Sunn SR8520P Mixer Amp

[spudler]

Hi All new here. I found link at guitarnuts. I have a question about a sunn SR8520P mixer amp. I was given the amp by an old friend and I was told it blew a fuse. I looked a little and I noticed some of the power transistors had been replaced not long ago. I also noticed when I did a little probing that it has a direct short on one of the power lines that hook to the heat ( I cannot remember the name but they shut off power if it overheats). anyway I did some testing on the transistors and I believe 2 are bad. I am curious I found an article where it recommended replacing the motorola ones I have with a upgraded version MJ15024 and MJ15025 There are 4 of each. I cannot recall the actual #'s that are in it but I do know when I looked them up I found the above #'s as their replacement. I have never done anything like this before but I cannot believe a shop wants $150 to replace less that $30 worth of power transistors. and thats for 5 of each. The $150 quote I got was just to replace the bad ones. I am curious as to what else I should check to make sure thats all I need to replace? I tried Fender but they would not send me any schematics or specs on resistors etc. thanks

[newey]

Spudler-

Hello and Welcome!

I don't have any answers for your specific questions, but someone will likely be along who can help.

The Fender website lists the Sunn products for which they have schematics, manuals, and so forth- and your model isn't among them. However, I noticed that there was a "Fender SR8520P" model powered mixer, I'm wondering if that model is actually the same as yours and if you might have more luck finding a schematic for that amp (Fender doesn't list that either, but it may be around on the web somewhere).

As far as the shop charging $150, just like your auto mechanic, it's the labor which costs money, not the parts. Doesn't sound unreasonable to me, but if you have the capability to DIY, that will always be cheaper- emphasis on "if you have the capability"

[spudler]

hey sorry I took so long to respond. I even contacted the tech dept. at fender and they acted like they could fix me right up. the tech sent me a schematic alright. It is the exact same one printed into the back plate which shows basic wiring so you know how to connect speakers, wire up the effects. I was very dissapointed. I contacted them again and they said there was no other schematic they could send. So either they don't allow consumers access to the internal schematics, or the people who built them did it from memory no paperwork recorded. lol

Ya I seen the Transistors online I just want some info so I know which are blown and which aren't, wait I should just replace them all like I had thought before. I have been doing alot of soldering lately and I am thinking of buying one of those really sweet weller setups with adjustable power. I have the solder vac thingy for removing solder. I think I will just go ahead and dig in. I'll post the steps and the results if I am succesful when I finish it

[newey]

Even the brute force rebuild approach would benefit from having a schematic- if only to give you the parts nos. and values, so you're not trying to decipher them from the board.

I suspect some further searching could disclose a schematic. I didn't find one, but I didn't look too deeply either. Maybe there's a Sunn Owner's group somewhere.

Do let us know how it works out. Back "in the day", as we codgers like to say, a Sunn Coliseum Lead was every 12 year old's wet dream- me included. So it's good to see some of these old Sunns are being resurrrected.

[spudler]

I got so lucky! I have a Digitech GNX4 guitar workstation, and I wanted updated amp and cab models and presets of the metal style. MFXsupermodels creates these files and sells them, they also happen to do amp repair back east somewhere. He answered my questions on what to chack and what to expect for readings. I found the PNP and the NPN upgraded/revised Power Transistors for sale. I hope I find a couple dead which I believe I did when I was just testing everything I could think of. If so I need to replace all of them, double check to make sure no shorts in output side of amp and I am good to go. It will be nice to crank those 15" super modded PA speakers again. Although last time I gave them a good workout I was in a concrete basement with 24" of concrete above me and all around besides being under ground too. I know my neighbors are gonna love me. teach the ones on the right a lesson for letting their kids out back with their Gansta music. Ill find a good metal one at about 200 tempo and the drummer doing double time on the double bass through 80 % of the song.
I think when I tested I coud get down to 20 HZ 40-60 and my eyes would water and my ears tickle.

[Teleblooz]

Mar 23, 2010 7:04:23 GMT -5 spudler said:

I know my neighbors are gonna love me. teach the ones on the right a lesson for letting their kids out back with their Gangsta music. I'll find a good metal one at about 200 tempo and the drummer doing double time on the double bass through 80 % of the song.

Serves 'em right.

[spudler]

well so far I have found some of teh schematics and I am able to identify almost every single item on teh board and I will be able to test them all. plus the Guy fom MTprosound has been a huge help. I hope to get the power transistors in a week and if none of the 33ohm resistors that go with those are blown I should be up and running soon.