Ampeg B5R sine wave on output with no input

[g1]

This schematic should be close: http://music-electronics-forum.com/a...ampeg-b-4r.pdf
I believe what 52Bill mentioned is on page 6 or 9.
I would be hesitant about installing those wire jumpers. I find it hard to believe someone would remove them. Is there maybe a layout drawing available? Can you ask Loud about JW138 & JW139 ?
Could you show photos of the rest of those ground traces?

The reason I am hesitant is I think the circuit ground may still be connected to chassis ground elsewhere, like a jack or something, and that connection may be missing. They may have had ground loop issues with JW138 and 139 and revised the circuit.

[J M Fahey]

Maybe .... and also maybe an earlier user had ground problems within his rack setup and carefully pulled all internal grounding but one ... and yours is not that one.
*I* would replace those jumpers and retest.
If now I have hum problems, instability, oscillation, whatever, I just pull them out again.
If I've solved the mystery with no new problems .... cool !!!!
Experimenting rulez

[audiopete]

Yah I will try that but one thing that occurred to me late last night (and has been driving me nuts since) was the signal ground could be through the input jack sleeve via the chassis and because I have the pre board out - no ground. I checked the pots but neglected to consider the input jacks.

Of course nothing would surprise me at this point.

[audiopete]

That the exact schem package LOUD sent me, g-one, thanks for posting.

haha you can't ask LOUD about anything technical...they just refer you to a service depot. Parts and schems....that's it.

[g1]

one thing that occurred to me late last night (and has been driving me nuts since) was the signal ground could be through the input jack sleeve via the chassis and because I have the pre board out - no ground.

Right now I have a Hartke GT100 to look at. The circuit ground connects to chassis via a metal grounding washer at the input jack, which is soldered to the jack ground. I had the preamp board loose and was having similar issues so I thought I'd mention the jack.

[audiopete]

Right now I have a Hartke GT100 to look at. The circuit ground connects to chassis via a metal grounding washer at the input jack, which is soldered to the jack ground. I had the preamp board loose and was having similar issues so I thought I'd mention the jack.

We have a winner. That was exactly the problem. Man, sometimes you get spun around and it's good to have so many excellent people to help you sort it out. Thank you everyone for your effort.

So in conclusion I did find that the ground leg of the IEC connector was cold soldered and cracked. As well the two ground jumpers that are under the chassis screw were cold soldered. When this amp freaked out it was connected to a PA system thru the DI out. I am thinking that when this amp lost it's main ground due to heat expansion this set up a loud ground buzz with the PA and if the bad solder connection was intermittent it would probably sound pretty gnarly.

I am testing it now with a EV power speaker hooked to the DI out and it sounds wonderful.....except.....

There is an interesting gating effect on the overdrive channel when the signal dies down to almost nothing. The buzz from my Jbass is enough to keep it open but as I dial down the volumes the signal cuts at about 15% volume and below.

I haven't investigated this yet and it was there from the get go but I was sidetracked on phantom grounding problems. I doubt it will even be an issue to the client as he never uses that channel and I don't thinnk he wants to invest any more into it.

Any ideas? I am thinking maybe an opamp or transistor in the OD circuit cutting out. Maybe a Jfet...

[MarkusBass]

The problem with the IEC connector is very common to Ampeg amps - I fixed many of them with exactly the same problem. And it's only Ampeg that has this problem .
You haven't said how exactly the preamp ground is connected. Is it a washer under the input jack? Other Ampeg amps (I don't remember how this is done in B5R/ B4R?) have a dedicated chassis screw with two jumpers on the preamp board. So maybe when you removed the preamp from the amp, you disconnected the ground.
Regarding the problem with the overdrive channel - you haven't posted a schematic for the preamp. You posted only schematic of the power amp and the power supply.

Mark

[audiopete]

Hi yah star washer under the input jacks. The other clue was that area on the chassis had no paint.

I can't post the schems due to the waiver. Another poster put that up. Perhaps g-one has the pre schems.