broken bass pot on my 1971 150-2 [message #14026] |
Sun, 11 July 2010 04:00 ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
drjymmiphreak
Messages: 6 Registered: July 2010
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Junior Member |
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I traded a guy a computer for this amp almost 10 years ago now. used it for everything from vocals, to guitar, bass, and even used it as a pa! It always had a bit of crackle in it. At first, it was the cables, then the pots. I took the cover off only to discover I'm probably the first person to open it up since 1971! After a little WD-40, everything came apart nicely, exept the bass pot on the first channel... The nut froze and spun the pot in its case, shredding the internals... Can any one help me get a replacement? I'm not worried about period correctness, just functionality. I love this amp! The good news is that it plays like new again, except I cant adjust the bass on channel 1... Any help appreciated. I'm not an electrician, just a fix it all kinda guy. I can probably solder the new one in if I can find one...
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Re: broken bass pot on my 1971 150-2 [message #14257 is a reply to message #14026] |
Wed, 01 September 2010 10:00 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) |
stevem
Messages: 4764 Registered: June 2004 Location: NY
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Senior Member |
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Hello and welcome!
I will guess that the board you are speaking of is the #5033 board on the floor of the amp.
This is the driver board with the two box mounted/heat sinked driver transistors.
If your amp is blowing fuses and or producing a 120 hz hum like mad out of the sepaker(s) then your problem may be on this 5033 board with one of those two driver transistors and or one, may be both of the output transistors mounted on the bottom of the amp.
It is possible to have a bad driver transistor and not have a shorted output transistor, but most times not the other way around.
also on the driver board are two 5 watt box type resistors of under 1 ohm in value,if you have a dead output transistor than one those resistors may be open also.
Keep us informed on your progess.
New transistors to replace these driver transistors will not have the boxed heat sink, so what I do is reinstall the box on the new one and also add a slip on metal star type heat sink also as once you unload the original from the box you never get the good metal to metal contack/grip that the orignals had, and poor cooling will fry these transistors if you drive the amp hard.
The orignal numbers on these transistors where 38736 and 38737, replacements for these are NTE brand NTE 128 and NTE129 respectivly.
The output transistors are both the same type.
A commom replacement will be a 2N3055, or a NTE131, or the better built NTE181.
If one of your outputs is toast I would replace them both with NTE131MP which is a matched pair of the 131`s.
Also note that the smaller transistors on the driver board should be checked as well, although if they are toast the amp will not alway blow the fuse.
The small round recitifer bridge mounted on the floor of the amp near the power transformer with the two black and one red and one green wire coming off of its tower should be checked for shorts also, as some times a blown output stage that kills the fuse may creat enough of a currnet draw to short that out also.
Also
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