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Old 8th May 2009, 05:43 PM   #1
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Default Solid state guitar amp

Hi i have a randall 300w solid state guitar amp and am having some issues. If anyone has any ideas it would be much apreciated.

1. When i turn it on it pops?

2. When i turn it off it holds power then pops then LED slowly goes off?

3. My presence POT needs a good flick to work as well as my volume POT?

4. I took the covers of and had a look at the boards and found that the R35 resister is burnt on one end of it and has crumbled a little?

I have the schematics for this amp but doesn't mean to much to me.
I have worked as a forklift tech for a few years so have some skills just need some guidense on what to do.

Thanks
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Old 8th May 2009, 11:34 PM   #2
norazmi is offline norazmi  Malaysia
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Hi there,

There is maybe someone could help you, try change burn resistor and refer to the schematic the exact value of it, maybe its good if you post the schematic here and maybe someone expert willing to check it for you.

This is my suggestion, the exactly problem with your amp maybe need more information about it.

Azmi.
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Old 9th May 2009, 04:07 AM   #3
jaycee is offline jaycee  United Kingdom
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At a pure guess R35 will be something to do with the protection circuit that should stop the switch on/off pops. You'd have to check though.

The pots are probably just worn with age or so and need replacing.

To be honest if the schematic is not making sense, get it serviced by someone experienced.
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Old 9th May 2009, 09:56 PM   #4
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Default Randall schematics

Thanks for replying i do understand the schematics a bit just need to be pionted in the right direction.
Attached Files
File Type: pdf wh-300_pre.pdf (49.9 KB, 102 views)
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Old 9th May 2009, 10:01 PM   #5
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Default Randall schematics

Here is the Power schematics
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File Type: pdf power.pdf (38.1 KB, 88 views)
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Old 9th May 2009, 10:35 PM   #6
norazmi is offline norazmi  Malaysia
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I think Jaycee is right R35 something like protection circuit, its connect to 7805 Q17 at input i think the resistor R35 burn and lost the connection totally. You must change with exact value R35 150 Ohm with 3 watts rating dont try with less dissipation, more is better like 5 watts type. Try change it first and try turn on you amp, i think your amp will not easily damage because it has circuit protection. Its good amp actually.

Regards,
Azmi
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Old 10th May 2009, 09:33 AM   #7
AndrewT is offline AndrewT  Scotland
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I see a mixture of FETs, BJT input opamps and FET input opamps.
That alone tells me the designer has thought about what he is trying to achieve
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Old 18th May 2009, 04:01 PM   #8
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Default Randall amp

Hi guys thanks for your feedback, I bought a 150 5w resistor and installed it everything is still the same. Still pops when i turn on and power stays there when turned off, then pops and led light goes out. With a solid state amp when you turn power of does power always disipate or is it suppost to be instant?
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Old 18th May 2009, 04:13 PM   #9
AndrewT is offline AndrewT  Scotland
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the amplifier continues to amplify as long as there is sufficient voltage on the supply rails.

Then weird things start happening when the voltages go below the operational voltage range of the amplifier.

If you want to avoid those weird effects then extra circuitry must be added to avoid the symptoms.
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