Burnt resister help with 70's Ampeg ???

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bcf1
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Post by bcf1 »

I acquired a late 70's Ampeg B 115 bass amp, upon plugging it in I was only getting a buzz from both of the 2 channels. Opened it up and saw obvious signs of burnt resister(s). See pic.

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So I replaced R221 (blue) since I had one and it was totally fried. I powered it up again and channel 1 now works, both hi and lo inputs no buzz, it's good. I get no sound from channel 2 just a constant low hum. I plan on replacing R214, R219 and maybe D210 because they look as some heat issues were present. I'm not really experienced with this kind of thing, however I'm not blind. Question : If I replace the above mentioned items would that fix channel 2 or can you see other things that are problems ??

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Any info is much appreciated. Thank you in advance !!

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phatt
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Post by phatt »

Even if you replace burnt parts it may just burn again, so until the cause of problem is found you maybe chasing your tail.
Number one test; with Meter in VDC mode,, Is there any DC voltage at the speaker terminals?

A healthy Power Amplifier circuit like this should read close to Zero VDC at speaker with no signal applied (idle)
A small amount of DCV like +/- 100mV is normal offset and fine but more than 1 Volt is a clue to deeper issues.
Best I can make out is that the bias might be stuffed and the pre-driver circuit has been over heating.
These circuits are DC coupled so when one part blows it often takes out more than one transistor or resistor.
Overheated resistors can change value quite dramatically sometimes and cause new parts to blow,, often instantly :shock:

My advice is Build yourself a *Light Bulb limiter* and then take some voltage readings,, looks like test points are marked on the schematic but hopeless to try and read them. If you can find a higher resolution schematic would be good :thumbsup
Some good postings on how to make and more importantly USE LBLimiters over at SSguitar. http://www.ssguitar.com/index.php?topic=2093.0
Phil.

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