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Mesa/Boogie Rocket 44 tube amp repair
Mesa/Boogie Rocket 44 tube amp repair
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Old 8th September 2018, 02:50 AM   #1
cmjohnson is offline cmjohnson  United States
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Default Mesa/Boogie Rocket 44 tube amp repair

I picked up an "as is" Mesa/Boogie Subway Rocket that was salvaged from a storage unit that had some flooding.

Initial inspection shows really no water damage to speak of. The high water mark is a full foot below the amp chassis so there was no direct water intrusion. Even the speaker survived and even the cabinet is none the worse for wear. I think it got a few inches of water that quickly receded.

However, although the amp ran OK when first tested, which only happened AFTER a teardown and inspection for corrosion issues, (It's very clean, really pristine actually.) after a couple hours of operation it developed an interesting problem. The whole power supply sags badly, and the bias supply sags worst. Normal B+ to the power tubes should be about 460 volts and bias, around 12. After several seconds on, bias drops to about 6 volts and B+ is down around 380.

Note, this happens even with NO tubes in the amp. So it's not a shorted tube.

Since all the capacitors in it are of 1999 vintage by the date codes, it's time for them to go. So I ordered a full set and swapped them all out with fresh caps.

Tests repeated. Results...SAME. With fresh caps.

Basics of the Rocket 44: 4 12AX7 tubes in the preamp stage, 4 EL84/6BQ5 output tubes, 44 watts rated output. Fairly conventional topology.

Subway Rocket is nearly the same amp.

Aside from hot tubes I'm not getting any hot or burning component smells.

So, the question I'm asking and trying to troubleshoot my way to an answer is, what is dragging down the power supply, and the bias supply most of all?

I tend to believe it's not a transformer issue. If it had a bad transformer it shouldn't be coming up to rated voltage at all. Well, if I'm wrong, a new transformer for this model is not expensive.

I have had an amp with a bias power supply problem before that was a bad bias coupling cap, an Orange Drop that shorted after that amp was recapped. The increased voltage from fresh healthy caps killed the Orange Drop so that was replaced, no further problems.

This is kind of a challenge. Usually when I see a problem with high voltage supplies that are being dragged down like this, there's evidence somewhere on the board. Some component(s) are expected to get HOT and be pretty easy to find. This one...not so much.
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Old 8th September 2018, 06:51 AM   #2
Enzo is offline Enzo  United States
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Quote:
Tests repeated. Results...SAME. With fresh caps.

That is why we try to find and solve a problem before wholesale parts changing.

SInce it does this without tubes, leave them out. ALL your voltages are falling, right? SO start at the power transformer. Is there a stable 120v right at the primary wires? Or at least is whatever is coming out your wall sockets also right at the primary? We could have a fuse holder or power switch going resistive. We could have a failing inrush limiter.

A schematic would really help.

Check the secondary voltages. Are they stable? or do they fall? If the transformer AC is falling, we can't fault the DC supplies for following along.

We need to isolate the problem to one stage in the process.
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Old 8th September 2018, 09:56 AM   #3
cmjohnson is offline cmjohnson  United States
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The cap changeout was routine. That's going to happen ANYWAY on a 20 year old amp and it rules out any issues related to the caps. It's not a case of throwing parts at the problem. it's a case of "I always recap a 20 year old amp.".

First step from this point is to isolate the bias supply from its load. Determine if there's a load on that supply that's dragging down everything else.

I have to also consider that though the board is clean, this has been exposed to water and the possibility of hidden corrosion under components has to be taken into account.
Closer inspection and some cleaning may be in order.

The schematic for the Subway Rocket is close enough to be useful but some component values are different. Both amps are built on the same preamp circuit board.
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Old 8th September 2018, 10:00 AM   #4
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Click the image to open in full size. Mesa-Boogie_Rocket_44.pdf


Hope this helps
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File Type: pdf Mesa-Boogie_Rocket_44.pdf (572.8 KB, 25 views)
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Last edited by JonSnell Electronic; 8th September 2018 at 10:05 AM.
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Old 8th September 2018, 10:11 AM   #5
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Sounds like a faulty mains power switch. Check the input to the transformer first.
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Old 8th September 2018, 10:17 AM   #6
cmjohnson is offline cmjohnson  United States
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Yes, that helps! It's nice to not have to guess about the output stage now.
Thanks.
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Old 8th September 2018, 10:30 PM   #7
cmjohnson is offline cmjohnson  United States
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Well, I wanted to be sure but I was suspecting it all along. The 75 VAC output of the power transformer (blue wire) isn't making even 50 volts with no load.

Looks like I need a new transformer. Fortunately they're not expensive for this model.


Or... I could just go insane and tap B+ and drop that tap to 75 V with a suitable dropping resistor.

Think I'll pass on that.
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Old 9th September 2018, 02:12 AM   #8
PRR is offline PRR  United States
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There's 15V bias to the tubes. Why is the raw AC near 50V?? I squint "20V", but that's a bad fax.

Aside from a low-current bias, you need substantial currents for the +/-15V opamp supply. You do not want to drop this off B+ supply.
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Old 9th September 2018, 02:38 AM   #9
cmjohnson is offline cmjohnson  United States
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Hey, I already said I'm not going to actually try to tap B+ to make a bias supply. It probably wouldn't work right anyway due to current draw requirements. Why even bring it up?

It needs a new power transformer. The 70 volt blue output is not making 50 volts with no load on it. Seems pretty cut and dried at this point.
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Old 16th September 2018, 08:27 PM   #10
danFrank is offline danFrank  United States
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Quote:
Originally Posted by cmjohnson View Post
Hey, I already said I'm not going to actually try to tap B+ to make a bias supply.
Why not? It works quite well and it will save you some money. You just have to experiment with a few resistor values in order to drop the HV down to bias supply levels. Try anything from a 75K to 150K resistor. Look at a Marshall 1987 or 2204 schematic.

Never mind... You need an opamp supply also... Didn't see that till now.
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