Getting rid of noise in my MP-1

[hansgraudal]

I'm glad you're enjoying the journey too rnolan :) This has been on my to-do list for a very long time now...  ;D

[rnolan]

 :thumb-up:

[hansgraudal]

Quick update on my progress so far:

All resistors on the tube board along with the resistors around the discrete input amplifier have been replaced with 0.6W metal film.

All ceramic and electrolytic caps around these parts have been changed to fresh electrolytics, MLC's and MKT's depending on their function. This was probably not necessary, but I don't really trust old ceramics in the signal path.

With OD1 & OD2 @ 10 on the dist voicing and nothing plugged in to the unit, the noise floor has gone from being beyond unbearable to being very low.

When i plug a jack in the input socket, there's still a high pitched squeal, that changes frequency with changes in gain settings. It disappears when there is a signal present, and reappears almost immediately after the signal is stopped. When both OD1 & OD2 are below 5, the squeal disappears. Gotta figure out where the squealing originates :o

Any ideas?

[rnolan]

There's another thread about the squeal, some of the guys reporting it (http://adadepot.com/index.php?topic=1992.msg24150#msg24150) have put the MDRT transformer in their units and are wondering/thinking is it that  :dunno: , anyway, it seems some oscillation is occurring so if you can work out what is causing it (and fix) there will be some smiles....

[MarshallJMP]

When you say "I plug in a jack" is there anything connected on the other side like a guitar? If so is the guitar vol open or closed?

[hansgraudal]

The starting point was this:

DIST voicing. OD1, OD2 & MASTER @ 10. Tone controls @ 0  -->

With nothing plugged in: Some humming.
With a jack plugged in the front input but nothing connected on the other end: Loads of hum and squealing.
With a guitar plugged in the front input, guitar volume all the way down:  Some humming.
With a guitar plugged in the front input, guitar volume all the way up: Loads of hum and squealing whenever strings are muted. Hum and squeal disappears when the strings are played, and "fades back" on note decays.

Squealing was somewhere in the 7-800Hz range.

The clean and S.S. channel behaved notably better. Still with some hum though.

[hansgraudal]

When I'm in the lab at work, I can't bring guitars, so i simulate a guitar pickup at the input with a 6K resistor in series with a 1mH inductor in series across the input terminals. Signal can then be injected in the junction between the two. This yielded results similar to my previous post.

The results of my work so far resulted in the humming being almost gone, and the squealing frequency moved up to around 12KHz.

Yesterday i replaced the BJT and JFET in the input stage, and that cured the squealing somewhat. With nothing across the front input terminals, there is still squealing, but as soon as there is a pickup (or my pickup simulation contraption) connected, the squeal disappears.

There is still some humming at high gain settings on the dist channel, but i think this might be a mechanical issue, as the transformer is humming quite a bit, and i can literally feel the vibrations from it in the chassis. It might be time for a replacement transformer?  :P

[herbyguitar]

With nothing plugged in: Some humming.
With a jack plugged in the front input but nothing connected on the other end: Loads of hum and squealing.
With a guitar plugged in the front input, guitar volume all the way down:  Some humming.
With a guitar plugged in the front input, guitar volume all the way up: Loads of hum and squealing whenever strings are muted. Hum and squeal disappears when the strings are played, and "fades back" on note decays.

Squealing was somewhere in the 7-800Hz range.

The clean and S.S. channel behaved notably better. Still with some hum though.

Exactly what I'm going through. Same problem. I've done the SS Mod, Noise Mod, MDRT Mod.

[hansgraudal]

This is not so much of an update as it is a pledge of commitment:
I still haven't solved the issue, but I have been working off-site for a client for some time now, and won't be back in my regular work space until the end of august.

I'm looking forward to tinkering with audio electronics again instead of developing power supplies for MRI scanners. High current capacity SMPS converters can be quite "exciting" to deal with, when you're tiptoeing on the edge of what's possible with cutting edge technology. :crazy:

[hansgraudal]

I took some pics the last time i worked on my unit - here's the state of it. There's Red circles in the diagram whenever I've replaced something, and I keep a neat little box of stuff I've pulled from the unit - just in case :P

[Iperfungus]

Some aspects of these issues are very similar.
It will be interesting to discover if they will have a common solution.  :thumb-up: