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[Cpt Adama]

No I didn’t measure the input signal at the Jack. I feed the second signal channel directly to the oscilloscope just for reference. But I’ll do that when I get back.

[MarshallJMP]

Ok let me know. I use a BNC T connector, one side to my scope the other side goes to UUT (unit under test), this way you can make sure it's correct.

[Cpt Adama]

I have set up only one signal which comes from my signal generator (BNC-1/4") to the input of the MP-1. I have 2 channels on the oscilloscope, channel 2 measures the signal at the back of the input jack from the solder terminals, channel 1 is connected to U4 pin 1. This time I have all the connections made BEFORE I turn on the MP-1. Previously I turned on the MP-1 and then made connections which took about 1 min.

When I initially turned it on I had about 16Vp-p, I adjusted T2 to 15Vp-p. After about 45 secs the voltage starts a slow but continuous drop to about 11.2Vp-p without touching it. That takes about 2-3 minutes. So I'm thinking that this is heat related as the tubes warm up the area something is heat sensitive and it drops the voltage. Before by the time I connected everything up the voltage had already dropped to below the 15Vp-p that it initially can make, so I never saw that. It seems it should be able to hold a steady voltage at 15Vp-p though.

[MarshallJMP]

Try replacing D16 with a 1N400X (X= 1-7), these are not so heat sensitive as the 1N914 or 1N4148 diodes in there.

[Cpt Adama]

How about D44, D45, D46, and D8? These are closer to the tubes.

[MarshallJMP]

I don't think these are the problem, these form a clamp circuit and your sine wave does not get clamped.

[Cpt Adama]

I replace D16 with a 1N4007 and kept it up a little for cooling as in your picture. That has indeed solved the heat drifting problem and I can now set the 15Vp-p.

Of course fixing one problem leads to another.

Now at step 2 in the alignment instructions, I can only get OD-2 at 0 down to 15.4Vp-p with the Trimpot 1 turned all the way down. The sine wave is also in almost full clipping.

[MarshallJMP]

First measure the voltage at R4 or R1, one side should be 190V, then check R6,5,3,2 with an ohm meter, they should be 100k (mp-1 powered off)

[Cpt Adama]

First measure the voltage at R4 or R1, one side should be 190V, then check R6,5,3,2 with an ohm meter, they should be 100k (mp-1 powered off)

I'm getting 194.8V after the rectifier section, all the resisters on the tube board are within 5% tolerance.

[MarshallJMP]

Voltage is a bit high, could be that R9 is going up in value, should be 240k. Check if R48 and R18 is connected to the vactrol.

[Cpt Adama]

Voltage is a bit high, could be that R9 is going up in value, should be 240k. Check if R48 and R18 is connected to the vactrol.

Actually I didn't find a resister that was over 2.5% tolerance values, I was just using the resister stated tolerance values for comparison. R48 and R18 are connected to the Vactrol. As for R9 I don't remember the value (it was within 2.5%) and I can't test that one on the board. I removed all resisters that couldn't be tested directly on the tube board and verified values. I could pull that one off again and give you the actual value if you think that's the culprit.

The other major player here I thought was op-amp U2 (TL072CP). Ok I put in a new op-amp with little to no change, put in a socket and replaced with the original op-amp.

I just got in some new matched EH 7025 tubes and threw them in, I am now able to get the OD2 voltage at 10Vp-p, BUT trimpot 1 is still at its min CCW position to achieve this. In Step 3 of the alignment I get 21.4Vp-p with OD2 set at 10.

So even though I still think something is wrong having to set trimpot 1 at it's minimum level to get the proper setting, I moved on to Step 4 with the amp in Solid State mode and checking the voltage at U8 pin 1, I get only 7.5Vp-p, should be around 11Vp-p. Disturbingly Trimpot 3 has NO effect of the voltage reading at U8 pin 1. Uugh! This amp is driving me crazy As a side note as it's not part of the alignment process but checking test point F on S.S. I get 21Vp-p with a fairly square wave distortion.

[MarshallJMP]

OD2 is common for the SS compressor and the OD2 setting in the tube voicings that's why you have 21V at point F.

Now what is trange is that all 3 settings you had/have problems with are done by U14, not sure but did you tried to swap U13 and U14?

[Cpt Adama]

Now what is strange is that all 3 settings you had/have problems with are done by U14, not sure but did you tried to swap U13 and U14?

Yes that was the very first thing I did when we started this long and winding road, and because the Vactrol was bad and a heat sensitive diode it masked any problems there. So, I was very fortunate to be able to find replacements for U13 & U14 and have removed and replaced the old ones. The good news is that the tube section is now working properly, both OD1 & OD2 are adjusted properly and the Trimpots are close to centered now as they should be. Yeah! I feel like I can see the light at the end of the tunnel now.

The bad news is that the S.S. section is still reading low at about 7.5Vp-p and T3 has no effect when adjusted. So looks like this is using one side of the of U14 which is now new and working properly on the tube side so I would say this chip is eliminated as a problem. That would leave U6, U7, U8, the clipping diodes and a bunch of resisters and T3 to check.

I noticed on the schematic that R46 in circled and on my unit R46 is not installed from the factory.

Another schematic difference is that the preamp schematic shows T3 to be 5KΩ but on the V2 layout map, it shows 2KΩ. Without removing from the board my T3 shows 1.7KΩ

[rnolan]

Great news about the tube side 

[Cpt Adama]

Follow up to my Solid State channel alignment issues. Well not sure I have an issue now or not. The alignment procedure doesn't really mention what the OD2 setting should be when this measurement was taken. I have since measured U8 with OD2 at zero (6.4Vp-p) and with OD2 at 10 and I get the 11.4Vp-p as per the alignment procedure. However T3 still has no effect that I can see on the p-p voltage of the S.S. channel.