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I am working on a Marshall MF350 that I was told would not power up. When I got it on the bench I took the chassis out of the cabinet to eyeball things. The fuse was not blown, but three of the multi pin connectors that connect to the main board were disconnected. Obviously someone had been in there already before it got to me. I plugged them in, turned the power switch to the off position, and plugged in the power cord. Immediately, the fuse violently blew, like there was a dead short somewhere. I immediately disconnected the power, then realized someone had installed the power switch upside down, so when I thought it was off, it was actually on. I checked the switch wiring and it seemed to be wired correctly, so I'm wondering now if the power transformer is shorted. I haven't had a chance to do a coil resistance check on the transformer primary yet, but I was wondering if any of you might have access to a schematic that you could e-mail me, and also the resistance specs on the transformer windings. Also, since I am new to these amps, if there are any of you out there that are familiar with these amps and have some advice on any quirks these might have would really be appreciated. Thanks!!
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WAAAYYY too familiar, I am.
There is indeed a dead short, it is the power modules you plugged back in.
Unplug all four power modules and THEN power up the amp without them. If it still blows fuses, then I might think about the power tranny. I bet it holds.
Your clue was that someone had disconnected the things. SHould have fired it up without restoring those connections first.
There are four of those power modules, the chip is a TDA7293V, Mouser sells them reasonably. If one is blown, replace all four anyway, seriously.
This amp has the four power amplifier modules, and they are working in bridge mode. NEITHER TERMINAL OF THE SPEAKER OUTPUT IS GROUNDED. DO NOT CONNECT YOUR SCOPE GROUND TO THE SPEAKER LEADS. There are really two pairs of them, one module has the other servo-ing off it, a power follower maybe is the word, then the two pairs are bridged.
No one publishes transformer winding resistance specs. If you suspect a transformer, disconnect its secondaries and power it up with the primary ONLY connected. If the mains fuse blows, your tranny is bad. You might find the transformer tester interesting over at RG's Geofex web site.Education is what you're left with after you have forgotten what you have learned. -
Hey Enzo, just want to thank you again for the schems and all the info. I have the IC's ordered from Mouser and I'll let you know how it turns out.
Thanks!!!!Comment
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Installing IC's in Marshall MF350
Since I usually repair old time tube radios and this is the first time I have worked with IC's, I have a question I hope you guys can help me with. The IC's in this amp have been replaced before, and the previous repairer had smeared alot of some type of white paste on the heat sinks where the IC's mount. I have cleaned it all off because it looked like a mess, but is this paste something that is supposed to be there, and if so, what is it called so I can get some? Thanks guys!Comment
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White paste
It's Silicone Heat Transfer Compound. It needs to be smeared on the flate side of the IC and on the heat sink before you mount the IC. Mouser will have this. As Enzo said, replaced all 4 of them. don't plug in or unplug the speaker when the unit is on. These things blow up easily!Comment
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You will often see heat grease on power semiconductors - the output stages of solid state amps. They get hot, and this goop helps transfer heat from the transistor or IC to the heat sink.
Be aware too, that depending upon the model, some of the heat sinks are live, NOT GROUND.
I think most electronics suppliers have the stuff. A search in any catalog for "silicone" will usually get you close. Heat Transfer COmpound works, Thermal COmpound sometimes, even "heat grease." You'll find it. Not cheap. Too bad, the excess slop from the last guy would probably have been enough. Might even still have it at Radio SHack, though I wouldn't bet on it.Education is what you're left with after you have forgotten what you have learned.Comment
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Fried all four new ICs
Hi Guys,
I got the new ICs and heat sink grease from Mouser and very carefully replaced all four. The boards looked OK. Before plugging in the boards I disconnected the power transformer secondary leads, installed a new fuse, and turned the power on with no problems. I then decided (probably stupidly) to plug in each board in succession, powering the amp up after each connection, figuring that if I had any faulty boards I would ruin them all. I plugged in the one closest to the transformer and powered it up, and everything seemed OK. I disconnected the power, then plugged in the one next to it. As soon as I turned the power on, that IC fizzed out with a blue flash. To make along story short, I ended up burning up all four before I was through. Should I have plugged them all in at the same time? Also, I didn't have a speaker cab plugged in to it and was wondering if that was a no-no.
Thanks for the hep!!Comment
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NO NO NO NO NO
When I wrote this, I was not very clear I guess. ALL FOUR amps are in one circuit, it is not like a row of 6L6s. The four are in two pairs, but none are parallel. Each pair is sorta servoed. One follows its mate. Then the other pair does the same but at opposite polarity. SO when one goes pos, the other goes neg. One pair drives the speaker + and the other pair drives the speaker -. So between them, the signal sneds the speaker one way then the other.This amp has the four power amplifier modules, and they are working in bridge mode. NEITHER TERMINAL OF THE SPEAKER OUTPUT IS GROUNDED. DO NOT CONNECT YOUR SCOPE GROUND TO THE SPEAKER LEADS. There are really two pairs of them, one module has the other servo-ing off it, a power follower maybe is the word, then the two pairs are bridged.
Yes you can fire it up with out a load, in fact I recommend that until you know it works that way. ONLY THEN do I connect a load.
YES, ALL FOUR AT ONCE.
A variac pays for itself working on this amp.Education is what you're left with after you have forgotten what you have learned.Comment
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Success!!
Enzo, I wanted to thank you for all your help. I installed another set of ICs, powered them all up at once, and everything works. The amp sounds great in all four modes and I can dial in whatever sound I want. You guys are the best!Comment
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Now sell it quick, ship it to Nebraska, do something so it never darkens your door again. We all live in fear, those amps are like women we broke up with. Hell hath no fury...Education is what you're left with after you have forgotten what you have learned.Comment
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Got a MF350 on the bench, so reanimating this excellent thread. Schematic is available now.
Enzo, can you explain the usefulness of variac in testing? Do you still need something in-line with mains AC to monitor current? Do you also use bulb limiter simultaneously? And what might you see as you increase voltage from zero on the variac?
Thanks!Comment
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I disconnected all PT secondaries, and the PT is drawing 25 watts according to my Kill-a-watt. Is this normal? I know that my tube-amp PTs draw 5 to 10 watts even when connected to mains only, just due to efficiency losses.
With the secondaries connected, current does not increase. I get +/- 33vDC on the HT rails, but no DC on the LT (though the secondary is supplying 19-0-19 AC). So maybe that rectifier is burned.Comment
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You do realize I have repeatedly stated on this forum that I do not accept that model for repair. I won't put my name on that amp and call it reliable.
A variac is not very useful without a current meter. The ONLY thing I watch on the variac is the current meter. If I dial the mains up to say 60v, the voltage readings in the circuit are next to meaningless. As I increase the voltage from zero, I expect to see a SMALL current surge as the caps charge up, and at full voltage, les than half an amp more or less. If I see current ramping up as I turn the variac, I back right off. This is essentially what the bulb limiter does, if current ramps, it lights instead of powering the circuit. I personally do not use a bulb, but I highly recommend them, especially for the novice.Education is what you're left with after you have forgotten what you have learned.Comment
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Of course, NO, I didn't read that bit until after I accepted the amp. But I have not estimated the work yet.Originally posted by Enzo View Post
Your reply is perfectly clear and answers all my questions. Thanks!Comment
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How can an unloaded power transformer pull 25 watts?
That does not sound like a healthy PT to me.
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