Okay, first of all, thank you for this list. Since ADADepot went down I
haven't had a good source for discussion or help with the MP1 and I
think this list is going to end up being a great resource.
I have ruled out the tubes, the tube board and its components and the
switching as the problem is just as bad as with the ss channel as with
the tube channel.
There is no difference between the results with the front and back input
jacks, the two outputs, or the headphone out. The headphone out is a
little weirder than usual but I believe that's because when I was replacing
ICs I stuck an OPA2604 in the socket I put there and I think the gain is
way too high or something.
The effects loop dummy trick didn't result in anything different though I
forgot to check whether or not a signal that's just through the loop is
affected. The noise level is affected by adjusting OD1 and all the rest of
the controls but doesn't really disappear until it's below 4.
I'm leaning towards the problems in the input section and here's why:
Whilst exploring that section with and LED flashlight I was able to see a
few resistors that looked burnt and discolored, something I'd missed in
my last two run-throughs. They're the same four resistors listed in a copy
of the version 2 of the noise mod I found. The schematics I have list them
as R89, R90, R91 and R92. Three 1M resistors and one 510R. I replaced
all four with metal film resistors and while I was in there I replaced two 2K2
resistors as well (my schematic looks a little different but I'm pretty certain
they are R69 and R70). After that the hiss and hum seemed to be a little
less overwhelming but still very present and the lower frequencies are still
absent.
I believe both input jacks to be fine but I'm not 100% sure about the input
volume pot. It's alway been a little noisy but I know a surge could ruin it
too. It's got to be something right after the input. I noticed another
symptom as well. The noise reacts significantly to touching the cable.
Even touching the metal casing of my test signal generator adds to the
noise. Some sort of grounding problem I guess.
What parts are contained in this JFET transistor stage and bipolar transistor
stage? Could you tell me what parts I'm looking for so I can test or replace
them. In case they're part of these stages, I replaced the 2SK364s shortly after
the overvoltage (Q3 and Q4 on the tube board which aren't the problem and
Q5 and Q7 which I'm assuming are part of what you're talking about).
Thanks again for all your help.
B
On Nov 27, 2012, at 3:00 PM, Kaz Kylheku wrote:
>
> On Mon, 26 Nov 2012 23:31:11 -0600, Benjamin Catchings
> <jonahcrock@???> wrote:
>> I have gone through the whole thing on a couple of occasions and
>> replaced my mods (the three caps in the power section from the noise
>> mod, the 3 caps in the tube section, 3 or 4 transistors from the tube
>> section and a couple on the main board in the same area, about 6 of
>> the opamps including the headphone, effects loop, etc - U4, U8, U9,
>> U10) and did a little better quality work on it than I did the first
>> time.
>
> Hi Ben,
>
> I feel your pain!
>
> So you've done some homework already. New IC's, filter caps and such.
>
> For this kind of troubleshooting, it benefits you to have an
> oscilloscope, or at least an audio probe (a small audio amp
> with a speaker or headphones, and multimeter probes for poking
> into circuits). But you can do a lot without the tools.
>
> The only things I have yet to replace are the tubes (both are
>> lighting up fine and don't appear to be damaged at all from visual
>> inspection), the 6 diodes in the power section or the 4 in the tube
>> section and the transformer.
>
> You can largely rule out the possibility that the problems are
> confined to the tube board, by reproducing the problems using
> the S.S. channel. (Unless the channel switching itself is affected.)
>
>> Is it likely that my problem is the transformer or would it even be
>> coming on if the problem was there? Are the diodes more likely to be
>> the culprit? Or is there something else that more commonly causes this
>> symptom?
>
> I would investigate this as an overvoltage event affecting the
> front jack's FET buffer circuit.
>
> The front input of the MP-1 goes into a JFET transistor stage, which
> is followed by another bipolar transistor stage. After that stage,
> the OD1 level is applied, and the signal goes into the buffer provided
> by an op-amp on the U4.
>
> People don't seem pay attention to that JFET circuit; it's not
> the target of any popular mods or upgrades that I know about,
> and the "stock gain mod" even talks about bypassing it, which is
> a dumb idea that will ruin the unit for any guitar with passive
> pickups.
>
> An over-voltage into the front input jack would likely fry the JFET
> transistor.
>
> Here is a test: does the volume level of the noise respond to
> changes in OD1? What if you turn OD1 all the way to 0.0?
>
> Another test: does it make a difference if something is
> plugged into the front jack or not?
>
> Another test: what happens if you plug into the rear jack?
>
> Another test: what if you put a plug into both at the
> same time?
>
> When nothing is plugged into the front jack, then the
> FET buffer is disconnected; it does not feed into the OD1
> stage. However, the rear jack circuit *is* connected at
> that time. The rear jack is based on the U11 op-amp. That
> might be fried, because as you insert a plug into a jack,
> the momentary contact could send the voltage there.
>
> The rear jack itself is also switched; it is shorted out of
> the way when nothing is plugged in. But that is before the U11
> op-amp. If the U11 op-amp is fried, it could spew noise.
>
> When you plug into the front, the output of U11 is then
> disconnected; it does not feed into OD1.
>
> If the noise goes away when you plug something into the
> front, that would indicate that U11 is fried and spilling
> noise.
>
> If the noise is only present when you plug something into
> the front, that points to the FET stage.
>
> If the noise is different, but OD1 turned to 0.0 can
> quiet it, that suggests both the rear jack U11 stage
> and the front jack FET circuit are toast.
>
> Here are some other ways to subdivide the MP-1 externally
> (without probing for signal into circuits).
>
> * Enable the effect loop and then plug a dummy plug into
> send. Does it quiet down? Then the noise is not in
> the output. You can further verify this by injecting
> some signal into effects return and see that it cleanly
> comes out on the output.
>
> * Always check whether a noise comes out the same
> way from all possible outputs: output A, output B,
> headphones, effect send.
>
> * Always check which control the noise responds to:
> OD1, OD2, master? Rear pots? Solid-state versus
> tube?
>
>
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