AIDEAN MESIA 100 troubles with hum and reverb

[Gerhard Grass]

Hi to all!

A customer brought me an AIDEAN Mesia 100 Boogie clone to have a look over it.
First I recognized was that someone made a Footswitch-Mod. A missing reverb-tank
some minor fixings an a changed reverb-transformer.

The thing would work fine and sound good if there were not the strange hum-problems.

All gain and the two master turned down and the thing is quiet.
Turning up the master-Vol makes the amp hum. (not a solid hum but some harsh, brizzle
and disturbing sound)

I added a new reverb-tank. I found Mesa MKII used a tank type 9AB2A1B so I took this model
.
Now noise is much stronger if you turn up the reverb.
If I disconnect the INPUT of the tank the hum from the reverb stops. If I disconnect the OUTPUT the
hum changes a bit but persists.

I am rather confused with this thing. Not to mention there that I was unable to find a schematic or manual for this thing.
The schematic of the Boogie MKII seems close but that does not help a bit...

Any help, hint or suggestion is appreciated.

Thx in advance and greetings from Austria

Gerhard

[Mick Bailey]

The grounding scheme of reverb tanks varies according to model. Some have isolated connections, some have one or both connected to ground. You may have a ground loop caused by the reverb grounding - the 5th character of the model indicates the input and output are connected to chassis ground. Take a look to see if the ground connection to the folded steel tank can be experimentally lifted on the input connection. You still need both black/green leads connected to the drive transducer, though.

A disconnected output can hum because the recovery stage is pretty high gain and the lead acts like an antenna - does it still hum with the reverb control turned off?

The other thing that affects reverb hum is the tank orientation. If the recovery transducer is close to a radiated magnetic field (such as from a power transformer) it will pick up hum that gets amplified by the reverb recovery stage. Sometimes flipping the tank cures this.

[Gerhard Grass]

Thx for help,

What I checked:

reverb-tank connections:

connected: IN / OUT -> hum
disconnected IN / connected OUT-> No hum at all
connected IN / disconnected OUT -> hum ++ (with shorted cable to the amp no hum)
IN with open GND / connected OUT -> hum --
connected IN / OUT with open GND -> hum

The hum can be completely removed when turing down the reverb-control

[Jazz P Bass]

Your tests are showing a tank input issue.

Attached is the MKII reverb circuit.

MKii Reverb Circuit.pdf

Is it close?

Start with the B+ node at the 10K 2 watt resistor.
Is there Vac ripple riding on it?

[Gerhard Grass]

Thx for the schematic!

Without a layout of the AIDEAN it is nearly impossible to locate any part in this amp.
Please see attached picture.



My hope was, the problem is located anywhere, where you can find it without taking the whole thing apart.

In addition I have to say the amp is rather noisy. When you turn up one of the master-volumes it hums too, but not so bad.

[Mick Bailey]

It appears that your recovery stage is behaving normally.

There's a way to test the tank drive circuit; it's a small power amp which will produce about 1/2 watt or so into an 8 Ohm load. pull the input to the tank and connect it to an 8 ohm speaker and you should hear the reverb drive signal when you play your guitar and turn the reverb control up. Not exactly hi-fi, but good enough to check for hum.

[Gerhard Grass]

Hi Mick

it was just as you wrote. The reverb-tank I used had both sides grounded and this appeared to be the problem.
Removing the ground on the input connection and all was good.

Thx very much
Gerhard

[drewl]

That's one good thing about the newer reverb tanks, you can configure the RCA jacks to be grounded or not.