Marshall 3315 Solid state attempted repair questions - diyAudio
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Old 12th September 2013, 12:40 AM   #1
defec is offline defec  United States
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Default Marshall 3315 Solid state attempted repair questions

Hi, I have a solid state Marshall 3315 head that I'm attempting to get back up and also use as a learning experience. One day years ago I turned it on and it just didn't work.

I replaced the 10amp pico fuses but they burnt right up. Should have known better. I then checked the diode bridge and saw that the negative and positive rails were shorted so I took it out of the circuit but it didn't help as the short was still on the pcb at the same point. I did notice if I disconnected either of the emitter leads of the power transistors MJ11015/MJ11016 that the resistance between the power supply rail goes to 10k.

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what should I try next? I am mostly a tube guy so I lack any real experience in dealing with solid state power amps. any help is greatly appreciated.
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Old 12th September 2013, 01:57 AM   #2
defec is offline defec  United States
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Checking both the the MJ11015/MJ11016 with the emitter wire lifted and the collector to emitter is shorted of the devices themselves. I'm guessing that this isn't normal. What else should be checked while I'm at this?

If I were to order new outputs and it fixes the issue is there any dirty method to adjusting the bias of the amp without a scope?

Thanks
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Old 12th September 2013, 02:35 PM   #3
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If you're lucky, just the outputs have blown - but really it depends what caused them to fail - in a DC coupled amp almost anything can do that.

You normally adjust the bias for a specific current, as specified in the manual - it would be pretty rare to adjust it using a scope.

But 'crudely', if the bias is too low you will hear crossover distortion at VERY low power levels, if it's too high the output transistors will overheat and may blow again.
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Old 12th September 2013, 03:29 PM   #4
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I suggest to replace from t3 and t7 to the output stage. Check or replace all signal diodes and zener in the short circuit protection circuit, tr5 and 6.

When start the testing with power, once again I suggest to use a lamp of more or less the same power as the DUT (apl in this case), so a severe overload or short circuit can be viewed in the bright of the lamp in place of replacing fuses continually. Also be sure that the entire amplifier isn't self oscillating, a new blown of the stage may occur.
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Old 12th September 2013, 07:18 PM   #5
defec is offline defec  United States
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Thanks for the replies/help. I'm actually enjoying what I've learned so far from this.

Nigel, unfortunately I can't find the service manual for this amp for the suggested factory bias range. If I measure the voltage drop across R22/R24 what range would you suggest I try. I think this is how you can adjust bias from what I've read so far but I can be completely wrong.

Osvaldo, I assume T3 to T7 are TR3 to TR7? I can't find t3 or 7. While replacing TR4-7 do any of the pairs need to be HFE matched or do I just install new ones?

I need to build the lamp protection circuit. It will come in handy. I was going to use a variac set to 20 volts and keep an eye on if the current goes down after the power supply capacitors charge/stabilize.

Is there any way to check for self oscillating without a scope? It's getting to the point that I should look for a cheap used one.

Thanks again guys. I'm going to have to order a bunch of parts so that I can get to the next step. I kept the amp around forever figuring that the worse case was that I'd have a great chassis/box for a tube head but I figure I should at least hear it again fixed and that I'd also learn something in the process of doing so.
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Old 12th September 2013, 07:24 PM   #6
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Quote:
Originally Posted by defec View Post
Osvaldo, I assume T3 to T7 are TR3 to TR7? I can't find t3 or 7. While replacing TR4-7 do any of the pairs need to be HFE matched or do I just install new ones?
No, although if matched, better.

Quote:
Is there any way to check for self oscillating without a scope? It's getting to the point that I should look for a cheap used one.
Yes, load the amp with a lamp or resistor in series with a big capacitor(non pol), and leave the input to the amp unconnected or shorted together. Then, if lamp glows or resistor becomes hot, amplifier IS oscillating. If not, not sure if oscillations appears sometimes and the amp is conditionally stable.
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Old 12th September 2013, 09:18 PM   #7
defec is offline defec  United States
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Great, that's really good to know. How large of a capacitor do I need? Something farads or would something smaller do?

Thanks again.
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Old 13th September 2013, 08:06 AM   #8
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Quote:
Originally Posted by defec View Post
Nigel, unfortunately I can't find the service manual for this amp for the suggested factory bias range. If I measure the voltage drop across R22/R24 what range would you suggest I try. I think this is how you can adjust bias from what I've read so far but I can be completely wrong.
Easiest (and safest) option is to set RV1 to maximum resistance, this gives minimum bias, then when you have it working see how it sounds at VERY low volumes.
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Old 13th September 2013, 08:38 AM   #9
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If you must replace TR1 & TR2, they must be matched with an hFE of at least 100 but they will not be damaged. Check output and drivers plus check the current limiters, TR4 & TR5 for damage. When the amp is up and running with RV1 at maximum resistance, with no speakers connected and no signal, set a DC voltage of 5mV across R22 or R24. That is the correct bias.
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Old 13th September 2013, 06:38 PM   #10
defec is offline defec  United States
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Nigel, JonSnell

Great. I'll do all this when I get the parts in and post how it goes. Keep your fingers crossed for me.

It's fun to learn about this stuff.
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