Marshall JMP1 rack pre, dead display

[ndc666]

Hi all, I have a JMP1 saga ...

It blew a fuse while in use, and some discoloration on the power transformer. The current across the fuse socket was ~200 mA without preamp tubes so I checked it unloaded, and yeah, the short was internal. Nothing else looks crispy.

Power transformer has been replaced with a new one from Plasma Music LTD, and the current through the fuse socket is a much more reasonable ~40 mA. I replaced the battery as well while I was in there. But now there is no display and buttons / rotary encoder do nothing. The unit still passes signal from input to the headphone jack on the front.

I've checked everything I can think of in power supply world. Linear regulator outputs all look correct, I've had a scope on them and there's no bounce or anything so apparently filter caps are fine. I've done the reset, holding OD1 + CL1 and restarting. No change. IC power pins are getting proper voltage. It -seems- like the transformer swap was successful and something else has gone wrong.

What else is worth poking here? If I somehow killed the EPROM / SRAM / processor, is there anything to be done there? I'm open to suggestions. I like this thing, would prefer not giving up!

Thanks

N

[The Dude]

Double check connections from main board to front panel board. Maybe you got a connector off a pin, bent a pin, or something of the sort.

[ndc666]

Yeah it's a fair thought but the connections check good with a meter set to beep ...

[Jon Snell]

Have you got VDS?
Does X1 oscillate?

[ndc666]

Have you got VDS?
Does X1 oscillate?

Hi Jon, yes, I have checked VDS in a few places all around the board, and haven't seen anything weird. Steady 5V ish.

Hadn't thought of checking X1. So right now I have a scope on one leg, the one tied to the 8031 on-chip oscillator's output, and it shows a 12 MHz sinusoid, about 1V PtP, riding about 2V of DC. Does this sound right? I know what this is conceptually but not sure what it should look like.

[Jon Snell]

That sounds about right.
Next take a look at IC26. Have you got clock and data?
VDS on both sides of LC7?
Q2 - Q6 data change as you adjust controls (IC26)?

[ndc666]

That sounds about right.
Next take a look at IC26. Have you got clock and data?
VDS on both sides of LC7?
Q2 - Q6 data change as you adjust controls (IC26)?

Yes, both sides of LC7 around 5V.

IC26 has power, clock is stuck high. The reset pin 15, coming from the 8031 is also stuck high. Nothing is moving at all. So I guess we're getting warmer ...

I poked around IC28 also, because front panel buttons are doing nothing ... one of the ~enables is high, pin 1, coming from 8031, the ~RD signal I think? The inputs from the buttons, pins 2-9 on IC28, are floating very close to VCC. If I hold a top row button, it does move up a little, and bottom row button does drop it to GND, so the buttons are connected. But this seems odd.

On IC13 itself, the sysreset coming into pin 9 is low, and both ~WR and ~RD pins are high.

So does it seem the processor is not starting, or firmware is not loading, or something, and so these other ICs are not getting enabled at all?

I appreciate the guidance this far!

N

[Jon Snell]

Try forcing a reset.
The Johnson Decade Counter is a simple beast. I attach a schematic of it.
Logically you should be able to follow the way it works and 'twiddle' with the inputs to get outputs to move.
Unfortunately you may find, if all else checks out, the 2764 has lost its program. Fingers crossed ir electrically erasable prom hasn't.

[ndc666]

Try forcing a reset.
The Johnson Decade Counter is a simple beast. I attach a schematic of it.
Logically you should be able to follow the way it works and 'twiddle' with the inputs to get outputs to move.
Unfortunately you may find, if all else checks out, the 2764 has lost its program. Fingers crossed ir electrically erasable prom hasn't.

Thanks for the reading, I think I get the basic idea how the counter works to drive the display. But it seems the counter is getting junk inputs from the processor, or do I misunderstand? So by "forcing a reset" ... reset what? Processor, with the RST pin, while it's running? Doing the factory reset on the front panel has no effect ...