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Op amp ic

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Breewell

New Member
Hi,
I would like help confirming that the pin outs and characteristics of a JRC4558D op amp will be the same as a NTE778A.
There are three JRC4558's in my boyfriend's guitar amp, and he blew at least one of them last night. I would like to switch one or all of them out, but don't want to ruin the amp.
I have found the specs for the NJM4558 and the NTE778A and they appear to be the same. I also called NJR today, but I am still unclear if JRC4558D and NJM4558 will be the same.
If you can just point me to this information I would be happy to do the searching myself
Thanks in advance,
Bree
Also, if anyone knows of any electronics distributors in the NYC area (other than Anchor, they are a wonderful store and great guys but they are so far out from the city) I would love that kind of info. Thanks
 

Breewell

New Member
a follow up question, troubleshooting and draining caps

Thanks!
I replaced the op amp and the amp still doesn't work. When it blew there were sparks and smoke behind just one of the inputs (apparently the boys were checking to see if a second guitar amp that just has a head but no speaker was working and thought they would plug it into the good amp to see if there would be noise, and there was a lot of noise)

When I trace the circuit from the main input it makes its (no contraction :)) way to the first IC (that had its top blown off) through a few caps, a couple of transistors, a handful of resistors and one lonely coil. Is it possible that some of those components are also blown? And if so how would I check that? Also could the other ICs also be dead and just not showing any outward signs?
Thanks for any advice.
Also,
I think I know I should drain the caps before I do too much work on the circuit board, I found a method of doing this on www.schematicheaven.com (of course that site appears to be down today) under "Maintenance". It involved wiring an aligator clip to a 10w 1ohm (I think) resistor to a multimeter style lead. You clip one end to the ground and then go through and touch the leads of each of the caps. Does this sound good? Is it necessary? The larger caps seem to be hot glued in place (so you don't touch them accidentally I assume).

Well, sorry for the long winded post, I just wanted to be thorough. Thanks for any help in advance.
Bree
 

nettron1000

New Member
Whew ! appears your opamp troubles have grown considerably from your original opamp replacement. It now sounds like (excuse the pun) something that belongs in an electronics repair shop.

Yes, if there was enough of a fault to blow the top off one of the opamps, its highly likely that other components have bin damaged as well.

Repair shops usually trouble-shoot electronic circuits with a suitable set of schematics with accompanying service notes along with proper test equipment. Doing this on your own without this would be rather difficult.

Im assuming you have a fair knowledge of electronics ?
 

Breewell

New Member
cheap amp

Well, the amp was a cheap ebay buy, and is a no-name that I can't find schematics for, so I'm not sure I want to spend too much money repairing it. I have a feeling that it would cost twice what the amp is worth to take it into a guitar shop. I have decent electronics knowlege, but maybe not enough. I have a feeling this will be a long term project, or maybe I'll try to make friends with some amp techs!
Thanks,
Bree
 

Phasor

Member
You clip one end to the ground and then go through and touch the leads of each of the caps
Not quite correct - you should connect your resistor between each capacitor's terminals. For larger capacitors, you should leave it there for a few seconds.

Many people like to use a screwdriver to short the capacitor terminals. Fine for small capacitors, but bad practice on large ones! The value of the resistor doesn't actually matter, as long as it's fairly low.
 
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