Trouble Shooting Sunn SX8350 Powered Mixer

[Jesse Pearson]

Hi everyone, new member here. I have a 30 year old Sunn SX8350 powered P.A. mixer that I accidentally blew up while doing some troubleshooting. I purchased it in 1985 I think. I recently built a "Light bulb Current Limiter" to help keep the meltdowns to a minimum and have resumed troubleshooting the p.a. The original problem was it would shut down the audio after 45 minutes of use. I finally found a capacitor next to a hot sand resister which appears to be the culprit. I used a heater and freeze Spray to find the bad cap. Since the amp is 30 years old but in great looking shape, I've decided to replace all the Caps in it and have ordered 50 new replacement caps. My question is, does anyone know if this is a A/B amp? On my friends O.Scope the signal looks clipped like it is all over the place? Also, if anyone has any copies of a service manual, schematics etc. they could send me in a email I would be really grateful to you. This amp has a great sound for vocals. It's 350 watts rms mono into 4 ohms. I've replaced all the components that appeared to have blown when the meltdown happened like Transistors, resistors and a few caps. I plan on re-soldering any caps that are right next to the sand resisters, underneath the p.c. board to get them away from some of the heat from those sand resisters. Can't understand the logic of sticking caps right next to such intense heat? Anyone else who has worked on these amps before I would love to hear about what you found and repaired. Sometimes it's the same things happening over and over. I also plan on modding a cooling fan on the side to cool off the circuits on the p.c. board to help deal with the heat issues. Hope to make some new friends here and learn a thing or two about electronics repair. Thanks jwhillbilly@hotmail.com

[52 Bill]

Welcome to the place.

I won't argue the point about replacing all of the electrolytic caps in a 30 year old amp, but why do you feel the need to add additional cooling and to place the caps under the board?

It worked fine for 30 years and then one cap failed. With the additional work do you expect to get it to last 40 years? 50 years?

[Jesse Pearson]

Welcome to the place.

I won't argue the point about replacing all of the electrolytic caps in a 30 year old amp, but why do you feel the need to add additional cooling and to place the caps under the board?

It worked fine for 30 years and then one cap failed. With the additional work do you expect to get it to last 40 years? 50 years?

Hi Bill, thanks for the welcome. The unit has always ran very hot I think because it's a A/B amp, who knows what else. I put it into storage in the early 90' because the thermal shutdown issue stared and I didn't really know how to work on electronics very good back then. I'm into learning electronics repair at this stage in my life and I've been fixing small issues in guitar amps and mixer's for myself and friends this last year and studying pretty intensely non stop. I start summer school for "Electronics Technician" tomorrow, so you can say I got the repair bug now pretty good. A number of the caps are showing that their plastic tops appear to have shrunk and exposed the top metal surface and or stretched out away from the surface of the cap. I purchased the 50 caps online for $10.00 along with some more transistors I also plan on replacing. I figure this will be preventative maintenance plus give me more practical application in developing my soldering skills. Caps dry out with age and are a potential problem waiting to happen. As far as the installing a fan for cooling air, I see a lot of bass amps and class D amps doing this. It will make a difference. I also plan on modding a bass amp with a fan for the same reason. Haven't designed a circuit yet to best do this but that's coming down the road. As far as putting some of the caps that are almost touching the sand resisters underneath the p.c. board, since I have the clearance, it seems a better idea to me to not have a cap almost touching a very hot sand resister? It was a cap right next to a sand resister that was causing the thermal shutdown problem to begin with.

[Enzo]

resist the urge to re-engineer the amp. The amp and all the ones like it have worked for three decades. That model does not have a reputation for blowing up or running hot.

I don't know what you mean by A/B amp. It runs in class AB maybe? it doesn't matter what class the amp is designed for, it was designed with thermal management in mind and will not overheat in normal operation. if it runs too hot, then the amp has a bias issue or a circuit failure.

As an experienced tech, I URGE you to repair what is wrong with the amp first. Once it works properly, only THEN should you go about wholesale replacement of parts. people like to say "as long as I am already in there" lets replace a bunch of caps. The problem with this is, that as great an opportunity as it may be to practice soldering, each extra part you replace is just one more opportunity to introduce new issues into the amp, which may mask the problem we are originally trying to solve. Imagine going to the hospital for an appendix operation, and telling the doctor "as long as you are in there", lets do some liposuction, a nose job, and fix the bunions on my feet.

Lots of amps of all applications and all classes have fans. Fans are used when other thermal management systems are not sufficient. When you add a fan to a system, you must make a complete "circuit" for the air. if you just cut a hole and stick a fan in it, all you will do is pressurize the cabinet, air won't flow. Forced air needs an inlet and an outlet. If you draw air out, then there needs to be a vent for makeup air. Or if blowing in, there needs to be an exit vent. and the air must also have baffles or something to insure the airflow crosses ALL the heated parts. Make sure not to have a fan blowing air that will go straight to its exit. But many amps are designed with convection cooling, and whatever you do with fans, make sure you are not in the same effort screwing up the convection system.

I had a customer once bring me a stack of Yamaha power amps that were all "overheating". The short story is, he decided to add a panel of three big fans to his amp rack, they blew a lot of cool air into the rack cab. Unfortunately, the Yamahas - nice amps by the way - all cooled themselves by drawing air into the front and blowing it out the back. or at least they tried to. By adding the unnecessary cooling fans, he had them blowing into the rack, just as the Yamaha fans did, so his new fans exactly counteracted the amp fans, thus negating them. So they were unable to cool themselves. There was nothing wrong with the amps themselves.

The whole rear panel of your amp - thick slab of aluminum - is your heat sink.

[Jesse Pearson]

Hi Enzo, nice to meet you and thanks for your input. The amp has always been way hot from the get go and started having a thermal shutdown issue after I let a friend borrow it for a gig. It blew up just recently because I took the screws out of the plastic protectors on the outside power transistors connected to the heat sink so I could spray the transistors with freeze spray to see if one would came on while under power after I cooled it down, I got as far as the freeze spray test with nothing happening after the amp had shut down the audio after being on for 45 minutes. Then I tried to move the amp under power to the side on my work table and it started a cascade of component failures before I got the power off. I guess the screws needed to stay in the transistors even without the plastic protectors on because it was part of some circuit connected to the collector portion of the transistor. Maybe it caused a ground issue perhaps or some kinda short, not sure. I should have taken the plastic protectors off with the power off and reinserted the screws back into the power transistors before turning the power on. I then could of used a heater to quickly heat the transistors to make it fail if that had been the problem which I don't think it was now that I found the bad cap next to the sand resister. But hey, if you want to learn to swim you gotta get wet right. After replacing all the blown components that were visible and the ones that showed up weird with the DMM that I double checked by pulling them out of circuit to test, the amp is acting like the internal limiter is on all the time (faint distorted audio when the volumes are turned half way up). With the correct speaker load connected the limit light stays on for a good minute before it turns off on power up? Without a speaker load it just blinks on during power up and then turns right off, still no audio however? Anyway its clear that the power transistors aren't doing anything because of the absence of any heat coming from them. I tested the transistor collectors to ground for shorts and also the two internal fuses to ground and that looked good, no shorts? I haven't replaced the cap in question yet or the transistor I got from Fry's which is testing odd now. Just waiting for the new parts to come in. What I really could use is a service manual pdf. for this Sunn SX8350 because my friend has a O.Scope but says he needs the paper work on the unit to help me out. I love the mystery of trying to fix this amp and just want to get the unit to be rock solid and maybe last another 50 years Ha!

[Jazz P Bass]

Quote":What I really could use is a service manual pdf. for this Sunn SX8350 because my friend has a O.Scope ".

At this point in the failure, an ohmmeter will suffice.

I don't know about any Service Manual per se but the schematic can be found here: Miscellaneous - Sunn SX6350 & SX8350 power amp - ampix

Oh, they still have the fatal error going on.

T-Boy!!!

[Jesse Pearson]

Thanks a bunch Jazz P Bass. Not sure how the Fender SR-8520 is connected to the SX8350 other than the pre-amps looks real similar to each other except I have a reverb spring and the SR-8520 has a digital reverb. Here's a cleaned up pdf. of the SX8350
Sunn SX8350 full SCHEMATIC Power Amp Section.pdf

[Jazz P Bass]

Well, here is the power amp schematic.


Sunn 6350_8350-schematic.pdf

Not sure how close it is but here is the 8520

Fender SR-6520, SR-8520 Powered Mixer Service Manual.zip

[The Dude]

Hi Enzo, nice to meet you and thanks for your input. The amp has always been way hot from the get go and started having a thermal shutdown issue after I let a friend borrow it for a gig. It blew up just recently because I took the screws out of the plastic protectors on the outside power transistors connected to the heat sink so I could spray the transistors with freeze spray to see if one would came on while under power after I cooled it down, I got as far as the freeze spray test with nothing happening after the amp had shut down the audio after being on for 45 minutes. Then I tried to move the amp under power to the side on my work table and it started a cascade of component failures before I got the power off. I guess the screws needed to stay in the transistors even without the plastic protectors on because it was part of some circuit connected to the collector portion of the transistor. Maybe it caused a ground issue perhaps or some kinda short, not sure. I should have taken the plastic protectors off with the power off and reinserted the screws back into the power transistors before turning the power on. I then could of used a heater to quickly heat the transistors to make it fail if that had been the problem which I don't think it was now that I found the bad cap next to the sand resister. But hey, if you want to learn to swim you gotta get wet right. After replacing all the blown components that were visible and the ones that showed up weird with the multi-meter that I double checked by pulling them out of circuit to test, the amp is acting like the internal limiter is on all the time (faint distorted audio when the volumes arebturned half way up). With the right speaker load connected the limit light stays on for a good minute before it turns off on power up? Without a speaker load it just blinks on during power up and then turns right off, still no audio however? Anyway its clear that the power transistors aren't doing anything because of the absence of any heat coming from them. I haven't replaced the cap in question yet or the transistor I got from Fry's which is testing odd now. Just waiting for the new parts to come in. What I really could use is a service manual pdf. for this Sunn SX8350 because my friend has a O.Scope but says he needs the paper work on the unit to help me out. I love the mystery of trying to fix it and just want to get the unit to be rock solid and maybe last another 50 years Ha!

Those "plastic protectors" isolate the transistors from the heat sink. You should not have taken them off period- amp on or off. Not to mention; the amp is designed with transistors mounted to the heat sink. Of course they are going to get hot if you un-mount them. I'm not sure what a measured confirmation of this proves? If the amp is/was running hot, there is a problem- not a design issue. I've repaired these and been in bands with people who used them. Never had a problem using them "as is".

[Jesse Pearson]

Hey Dude, nice to meet you. The amp was way hot right out of the gate when I bought it new. The plastic covers on the 85' modal only cover the outside, not underneath the transistors. There is a gasket between the Power Transistor and the heat sink. I reapplied fresh heat sink grease to all of the power transistor gaskets. Some of the power transistors fried and had to be replaced and others tested good after my initial mistake. O.K. so it sounds like the unit was messed up from the get go as indicated by the excessive heat. Yeah, it would get pretty hot even in idle mode? Hopefully I can get this sorted out. Dude, would you by chance have any of the Test Point Measurement documentation for the 85' modal SX8350 Power Amp section laying around that you could scan as a pdf. and send? That sure would make this easier. But I realize that's a pretty big long shot

[The Dude]

I don't have any documentation. I'm not sure I ever did. Most of the stuff I repaired in those days was without manuals. I still rarely use manuals, although it's much easier these days with the internet. In the early days, it just wasn't feasible to order manuals for every repair. You could have twenty to fifty bucks wrapped up in a repair before you even started. Add that to the cost of parts and labor, and it doesn't leave you much room. Anyway, I forgot to welcome you to the forums, so welcome! If you had shorted outputs, be sure to check the emitter resistors. Jazz has the service manual posted above.

Edit: FWIW, most of the time those things get hot it's because someone is running the ohm load too low. Many did not understand that this is a mono amp and the speaker jacks are paralleled. Two jacks does not necessarily mean twice the speakers.

[Jazz P Bass]

Thanks a bunch Jazz P Bass. Not sure how the Fender SR-8520 is connected to the SX8350 other than the pre-amps looks real similar to each other except I have a reverb spring and the SR-8520 has a digital reverb. Here's a cleaned up pdf. of the SX8350
[ATTACH]34187[/ATTACH]

That is a whole lot 'cleaner' than the one I had.
Thanks.

[Jazz P Bass]

Getting real hot at idle can be caused by 'too high' bias issue.
A mains current reading at idle is a good item to check.

These amps are technically Class AB.
A small bias voltage to just turn on the output transistors is what takes it from Class B into slightly Class A.
Thus the Class AB moniker. (Class BA would be more appropriate )

Enzo recommends a cute trick were you measure the current that the amp is pulling from the mains at idle.
(I use a Kill-A-Watt meter)
With the bias set stone cold (Class B) as you turn the bias up eventually the mains current will start to increase (Class AB).
At this point turn the bias back a tad. (Closer to Class B)
Done deal.

[Jesse Pearson]

Yeah Dude, I wondered about that when I first bought it. The two jacks are in parallel, so two 8 ohm cabs make 4 ohms mono which is what the amp wants to see. I know if the ohms are too low it will make the amp run hot and I think if the bias is off as Enzo mentioned. One thing I just figured out a couple of days again when checking the speakers out to make sure I didn't blow those up too, was the difference between DC 8 ohms measured on the multi-meter and the impedance ohms of the speakers which are different readings. The speaker impedance is the freq. of the whole speaker circuit in AC if I'm correct and the multi-meter reading is DC measurements which will show between 5 and 6 ohms on the meter when the speakers are supposed to be 8 ohms impedance. Had to do some detective work to figure that one out.HA...

[Jesse Pearson]

Thanks Jazz P Bass, I'll do that.