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Lets get Technical > All Things Tube All about preamp and power amp Tubes.

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Author Topic: MP1 V2 (no top switch) All original excluding battery mod  (Read 3781 times)

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ScottGray

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Greetings!
I have decided to troubleshoot a misbehaving MP1 V2 (no top switch) which I believe to be all original/stock excluding battery mod (Thanks for the quick shipping!) etc.

The current eprom is 2.01 and the problem is complete loss of gain from any tube voiced patches.
I have reset the patches and using patch 1 as an example. Setting OD1/OD2 to maximum gain does nothing. I verified using headphone output as well as output A and B.

I have verified the existing tubes (Groove Tubes 12ax7) were glowing but replaced with a known good set pulled temporarily from a working unit. The SS voiced patches appear normal and I don’t hear any anomalies such as noice, feedback whether using SS or tube patches.

Everything else seems fine and functions they way they should. I don’t have a ton of experience troubleshooting circuit boards but somewhat handy with my soldering iron and multimeter.

I began by taking lots of pictures of the board (front/back) and closely examined each component for discoloration, burn marks etc., nothing I can see out of the ordinary. As I stated earlier, I believe this is all original with no modifications excluding the battery mod. I would assume that as old as the unit it, I probably have a faulty capacitor etc., they look fine with no bulging or any appearance of being faulty but that is as far as I have gone.

Can one of you brilliant members point me in the right direction for things to try/diagnose? I would expect this to be a common problem but after skimming most of the threads, I can’t seem to find anything specifically matching my issues so any help would be greatly appreciated.
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MarshallJMP

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Well it seems you have a problem with the tubeboard. So first thing I suggest is that you try to measure the high voltage. On the tubeboard you see on top 2 resistors placed horizontally, both are 10k ohm resistors (color code brown black orange gold). Take a multimeter, set it to DC volts (200v or more), put the black pin into one of the screwholes and with the red one measure the right side, then the left side of the resistor and let me know. One side should be around 190V the other side 170-175V.
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ScottGray

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Had a minute to check and reading almost 194v/182v. Both resistors are giving almost identical readings:

First resistor (R4): Lead closest to front side of MP1 (and R5/R6) is reading 182v, the other lead (next to C7) of resistor reads 194.5v.
Second resistor (R1): Lead closest to R2/R3 is reading 181.5v, the other lead (next to D1 through D4) of resistor reads 194.4v.

Thanks!
Scott

Tube board pics attached - hopefully useful.
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MarshallJMP

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Ok that looks good, maybe a bit high but nothing to worry about now. Next to the second tubesocket is a brown shielded wire, can you take a pic of it, can't see the connection on the other pics you posted.
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ScottGray

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Hopefully this is clear enough, I tried to get the tube board and main board termination in focus.

Thanks!
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ScottGray

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Just realized you may have been talking about the other shielded link so I grabbed some pics of that as well.

Thanks, I truly appreciate the help!

Scott
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ScottGray

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Using ADA Depot's tube board layout and in attempt to do a bit more troubleshooting, I verified the seven (7) red wires A,B,C,E,G,J and L appear to have good continuity from the tube board to the main board.

I also verified the three (3) shielded links N,G and H appear to have good continuity as well.  :D
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MarshallJMP

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Ok don't see any short on those cables. Next, power off, tubes out and measure R2,3,5,6, they should be 100k ohm.
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ScottGray

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R2: 101.1k ohm
R3:   99.3k ohm
R5:   99.1k ohm
R6: 100.1k ohm

I'm not sure if that little of a variance would cause issues? Note, I am using a 10y old Sears Craftsman auto-ranging multimeter so I am not 100% confident of its accuracy but it appears to get close.

Hoping the values for the following will be beneficial:
R7:   50k  ohm (should be 100k?)
R8:   10.02k ohm
R9:   246k ohm
R10:   3.2k ohm
R11: 586k ohm (low should be 620k?)
R12: 609k ohm
R13: 745k ohm
R14: 277k ohm
R15: 1.08
R16: 1.08
R17: 428k ohm
R18: 0.5M ohm (should be 1M ohm? <-smoking gun??)
R19: 509k ohm
R20: 741k ohm
R21: 265k ohm
R22: 15.86k ohm
R23: 62k ohm (should be 100k?)
R24: 4.6k ohm
R25: 15.9k ohm
R26: 3.86k ohm

While I have been gentle with the board, last time I checked the last resistor, the shielded link "N" disconnected.
After looking closely at the pad, there is very little of the wire sticking up and possibly a weak solder joint?
Could that have been the issue? According to the schematic, "N" is signal in? Does that indicate if it was a weak connection it could result in no tube voicing/gain/no sound etc? I might try to solder it back and see if it does anything but I will wait for your analysis MJMP  :D



Thanks!

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MarshallJMP

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Resistors look ok except R7 and R23, these should be around 50 ohm. But this won't be the problems you have.

The N is the input so yes could be the problem. But on the other hand then you should get some noise or hum in the tube modes. Repair it and let us know how that turns out.
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ScottGray

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Will do!  :thumb-up:

I doubt it will be something so simple, not with my luck!
Fingers crossed!  :banana-rock:
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ScottGray

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I was hopeful but that wasn't the source of the problem OR there may be more than one issue.

I can hear the signal now but still have no control over volume/gain when using the tube voicings. I didn't think I heard anything before but I hear the signal now.
I can change the patch to add/subtract the EQ (Bass, Middle and Treble) and hear the audible difference with EQ. I can hear chorus as I adjust that as well but the guitar signal sounds the same with regards to volume and or gain. In fact, I can turn the OD1, OD2 and Master to 0.0 and still hear the same guitar signal, muffled distortion and no change in volume. I don't hear any noise currently, no humming or related. The signal sounds quiet with regards to noise which is surprising but maybe expected if none of the signal is traversing the gain stages?. I also used a few different guitar sources for signal with no apparent changes.

Man, I was hoping that filtered link was the smoking gun.  :dunno:


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MarshallJMP

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Does the SS mode work correctly, if you raise OD1, more volume, if you raise OD2 more compression? Master volume works too?
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ScottGray

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When I thought I had verified SS working normally prior, it must have been due to EQ and chorus working within the different patches. I didn’t try to change the OD1/OD2 etc., on SS patches as they appeared okay but sadly that isn’t the case. When I change to a SS patch and edit for more OD1/OD2 or master, nothing changes. Apologies, I hope I didn’t send you on a wild goose chase and I should have verified more prior to posting.

As of current, when I change thru the patches from tube to SS voicing, I hear differences only due to the varied EQ settings. It does not appear that OD1/OD2 or master is working at all for SS or tube voicing.  :(

The above was verified using headphone, A and or B output, front input or rear input.
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MarshallJMP

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So the EQ works, you can hear when you edit the EQ settings?
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