Marshall YJM100 schematic?

[Nyqusit5]

Hi all,

I am repairing a Marshall YJM that has a faulty Hi channel I input. This is the input that is routed through a switchable boost (probably a DOD 250 circuit). This Hi input is very dull while the low input of this channel is bright as it should be. As a consequence, the issue is probably located on the "boosst" board which is all CMS. the switching is done through a relay which seems to be fine. Also, i can test continuity from the input jack (jack inserted) throught to the first grid 68k resistor located at the side of valve 1. Sale for the ground connection: no problem here. The input impedance seems to be fine as well (the dullness doesn't seem due to a drop in input impedance as it is higher than 1MOhm).

If anyone has the schematic, it would be of great help!

Thanks in advance!

[Gregg]

On other forums couple of people admitted of having it didn't want to release it. It's a regular plexi circuit the rest is like AFD100.

[Nyqusit5]

Thanks Gregg. But while the valve circuit is kind of a regular superlead circuit (apart from the loop), the boost and associated circuit aren't and it is where the problem lies. The layout isn't your typical 1959 either...
So... still looking.

[Enzo]

When you contacted Marshall to ask them for it, what did they say?

[Nyqusit5]

I have but I am still waiting for the reply.

[Mick Bailey]

Authorised Marshall partners have direct access to the schematic, but it's in breach of the agreement terms to release it without permission. It's hard won becoming an authorised partner - you have to provide evidence of qualification and established trade references. To lose this would be like a doctor getting struck off.

It's another thing for Marshall themselves to release the schematic.

I can tell you it's nothing like an AFD100.

[Mick Bailey]

When you say the Hi input is very dull, what exactly do you mean? Is it a lack of treble, or is the volume low as well? The dynamic of the boost is controlled by a THAT4305 IC, which is a VCA controlled by a side-chain RMS analyser - as you say, in CMS (SMD).

The main task is to identify which function block is faulty, then troubleshoot within that block. The tube side is all pretty standard, but the signal gets routed through the boost/expander board and then on to the digital board. It would be a good idea to check tube signals and voltages first and at least eliminate these.

I don't know what you're equipped with, or your technical level. If you don't get the schematic from Marshall I can help you with fixing your amp, but you need to be able to work with CMS if the fault lies within the boost circuit.

[Jazz P Bass]

Busy chip.
Here are some datasheets on the IC.
http://www.thatcorp.com/datashts/THA..._Datasheet.pdf
The Theory Of Operation is covered in the single block IC's.
http://www.thatcorp.com/datashts/THA..._Datasheet.pdf
http://www.thatcorp.com/datashts/THA..._Datasheet.pdf
The manufacturer also provdes application notes: THAT Corporation 4305 Low-cost Analog Engine - Dynamics Processor IC

[Nyqusit5]

I am well equipped and have plenty of experience, no worries.

The fault comes from the boos board (the tube signal chain works perfectly right from V1 input through to the PA). My problem isn not really the circuit itself but more the way it is implemented (SMD) and the fact that I don't have aschematic to have an overview of what's going on on this board...

Thanks for your help

(I typed a reply this morning but it seems it didn't go through)

[Mick Bailey]

Can you clarify what the fault is - lack of treble, lack of volume, or both?

[Nyqusit5]

Lack of treble only

[Jazz P Bass]

Please clarify also that the only input difference is from the Hi (lack of treble) to the Lo (sounds good) input jack.

[Nyqusit5]

Well, this is exactly that. To quote my first post: the only faulty input is the Hi input on channel I, the only input routed through the boost circuit. Again, the boost circuit is where the fault seems to be as the Low input of channel I goes through the same signal chain EXCEPT the boost circuit and it sounds as it should (I have several original Marshall Superlead and have worked on countless others).
Thanks for your help.

[Nyqusit5]

Got it this morning.
The Hi input is going to the boost board where it sees a 4.7MOhm SMD resistor to ground and a relay for the boos switching. But it also sees a 47nF (or 470nF) leading to the second "+" input of the 4558 used in the boost circuit. My guess is that it is used as a buffer for the control input of the gate circuit. Anyway, the 4558 was faulty giving a low impedance to ground and therefore cutting all the hi-end together with its input cap.

To be honest, this thing is not far off a Bugera as far as build quality and manufacturing methods are concerned (don't start me on the tone side, it is always very subjective). No wonder Marshall is not the hot brand it used to be.

[Mick Bailey]

The second 4558 is the buffer to the RMS detector side of the THAT4305.