Fender Princeton 112 Blowing Fuses

[wlw-19958]

Hi There,

I bought my son a used Fender Princeton 112 Amp a little over a year ago
and now it is blowing the 2 amp Buzz fuse every time he turns it on. I'm
thinking that it is either the Cap.s or the rectifying Diodes but I thought I
would ask before tearing into it.

I'm not a electronics geek per say, but I have built a few "kits" in my time
(like Heathkit, Archer, etc.) and can identify and de-solder components and
stuff. So I'm not afraid to get into trouble.

So far, all I have done is disconnect the transformer and check it for shorts
but it found nothing (I know it was a long shot but what the hey).

I would really appreciate getting a hold of a schematic/circuit diagram for this
amp.

Any help would appreciated. Thanks in advance!
-Blue Chips-
Webb

[Tom Phillips]

Webb,
Here is a link to the schematic.
http://www.bnv-gz.de/~ooehmann/schematics/fender/Princeton%20112%20Plus.pdf
Regards,
Tom

[Enzo]

In sincerely doubt it is the caps, and while rectifier diodes could be shorted, they are way down my list. The most likely thing bad here is the output power transistors. The two tab types on the heat sink - TIP142 and TIP147. Legs 2 and 3 are probably shorted together. If one is bad, replace them both.

Make sure neither .47 ohm ballast resistor is open - R65,66. ANd see the eight diodes to the left of the output transistors on the schemo? MAke sure none is open or short. In the case of the zeners, if they act like a diode, they are OK. WHile you ar at it, might as well verify that R61-64 there are close to their values.

[wlw-19958]

Hi There,

Tom, Thanks for the link. That is a big help!

Enzo, 4 of the eight diodes you suggest testing (and may need replacing)
are listed as BYV26D. This part number doesn't come up on Digi-Key. And
Mouser lists part that doesn't have the same case dimensions/material.
Do you have a source for these?

Thanks for all the help!
-Blue Chips-
Webb

[Enzo]

Allied has them from Fagor for 15 cents. But really all they are is fast recovery diodes. They are 1 amp parts with an astonishing 800v rating. They must use them in another product since this solid state amp runs on +/-33VDC. Good luck generating an 800v+ transient.

The glass package is not necessary, but glob some heat grease in the holes around them. You should be able to test them in their circuits, just remove the shorted power transistors first. They are shorted, open, or good. Very rare to find a half assed one. Look into the power transistors first, the diodes are probably OK. Just check them to be sure.

[wlw-19958]

Hi There,

Enzo, Thanks again for your help. Luckily, I found a seller on eBay that had
some of the BYV26D diodes. It looks like the power transistors are bad as you
predicted.

Thanks Again!
-Blue Chips-
Webb

[hollisdevillo]

i had the same problem with a deluxe 112, almost same schematic. the power output transistors were bad. but when i replaced them, the volume was very low. and some intermittent noises were coming through. both channels were the same. i changed the .47ohm ballast resistors, as enzo called them, and as far as i can tell the diodes are all good. any ideas? i know enzo knows exactly whats going on. thanks guys, glad the site is back up!

[Enzo]

HArd to say, more is wrong. Is the preamp out signal OK? Otherwise, when your outputs go bad, you need to check ALL the transistors in the power amp. A shorted little limiter can kill your signal as easily as something large.

[hollisdevillo]

the owner said the amp was working fine and then he unplugged the reverb pan and it touched against metal, which shorted the output and fuse so they blew. does this narrow it down at all? i have to stick to tube amps.