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Author Topic: Pooched my MP-1! Help! (yes, I'm embarrassed!)  (Read 5094 times)

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shredbaron

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I was so excited this morning to re-assemble my MP-1 after completing my re-cap and component upgrades...only to fire it up and feel the pang of disappointment.

Whereas prior to my work, my dist/clean tube channels were reversed, now when I fire it up, nothing happens other than a few lights on the main display.

Based on what I've read I'm thinking it could be any of the following:

1) I screwed up a solder joint on one of the many caps I replaced or another component.

2) I damaged a diode. In particular, there was a diode I mistakenly de-soldered (and likely damaged from heat) and had to put back in. It's the one with the white arrow in the pic below. I'm going to get a new one and see if that fixes the problem.

3) The 2SK364 transistor I replaced was/is faulty *or* the 10uF cap beside it is faulty and fried the new one I put in. Serves me right, wasn't thinking when I put it back together I needed to wait to get a new cap.

Any other ideas? I really can't stand the thought of using this for spare parts after spending $100 on upgraded components, there has to be a logical way to troubleshoot this - any suggestions would be greatly appreciated.

thanks!

George



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MarshallJMP

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Looks like a digital problem, did you do anything on the right side? Also measure the voltage over those 3 big caps near the tranny.The 10.000 µF should have around 12.5-13.5V the other 2 around 20-22V
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shredbaron

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I'm getting 13.7 volts on the 10uF and 22V on one of the 2200uF caps, 23v on the other

I didn't touch anything on the right side other than replacing some electrolytics caps (the ones that are between the 2200uF caps and the transformer, and the ones between the 10uF cap and transformer)

no sound at all (I tried just in case it was only a display issue).
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MarshallJMP

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Next try to measure the 5V, best is to use one of those 74LSxx chips, choose one ,get the datasheet and see what the 5V pin is.
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shredbaron

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Next try to measure the 5V, best is to use one of those 74LSxx chips, choose one ,get the datasheet and see what the 5V pin is.

Before I saw this, I went and tried measuring voltage on pin 26 of the z80 (reset), getting 5V there - would that suffice for the above?
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shredbaron

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Next try to measure the 5V, best is to use one of those 74LSxx chips, choose one ,get the datasheet and see what the 5V pin is.

getting 5v on the 74LSxx chip too
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shredbaron

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WORKING!

not sure what happened - I pulled the ribbon cable and re-seated it, pulled the battery and replaced it, and all of a sudden it fired up

now to see how much better it is once plugged into my Marshall 9005

thanks for the help MJMP!
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shredbaron

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...and...

back to where I started - sort of

unit is working

sounds great

BUT...

the tube clean channel has WAY more distortion than the tube distortion channel - even at the same patch settings - like my original problem, it's as if the channels are reversed

now...I did replace the 2SK364, but I (foolishly) failed to order the replacement cap that goes with it - do you think that could be the culprit?
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MarshallJMP

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If you measure on point G does the voltage switch between -15V and 0V when you switch from clean to dist tube?
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shredbaron

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If you measure on point G does the voltage switch between -15V and 0V when you switch from clean to dist tube?

On tube dist mode, point G is 14V

on clean tube mode, point G is negative 4.9V

the noise level in clean tube mode is crazy too - even at low gain settings

same problem I had before I replaced all the components - same voltages - but I didn't replace that cap near the 2SK364. I'll replace it and the resistor close to it and report back once my Mouser order arrives.

thanks for the guidance, if it turns out to be the cap I forgot in my order (the whole reason this thing started in the first place) I'm going to laugh.
« Last Edit: Time Format by shredbaron »
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MarshallJMP

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Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
« Reply #10 on: Time Format »

That's strange, normally dist should a little over 0V,clean should be a little less then -15V.Can you desolder the G wire going to the tubeboard and measure again.Maybe one or both 2SK364 FET's are busted on the tubeboard.
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shredbaron

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Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
« Reply #11 on: Time Format »

That's strange, normally dist should a little over 0V,clean should be a little less then -15V.Can you desolder the G wire going to the tubeboard and measure again.Maybe one or both 2SK364 FET's are busted on the tubeboard.

I'm reading -13.98V on the board with clean tube, and 0.14V with distorted tube with the wire removed that goes to the tubeboard.

That suggests to me there's an issue on the tubeboard, no? I did put new MPSA92s in 2010 but was never able to test at the time because I wasn't getting any sound (which I traced a few weeks ago back to a bad connection on one of the tubeboard wired pads). Think I may have damaged them when I soldered them in, or am I just guessing and the MPSA92s have nothing to do with this issue?
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MarshallJMP

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Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
« Reply #12 on: Time Format »

That looks a lot better. Now the MPSA92 are not the culprits, these are used in the high voltage regulation circuit and have nothing to do with this. On the tubeboard are also 2 2SK364 fet's (they look the same as the MPSA92) that switch the gain of the first tube. And one or both of these is the problem. Best is to replace both of them.
These are located next to the first tube, on the left of the tubeboard.
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shredbaron

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Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
« Reply #13 on: Time Format »

That looks a lot better. Now the MPSA92 are not the culprits, these are used in the high voltage regulation circuit and have nothing to do with this. On the tubeboard are also 2 2SK364 fet's (they look the same as the MPSA92) that switch the gain of the first tube. And one or both of these is the problem. Best is to replace both of them.
These are located next to the first tube, on the left of the tubeboard.

Well...this is interesting - all of the FETs on the tube board are MPSA92s. There are no 2SK364s.

I ordered 4 2SK364 last night, they should arrive early next week. It's Q3 and Q4 that need to be 2SK364, correct? (photo attached)

« Last Edit: Time Format by shredbaron »
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MarshallJMP

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Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
« Reply #14 on: Time Format »

REALLY? that why it doesn't work. A transistor is not suited for this and that's why your voltages on G were out of spec.
Now I've seen this before but on the input section, the FET goes down so let's put a transistor in it, that would work fine , euh NOT. You can't believe what some tech's do.
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