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Marshall JMP-1 Service

Marshall JMP-1 in for service at Plasma Music

Here’s a Marshall JMP-1 MIDI valve pre-amp that I’ve just serviced.

I’ve changed the valves and power supply capacitors, cleaned up the metal work and implemented my ‘Live-Forever’ battery mod’. The original battery is mounted bang in the middle of the main PCB, so you really don't want this to leak.

The serial number indicates that it was built in early 1992 which not only makes it twenty-eight years old but one of the first JMP-1s off the Marshall production line.

An early  MIDI valve pre-amp although following the likes of similar pre-amps like the ADA MP-1, the Marshall JMP-1 has always been a really under rated bit of kit, despite the fact that named artists such as Billy Gibbons (ZZ Top), Phil Collen (Def Leppard), Iron Maiden, Megadeth and many more, still use them even today. In production for well over ten years, also says something about this little gem.

The JMP-1 is not a modeler like modern stuff; it’s the real thing, just with MIDI. Run into a valve power amp (or two), it just sounds amazing and the quality of the speaker emulated outputs is definitely good enough to go straight into the board or your DAW.

With most of the distortion being generated in the semi-conductor domain (some of which is highlighted in the image below), many purists might shy away from this beast.

Marshall JMP-1 semi-conductor distortion

On the other hand, classic Marshalls such as the legendary Silver Jubilee series and the monstrous JCM900 Dual Reverb, also use semiconductors to produce some of their distortion tones.

And then there's the clean channel of the JMP-1 which is based on the classic Fender design... with a few tweaks, of course. Anyway, at the end of the day, it's the sound that counts and as I've said, these things have still got it, even today.

RATTLE A' HUM

Lots of JMP-1s hum. This doesn't come through the audio outputs but it can be annoying and I've been trying to get to the bottom the cause for years.

Now, instead of looking at the JMP-1, I'm going to look at what the JMP-1 is plugged into…

Marshall JMP-1 Power Inlet

We all know that mains voltage is AC (alternating current). The voltage varies between 100V and 240V depending on where you are in the world. The frequency also varies and can be either 50Hz or 60Hz. We convert this AC voltage to DC (direct current) to power appliances, gadgets and other systems that now dominate our lives. So why is the supply AC and not DC, then? Well, the answer to that is a little involved as there isn’t a single reason. Predominantly however, it’s a matter of safety. If you are in contact with a mains supply (eek), the alternating nature and the frequency of the supply gives the human body enough time to react and pull away. There are other reasons to do with energy but safety is the main reason that the electricity supply is AC.

Unfortunately, it’s not pure AC, though. There’s a DC element present in the mains which has actually grown over the past twenty years or so due to the fact that the number of appliances that use what’s known as half-wave rectification has increased. I won’t go into just why half-wave rectification dumps DC on the mains suffice to say that it can be an issue for some devices.

So how does that affect my Marshall JMP-1?” I hear you ask.

Well, the other downer is that some appliances contain components that are particularly sensitive to the DC offset on the mains supply and it just so happens that the transformer in the Marshall JMP-1 is one of them. 🙁

Anyone with a little electrical know-how will now wonder how that can be. Transformers are inductive components and so filter out DC. Yep, that’s absolutely correct and in fact the DC that’s on the mains doesn’t get through to the rest of the power supply in the JMP-1.

What the DC component can do however, is saturate the core of the transformer, thus causing the laminates to oscillate. The gap between the top of the transformer and the inside of the JMP-1's top case is very small so you can guess what happens. That's why the hum disappears when you remove the top-case!

Marshall JMP-1 Transformer
The core of the transformer in the JMP-1 saturates due to the presence of DC on the mains supply.

What pointed me in this direction was that after being on for about an hour or so, the laminates on the transformer get seriously hot. I mean SERIOUSLY hot. The JMP-1’s regulators however, don't. In fact, they don’t even have any additional heatsinking. This paradox was intriguing. I concluded that the transformer heats up as a result of oscillation. As mentioned, the heat is substantial and the transformer is heavily stressed as a result. Over time, this will reduce the life of the transformer. I'm afraid it's just a matter of time before the transformer fails. 🙁

Marshall JMP-1 Regulators
The regulators in the JMP-1 don't have any additional heatsinking because they don't run hot.

I should mention the obvious; the JMP-1 has got a couple of valves in it! Although running at about 300V, much higher voltage than the semiconductor parts of the analogue circuitry, they consume very little current and hence, the supply for the valves isn’t regulated. The valve heaters run at about 6.3V and although they ‘heat’ (obviously), it’s not normal to regulate the supply to the heaters, either.

Marshall JMP-1 Valves
It's very unusual to regulate the voltages to pre-amp valves.

Familiar with DC offset on mains being a problem on high-end audiophile equipment, all the evidence seemed to be mounting up. 🙁

You can suppress the hum with cushioning between the top of the transformer and the case but this will only keep the transformer from dissipating heat that's generated as a result of the problem and hey, it doesn't actually fix things, does it?

If asymmetrical voltage supply is indeed the problem, then the only real and proper solution is to get rid of the DC component all together.

I'm pretty determined to get to the rout cause of this and will keep everyone posted on how my pursuit for the quiet Marshall JMP-1 progresses.


Marshall JMP-1s now go for between £400 and £700 (yes, £700). If you can pick one up nearer £400, then I personally think you've got yourself a bargain and a seriously good bit of gear.

If you want to find out more about the JMP-1, then there's loads of resources on-line and of course there are Marshall JMP-1 groups on Facebook like this one: https://www.facebook.com/themarshalljmp1

I know this box inside-out and I absolutely love it and love working on them, so if you have a JMP-1 that's in need of a little TLC, then don't hesitate to contact me.


MARSHALL JMP-1 FACTORY RESET

WARNING: Implementing this procedure will permanently delete all user-made changes to any patches. If you want to save your JMP-1's memory, do a MIDI dump first.

  1. Switch off JMP-1 via the power button on the far right.
  2. Hold down the OD1 button and the CLEAN 1 button.
  3. While holding down these buttons, switch on the JMP-1.
  4. Wait a few seconds while the display flashes and then release the OD1 and CLEAN 1 buttons.

Marshall JMP-1 Factory Reset


UPDATE - 4th January 2021

Marshall JMP-1 Knobs Nuts and Bezels

If you want to read more, my latest Marshall JMP-1 post is here. I now provide replacement Marshall JMP-1 Knobs Nuts and Bezels sets which you can read about here.

I've also developed something called Eclipse; a bounce eliminator which can be fitted by anyone with a little technical know-how and which gets rid of the data entry knob skipping thing! You can read about Eclipse here.

Eclipse Bounce Eliminator for the Marshall JMP-1
Eclipse bounce eliminator for the Marshall JMP-1 stops data entry knob skipping once and for all.