(Attempted) Resurrection of a Mesa Boogie 400+

[chevaliernoir]

PROLOGUE

I have a Mesa Boogie 400+. I've tinkered with it a bit but haven’t done any major work, except for building a nice box to protect it.

PURCHASE DECISION

I’ve been talking for a while with a seller in Toulouse who has a broken 400+. His "buddy who knows his stuff" determined that the output transformer is shot. The part is nearly impossible to find and super expensive. The tubes installed are Mesa Boogie, so they’re probably original, meaning at best, I’ll need to replace the output transformer (€400) and retube it (€450). Even though these amps typically sell for over €1000, repairing one in this condition isn’t really cost-effective, especially since there could be other hidden damages (you see where this is going?).

At the same time, I spotted another one on Reverb, from a shop in Lyon, with some burnt components and no tubes, but priced at the cost of a new output transformer. So, in the same week, I received the one from Lyon and went to pick up the one from Toulouse. My car trunk is a mess...

DIAGNOSTIC

Today, I’m taking a closer look... The Lyon amp is in really bad shape.

A broken pot, missing EQ knobs, and, most importantly, badly damaged electronics and cut wires:

In comparison, the Toulouse amp has a damaged front panel, but the electronics look brand new:

I’m going to try to restore the Toulouse amp and make it as pristine as possible, including the aesthetics, and we’ll see if the other one has any chance of revival or if it will just serve as a donor for parts and be sold piece by piece online.

DIAGNOSIS

Since the output transformer is supposedly dead, before plugging anything in, I remove all the tubes, measure the transformer’s impedance, and... the transformer seems just fine! In fact, the other amp has the same impedances.

After removing the tubes, I see that there are indeed only Mesa Boogie tubes, but some with gold lettering, others with silver, and the power tubes aren’t matched... Clearly, this amp has had some use.

From what I understand about Mesa Boogie power tubes, they’re identified by color (red, gray, green, blue...) so you just need to install tubes of the same color to avoid any matching issues. You can order JJ tubes with the corresponding Mesa color code. Great. If you see things differently, let me know, I’m eager to learn.

After a visual inspection where I don’t detect any tampering, and everything looks clean and original, I put in a new fuse, plug it in, and switch it to ON for a second. The fuse blows instantly.
I desolder the transformer outputs that supply high voltage to the power stage (180V AC) and power it on again. The fuse blows instantly (again and again).
I desolder the transformer wires that provide the negative power supply (60VAC). The fuse blows instantly (again and again and again).
I desolder the transformer wires that power the tube heaters (6.3V AC). The fuse blows instantly (again and again and again and again and again).

In short, the transformer is now powering nothing, so it must have an internal short circuit. It’s not the output transformer but the power transformer that’s dead. Too easy.

PLOT TWIST

While searching for documentation on the power transformer, I came across this great document describing the 220V and 110V configurations...

So I check my amps and... the two amps don’t have the same configuration!

And it’s the one in good condition, the Toulouse one, that seems to be set up for 110V.

I’m going to switch everything back to 240V, but I’m wondering how well the fuse protected the electronics or if I’m about to discover a bunch of broken components everywhere!

Stay tuned for the next episode...

 

[woodbutcher65]

If you decide to give up on the heavily damaged amp, I may be interested in buying it. I understand that you are on the other side of the Atlantic Ocean and shipping costs would not be cheap.

 

[chevaliernoir]

Wired in 220V this morning and... it works. Maybe I got lucky because I didn’t take any prior measurements.

I'm still monitoring it with the thermal camera. The hot spots are identified, reasonable, and "normal."

I have the correct AC and rectified voltages, great!

506V instead of 480V
-64V instead of -60V
6.7V AC instead of 6.3V for heating
(I'm supplying 240V AC instead of 220V, and there is no load connected)

I tried wiring the 240V transformer wire instead of the 220V one, but it didn't work as expected. I quickly went back to the 220V wire.

 

[chevaliernoir]

Voici la traduction en anglais de ton texte :

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Well, I installed the four Mesa 12AX7 tubes to see and...
When I put sound through input 1 or 2, it comes out well through the effects loop. No crackling and no noticeable distortion. Awesome! (again)
The tone controls work, the EQ too, and the Bright switches and frequency shifts (by pulling the bass and treble knobs) also work.

So, I took a look at the Mesa power tubes that were installed:
6 tubes with the GRAY index
2 tubes with the GREEN index
2 tubes with the BLUE index, with gold writing on the glass
2 tubes with the BLUE index, with silver writing on the glass
A nice mix.

My understanding of the setup doesn't quite match what I read online, but I'm not an expert at all. So, I’m sharing my reasoning:
This isn't a push/pull Class AB setup, like in a classic transistor amplifier.
My understanding is that each transistor operates in Class A.
There are 6 in parallel that process the amplified signal, and 6 others that handle the opposite signal. The output transformer uses the difference between the two.
What seems important to me is that the 6 tubes in parallel should have the same characteristics so that they all supply the same current, to avoid some contributing more than others. If the bank processing the opposite signal doesn’t have exactly the same characteristics, it will at worst generate some common mode noise, but I imagine it's not critical. So, I think that if we have different tubes, we need to ensure that each bank of 6 is made up of tubes with the same characteristics.
I’ve often read the opposite. What do you think?

I installed 2 tubes to check and... if I didn’t see anything, I definitely heard it... a sweet melody, no hum, no crackling, no distortion.
It works! Hallelujah! Glory to Pino Palladino, John Britten, and Nina Simone (my idols)

When I installed all the tubes, it still worked, but it sounded less nice. I didn't insist.

Behind each tube socket, there are two resistors—one for the grid and one for the screen. On one of the tubes, one resistor was burnt out, and the other wasn’t original and was way off the right value
That’s been fixed. It’s nice to have a parts donor right next to you.
New test in progress... things are heating up, ...

I'm now thinking that, since I only borrowed two tiny resistors from the burned-out amp, it might be worth trying to get that one working too...

 

[chevaliernoir]

I left the amp running, with all the tubes installed. I turned up the volume a bit (nothing extreme).

After a few minutes, I had the feeling there was a slight hum... then distortion, and finally the fuse blew.

Well, I had only put in a 2A fuse, whereas I should have used something higher. But before powering it up again, I’m going to buy an autotransformer/variac/alternostat to bring it up more slowly and compare it with my working amp.

So, I turned my attention to the other amp, the one that burned out from Lyon.
The output transformer seems OK (at least the preheating and -60V are fine). I still need to check the high-voltage outputs. If everything checks out, it might be worth trying to restore it.

I removed the burnt parts and the electrolytic capacitors nearby, as they had overheated.
In the end, many components look bad, but that's mostly due to soot deposits, not overheating.
Now I just need to finish cleaning with isopropyl alcohol, check that the power transformer is OK, and order/replace the components... probably in 2 or 3 weeks.

 

[chevaliernoir]

Hello. My "Toulousain" amp seems to be in perfect condition after its conversion to 220V. The output sound is good without any parasitic noise. However, as I mentioned earlier, after playing music for a while, the sound quality degraded, and two seconds later, the fuse blew.

The 110V version suggests using an 8A slow-blow fuse, so I imagine that in 220V, a 4A slow-blow fuse would be appropriate. There was a 3A fuse installed. This could be the reason for the previous blowout, but it might also be hiding another issue.

So, I just equipped myself with a 1kVA variac, allowing me to control the input voltage. I also have the means to measure the electrical power supplied to the amp, the current, and even the phase shift.

I recorded the power consumption relative to the input voltage. In standby (blue line), it rises to around 100W for a 220V supply. With everything turned on (orange line), consumption reaches about 250W.

What’s great is that I compared it to my reference amp, which I’ve been using (occasionally) for a few years, and although the tubes are different models, I found exactly the same values (black dotted lines).

This is a good sign, but it doesn’t explain why a 3A fuse would blow...

I left the amp plugged in for a while, with the power measurement displayed, and sometimes (a few seconds after powering on, sometimes after a long period), the power instantly jumps from 250W to 525W. (This is the red point on the previous graph).

I quickly measured the internal power supply voltages under these conditions. The voltages are very close to the expected values.

When the amp starts acting up, they drop by "only" 20V. I didn’t spot any overheating component on the thermal camera (but when this happens, I only allow myself a few seconds for diagnosis before turning everything off). However, 275W of parasitic consumption should be noticeable, right?

Could it be a faulty tube? A short circuit in the power transformer? Any other ideas?

(I just ordered a new set of tubes to start from a solid base. After comparing various websites, I placed my order with BTB-Elektronik.)

 

[chevaliernoir]

Quick question for the electronics experts... what is the purpose of the 3 Ohm / 5W resistor?

On the amp that seems to work, it heats up a lot (I understand why a 5W resistor is used), but I don't see why there would be a significant current.

On the amp that burned out, it seems that the problem originated from this resistor.

 

[chevaliernoir]

.. by the way, the two 62k resistors are also very hot (~100°C).

Nothing abnormal or alarming since they each seem to have 240V applied to them.
Except they are almost in contact with one of those large 220uF electrolytic capacitors, which probably doesn't appreciate it too much...

So, I replaced the 2W resistors with 3W ones (which, strangely, are the same size) and moved them away from the capacitor.

Unsurprisingly, they still heat up just as much, but this should protect the capacitor. I’m surprised that this design choice persists.

In my next order, I think I'll replace this capacitor as a preventive measure and move the resistors like above... on all three of the 400+ amps that I have at home.

 

[chevaliernoir]

Voici la traduction en anglais de ton texte :

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And the burned-out amp? I removed the components that were burnt or nearby... some were completely charred.

After a good cleaning, the PCB is still perfectly usable.
That’s great news, especially since I only ordered around fifty euros worth of components.
Before / during / after (I'm still waiting for two capacitors).

The missing capacitors aren't critical, so I powered it up, and... everything seems OK. There are no tubes installed, so there’s no power consumption, but the voltages are exactly as expected. Things are looking good.
Feeling confident, I took a look at what remains to be done, and there’s this push/pull treble adjustment pot to replace (around 200 to 250k, logarithmic).

I haven’t found the right one yet (mainly because of the smooth shafts that seem to be no longer available), so if you have any good suggestions, I’m interested.