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?

 

[east electronics]

well ...not really

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]

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.

 

[sandyK]

Planerguy
When you construct a new amplifier, one of the golden rules is that you check your power supply voltages BEFORE you connect it to your new amplifier. You do not appear to have verified that your Power supply without load, has the correct DC voltages, without excessive ripple components. There must be some way that the amplifier can be disconnected from it's PSU for fault finding purposes? If the amplifier does have a fault, there is a small chance that the PSU itself may have suffered further damage.

SandyK

 

[wg_ski]

There must be some way that the amplifier can be disconnected from it's PSU for fault finding purposes? If the amplifier does have a fault, there is a small chance that the PSU itself may have suffered further damage.

SandyK

PL400s will operate properly into a high impedance load with the rail fuses removed. It just removes collector voltage from the OPS and you can't generally damage it. Output voltage should center on zero and be able to pass a signal.

 

[d3imlay]

A scope photo sure would help. I specialize in these amps and haven't seen what you're describing. I would avoid wholesale parts changeout as it's more likely to introduce new problems.

Here's my web page where you'll find schematics and service bulletins.

Phase Linear Site

On the driver board, R101 (7.5K/2W) likes to change value and eventually open up. I replace these without even thinking.

On second thought, you can not bring this amp up with a variac/light bulb AND a load attached. At around the 50 volts you're talking about it does exactly what you're describing until the front end starts working. With a variac, as you bring up the voltage the output goes DC. Then with a little higher input voltage the output breaks into oscillation, around 40-50 VAC input. Then if you keep going it stabilizes by 60VAC in.

With a light bulb it will never come up with a load on the output because of the DC offset at low AC input voltages. The bulb just gets real bright.

This is a non problem if the amp is fitted with an output relay. PM me for more info on my design.

 

[Planerguy]

d3imlay -

Thank you - This is exactly what I was looking for!

 

[Planerguy]

Well - tonight we had music! The amplifier behaved just as d3imlay said. Initially DC, then oscillation, then steadied around 60 to 70 volts. Power rails are fine, output devices are fine.
I warmed it up for 1/2 hour or so on a dummy load, and confirmed bias - only needed a small tweaking. DC offset was lower then factory spec by about half.
Then I hooked it up to a cheap set of speakers(nervous even then) and played music for about a half hour. On shut down, she exhaled thru the speakers like a pig for a few seconds, almost a squeal, not quite.
A few minutes later, I hit the power on button, and amp latched up. .woof......smoke time!
Looks like a couple of 10 ohm, 1 watt resistors didn't like the surge. Hopefully thats all that went.
I still have to find out why it latched? Clearly we have some more
work to do, and I'm hoping nothing else went. It also looks like I'll be ordering a relay board for it.
Oh BTW - it sounded really good, while it lasted.
Thats the update.

 

[Netlist]

Sorry to hear.
They are tricky devils and I recall replacing ALL diodes on the driver board along with the whole battery of output transistors and many emitter resistors.
Luckily, the 1N4148 are cheap and it doesn’t take that much work.
You will need to measure every single transistor as well and make sure that none of the collectors of the TO3's touch the heatsinks. (Measure with ohmmeter and replace insulator if needed)

Oh, and were both channels motorboating?

/Hugo

 

[d3imlay]

The normal shutdown is a thump which is really a DC offset that uses the speakers to discharge the filter caps! There is an upgrade that replaces the front end opamp to reduce the thump.

I'm surprised the 10 ohm resistors burned. They're across the base/emitter junctions of the outputs which should limit the voltage across the resistor. That seems to indicate that all of the .33 ohm emitter resistors blew out. I don't recall seeing this happen, even by the knuckleheads who wrap foil around the fuse.

At this point you go through and check every P/N junction on the board. Also look at the 7.5K/2W resistor on each channel. They like to change value or even open up. Are the outputs all originals or a mixed bag?

You gotta use the 100 watt light bulb the next time you bring it up. Attached is a pdf of a light bulb test set I've built in a 4" box.

Did the speakers survive?

 

[Planerguy]

Actually, 4 of the outputs went on 1 ch. too. That's what cooked the 10ohm resistors. The MJ15024's were labeled Motorola, so they were old, and one of them had a loose cap. I don't know if they are supposed to be in a vacuum? In any case, I HAD GLUED IT down, and I suspect it failed and took 3 others with it.

Once i get it up and running, I'll look into the relay. Does that give it a soft start? Do anything about the thump?

 

[d3imlay]

The top of the T03 was loose? I've never heard of, or seen this. I don't think a vacuum would make any difference. The relay board is a delay on turn on and immediate turn off. There is no soft start on the power supply. I don't think it's necessary to have a soft start on these amps. You can't wedge enough caps in the amp to need it. If you do an outboard mod then you put the soft start there.

 

[Planerguy]

Yep - the top cap literally fell right off. They are just glued on. I have seen this in Rf transistors, such as Motorola MRF421, or Toshiba 2SC2290's.
This has always been do to overheating, and those are an entirely different case design too.
With the TO3 in question; it was a supposedly new transistor, and it tested ok prior to being mounted. I say "supposedly new", as it came from a local surplus electronics outlet, and sometimes they sell pulls, although they will tell you that IF they know.
I'm thinking, if nothing else, it was defectively manufactured, and that led to it's failure, which cascaded to others.