I need help identifying a part! (Sunn Concert Bass content)

[aaron gein]

I bought a 1974 Sunn Concert Bass from a friend of mine awhile ago and it recently started cutting in and out while playing and then a few days later stopped working altogether.  I took the amp cover off and looked around a bit and noticed a slight burning smell and some heat coming from one side of the board when I plugged the amp in.  I unplugged it and took a closer look and found the problem.  I am fairly electronically inexperienced...i don't have a clue how to read schematics or anything, but I know enough to recognize a severly fried part when I see one.  I just have no idea what the part is!  
If someone could help me identify the part and maybe give me a replacement part number I would be very greatful!

I have taken a few pictures and posted them up here... www.ed-gein.com/friedsunn.html

Please help!

[animosity^2]

Easy call, it's the exact same resistor as what sits next to it.

It's a 22ohm 2W (+-10% tolerance) resistor
I think that's accurate anyways, from the schematics

If only this amp was a few years newer, I have the newer model and the wires to the power amp board are all out of place and your pictures would be a lifesaver if it was the same model!

[Isaac]

But what caused it to burn out?

[aaron gein]

I found a website that explains the color bands on the resistors.  So yeah, it is a 22ohm +-10% tolerance....but how do you know that its 2w?
And where can I find a replacement?  Like I said, I don't have much experience with electronics!

[animosity^2]

Isaac, if I knew that, I could figure out why mine is blowing resistors and such as well and maybe get it going again.  It's been dead for over a year.  It was hit by a power surge that got 3 different amps, and I've been trying to get it to waor again ever since!


Aaron_gein, I've looked over the schematics for your model amp trying like crazy to figure out my blackface model, and there are four of those, each going to a different output transistor.  The schematic on this site is for your model, you can pull it up and find out the specs for each component if you can follow it.

I think Radio Shack would have these handy, or they could order you a few and have them delivered to the store, I buy components for all kinds of things there.  If not, you could EASILY find a supplier on the internet.


ANYONE HAVE PICS OF THE INSIDE OF A BLACKFACE MODEL?
specifically, the power amp board and wiring connections on it...

[Isaac]

Either it's a bad design, which seems unlikely, or something is causing excessive current through the resistor. Does anyone have a scematic for the amp? Can anyone come up with one, even for just the applicable part of the circuit? Maybe we can figure this out. Mine's a silverface, so I doubt it's the same.

[animosity^2]

Isaac, I thought that this was a silverface based on the pics of the inside.
(side note:  I know you posted on my topic about crossed wires, I ended up finding 3 crossed pairs coming off the transformer, still have components to replace before I can find out if she works)


Aaron_gein:  is this amp a silverface or blackface concert bass?

The 4 like resistors sit between the transformer and ground in the unit.  The schematics on this site are, I believe, for this very model.  As far as I know, these big fat old 2W resistors only have a life of about 30 years, then they start breaking down and cracking, then burn up.

There may be a bigger issue at hand, let us know if anything else burns up.

[animosity^2]

Apparently, finding a supplier is not as EASY as I had imagined.

Radio shack cuts off at 1W resistors, and about every site I find for a supplier wants to limit orders to bulk.  I only need a few, so this didn't seem a viable option.

Anyone know any good suppliers that carry 2W resistors?  My amp takes 24ohm +-5% type.

[aaron gein]

the one with the fried resistor is a silver face model.  

i also have a black and red model that is in working condition.  animosity, if you still need pics of the inside of one i can bring mine home from my bands practice space and take some shots for you.  let me know.

[Isaac]

An exact match might be difficult. Parts Express has 5-watt, 25 ohm or 15 ohm resistors, and they sell individually. Or, you could put two 43 or 47 ohm resistors in parallel.

[animosity^2]

aaron:  you would be my very own personal savior if you could get me shots from the inside.  I'm mostly focusing on which wires go to which pin on the power amp board, I think I have it figured out after studying schematics and understanding the changes from the silverface to the blackface for about 5 hours, but I am not 100%.  It's hard to tell since these seem to be like 2 power sections tied together, and I'm not sure if i have half a section crossed or anything.

isaac:  would it be safe to put a pair of 5W 47ohm resistors in parallel in place of the 2W 24ohm?  Am I right to understand that this would result in a 2.5W 23.5ohm resistance (close enough for me)?

This thread has been a major help for me, thank you aaron for starting it.  If my amp gets to work again because of you two, I'll be forever greatful!  I already sing your praises.

[Isaac]

Close, but better. The impedance you calculate is correct, but the power handling goes up, not down, to 10 watts. Yeah, that should suffice!

[animosity^2]

Isaac:  Looking at my amp, the blackface has a pair of resistors, a 2W 24ohm and a 5W 1.5ohm, not just the 2W 22ohm.  Is there anything in series after this on your amp?

God bless the genius that figured this out first that produced it!

[animosity^2]

anyone?

If memory serves, the silverface has a pair of resistors, each going to a different ground rail.  One is the 2W 22ohm and the other is like an fractional-watt 810ohm or something.

Why is that so different than the blackface?  Are the ground and lead rails a different voltage?  Or did they just want a whole lot more power handling?