Mesa still Blowing fuses.

[Jerm]

Hey guys! Need some wisdom and direction. I'm still learning a good bit about tube amp repair so go easy on me. I'm repairing a mesa single rec. 50 watt head. the owner said he saw smoke and thought it was the transformer. Note* he had a 10amp fuse in instead of the recommended 2.5. Is the transformer still a concern? Not exactly sure of a way to test it. I disassembled the amp and saw the the entire power board was charred. I cleaned up what I could, replaced the resistors and tube sockets and ran traces.. The amp is still blowing fuses. What should I do next? Thanks in advance.

Jeremy

[Mick Bailey]

When I get an amp where there's been serious damage the first thing I check is whether the transformer survived. If not it can be a deal-breaker whether the owner wants the amp repaired and you can be in a position to give a more accurate estimate of the repair cost up front. Think of it like a car mechanic telling you he's fitted new plugs, leads and injectors. Then later on he calls to say that didn't fix the problem and there's a crack in the cylinder head and it needs replacing. You'd want to know all this up-front.

You need to rig up a light bulb limiter. This will offer some protection to the amp by limiting the current, as well as saving money in blown fuses. Check the transformer by disconecting the secondary and properly isolating the leads. Now power the amp with the bulb limiter. If the bulb glows brightly you have a short somewhere. I always make sure to differentiate between a transformer short and a mains-feed short by disconnecting one side of the primary and re-checking. If the bulb doesn't glow when you do this, then you've got a shorted transformer.

So, then you need to find what shorted the transformer. Sometimes they go on their own. More often it's something that can draw a heavy current - shorted main filter caps or output tubes. If the board is toasted you can see from the damaged area and tracks where the short happened. A 10A fuse will ensure the damage is greater than it would otherwise have been.

Why did you replace the tube sockets, and which ones? I'm assuming power tube sockets, but more info and a picture of the board would be helpful.

I'm thinking a tube shorted, pulled a lot of current, fried the tracks and shorted the HT winding because the fuse was too large. But I may be a mile off.

[Jerm]

Hey Mike, good morning and thank you for your input. I will follow those steps as soon as I get a chance. The reason I replaced the sockets (power tube sockets)is because they were cooked beyond the point of saving. Pins 2 and 3 seemed to have been where the arching happened. attached is a picture of the power board after repair.

[g1]

Make sure you take some pictures of the transformer wiring before you undo any wires. Or draw a map or label them or something. Mesa schematics do not always show the wiring correctly for the revision of the amp you may have.

[Mick Bailey]

Interesting that both screen feeds are affected. A shorted tube would usually only take one side out. In most amps the resistor would blow before the track burnt up, but look how thin those tracks are - they'd let go a long time before the resistor went open. A lower wattage fusible resistor would have been a better design choice.

Does the amp blow fuses with the output tubes removed? I take it you replaced the tubes with new ones (or a good test pair).....

[Jerm]

Interesting that both screen feeds are affected. A shorted tube would usually only take one side out. In most amps the resistor would blow before the track burnt up, but look how thin those tracks are - they'd let go a long time before the resistor went open. A lower wattage fusible resistor would have been a better design choice.

Does the amp blow fuses with the output tubes removed? I take it you replaced the tubes with new ones (or a good test pair).....

Only one socket was charred, but I went ahead and replaced both. The 1k 2watt resistors tested in range but I had some to spare so I threw them in. The traces were fine except for the eyelets around the legs of the resister. They arched and burned through the board. I tried to clean it up as best as possible but still lost alot of the board. As you can see, I made what I had work. Traces are run exactly the way the originals were. Yes it still blows fuses with the tubes out. A good tested pair is waiting to be put in. I don't have a bulb limiter but I will be building one tomorrow to do as you suggested.

[Jerm]

Alright, I have a little bit of info. Feel free to be as descriptive as you'd like. I'm here to learn. I did as you suggested. Bulb glows brightly with the secondary unplugged. notice the two plugs. I disconnected the multi-colored one and left white and black plugged in. Thats when the limiter glows brightly. the two orange wire go to a transformer and there is still the output transformer which is not in this picture. Whats next guys? Thank you so much for your much appreciated help. Edit. I had a feeling that the white and black is still part of the secondary so I disconnected both connectors. No light. I may have a short in the transformer yes????? Off to work. later guys.

[Mick Bailey]

The Black and White wires are the primary connections. Theres a MOV connected across the supply and this could be shorted. Unlikely, given the fault condition, but it needs eliminating. Unsolder one leg and lift it off the board and retry your bulb limiter with the multi plug still disconnected, but the black/white connector plugged in.

Does the bulb still glow bright?

The orange wires run to the choke.

[Jerm]

Mick, no dice. it still glows brightly.

[Mick Bailey]

Ok. You may have a shorted HT winding as this is the most likely to fail. The two red leads are the ends of the winding and the red/yellow is the centre-tap. With the connector unplugged, measure the resistance from red to red, and from red/yellow to each red lead.

This won't always show up as anything wrong. A shorted turn or two will blow a fuse but meter out fine. The definitive test is to use the RG Keen neon trick;

The Super-Secret Transformer Tester

[Jerm]

Red to red reads 20 ohms. yellow/red to furthest red is 0.9 ohms. yellow/red to the closest red is 20 ohms. I will do the "super secret" test this afternoon. Have a great day Mick!

[Jerm]

Not sure if this matters. Just wanted to clarify something. The connector that I said has the "black and white wires" the wires are actually a black/white and white. there is also a single black wire coming out of the opposite side of the transformer that is soldered directly to the board, labeled "xfmr"

[Mick Bailey]

The transformer has a tap on the primary. Black is the beginning of the winding then there's a black/white tap (possibly 110v) and then the other end of the winding is white (possibly 120v). Maybe the rating plate will tell you the exact voltages.

You have a shorted half of the HT secondary. The yellow/red should read the same to both red leads, as it's the centre tap. It's well shorted, so no other test really necessary.

[Jerm]

Thanks so much for your help. Transformer is on order and I'll keep you posted on the repair. Are there any websites you can recommend that have some pretty good and informative step by steps on tube amp repair?

[Mick Bailey]

This is the best site for amp repair because it not only addresses the theory, but has a strong practical side. Tube amps aren't really a good subject for step-by-step instruction - It's not like baking a cake. The reason is you need to understand, not follow a list. Each fault situation can have a number of causes and implications. So my starting point would be to look at the theory first.

The other point I'd make is that relatively few 'tube' amps are all-tube and you need a working understanding of semiconductors as well.