Salvging a Fender Stage 112 SE

[gavnook]

I've had this amp for 12 years and it's been good until recently. I never cared for the drive channel, but the clean was very nice. Anyway, it's been giving me a hard time. I just replaced all 6 power transistors. I knew at least one of them was bad. Now it blows the fuse. I'm guessing I did something wrong, but I don't know what, and I don't really feel like trying to figure it out as I think I may have damaged the board itself. After coming across this site, I'm starting to think it may be time to scrap the board and try to re-use the larger components to build something new.

The amp is rated at 105w into 8ohm and 160w into 4ohm, so I'm looking to build a power amp that will match or nearly match that output reusing the old power transformer. According to the schematic, the power supply puts out +/- 45vdc and +/- 16vdc. I'm very noob with this stuff, so I don't what would work well with that, besides the old power amp, which uses 3 TIP142 and 3 TIP147.

[gavnook]

The schematic for the old amp is here in pdf.

[J M Fahey]

Hi Gavnook.
That old workhorse is a very good amp, and a powerful beast to boot.
Try to keep i alive, I guess repairing it will be *much* less effort than building something new with its carcass.
Post what you did so far and we´ll try to help.
Bye.

[gavnook]

Several months ago it died. I opened it up and found a single fried resistor and a one or two bad solder joints. I re-did the replaced the resistor and re-did the joints. This worked fine for a while, but it wasn't completely reliable. I sometimes had to smack the amp to get it to make sound. Some time later it stopped making sound altogether. I opened it up to see if I could see what was wrong. I didn't see anything, and tried playing opened up. This was a stupid move because the power amp transistors where off their heat sink. I realized this when I saw one of them smoking. Later I noticed that the speaker had become disconnected. I reconnected the speaker and I think fed it DC current when I turned the amp on. Just yesterday, I replaced all the power transistors. At first, it powered up and made almost no noise, but the fuse blew after about a minute. Subsequent fuses blew within second.

There are many things that could be wrong. I was wondering if the pinout on one TIP147 could possibly be different than the pinout of another TIP147 by a different manufacturer. I could've connected something wrong. I might have damaged the new transistors or the board itself beyond repair with my weak soldering skills. I really don't know.

Whatever the case I really do want to build my own amp. I also have a dead Line 6 Spider II HD 150 that I bought to replace the Fender. 

[joecool85]

Well, if you want the amp gone, I'm interested 

Sounds like you connected one or more of the transistors in backwards.

[Enzo]

All the TIPs will have the same pinout.

Did you leave out the mica insulator under any of them?  Go down the row of power transistors and see if the tab is shorted to the heat sink.

[gavnook]

All the TIPs will have the same pinout.

Did you leave out the mica insulator under any of them?  Go down the row of power transistors and see if the tab is shorted to the heat sink.

I didn't see any mica insulator. I thought it was just thermal paste between the transistors and the heat sink, so that's all I used. If they're not allowed to be touching that could definitely be the problem. The replacement transistors are all metal on the back.

[joecool85]

All the TIPs will have the same pinout.

Did you leave out the mica insulator under any of them?  Go down the row of power transistors and see if the tab is shorted to the heat sink.

I didn't see any mica insulator. I thought it was just thermal paste between the transistors and the heat sink, so that's all I used. If they're not allowed to be touching that could definitely be the problem. The replacement transistors are all metal on the back.

Were the old ones coated in plastic?  If this is the case, that's your issue right there.  They need to be insulated from the heatsink.

[J M Fahey]

Hi Gavnook.
Please be *very* careful when desoldering, pulling, replacing and re soldering parts, take good care of the board, avoid stripping tracks & soldering pads. Any component can be bought except the board itself. In a way the board *is* the amp.
Use the best solder sucker, desoldering braid, etc.
Also use a series lamp fixture, as has been posted here elsewhere , both as a protection and so you can measure *something* before the fuse blows.
Bye.

[EDWARDEFFECT1]

i fixed one with the same problem.it is a very common problem with this amp. the resistor burns out due to bad solder joints on the heavy heatsink next to the resistor.the heatsink lets the resistor take all of the current instead of sharing.....good luck....ed!!