Corey Y's Sunn Concert Bass

Hemisaurus · September 09, 2011, 08:35:12 PM

Was I right in thinking the buzz is affected by the EQ?

Have you tried preamp out into a different power amp, and vice versa? If the hum is affected by EQ, it probably originates in the preamp. Does it hum with a shorted jack in the input?

VOLVO))) · September 09, 2011, 08:49:58 PM

It isn't the preamp, none of the controls effect it. All i did in the video was turn the volume up and down. The bias pot effects the hum, higher on the dial is louder, lower on the dial (to a certain point) makes it quieter. It's a 100k linear trim pot. (R310) I've ran preamp out into my peavey poweramp and it was a clean signal. No buzzzzzz. It's only when it's plugged into a cab that it has to power. otherwise, solid signal with no noise from the preamp.

I replaced all the output transistors. That's what originally gave me my output back.

It doesn't sound like a grounding issue, either... I don't know : \

Hemisaurus · September 09, 2011, 09:02:01 PM

The volume on what? The volume on that beast is in the preamp, have you tried bypassing the preamp and plugging into the power amp jack with a boost pedal?

Have you checked for a DC level on the output, with a meter if you've tweaked the bias pot?

VOLVO))) · September 09, 2011, 09:08:25 PM

I did use the poweramp in and an active bass to check the poweramp. It buzzes. The preamp does not buzz when run into a separate poweramp. I marked the bias pot before I moved it, it's where it was when I got it. How do I meter the output? Put it to the + side of the output, and ground?

What would you do? I don't really have a developed process, since I'm not experienced enough to have one. I usually just clean shit before I do anything else. I did that. Cleaned every jack, pot, switch, etc. Looked for cold joints, and suspect looking shit, fixed all of that.

How would you process? How should I go about diagnosing? I agree with not throwing parts at it, but I had a pretty educated guess as to those transistors being part of the problem. (since they're usually the main cause of it not having sound.)

(LISTEN UP FUCKERS, TIME FOR YOU TO LEARN SOMETHING.)

spookstrickland · September 09, 2011, 09:20:07 PM

I'm digging this thread. you guys are really giving me a good education.

Thanks please keep posting all the pics and vids you can.

Hemisaurus · September 09, 2011, 10:14:20 PM

You have a meter, yes?

Check for AC voltage, on the DC power rails first. Honestly at this point I'd be using a scope and seeing exactly where the hum starts in the circuit, usually it's bad power or a bad ground. I'm not good at this fault finding by proxy

Oh and keep a shorting jack on the power amp in, until you've fixed the hum, it'll help

Have you done the biro test? Poke each component with a non conductive tool (including those spade terminals) whilst the amp is running and see if anything makes it better or worse (ONE HAND ONLY, KEEP OTHER HAND CLEAR, SIT ON IT!) and be suspicious of any tantalum capacitors in the problem area, they fail in weird ways. Without a scope these are methods I'd try.

Hemisaurus · September 09, 2011, 10:29:57 PM

Oh, and that cap was replaced by a lazy tech, or someone in a hurry. Snip out the old component, leave the leads free, twist the new component leads to it, solder in place, and the glue for a bit of extra strength, it works, saves you disconnecting a bazillion wires. It doesn't make it more suspect, except for the fact it's a new repair.

VOLVO))) · September 09, 2011, 11:05:18 PM

I'll put it to you this way, I'm somewhere between total retard and complete prodigy. I can do most simple things, some advanced things.

I've have indeed done the Biro test. Nothing I've poked changed anything, save for popping off that white wire by accident. That changed/eliminated the hum. I eliminated the spades terminals, direct soldered them to the jack. Basically, anything that looked like complete bullshit, I've fixed already, and it has only done ~good~ to the amplifier. I'm really good at cleaning up circuits, odd looking wiring jobs, and that type of shit. (you should see what I can do to those low rent Korean 49CC scooters

) I have to get a scope, if I'm going to do this, I understand that, but for now, I can't afford one/don't know how to work one. I've got these for reference, as well as the original schem, Via the Sunn forum. There are some discrepancies than I can't explain between the CB I have and these layouts. Mine is missing things, but there's no place, and no traces for them. (for example, C303 isn't on my board, at all. No place for it, either. In it's general area, R313/R311/R314, and C311 are there, but no C303.)

Am I checking for AC to eliminate caps from my search? Where should I poke the probe? EDUCATION! All of us need it... haha. Also, just leave a jack in the poweramp in?

Hemisaurus · September 10, 2011, 09:52:47 AM

You're checking for AC on the DC for noise, which would point to bad power supply caps.

The jack in the power amp in should be a shorting jack, get a jack solder tip to sleeve, you now have a shorting jack, don't use a cable or a regular jack, the point is the short won't pick up stray noise from outside, and in this case isolates any hum in the preamp, even if you think there isn't any, it's good practice

VOLVO))) · September 10, 2011, 01:51:59 PM

Engh. Now one of my beta leads is acting up. It's intermittent and a slight tap on the amp stops it.

Anyways, where do I test the DC from? The Tranny, last component in line, eh?? Somewhere on ze board?

Discö Rice · September 10, 2011, 01:58:16 PM

Test it on your heart. It will show you the way - and if it does not, I cannot help you...

This will be my last post...

...until my next one.

Corey Y · September 11, 2011, 01:48:53 AM

I've been on vacation, so I just read all the way through. I'm glad it got there in one piece. My packing was so-so, but it couldn't get much more broken. What an epic saga, fraught with twists and suspense. I'm interested to see if you can bring the old girl back to life.

VOLVO))) · September 12, 2011, 07:50:51 PM

Herb, when I'm ordering caps (the shack, and the local electronics surplus store doesn't have dick, for anything I need,) what tolerances should I be using in the poweramp? I looked on Weber, and there's no 1000MFD/50V caps, so I'm reduced to Mouser, which is convoluted and is going to end up fuckin' me up. I'm gonna replace that lazily done cap job, and those two next to it. It's old, it needs to be one anyways, so I'll just do it. as for those monstrous metal coke can caps... where can I dig those up?

Hemisaurus · September 12, 2011, 08:19:13 PM

Right, the voltage is a working voltage rating, so when replacing you normally need that voltage OR ABOVE, unless you know the working voltage of the circuit is under that. So don't be scared to use 1000uF @ 80V or whatever or 1000uF @ 700V if you have such things.

The shack seem to have them? It's in stock in all the Shacks around Jacksonville, FL

http://www.radioshack.com/product/index.jsp?productId=2103627&filterName=Type&filterValue=Electrolytic+capacitors

Also you can double up, capacitors in parallel are additive, so 2 x 470uF = roughly 1000uF in a pinch.

VOLVO))) · September 13, 2011, 12:44:12 AM

I can over-rate voltagewise as long as the farad is the same? Sweet. life is easier... Those things are huge, though, size doesn't matter?

Hemisaurus · September 13, 2011, 08:53:16 AM

Capacitors have gotten smaller as manufacturing methods have improved. Todays 1000uF 50V capacitor will be considerably smaller than those from the 70's, and yes, long as you keep the new cap at or above the working voltage of the old cap.

spookstrickland · September 26, 2011, 05:44:35 PM

Any new progress?

VOLVO))) · September 26, 2011, 06:10:33 PM

Ordering parts tonight!

Hemisaurus · September 26, 2011, 07:42:23 PM

Oh and caps are the opposite of resistors, caps in parallel increase in value, so 2 100uF caps in parallel = 200uF

VOLVO))) · September 26, 2011, 07:48:31 PM

Do voltages stack? Are caps in series subtractive? 300 in series with 40 =260?

Hemisaurus · September 26, 2011, 08:46:39 PM

No it's not addition and subtraction, it's C1xC2/(C1+C2) like resistors in parallel.

So if you wire 4 1000uF 50V caps in series parallel you have 1000uF 100V, but it's best to add a bleeder network to make sure the voltages are evenly distributed. I don't know the math for DC if you use different Farad values in series off the top of my head.

spookstrickland · October 23, 2011, 04:50:10 AM

Anything new? I'm really waiting to hear that Sunn come to life again!

spookstrickland · November 18, 2011, 12:38:51 AM

Did Corey's Sunn ever get fixed?

??

VOLVO))) · November 18, 2011, 12:40:49 AM

It's put back together, and was working. Then died again. I'mma drop it off at the local shop for shits and giggles, he does diagnostic work for me for free because I occasionally do work for him. I'll drop back in once I know more!

jibberish · November 18, 2011, 08:22:26 AM

have you looked the ground plane over? maybe some filter components are cold soldered or failed open to the ground.

have you rung all the test points? like bias levels and the collector/emitter voltages wrt the rails? you will find a leaky component that way if the voltage is pulled up or down too far as the dividers arent working correctly from a leak. bogus biasing could monkey up the gain

SS is all dc except for the signal. hums are almost always ground related

is shielding or a grounded shield removed anywhere? was there a metal piece or some farraday cage thing that has been removed?

is pretty crude circuit there actually, especially that 15v zener clamp heh. there's brute force for you.

edit: check those diodes on the op transformers too.

editz: maybe im way late to the party heh