here the amp will misbehave badly. The usual cause is the 1.8k 5 watt resistors or the pair of 1N4744's cooking the PCB over time and going open circuit somewhere. Replace, and beef up the traces on the board with solid wire if that's the case. The other possiblity is an open Vbe mutliplier. The light bulb limiter will go bright if that's at fault. The leads like to break off the stupid little T0-92-HS device they used.

[bear]

got scope - check for DC offset first.
the previous post also has good insight.

RF is the same as audio, just lower freq, turn the timebase dial on ur scope.
Class A or AB not C.

_-_-bear

[eyoung]

hello again...

Go back to the begining check your p/s at the filter cap look for a/c component(a schematic would be nice),maybe an open/shorted diode in the bridge rectifier. some amps have separate p/s for opamps, in which case p/s diodes and filter caps are suspect ck for same. good luck...

Regards, Elwood

[sandyK]

Planerguy
Remove any onboard fuses (O/P stages) before switching on and winding up the variac, then check your DC supply rail voltages are present and correct.
I assume the problem existed before the Bridge rectifier was replaced ?

SandyK

[Planerguy]

The idea's and suggestions are coming so fast, i'm having a hard time reading up on everything. Thanks to all!

How do I measure the DC offset? I'm starting to consider just replacing every single component on the input and driver board?

In response to a few posts -
The problem existed prior to rectifier replacement. I assumed(naively) that the power supply caps were the cause, as there was some obvious leakage. I replaced the bridge rectifier just because it was old, and I was already in there and had a new one in my parts bin.
I replaced the original outputs with new Motorola MJ 15024 as a couple of the originals had some funny junction readings, although no shorts. Again, I had these parts already.
I believe the power supply components, and all outputs and drivers mounted to the heatsink are 100%. The inputs measure the same resistance, roughly 30k, AMP OFF, and the grounds are good.
That leaves some 40 misc. diodes, resistors, and transistors, along with 2 opamps to look at.
This is a Series 2 amp. and the only schematic I have available is for a series 1. They are not really very similar.

So, I'm now reading up on DC offsets. One problem is the dearth of electronics suppliers in my area anymore. 10 years ago, we had 9 electronics dealers. 5 years ago, we had 4 suppliers.
Today, only one, and it's all surplus, and pulls, if the "shack" isn't included, of course.

Sign of the times I guess.