FX Loop Buffer Circuit Hum (Not a Ground Loop) (1989 Laney AOR 50 Series II)

[Helmholtz]

The shield is only connected to the ground at the jacks, not at the PCB as well.

That's the correct way to wire cable shields.


What happens if you short R46 with nothing plugged into return?

[neptoess]

So I haven't picked the amp up yet, but the tech gave me a call. He spent an hour and a half scoping the amp, and ended up replacing IC3 (the TL072CP). He said this fixed the problem. Looks like pdf64 was correct that the chip got damaged somehow. He did let me know though that a guitar directly into the return is still noisy, but it's quieter now. I guess it makes sense, since the input impedance of the return is so much lower than the output impedance of a guitar with passive pickups. He tried a few pedals in the loop though and said they're now dead quiet, and they all had the hum before, so I'm going with the conclusion of "he fixed it".

[neptoess]

I picked the amp up. The tech was right that a pedal in the loop can be quiet. Where, before, just having a Boss pedal in the loop was constant hum, it's now dead quiet, with the pedal off. However, my weird differential situation still exists, i.e. if the level coming into the return is different than what's going out of the send, I get hum. I've contacted Laney to figure out if I'm chasing ghosts or not.

[g1]

I'm not sure if this has been covered already or not, but most FX loops, especially from that 'rack mount' era, are designed for line level rather than pedal level effects. (+4 vs -10db).
May not be relevant to this particular issue but I thought I'd mention it.

[neptoess]

+4 dBu and -10 dBV are both line levels. This amp was designed with a -6 dBV loop, per the manual http://pdfstream.manualsonline.com/f...db7f3a4e80.pdf. That's about 0.5 V RMS. Hotter than a passive pickup for sure, but I don't hear any of the pedals I've tried in the loop clipping. This is a hum issue that can be reproduced without any effects units.

[Helmholtz]

I might have mentioned it before, I think this amp has a poor grounding scheme/layout.

It seems that there is some hum produced before and after the loop and that these 2 hum signals are out-of-phase.
So hum will be lowest if both hum contributions are equal in amplitude at the return input.

[neptoess]

Helmholtz
That may very well be the case. There are a few points in the amp where the signal common is just a rivet or nut going into the chassis. I don’t know what the typical approach for amps is, but, for industrial stuff, we shoot for keeping the 0 VDC and AC common from our power supply secondaries separate and tying to the ground bus (which has a chassis connection and the supply ground connected) at only one point.

If this is an inherent design issue though, I hope Laney gets back to me and tells me that. Then I won’t feel as bad moving things around lol.

[g1]

If this is an inherent design issue though, I hope Laney gets back to me and tells me that. Then I won’t feel as bad moving things around lol.

I think the odds of anyone there being aware of the issue may be slim. Unless there was a service bulletin or something and they keep them this long.
If you can get in touch with anyone else who owns one, like you say, the fault can be reproduced without even using an effect. That should make it easy for another owner to verify for you.
Sounds like there is a laney amp forum group on facebook.

[neptoess]

So, Laney service helped me out. They had access to a series II AOR, but it wasn't identical to mine (theirs had reverb), and they verified that the amp is dead quiet with a shorted plug in the return jack. He also noted that the AOR he tested with had the return jacks grounded via the shield, instead of via the chassis, as mine is. As a quick test, I tried grounding my jacks via a different ground point within the amp instead of the chassis.


No dice, so I went back to grounding it to the chassis. Also, when lifted, there was no continuity between the jacks and ground, as was expected.

Then I started probing around the amp with this cheapo scope I got off Amazon.

All of these are taken with the amp on, shorted jack inserted, amp humming, referenced against the return jack ground, AC coupled (since some of the measurements around the op amp have -15 VDC present)

Return jack tip


C30 / R41


C29 / R44


R44 / C32



TS6 / R46


TS6 Grounded Leg


C36 / R58


So, it looks like there's no significant hum on the op amp inputs, but there is significant hum on its output. Anyone have any ideas?

[nickb]

Try this. Disconnect the wire than goes to the return jack hot from the PCB. With a short wire, connect that node (R41a / C30) to C28 positive end. Hum? Now try connecting it to the negative end of C28.

[neptoess]

Removed the wire from the return tip (R41A / C30 node) and shorted it to both positive and negative side of C28. No hum in either scenario. Nothing was plugged into the return jack for these tests (though that shouldn’t really affect anything for this test).

[nickb]

That experiment tells us that it's the ground to the jacks that's the problem. Guessing that it's due to magnetic induction, let's make the loop smaller. Reconnect the shielded cable that goes to the jacks, remove any grounds other than the shield that got to them and ground the shield of the cable on ground ( positive) end of C28. Now try the short in the return jack test.

[neptoess]

Thanks to everyone who helped out. The problem is completely fixed by re-routing the ground for the footswitch, send, return, and line out jacks to be the grounded leg of R43 instead of the chassis ground lug originally used.


I was going to run this under the board and solder it on the bottom, but I realized when lifting the board that I would need to unsolder a few wires to actually get good access to the bottom, so I decided to just run it on top. I chose R43's grounded leg instead of C28 for two reasons: easier to access, and a resistor is much less likely to need replaced in the future than an electrolytic capacitor.