ADA Depot - A Forum To Support Users of ADA Amplification Gear

Let's Get Technical => All PREAMP Tech Tips & Support => Topic started by: shredbaron on March 27, 2017, 09:04:23 AM

Title: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: shredbaron on March 27, 2017, 09:04:23 AM
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



Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: MarshallJMP on March 27, 2017, 09:18:53 AM
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
Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: shredbaron on March 27, 2017, 10:31:05 AM
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).
Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: MarshallJMP on March 27, 2017, 10:33:10 AM
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.
Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: shredbaron on March 27, 2017, 10:36:17 AM
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?
Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: shredbaron on March 27, 2017, 10:49:21 AM
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
Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: shredbaron on March 27, 2017, 11:00:56 AM
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!
Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: shredbaron on March 27, 2017, 11:21:39 AM
...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?
Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: MarshallJMP on March 28, 2017, 04:57:25 AM
If you measure on point G does the voltage switch between -15V and 0V when you switch from clean to dist tube?
Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: shredbaron on March 28, 2017, 06:39:29 AM
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.
Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: MarshallJMP on March 28, 2017, 11:12:21 AM
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.
Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: shredbaron on March 29, 2017, 10:20:42 AM
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?
Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: MarshallJMP on March 29, 2017, 03:32:37 PM
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.
Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: shredbaron on March 30, 2017, 05:59:43 AM
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)

Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: MarshallJMP on March 30, 2017, 11:23:51 AM
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.
Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: Kim on March 30, 2017, 12:40:06 PM
 :o 

Just when you think you've seen it all... :facepalm:
Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: shredbaron on March 31, 2017, 04:56:09 PM
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.

Yeah, I realized this MP-1 had been modded before, there are sockets on some of the ICs that I didn't put in that don't exist on my other MP-1.

No biggie. Got everything finished up today except the replacement of the tubeboard 2SK364 FETs. Military spec 1% metal film resistors in the audio path, replaced all the caps (electrolytic, NP and ceramic) with upgraded versions, did the bass warp pot, mid-scoop pot, replaced the TL072s I had in U11 and U15 with NE5332 op-amps...this things is better than new inside.

Only thing remaining is to get those FETs sorted out and then figure out why the chorus isn't working. I read the other chorus thread, tried a few things, still nothing - so I'm thinking either the trim pot is bad, or the IC has failed. We shall see.

Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: shredbaron on April 04, 2017, 09:28:08 AM
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.

2SK364s arrived today - just installed them - BOOM! everything works great!

just going to replace the volume pot and see if I can sort out the chorus, then it's going back into my rig for some full-volume testing - from what I could tell playing it through my small practice amp, it's a LOT quieter and the EQ is more responsive after replacing all the internals, but I'll know better when I get it going through my tube power amp and 4x12

thanks for the amazing help, it's been very much appreciated
Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: MarshallJMP on April 04, 2017, 10:06:45 AM
Aaah great !!!! let me know if I can be of any assistance with the chorus
Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: shredbaron on April 04, 2017, 01:43:15 PM
Aaah great !!!! let me know if I can be of any assistance with the chorus

Thanks, I'm thinking it's a bad trimpot. I was able to get some chorusing but it cut in and out with the pot. I'm going to replace it with a new 20K pot (if I read the schematic correctly), hopefully can figure out which one will fit via Mouser.

cheers

george
Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: MarshallJMP on April 05, 2017, 03:43:51 AM
531-PTC10V-20K  This one should be the correct one (I hope  ;) )
Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: shredbaron on April 05, 2017, 07:17:08 AM
531-PTC10V-20K  This one should be the correct one (I hope  ;) )

Thanks! I was vacillating between a couple of different options last night and thought I'd sleep on it (this was one of them) - really appreciate the heads up!

Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: MarshallJMP on April 05, 2017, 08:50:52 AM
No problem!
Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: Iperfungus on April 11, 2017, 02:43:21 AM
Another MP-1 came back to life!  :thumb-up:
Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: rnolan on April 11, 2017, 03:36:40 AM
Another MP-1 came back to life!  :thumb-up:
A new take on mouth-to-mouth resuscitation  :lol:
Title: Re: Pooched my MP-1! Help! (yes, I'm embarrassed!)
Post by: Iperfungus on April 11, 2017, 04:15:40 AM
Another MP-1 came back to life!  :thumb-up:
A new take on mouth-to-mouth resuscitation  :lol:

 :lol: :lol: :lol:
Well, I would prefer a redhead...