SWR pro ser 900 distorted

unclerny · 08-29-2009, 09:39 AM

Hi forum, I had a SWR 900 head come in with a bad power switch and a broken knob, a few were dirty also.

replaced the power switch, no issues.

The knob was a bass tone control. I used a simple panel mount with leads to replace it. seems fine.

When I reassembled the amp it now has a brief, 15 second, clip light at power up, not sure that's normal. I can't get the manual to download fromt he swr website.

My problem that is certain is that the amp seems to have huge gain and is distorted. Even with the gain down all the way the clip light will light and then the compressor. Plugging into the eff. ret. shows the PA is fine.

I figure I got some wires messed up since everything is soldered in place, I guess they've never heard of disconnects.

I'm hoping someone else has run into this problem during a rebuild.

It seems to come from the circuit around the gain pot as removing this opamp makes the biggest difference. removed the tube and all the rest of the opamps and the clip led still lights during startup. not so with IC2 as the schematic calls it.

UE

Enzo · 08-29-2009, 08:40 PM

Seems unlikely that someone else would make the exact same wiring problem. But maybe.

Are we talking SM 900? Here is the newer preamp. or is yours from 1997 or earlier?

unclerny · 09-01-2009, 12:39 PM

Thanks Enzo, I figured I'd give it a try. I'm at a loss as I don't have many reassembly problems. Pretty observant. I'm not sure what year but it is the SM900. I'll compare your schematic against it.

The owner has hauled it around unracked so it's aged a good bit and explains why several of the stacked pots are tweaked a bit but functional.

As before, I've pulled each of the opamps to track to a particular circuit and the only one that stops the over driven problem is the one right behind the gain pot, U2 I believe. All others seem to have no effect. I tried another tube also.

unclerny · 09-01-2009, 12:43 PM

Enzo, you're schematic appears the same and is better than the one I found.

Thank you, UE

unclerny · 09-01-2009, 06:55 PM

Enzo, this amp was built 11/27/97.

I don't know if this helps.

Tim

Enzo · 09-02-2009, 12:51 AM

It matches the schematic or it doesn;t, that is the test.

08-29-2009, 09:39 AM	#1
unclerny Junior Member Join Date: Jul 2009 Posts: 11	SWR pro ser 900 distorted Hi forum, I had a SWR 900 head come in with a bad power switch and a broken knob, a few were dirty also. replaced the power switch, no issues. The knob was a bass tone control. I used a simple panel mount with leads to replace it. seems fine. When I reassembled the amp it now has a brief, 15 second, clip light at power up, not sure that's normal. I can't get the manual to download fromt he swr website. My problem that is certain is that the amp seems to have huge gain and is distorted. Even with the gain down all the way the clip light will light and then the compressor. Plugging into the eff. ret. shows the PA is fine. I figure I got some wires messed up since everything is soldered in place, I guess they've never heard of disconnects. I'm hoping someone else has run into this problem during a rebuild. It seems to come from the circuit around the gain pot as removing this opamp makes the biggest difference. removed the tube and all the rest of the opamps and the clip led still lights during startup. not so with IC2 as the schematic calls it. UE

09-02-2009, 12:51 AM	#6
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 10,323	It matches the schematic or it doesn;t, that is the test. __________________ Education is what you're left with after you have forgotten what you have learned.

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09-01-2009, 12:39 PM	#3
unclerny Junior Member Join Date: Jul 2009 Posts: 11	Thanks Enzo, I figured I'd give it a try. I'm at a loss as I don't have many reassembly problems. Pretty observant. I'm not sure what year but it is the SM900. I'll compare your schematic against it. The owner has hauled it around unracked so it's aged a good bit and explains why several of the stacked pots are tweaked a bit but functional. As before, I've pulled each of the opamps to track to a particular circuit and the only one that stops the over driven problem is the one right behind the gain pot, U2 I believe. All others seem to have no effect. I tried another tube also.

09-01-2009, 12:43 PM	#4
unclerny Junior Member Join Date: Jul 2009 Posts: 11	Enzo, you're schematic appears the same and is better than the one I found. Thank you, UE

09-01-2009, 06:55 PM	#5
unclerny Junior Member Join Date: Jul 2009 Posts: 11	Enzo, this amp was built 11/27/97. I don't know if this helps. Tim

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