Sunn Concert bass troubles

ryognbass · #1 07-22-2011, 11:19 PM

Hi all,
I have a 70's sunn concert bass (4 knobs/ bright switch) that is on the fritz. The amp produces sound, however it peters out sharply when hit with a hard signal. if I dig in even the slightest at any volume level, a loud crackle occurs, and then there is still sound being produced, but it is very weak and distorted. the issue appears to be in the preamp, as I can run into the power amp via an outboard preamp (Tone hammer), and the sound does not cut out at such a low threshold, although if I crank the tone hammer and dig in, the same problem occurs. The plastic standoffs do seem to be intact though, as I know this is a concern on such amps.

I wanted the expert opinion of TB'ers before turning it into an absurdly priced tech guru, in hopes that this is a fix that can be made in a DIY sense.

Thanks in advance for your help!

T-Bird · #2 07-22-2011, 11:31 PM

Hi.

If by "a fix in a DIY sense" You mean blindly guessing and tossing stuff out and in, that can (but shouldn't) be done. Granted, sometimes it works but usually doesn't, creating more problems than solving them.

Keeping the safety as the top priority, chopsticking around the PCB and wires will reveal any loose components or cold solder joints.

Digging any deeper will require at least a o'scope and a decent multimeter, PLUS the knowledge to use 'em.

Good luck.

Regards
Sam

ryognbass · #3 07-22-2011, 11:42 PM

Hey Sam!
I have an oscilloscope and a multimeter that could get the job done. I don't want to blindly swap out components or anything, as that would be risky, costly, and potentially innefective. By DIY, I mean just that I'd prefer to fix it myself rather than outsourcing the work, although there's obviously a limit as to what I can do. Thanks for the suggestion on chopsticking though! I've visually inspected everything, but I havent dug into it at all yet.

Anybody out there with specific knowledge of sunn heads that could potentially shed light on this issue?

BogeyBass · #4 07-23-2011, 12:27 AM

no not really specific to the sunn

Do a visual inspection and look for the obvious
if the power amp is cracking too it could be more of a power supply problem not preamp problem

Could be power supply caps are on the way out.
if your in love with the amp it never hurts to invest in new ones

or measure higher wattage 10 watt and 2watt resistors in the poweramp and see if they are out of spec.

C1 C2 C305 C308 / R1 R2 R3 R4 big 10 watts R302 thru R307 are 2watts

Here is a schematic

otherwise voltages need to checked, check values or look for leaking or swelled capacitors.
and dont let anyone take guesses and change the transistors
the vintage 2n3055 sound different and their is no need to change them.

after whatever is faulty gets replaced , it would be a very good idea to check the bias.

be careful the caps can hold a charge and your going to need to safely drain them, no bleeder/safety resistors in that supply.....unless you know whats up with basic electronics or have a friend that does...take it to the tech

ryognbass · #5 07-23-2011, 08:52 AM

Thank you very much for that! I'll look into that now, and get back with what I find on it!

T-Bird · #6 07-23-2011, 10:51 AM

Hi.

Quote:

Originally Posted by ryognbass

Hey Sam!
I have an oscilloscope and a multimeter that could get the job done. I don't want to blindly swap out components or anything, as that would be risky, costly, and potentially innefective. By DIY, I mean just that I'd prefer to fix it myself rather than outsourcing the work, although there's obviously a limit as to what I can do. Thanks for the suggestion on chopsticking though! I've visually inspected everything, but I havent dug into it at all yet.

In that (very rare in TB universe

) case the troubleshooting should be pretty straight forward with the schematic BogeyBass provided.

If the power amp performs better than the pre, but both show the same symptoms, the first thing I'd check would be the power supply section.

Regards
Sam

ryognbass · #7 07-23-2011, 12:51 PM

Quote:

Originally Posted by T-Bird

Hi.

In that (very rare in TB universe

) case the troubleshooting should be pretty straight forward with the schematic BogeyBass provided.

If the power amp performs better than the pre, but both show the same symptoms, the first thing I'd check would be the power supply section.

Regards
Sam

Very good pointer, Thanks Sam! I got called into work but i'll slowly make progress onthis once I return home. I'm letting it "Burn in" while i'm out just to let everything warm up. It seemed like a good idea, but seeing as it's solid state, it is more than likely moot...

I'll keep you posted as to what I find!

nashvillebill · #8 07-23-2011, 04:40 PM

I had a Concert Bass for a long time. Here's some likely trouble spots:
-the caps are now 35 years old. All electrolytic caps should be replaced.
-the "preamp out/power amp in" loop jacks can get oxidised, run a patch cord from the pre out to the power amp in jack as a test
-All the boards connect with dinky little push connectors. On mine these had gotten loose and oxidixed. I cleaned them and soldered them, made the amp tough to work on later but it eliminated a problem area
-All the pots were oxidized, I continually had to use Deoxit or else they would get crackly
-The boards just "snapped" into place onto little plastic clips. after 30+ years the plastic was brittle and they had mostly snapped off, leaving the boards to flop around and potentially contact the chassis. I made new plastic standoffs and screwed the boards down
-Note the solid core wiring is thin and brittle, if a wire kinks anywhere it can break inside its insulation and that is a terribly difficult intermittent fault to find

I understand you have a scope? I also assume you have a sig gen and dummy load? Fire it up on the bench, and watch the output trace on the scope.

If you have a clean sine wave on the output, great. Try tapping components one at a time with something non-conductive (the old chopstick test).

If the sine wave is raunchy, work backwards from the output and find where it goes bad.

johnk_10 · #9 07-23-2011, 05:53 PM

it sounds like a cold solder/bad connection or a loose part to me. the choptick test should be able to find it pretty easily.

believe it or not, i have repaired a few with the chassis opened up, turned off the lights in the room and played thru it. often you'll see a tiny spark somewhere on a pcb while it exhibits the crackling, and it'll point you directly to the bad connection (or short).

some of the coupling capacitors can break loose from the their solder pads due to vibration and shock over the years.

of course, turn the lights back on before you attempt to work on it.

someparts · #10 07-23-2011, 06:28 PM

I recently had an Acoustic 370 head from the same era with one bad transistor. Not Sunn...but sounds just the issue I had.

ryognbass · #11 07-24-2011, 01:59 PM

Quote:

Originally Posted by johnk_10

it sounds like a cold solder/bad connection or a loose part to me. the choptick test should be able to find it pretty easily.

believe it or not, i have repaired a few with the chassis opened up, turned off the lights in the room and played thru it. often you'll see a tiny spark somewhere on a pcb while it exhibits the crackling, and it'll point you directly to the bad connection (or short).

some of the coupling capacitors can break loose from the their solder pads due to vibration and shock over the years.

of course, turn the lights back on before you attempt to work on it.

This is a very simple test it seems so I'm going to try this before all else. Sadly, I do not have a Dummy load, although I do have a signal generator. Without the dummy load, it is nearly impossible to problerly image the while circuit on the o'scope, so i'll have to test it otherwise. I'll prob set up the function generator through the amp and tap through the enitre circuit o find the weak link (if one exists).

Quote:

Originally Posted by someparts

I recently had an Acoustic 370 head from the same era with one bad transistor. Not Sunn...but sounds just the issue I had.

this gives me hope! How did you diagnose the issue or if a tech did it, how much did it run you?

nashvillebill · #12 07-24-2011, 02:06 PM

Although a non-inductive dummy load is preferred, if you can tolerate hearing the sine wave...and since you don't need to run the amp up to full volume...turn your bass cab upside down so the speakers are pointed into the floor and just use it. For this type of troubleshooting, a low volume would be fine. All you're trying to do is isolate the problem.

Note the gazillion pin connectors inside, that'd be one of the first areas to check IMHO.