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Old 8th November 2012, 09:56 PM   #1
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Lightbulb Marshall G100R CD

Hi, I have a Marshall G100R cd which I found in my friends garage, he said it had a problem and it kept blowing the fuse.

I checked it out and found tr9 and tr10 the darlington drivers were short, I replaced them and after further checking discovered 3 x 33 ohm resisters in the power stage which were also short circuit I replaced them and powered up the amp. All was well so I connected a cab and used a guitar on the i/p, the amp seemed to work fine but had very little power, it was only giving out about 10 watts, everything on the preamp worked as it should.

Is there someone who knows about these amps who could guide me, I dont have a scope or signal generator and have exausted my multimeter. All the rails seem fine, apart from 2 x 7watt resistors getting a little hot, I cannot see anything that could cause this lack of gain. Please help if you have any ideas, thanks, Peter
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Old 8th November 2012, 10:17 PM   #2
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Were where those resistors located?
Are you sure they weren't supposed to be .33 ohm?
It is highly unlikely that a 33 ohm resistor would be shorted.

jer
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Old 8th November 2012, 10:51 PM   #3
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if he replaced .33 ohm emitter resistors with 33 ohm resistor( thinking his replacements where solving a short his consequent lack of gain should should serve as a clue)
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Old 8th November 2012, 10:54 PM   #4
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Yep !!!
My thought exactly !!

jer
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Old 8th November 2012, 11:10 PM   #5
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Hi, the resistors are 7 watt and marked R33j. I have just checked the diagram and they are 0.33 ohms. I have still got the origional they seem to be measuring 60 ohms, but I think my multimeter cannot measure them. I will have them checked, I must thank you for recognising my error, may I ask the two resistors in the psu 270 ohm 7watt and on the -ive 330 ohm 7w are they supposed to get hot. Thanks, Peter
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Old 8th November 2012, 11:27 PM   #6
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Yes, They will get hotter do to the higher voltage drop across them from the current that is set to flow through them.
P= I*I*R

jer
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Old 8th November 2012, 11:34 PM   #7
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Bless you all, I cannot wait until tomorrow, Peter
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Old 8th November 2012, 11:38 PM   #8
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Pitagi
i would like to know what your using in terms of a multimeter make model
secondly when you measured these resistors where they still in circuit or had you removed them
i have a mental image of you holding a resistor (out of circuit) and with your thumb and forefinger pinching the lead of the resistor to your test probe and your other hand doing the same thing and looking at the reading on your meter with a puzzled look

what is the average resistance of the human body paralleled by a fractional ohm's resistor?

Last edited by turk 182; 8th November 2012 at 11:45 PM.
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Old 9th November 2012, 12:23 AM   #9
Enzo is offline Enzo  United States
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A good 0.33 ohm resistor will look like a short circuit to your meter. AFter all, one thrid of an ohm is not far from zero ohms. If the old ones measured 60 ohms, then they are bad, likely burnt out by the failed power transistors.
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Old 9th November 2012, 08:57 PM   #10
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Hi, sorry for the late reply, yes I am using a cheapo multimeter but having spent much of my early days working for Acoustic Research , Recording Studio Designs teac etc I do know how to use a mutlimeter. I think the problem is my eyes, they are not as good as they should be and to look at a 10 year old resistor for a decimal point is a easy mistake ( should have checked the diagram first). The problem is now solved and I have the gain that was missing. I have decided to invest in a fluke multimeter and at some time to get a signal generator and scope. Thanks for the interest you have shown in my post please keep in touch.
Ps I had a good sesion with the amp today, I think it is just as good as the valve state. Regards, Peter
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