Fender Princeton Chorus blowing fuses

[mustangcrf]

Hello,

I have created a problem. I have a Fender Princeton Chorus (black knob, no DSP). It was in good working order, but the collar from input jack 1 was missing and that jack would not work. Jack 2 worked fine. I disassembled the amp and went to replace the bad jack. I used a Fender 9 pin stereo jack. When I hooked everything back, up I blew a fuse right away. I also saw a lot of heat coming from the 10 pin diode in space U10 on the board.

What did I do? I thought at first, I had the red wires coming from the transformer hooked up wrong. I can't tell from the schematic exactly how they are supposed to go, but the one that is red with the orange trace is attached to CP6. I do not have the speakers hooked up and all the pots are turned as low as they will go.

Thanks in advance for any help.

Chuck

[nashvillebill]

Should look like this:

if I can get the attachment to work.




Well curses, the attachment function seems to be having difficulties. But U10 is not a diode, it's one of the output chip amps. You have cooked it, sorry. That's hard to find IIRC, Enzo would know better than me.

[mustangcrf]

Hello,

I don't see an attachment.

Thanks,

Chuck

[mustangcrf]

Do you think I had the wires from the transformer hooked up wrong and burned it out? You say that is tough to replace?

[mustangcrf]

I found this part number in another thread. NEC UPC1188H If that is the one, it seems cheap. Do you know where exactly, each red transformer wire is supposed to go?

[Enzo]

AES sells the 1188, here is a link:

https://www.tubesandmore.com/products/P-QPC1188H


You have three red wires? The striped one would go to CP6 the plain ones to CP5 CP7, per the schematic. CP6 is between the caps and other other two are next to the fuses.


The back of the 1188 is bare metal, and I cannot tell from the data sheet if it is connected internally. Is there an insulating mica washer or silpad between the IC and the heat sink? And is it missing under the hot one?

[mustangcrf]

Hello,

Thanks for that info. I think the first time I hooked the red wires back up, I had the red wire with orange stripe to either CP5 or CP7. Would that have fried it? I now have it on CP6 and it will power up for a second or two before it starts smoking on the 1188 and blows the fuse.

There is some type of white material between the 1188 and the heat sink. It doesn't appear to have been burned away.

[nashvillebill]

I was trying to post a picture of this area in question. On mine the two red transformer wires go to XF2 and XF3 and the red/yellow wire is CP6. CP5 and CP7 do not have a terminal (tab) on them on my amp, nothing is connected to them. But it's the same as CP5 or CP7--so having the red/yellow wire to one of them would be incorrect...

[Enzo]

There is white goo, which is silicone thermal grease, but there also needs to be a thin piece of mica between the IC metal heat sink. Use your meter probe or a small screwdriver to slide alongt the metal in the white goo to see if that little piece is still there.

Oh wait, had you removed that 1188? or is it still where it always has been. I was concerned you might have lost the mica, but if you never removed the IC, then it should still be there.

The uPC1188 is a power amp all in one IC. Each speaker has its own 1188. If one is getting real hot, it is likely a goner, so remove it from the board - you will have to do that to replace it after all - and put the amp back together without it and see if the other speaker still makes sound.

[mustangcrf]

I got the 1188 removed and hooked it all back up. It works with the one speaker!

My conclusion is that when I first hooked it all back up after replacing the 1 input jack, I created the problem that blew the 1188. I had the red with orange transformer wire hooked up to the wrong place (somewhere other than CP6). I conclude that hooking the up wrong the first time blew the 1188 and after that it was never going to be right. Even after I hooked up the transformer wires in the right place, the 1188 was sparking and burning.

With the bad 1188 out of the board everything works well. I am going to order another one of those and reinstall it.

I never found the mica washer you mention. The back of the 1188 is bare metal.

Thanks to everyone that helped. I will post an update when the new 1188 comes in.

[Enzo]

Scrape across the field of white goo that was left behind, is there no little ridge to catch?

[mustangcrf]

There is nothing there. The chip still looked like the factory install too. Is that washer required?

Also, where can I get a small amount of that goo, auto parts store?

[Enzo]

If the metal back is live it needs to be there. I was hoping someone else knew about that, the data sheet doesn;t seem to say either way. SO I can't say for sure.

The two mica insulators are listed in the parts list for the amp, so I assumed they were needed. The 1188s are the only things on the heat sink, so nothing else would use them. Could it have stuck to the back of the old part? Pick through the goo on the remaining one, is ther an edge to be felt over there?

The goo is specific to the electronics industry. it is not a lubricant, it is a silicone paste to promote transfer of heat. It help the IC or transistor to radiate heat into the heat sink. Auto stores would not have it. Radio Shack may still have it. Even a smal tube would last you a long time. A friendly electronics repair shop might give or sell you a small glob of it. A dollar for a glob is cheaper than $8 for a tube full you never use. They figure out how to get it home with you.

[nashvillebill]

Let me try to post my picture again. You can see the mica--on BOTH chipamps--in my picture.


edited.... Well it shows the picture is there, but it won't show up in real life. Dunno.

[g1]

Well it shows the picture is there, but it won't show up in real life. Dunno.

[ATTACH=CONFIG]30279[/ATTACH]

You must have posted it before at some point before the crash. You will need to resize the pic, just crop a tiny bit off, then it should work. Alternatively, you could zip it and post the zip as an attachment.