Ampeg B5R sine wave on output with no input

[audiopete]

Hello I have a Ampeg B5R that is driving me crazy. There is a sine wave on the output and audio ground with no load and no input and all pots at zero. The oscillation shows up on all 3 power supplies - power amp, +-15V and the relay power supply.

I have checked rectifiers and replaced some diodes and caps in the 15V power supply with no luck. The thing that is throwing me is the wave shows up even if I pull the power transformer connector from either power supply section or pull the fuses.

This amp originally came to me because it just died - no power. I found that the pc mount IEC AC connector on the main board did not have solder on the neutral lead to the solder pad but there was some evidence of heat from discoloration and the copper pad had lifted from the board at the neutral lead. I assume there was a small solder connection because the amp worked till now but I guess the heat from the resistance of the current eventually melted the little bit of solder away. I think this incident caused spikes and down circuit damage but the amp plays fine except for this sine wave hum on the output. I resolded the connector, replaced the circuit breaker that my client had taken apart thinking it was a fuse and everything worked. It came back after it's first gig where apparently the output got badly distorted and bad static or noise. They used a backup amp and all was well to finish but I can not re-create this distortion/noise issue and all I have is the oscillation on the output.

I found some cracked ground solder joints at the IEC connector and jumper wires used to connect the ground to the chassis ground via pc board mount screw.

Also the sine wave is more of a triangle or saw on the transformer side of the rectifier diodes.

Any ideas? Thanks for your help.

[Enzo]

So it "oscillates" even with the power transformer disconnected?

What frequency? Touch your finger to the tip of your scope probe - that is 60Hz. Is the oscillation the same?

I could be wrong, but I really don;t think you have oscillation if the circuit has the power disconnected.

Otherwise, if you do indeed have oscillation at RF, check for an open R168/268.

If you have some sort of sine wave all over everything, and it is at mains freq, then I suspect you are missing a ground. If your scope ground connects to chassis of the amp, and you still have a signal on the ground of the output, the output ground is not connected to the chassis.

[audiopete]

I apologize if I wasn't clear. There are two secondary harnesses off the toroidal power transformer - one feeds AC to the Mosfet rails supply and the other feeds the +-15V supply which also feeds the 6V relay supply. There is a 60 hz sine wave everywhere regardless if one power transformer harness or the other is plugged in. With both unplugged there is no power. Also removing one set of fuses or the other set on the secondaries has no effect.

I found cracked solders on the ground pin of the IEC connector and two jumpers that get sandwiched under a washer and board mount screw to ground to the chassis. Perhaps there are more bad ground solder joints to fix? I've looked over both boards under magnification but I could have missed or not been able to see one. Also the preamp board doesn't seem to be involved.

Do you think this would cause the weirdness I described of static or crackling or big distortion? I didn't hear it personally and I haven't heard in since I've had the amp.

Thanks for your reply Enzo.

[g1]

I was thinking the same as Enzo, that you may not really have an oscillation. You said you have not been able to get the amp to show it's fault ("apparently the output got badly distorted and bad static or noise").
So, aside from what you see on the scope, do you HEAR excess hum on the output? 60Hz oscillation is hum.
This sounds like the kind of thing that happens to me, then after spending way too much time I find out it's something wrong with my test set up.
Maybe a broken scope ground wire? Can you verify your scope is good with another amp or something?
If there was really 60Hz everywhere instead of supply voltages, I don't think the amp could work. Or is the 60Hz riding on top of the proper DC voltages?

[J M Fahey]

1) Where do you clip your scope probe ground clip?
2) You also said

There is a sine wave on the output and audio ground

*How* do you read a sinewave on the audio ground, if both hot and ground scope probe ends are clipped to the same point?
3) What do you see on your scope screen if you clip together ground to hot probes? "In the air", not connected to anything else.
Please answer all 3 questions.

[Enzo]

When I say missing ground, I am concerned there is an internal connection missing or burnt away between common and chassis in the circuit.

[audiopete]

Hi g-one - scope is fine as far as I know - use it all the time - yes 60 hz on top of supply voltages. Yes you can hear it in the speakers when you turn up the volume but the cab is a 210 so it doesn't produce it very loud (and it doesn't measure much on the AC voltmeter). This is not necessarily related to the original distortion/noise problem but I found this issue while checking the power supplies and I definitely found cracked ground solders. The Ampeg schematic has charts of test points and waveforms and I should see a flat line but I don't. The sine wave collapses when I shut the amp off.

JM -#1 I clip the ground from the ground post on the scope itself to the chassis. #2 not using the probe ground but the ground post of the scope to the chassis. #3 - transient spike then flat line

This amp would fall under the bad years for Ampeg quality control doesn't it? I'm getting the idea here that I have to keep looking and fixing cracked solder joints. I also thought to double check the transformer harness quick connect on the board because the center tap connects to all grounds. I am also going to try some continuity tests to ground from various points.

Other than this the amp looks good for voltages and operates fine. I really don't know what to say about this noise/crackling/distortion problem other than another bad solder affected by heating.

Thank you everyone for your help - much appreciated.

[J M Fahey]

Thanks.
Since this is a weird problem, I distrust *everything*
#3 tells me your scope cable is not broken (mine sometimes is, just inside the ground crocodile, then I see *strong* hum everywhere ... even shorting probes)
#2 and #1 suggest what Enzo suspects, that chassis ground and circuit ground somehow got disconnected.
It happens, because modern designers tend to join all circuit grounds as needed but in a "floating" way, and use a single wire to the chassis, often looking for least hum (or RF interference or whatever problem troubles them).
Said connection may very well have been lost, even because of a cracked solder or something.
I'd solder a #18 wire from the centerpoint of the big filter caps (which is usually considered the ultimate ground point) straight to the chassis ground lug and see if this solves the mystery.
Do not disturb the green/yellow outlet ground wire in the least.
If you want to play it real safe, just drill a small hole 1 inch away and add a new dedicated ground tab.
That said, some Rack mountable Pro stuff have a ground lift switch as a regular feature, *maybe* yours does and the ground lift switch is open !!!!

[audiopete]

The only ground point to the chassis is from one pc board mount screw to the chassis. It has a large washer that sits on two bare wire jumpers that are soldered underneath to the ground circuit and this connects via the screw and washer to a chassis board mount post by physical contact only.

I am going to look at the underside again for solder problems then put it back together and look for ground continuity from the circuit board to the ground prong on the cord.

I love ground problems!

[audiopete]

Some pix - this is the original repair - inadequate solder on the neutral leg of the IEC connector leading to mini-meltdown and total failure:


Next is the area of ground trace on the board. You can see the neutral repair to jump the damaged trace.


Now I traced out the paths - I have continuity from the ground prong on the cord to the yellow area but nothing to the green circuit ground.


The ground jumper wires on the component side that the screw and washer sandwich down:


and finally the chassis ground/board mount:


Now I guess the question is why no continuity and where would it have been made originally?

[g1]

Do you have the schematic? If so, upload it here. You have found the area that connects the IEC ground to the chassis. But this is not necessarily where the circuit ground connects to chassis. That could be through pot brackets or preamp or who knows where.

[audiopete]

Thanks for the suggestion. I double checked with the preamp board plugged in and no difference. The pots casings are not grounded and the mounts are insulated so no grounding there. The pre board circuit ground does show continuity to the main board circuit ground, but not back to the IEC ground.

[52 Bill]

Page three of the schematic shows that all three grounds meet somewhere around header J31 pin 2. This is where the power transformer secondary center tap connects to the board ground.

[audiopete]

Yes and that trace goes to the star ground arrangement of traces that the main filter caps are on (and all circuit grounds) but all that does not make contact with the chassis ground/AC ground. If you look at the area a shaded blue it looks like there may have been supposed to be a connection between the two ground traces. I am hesitant to connect it because I don't know for sure if it is supposed to be like that. Wish I had a trace diagram like I do for the pre board.

[audiopete]

I noticed there are 2 pads on the chassis gnd side and 2 pads on the circuit gnd side. On the other side these are labelled JW138 and JW139. Is it possible that these jumper wires were never installed from the factory? I supposed if they were installed factory then someone could have removed them down the road for some reason but I can't think of one.

Anyone think it's a bad idea to jumper them back?