ADA MP1 - Tube Board Problem and Resolution.

[MR THUD]

Hi Guys,
Just sharing this in the hope that it may help others identify suspect Tube Board related issues on their MP1.

PROBLEM IDENTIFICATION.
My MP1 was working great ..right up until it wasn't..
It started producing a 60cycle hum that over the course of a few hours got to be almost as loud as the guitar signal. A 60cycle hum isn't the same as 'Hiss' its more of an electrical 'drone' sound.

First I had to ensure that the problem was indeed due to the ADA MP1 and not something else in my signal chain.

So how did I do that?

- First rule out the Guitar and Effects as the possible cause.
I disconnected the guitar and all effects such that I only had the ADA MP1 plugged into a power amp and cab. (No guitar input, no effects in the loop or chain).

Step one, Connect a known good speaker cable between the Poweramp and cab. Power up the Power amp on its own.
RESULT: No Hum.

Step two. Connect the output of the MP1 to the input of the Power amp.
Power on the MP1 and Power amp.
RESULTS The loud Hum returns..

Ok, so the MP1 is the unit causing the hum.. so how to be sure it was due to the Tube Board..?

Easy.. Change the MP1 patch to a tube patch
RESULT: Loud Hum.

Change the MP1 patch to a SS (Solid State) patch
RESULT: Hum stops.

OK... so now we have identified that the MP1 has a fault and we have narrowed it down to the Tube Board.

RESOLVING the issue...
Thanks to ADADepot I was directed to user MarshallJMP (Phillipe) who is a great source of all things technical and had infinite patience with me as a novice repair guy!

On visual inspection of the tube board, I couldn't see swollen or 'burst' capacitors. (I can identify burst capacitors pretty well after years with Marshall Plexis!) But due to the Hum and a rather high resistor reading (R9 at 265V) it was recommended that I should upgrade the capacitors and a few of the resistors on the board.

I have very little experience with replacing components on a PCB, but with a good temp controlled soldering iron, with a fine tip and taking my time and with some care I was able to manage the task with ease. (So don't be put off doing the work yourself).

After replacing the components, the unit powered up and the 60cycle hum was gone! Yay!!
But, now I had a new problem.. bummer.

NEW DAY, NEW NOISE..
The 60cycle drone/hum was gone, but there was a new noise, this one sounded like a radio tuned between stations.. you know, that 'Static / Hiss'. (And that type of noise is a good hint as to the cause. More on that later) also the Tube patches were very low in volume, but the SS patches were without Static/Hiss and working at usual loud volume. So again, this was a problem associated with the tube board.

With great support from Phillipe, I checked and rechecked all my new soldered connections on the board, I also tested the Valves (tubes), ensuring there wasn't a corroded pin or corrosion in the tube base. Couldn't find a problem, all looked good and tested fine. (I was lucky to have other tubes to try, and rule out a bad tube as the problem).

Phillipe suggested that due to the type of noise I closely inspect the 'Shielded' connections on the Tube board.

If you look at the tube board you will see some thick wires joining it with the main board (There are 3, if memory serves correctly). In each case, inside the thick covered wire is an internal 'shield' and coated wire. The coated wire is connecting the main board with the tube board, but the shield is only connected at one side (either the Main board side or the tube board side depending on the wire).

The purpose of the shield which runs inside the thick wire and parallel to the coated wire is to try isolate that wire and prevent ground loops (hum!). Its important that at no point does the shield touch the coated wire where it is exposed and soldered to the main or tube board.
In my case the Shield near V2 (Valve 2) was bridging with the end of the internal wire and it was impossible to see that fault until I pulled back the thick outer wire cover a cm or so to reveal the problem.
Once I has separated the shield from the internal wire at the point where it is exposed and soldered the Static/Hiss stopped and my tube patches returned to usual loud volume. (To separate, I just inserted a screwdriver and pushed the wires apart).

N.B. To be clear I did this inspection and separation with the POWER OFF and plug removed as there is 100v transferred on that wire!
So anytime I was working/touching the board, the power was off. ..better for your health :)

Anyway I hope this little report helps someone.

Cheers,
G.

PS If you are looking at the "ADA MP-1 Stock Tube-Board Component Layout
R. Metzger 8/2003 ' Image/diagram. The shield areas are labeled 'N' and my fault was at 'H'.

[rnolan]

Hey G, excellent wright up, really appreciate you sharing that for us all 

[MarshallJMP]

+1