Mesa Subway Rocket burned up! Help! - diyAudio
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Old 31st July 2014, 03:12 AM   #1
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Default Mesa Subway Rocket burned up! Help!

My Rocket has burned up on me!
I was playing through it and all of a sudden the volume level started dropping. The power light went out and I smelled something burning. I unplugged it immediately. I checked the fuse and saw that it was blown. After replacing the fuse, it blew again. I knew that there was something seriously wrong, possibly the power supply, mains transformer, etc.

Later, upon further instruction, I removed all tubes (4 12ax7s and 2 EL84s) replaced the fuse and turned it back on again. Once again, the burning smell and power loss.

Well, I opened the thing up. I found three resistors that look like they might be burned up. Two are 1K resistors and the other is a 110 ohms value. When I checked the resistance on these they all read 244 ohms. Not sure why differing rated resistors are reading the same value......
I also noticed that the white wire, coming off of the power jack and leading to the circuit board, has a burned and discolored looking solder point now.

I was thinking about replacing the resistors but I am afraid that they will just burn up again!

Two questions:
(obviously....) Any ideas as to what the problem might be?
What is the mA rating for the resistors I will need to replace?

Thanks!
Corom
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Old 31st July 2014, 04:08 AM   #2
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Here is a picture of the burned up components. I drew yellow arrows to the places I noticed as messed up. Sorry the picture is so blurry.

Corom
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Old 31st July 2014, 04:48 AM   #3
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Disconnect the secondaries of the PT and leave them floating. Turn the amp back on preferably with a dim bulb tester so you don't keep wasting fuses.
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Old 31st July 2014, 05:26 AM   #4
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Hi,
Does resistors read 244 because all 4 are in parallel. Do you have the schematic? If not you can download here.
Link :Mesa Boogie Amplifier Schematics
To start I would check all the diodes for short specially does 2 next to the left of the resistors.
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Old 31st July 2014, 09:47 AM   #5
JMFahey is offline JMFahey  Argentina
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Sorry but given the perceived level of expertise I suggest you send the amp straight to a Mesa Boogie Authorized Tech.

The shortest cheapest way, because after you unsuccessfully mess with it, it will be more difficult and take longer to repair and bench time gets billed by the hour ... on *your* pocket.

What originally might have been a relatively simple problem will be hidden by your repair attempts.

Say, you replace the burnt resistors ... the Tech will see they are fine and search for someting else in another area.
That wasted time is on you
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Old 31st July 2014, 01:02 PM   #6
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JMF:
Yes, I see your point and I agree with you. It's just that I don't have any money for repairs right now and if it is something simple then I'd like to do it myself. But if it is not then I don't want to totally destroy my amp and lose out big time.

I appreciate your concern though. I will definitely take your post into consideration.

Corom
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Old 31st July 2014, 03:06 PM   #7
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by Corom View Post
Here is a picture of the burned up components. I drew yellow arrows to the places I noticed as messed up. Sorry the picture is so blurry.

Corom
Hi,

The first thing is don't plug it in again..you are probably doing more damage..

Next take a step back and listen and don't do anything until you get some feedback.

You need to tell us the component numbers printed on the circuit board (of the burnt resistors) so we can find them on the circuit diagram..again DON'T switch it on again at the moment..its probably not a tube issue so don't start plugging and unplugging things..

Next BE VERY CAREFUL TUBE AMPS HOLD POWER AFTER POWER OFF SO DON'T put your fingers on the circuit board just look!

Regards
M. Gregg
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Last edited by M Gregg; 31st July 2014 at 03:09 PM.
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Old 1st August 2014, 12:47 AM   #8
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That white wire that looks ugly at the circuit board may be fine. It looks like some solder flux was left behind (not a big deal). The burnt resistors will need to be replaced after verification of what their value should actually be, but it's highly likely that they are just indicative of something else wrong, which caused them to burn up. If the circuit is a highbrid (tubes and transistors or opamps), then it could still have been a tube intermittently shorting out and blowing up some of the solid state circuitry, which then burned up the resistors, so I wouldn't assume the tubes are innocent. I would definitely not go forward with this project unless I had a schematic. The rectifier diodes that may well be feeding those burnt up resistors may also be blown or damaged. I'd look closely on the schematic at what those burnt resistors are feeding voltage to, and take it from there. It may well be wise to take it to an authorized service tech as stated above. When you or whoever finally powers up the unit again, it should be brought up slowly with a variac, while AC current is being watched. That minimizes your chances of another blowout, in case some of the problem is still there.
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Old 1st August 2014, 10:33 AM   #9
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Quote:
Originally Posted by Corom View Post
Well, I opened the thing up. I found three resistors that look like they might be burned up. Two are 1K resistors and the other is a 110 ohms value. When I checked the resistance on these they all read 244 ohms. Not sure why differing rated resistors are reading the same value......
That's because they are all 1K, and all in parallel - so should be 250 ohms when measured (I'm presuming they are the ones in the low voltage supply rails).

However, this couldn't blow the fuse - so isn't going to be your fault (it's just ageing and discolouration of the resistors).

The white wire looks fine as well.

No need to take the valves out, simply switching it to standby disconnects them all from HT, leaving little that could be wrong (basically transformer, rectifiers, large capacitors).
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