SUNN 1200S Woes: Hybrid Amp Specialists? Help me out, Please?

[pickinatit]

Last summer my Sunn 1200S (the "modern" hybrid model) crapped out in the middle of an outdoor barbecue we were playing.  You can play for a few minutes after powering up and everything sounds fine.  Then there is a crackling noise like you would hear from a bad cable or something and then the volume drops WAY, WAY down and the sound is distorted. (Almost sounds as if the tube pre-amp section is being overdriven, but this persists at barely audible volume and you hear that crackling sound intermittenly).
(Even with Channel volume on as low as .5 to 2  and master volume on 1 or 2 so I don't think I could be clipping the amp. And I even tried turning the master volume up to 7 or 8 last night which should have knocked the walls of my house down or at least blown out the windows).

I took it into "Old Town Music",  Washington Township here in South Jersey.
They had it for a couple weeks.  Said my amp worked fine.  They couldn't find anything wrong with it.  Took it back home.....still the same problem as before.
The amp tech. at Old Town put a 2 ohm load on it, cranked it and left it powered up all day and nada...nothing wrong.  But could it be because he only
"Bench tested" it ?   He admitted they never played thru it or hooked it up to a cab to check what it sounded like.  At least they didn't charge me.

Well, now to the point.  I'm looking for a different tech to take it to.  And I was wondering if anyone on here would have suggestions as to what might be suggested they check/look for that might be causing the problem I described.

BTW, I went thru a careful process of checking the cabs I'm playing thru with other amp heads (they are fine),  the cables I'm using are fine (except the other amp heads aren't nearly as powerful as the 1200S which I was running with two 8 ohm cabs for a 4 ohm load. The 1200S is supposed to handle as low as a 2 ohm load @ 1200 watts).

Any suggestions for a good  South Jersey Amp tech.  and what they might be looking for ?

[loudthud]

Your description points to a "mechanical" problem, one that shows up due to vibration. It's probably a bad connection on one of the circuit boards or the connections between circuit boards. You've done a good job isolating the problem to the amp. Now see if you can isolate it further by checking the tuner out and effects loop(s) when the amp is not working. If you have another amp with an effects loop, try connecting the effects out on the good amp to the effects return on the 1200S. Make sure it works while the 1200S works normally, then play the 1200S until it fails, connect the two to determine if the 1200S power amp still works or the problem is in the 1200S preamp.

[mckinnon audio]

   Hi there,I've worked on a few of those amps,your 4 spkr.out jacks are all connected in parallel on a little p.c. board at the back of the amp,they're the plastic jacks too,check the connections on these and the jacks themselves.I replace them with the metal ones.Good luck,Mel.

[pickinatit]

   Hi there,I've worked on a few of those amps,your 4 spkr.out jacks are all connected in parallel on a little p.c. board at the back of the amp,they're the plastic jacks too,check the connections on these and the jacks themselves.I replace them with the metal ones.Good luck,Mel.

Thanks, Mel and Loudthud.
I don't think it's a speaker out jack connection because why would the amp always work fine for a few minutes when first fired up and then start it's crap.  Turned off for a few minutes and then powered back up the same thing happens, although the interval of time the amp seems to operate OK gets a lot shorter when it's only been off a few minutes then it is if the amps been off over-night for instance.
This is why I personally don't believe the problem is anything being loose,  but I have little to no knowledge of electronics/amplifiers.

I'm looking for possible leads to pass on to an amp tech. when I find one that I think I can trust.

BTW,  Mel,  the four speaker output jacks aren't labeled.  Does it matter which two jacks I am using to plug my two 8 ohm cabs into?
I'm using the two "inside"  jacks because that what it shows on the diagram in the 1200S manual.

Michael

[pickinatit]

BTW,  I'm at work so I have to check back from time to time. So I apologize for any delay in my responses.  I really am following the thread as best I can.

[pickinatit]

Your description points to a "mechanical" problem, one that shows up due to vibration. It's probably a bad connection on one of the circuit boards or the connections between circuit boards. You've done a good job isolating the problem to the amp. Now see if you can isolate it further by checking the tuner out and effects loop(s) when the amp is not working. If you have another amp with an effects loop, try connecting the effects out on the good amp to the effects return on the 1200S. Make sure it works while the 1200S works normally, then play the 1200S until it fails, connect the two to determine if the 1200S power amp still works or the problem is in the 1200S preamp.

Unfortunately, I don't have another amp with an effects loop. My other amps are all  Pre-1975 Sunn Amps (2000S, Solarus, Concert Bass) except for an old Peavey TNT 130 that I'm trying to sell.
But, I am going to pass this whole thread along to whatever amp tech. I end up finding.  There's supposed to be an "older" guy (he's 60 something) at the Hamilton Guitar Center that is supposed to be a real wizard,  but I kind of hate the idea of taking it there (don't know why really, I just do) Ha Ha.

[pickinatit]

So, the guys on the Harmony Central Bass Forum seem to form a concensus around the idea of bad solder joints which they say was a chronic problem for Fender equipment of that era.  Any thoughts ?

[mckinnon audio]

  Hi there,yeah they're known for their bad soldering,check the power supply and output stages,you'll have to disassemble just about the entire amp to do it,but it'll be worth it in the end,and no,it doesn't matter which spkr.jacks you use,they're all in parallel,good luck,Mel.

[Oli]

So, the guys on the Harmony Central Bass Forum seem to form a concensus around the idea of bad solder joints which they say was a chronic problem for Fender equipment of that era.  Any thoughts ?

I used the SUNN 1200S for eight years and had the same problem with the solder joints. There are four transistors right in the front of the main amplification connected with a simple "flying" cooling divice. The coolig divice is far too heavy, so the transistors tend to loose the connection to the mainboard. I repaired this and overdid all transistor solder joints and had no problem after this at all.

Greetings

Oliver     

[pickinatit]

I am printing out this whole thread and will present it to whatever amp. tech. I end up finding.
Thanks for all the help.  This is a great website if you own and love Sunn.