SWR 750x problem

[bigdrums925]

Hi all, This is my first post so please forgive me if I'm not up to speed with the how to's. I'm having a problem with a SWR 750x where all seems fine except when I put a load on the speaker output. When I do, I can see the current draw on my variac is excessive. There are no symptoms or oscillations from the preamp noticeable on the oscilloscope. I've checked all the devices on the power amp board and no shorts that I can find. Apparently the problem is only once the transistors turn on. Replaced the 2SC3264 power transistors and the drivers. It came in with all 6 of the power transistors shorted and the .5 ohm 5 watt resistor open at the power in jack. At present, I have 2 of the 6 power transistors installed, and a 150 watt bulb in line with the variac. BTW current draw is nearly nill without the load connected. Even the headphone out has an unclipped output as long as the speaker load is not connected. Without the 150 watt bulb in line when I ramp up the variac the current meter will suddenly peg at about 50%. All voltages appear normal

Any suggestions are appreciated. Thanks!

[nickb]

Hi all, This is my first post so please forgive me if I'm not up to speed with the how to's. I'm having a problem with a SWR 750x where all seems fine except when I put a load on the speaker output. When I do, I can see the current draw on my variac is excessive. There are no symptoms or oscillations from the preamp noticeable on the oscilloscope. I've checked all the devices on the power amp board and no shorts that I can find. Apparently the problem is only once the transistors turn on. Replaced the 2SC3264 power transistors and the drivers. It came in with all 6 of the power transistors shorted and the .5 ohm 5 watt resistor open at the power in jack. At present, I have 2 of the 6 power transistors installed, and a 150 watt bulb in line with the variac. BTW current draw is nearly nill without the load connected. Even the headphone out has an unclipped output as long as the speaker load is not connected. Without the 150 watt bulb in line when I ramp up the variac the current meter will suddenly peg at about 50%. All voltages appear normal

Any suggestions are appreciated. Thanks!

Hi and welcome. With any problem is a good idea to include a schematic. Hopefully, this is the one.

Leave the speaker off until you get this fixed or you could damage the speaker. It sounds like the output is not at zero volts so check the voltage on the output terminal to 0v. It should be in the mV.

That being the case (i.e not near zero) then something else is blown - the transistors Q3, Q16, Q5-Q8, Q9 and Q12 are most likely as are the associated components. There are a couple of things you can do. Looking only at the power amp section:

1) You can measure the voltages on the CB & E of each transistor, write them onto the schematic, post it and we can often deduce what is wrong from that.
2) Use your meter on diode range to check the transistor junctions. Also use the ohms range to check the resistors. Both of these can give misleading results due to parallel current paths. If you have the tools, it often easier to remove from the boards and test.

[J M Fahey]

SWR are known for "waking up stupid" (me too, so I can't complain ) if loaded, so if they are current limited (bulb) or very low voltage (slowly ramping up (variac) , they slam output against one rail and ... stay there.
So turn it on with just the bulb, full voltage, NO load.
If no DC voltage at the output, apply load (without turning off) , if normal play something at low volume.
If it sounds normal, it's fine.
If bulb blinks in rhythm with the music, it's fine.
Then you can unplug from the bulb and plug straight in the mains, it should thump, as always, but play fine.
Just don't drive it hard, you have only 1 pair transistors.
Then turn off, added needed transistors, retest as described and the amp will be fully repaired.

EDIT: if the amp is still bad, you'll have to continue troubleshooting as suggested above.

the schematic comes out as empty/blank, please repost it but ZIP it so server does not think "I already have it"

[nickb]

Thx for heads up Juan.

After several attempts with different file names I edited the post took out the attachment and changed it to a link. Something a bit odd seems to be going on with the attachments.

[J M Fahey]

Yes, 1 or 2 years ago there was a BIG server meltdown, but worst is that it lost the files themselves but seems to keep some kind of index, so if you try to reload it, it doesn't.
Reposting with different file size (usually by Zipping it) is enough, or if editable, adding a couple words or an extra line or two.
Oh well.

[nickb]

Thing is, this was fresh upload. I suspect there is still a gremlin lurking in the system that will no doubt bring about a final apocalyptic crash in the not too distant future.

Anyway bigdrums, how is the amp coming along?

[Enzo]

What happens is the file is empty but still there. Your fresh upload fits the description for the empty file, so the system thinks it already has it. So the upload doesn't work. LOts of us zip the file, and that fools the system. I usually go into the file and add some small text, usually my initials on a page somewhere. That also changes the file nature enough to get by.

[Mick Bailey]

SWR are known for "waking up stupid" ......

There are quite a few that don't like a bulb limiter and stick to one output rail; some Trace Elliots and Sessionettes are particularly bad. Maybe we could pull together a 'sticky' list.

[bigdrums925]

So I turned it on full power, no load and get 135mv on the speaker outputs very slowly creeps toward zero. Seems this is a bit high so will check the associated transistors as suggested. I will note the C,B & E voltages where possible and try to post the amended schematic if I can. Thanks!

[Enzo]

135mv is small, I don't worry with that. Especially if it trends towards zero as the thing warms up. And once we know the amp I snot making DC and seems stable, then put a load on it. Your little 135mv might just then disappear.

[bigdrums925]

Haven't figured out how to post the modified schematic with the voltages added. Can anyone tell me how? I have it now in .jpeg format on my computer

[nickb]

Haven't figured out how to post the modified schematic with the voltages added. Can anyone tell me how? I have it now in .jpeg format on my computer

Enzo already spoke to this above #10. 135mV, especially trending to zero, should not be a problem. Connect a load a try it out.


So, you probably don't need to post the sch but just use the little tree icon to insert an image if you do.