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

[neptoess]

Hey everyone. I've got a 1989 Laney AOR 50 Series II head (schematic: https://www.thetubestore.com/lib/the...-Schematic.pdf) that has an interesting issue going on in the FX loop.

1. The amp sounds completely fine with nothing in the loop
2. The amp sounds completely fine with just a cable between the send and return jack
3. The amp sounds completely fine with a 9V battery powered true bypass pedal in the loop, but only when the pedal is off

I get a hum (sounds like 60 Hz) in any of the following situations:

1. True bypass pedal in the loop, turned on
2. Buffered pedal in the loop, regardless of whether it's turned on or off
3. Either of the above plugged directly into the loop return, e.g. guitar -> pedal -> return, with nothing plugged into the amp's inputs
4. A guitar plugged directly into the loop return, with nothing in the amp's inputs
   1. The interesting part here is that I get the hum at full volume with the guitar volume on 0, but as I raise the guitar volume, the hum gets quieter, almost completely disappearing with the guitar volume ~50% up. Past that, the hum gets louder until the pot is at 10

To rule out any portion of the preamp, I ran the amp without V3 and redid the tests involving signals directly into the loop Return. The problem remained the same.
Another somewhat interesting observation is that, if I turn all the knobs on the amp to 0, and plug a signal into the Line Out, instead of the Return, the hum is still there, but it's quieter.

Some background

1. I've had the amp for about 10 years. I don't think I've ever used the effects loop
2. I replaced every electrolytic cap in the amp around Oct 2015 (overly paranoid, but hey, the amp sounded a hell of a lot better afterwards)
3. I just replaced the Send, Return, and Line Out jacks yesterday with brand new Cliff UK jacks to rule those out (it didn't change anything)
4. Inserting a cable into the loop return, I measure 100 kOhm DC resistance between tip and sleeve

Some pics from last night after my jack replacement

Overall shot of the amp's PCB


Close up of the section of the board containing IC3 (TL072CP dual op amp IC used for the effects loop buffering)



I'll be getting some TL072CPs in the next week or so, so I'll replace the IC, but it's sort of a last ditch effort.

Anyone else have any ideas?

Thanks in advance

[Jon Snell]

It cannot be the TL072 otherwise it would hum all of the time.
R41A is your 100k.
There is an earth loop. The send and return sockets are both ground when your effects are in use both by thew internal wiring and your effects pedals.
These Laneys were wired that way.
Swapping ground to switched ground on the send socket or disconnecting the send ground, cures that.

[neptoess]

I thought the same thing Jon. However, if you look at the inside of my amp, I only see one spot in the amp where there could be a loop. There's a piece of buss wire soldered from the ground bus to the center lug of V1. I marked this with a ? in this picture. If that lug is somehow already grounded, that's a loop for sure.



EDIT: My guess as to the origins of that wire is that, when I first bought the amp, V1 had one of those spring loaded shields on it. I put shields on the other 3. Not really sure why, considering my Class 5 combo has no shields, and those tubes are seeing a lot more abuse than the ones in my head are


EDIT 2: Another interesting tidbit is that I found a WordPress blog where someone took a picture of the inside of their series 2 AOR (a 100 W, but the preamps are identical), and that piece of buss wire _isn't_ there
https://ampstack.files.wordpress.com...eries2-004.jpg

I'm going to remove it and see if it improves anything

[Helmholtz]

That socket centerlug is just a mechanical post, doesn't connect to anything by itself. Easy to verify with an Ohmmeter. So no path for ground currents.
A ground loop is a closed loop for ground currents.

[neptoess]

That was my thought Helmholtz. Seems like the fact that it's grounded makes no difference really.
Jon Snell,
Do you still think it's a ground loop? Note that I can plug a guitar with passive pickups into the return, with the V3 tube removed from the amp, and go from dead silence to this hum. Even with guitar's volume pot on 0 (which should short the power amp's input to ground and give me dead silence), I get the hum. The super weird thing about that is that the hum is nearly gone with the pot in the middle.

[pdf64]

I wonder if disconnecting the jack ring terminal buss bar from the chassis lug may help?

[neptoess]

Hey it's pdf64, who's been helping me with this exact problem on TGP. I'm fairly certain that the chassis lug is the only way the entire bus is grounded (see https://music-electronics-forum.com/...856#post918856 for a picture of the other end of those shielded cables on the return jack)

[Helmholtz]

For a test wire a shorting plug and insert it into return. Hum?

[neptoess]

Helmholtz
The guitar I was testing with had no tone pots, just 2 volumes and a toggle. So, my test where I plug the guitar into the return with the volume pots on 0 is effectively identical to the shorting plug, right?

[Helmholtz]

Helmholtz
The guitar I was testing with had no tone pots, just 2 volumes and a toggle. So, my test where I plug the guitar into the return with the volume pots on 0 is effectively identical to the shorting plug, right?

Maybe not quite. There's still the guitar cable and pots often don't go down to zero Ohm.

[neptoess]

Alright I'll give the shorting thing a whirl. I have a footswitch I made for this amp, so it will work perfectly for this (the switch simply switches between an open circuit and shorting tip-to-sleeve)

[Helmholtz]

Alright I'll give the shorting thing a whirl. I have a footswitch I made for this amp, so it will work perfectly for this (the switch simply switches between an open circuit and shorting tip-to-sleeve)

That would still mean some cable with unknown shielding quality.

[neptoess]

Alright, I'll make a shorting plug (I actually don't have any spare 1/4" plugs around, but I'll find a cable I'm not using and pull its end off)

[Jon Snell]

I presume you are UK based. Is the hum 50 or 100HZ?
Did you realize the 072 has only one supply rail ... -30volts. It uses a virtual ground made up of R40 and R43. If there is an issue with C30, when an outside source that drags the return line to the chassis, it may hum.
Is there any DC voltage on the tip of the send or return sockets when nothing is connected to it? If there is, replace C25/30
Have you any DC on the return socket, there shouldn't be.

[neptoess]

US based. 60 Hz hum. The schematic is weird. IC3A and IC3B are both part of a single TL072CP IC. The schematic only shows the power supply for the IC on IC3B for some reason


I did not think to try measuring DC voltage on the jack. I'll give that a shot.