Factory bulletins on the Fender Princeton 112+? Nope but here are some mods I did

[Steve A.]

The schematic and layout drawings are at the Fender site but I was wondering if they had a bulletin concerning the loud pop when you switch off the amp, which seems to be common with many of the ss amps that they designed in the 90's.

http://support.fender.com/schematics..._schematic.pdf

Steve Ahola

P.S. If there are no factory bulletins I will start a thread in the Guitar Amps area. Thanks!

EDIT 4/7/13:
Here are the mods I have done...
> > > Replaced R47(470k) with a 500kB pot as a Master Volume
> > > Added 68k resistor in parallel to C14(470k) to cut gain in Drive channel for more control over low gain settings [opt.]
> > > Wired up a "standby" switch by interrupting circuit between junction of R48 & C32 and R49 (I used switch on 500kB pot)
> > > Will replace R10(470R)- mid resistor in tone stack- with 1.8k as in the newer Princeton 65

BTW I moved this thread from Schematic Requests to Mods and Tweeks. Tweeks? Isn't that what you do on Twitter after eating peanut butter?

[Jazz P Bass]

Two things come to mind.
#1: that is the way that it is
#2: the power switch (unplug the unit with the switch on, if there is a noticable drop in the loudness of the pop, you could try a capacitor across the switch)

[Enzo]

Why woulod we expect such a bulletin for the Princeton 112+? A zillion other amps do the same thing.

TONS of amps pop at power-off. I have never considered it a problem.

[Steve A.]

Why would we expect such a bulletin for the Princeton 112+? A zillion other amps do the same thing. TONS of amps pop at power-off. I have never considered it a problem.

The pop on the solid state Princeton 112+ is extremely loud. So much so that I'm surprised that the speaker hadn't been damaged on the 14 year old amp. To make things worse when the power is shut off it switches to the OD channel. Turning down the volume controls to 0 helps some but not a lot. I think it has something to do with the caps in the power supply, since it doesn't pop as much if the amp hasn't been on very long. (Hmmm... perhaps there is some defect in the power supply.)

In any case I did come up with a solution that works for me- disconnect R49(1k) from the junction of C32(22uF) and R48(100k) and run that to the SPST switch on the back of the 500k pot that I used for the MV by replacing R47(470k) the NFB resistor for U3A. Two birds with one stone- kinda lucky because the only 500kB pot I could find last night had the switch.

The other mod that I did to the amp was to add a 68k resistor from the wiper to the #1 terminal of R23(100k) which controls the NFB for U2A, the first gain stage for the OD channel (I actually mounted the resistor across C14(470pF) the NFB cap which was much easier to access.) I think that the OD channel is usable at very low gain settings- like halfway between 0 and 1 on the stock amp- so that works for me. I'm happy because those 3 little mods solved my main complaints about the amp! (It is extremely loud- 63 watts RMS- and the clean channel gets real loud real quick!)

Thanks for the quick replies (I didn't get an email notification for this thread so I didn't bother to look yesterday- my bad!)

Steve Ahola

P.S. BTW this amp has a great clean channel and is perfect for blues or country- maybe even jazz. Even though it is solid state it sounds like a Fender to me! (The tone stack has the same architecture as a BF, but with different values.) It is built like a tank with a very solid cabinet and a very strong chassis. The OD channel sucks- you are better off plugging pedals into the Normal channel. I think it handles the lower gain pedals better than the high gain ones which would rather not go through a notched-mid Fender preamp (it might not be a bad idea to add a mid-boost trim pot accessible through the front panel with a screwdriver.)

EDIT As they say, great mimes think alike! When Fender replaced the 112+ with the Princeton 65 in ~2000 one of the changes in the Normal channel was replacing the 470R mid resistor in the tone stack with 1.8k. Not a lot of differences between the two amps besides the name (I think that they wanted to cash in on the similarity to "'65 Princeton Reverb".) I had gotten the P65 at GC with it needing some repairs that I would diagnose within the 30 day return window. And then I found the P112+ at the GC used on-line store from back East. With shipping it was about the same as the P65 which I returned. One thing I liked about the P65 was that it had two inputs, hi and lo, basically the same as the Champion 30.

[Steve A.]

Comparing the schematics of the P112+ to its replacement, the Princeton 65, I see that a 0.1uF/250v cap was added between the "hot" PT secondary leads which I think will reduce the pop on shutdown considerably. As they say on TV full details at 11:00.

Steve Ahola

[IPolato]

Fisrt of all, this is my first post to this forum. I am a guitar enthusiast and player, and like to mod/tweak my equipments whenever possible.

I'm really interested in these mods. I have my Princeton 112 since 1994, and the pop always annoyed me. Is there any chances to get some pictures from these mods, especially from the standby switch? Where did you fixed it in the cab? In the front or in the rear?

Thanks in advance.

[Steve A.]

Fisrt of all, this is my first post to this forum. I am a guitar enthusiast and player, and like to mod/tweak my equipments whenever possible. I'm really interested in these mods. I have my Princeton 112 since 1994, and the pop always annoyed me. Is there any chances to get some pictures from these mods, especially from the standby switch? Where did you fixed it in the cab? In the front or in the rear?

I mounted the MV/Standby switch on the rear panel in roughly the same position as the reverb control on the front panel. I haven't gotten around to making a legible drawing of the mods I've done to the Princeton 112+ (I need to look at the schematics to see how that differs from the 112.)
I've also been working on mods for the Champion 110/30 amps and skipped the standby switch, just turning the MV down if I don't want to hear the pop. All of these amps seem to get way too loud way too fast on the Normal channel so adding a Master Volume is the most important mod.

Steve Ahola

[IPolato]

I'm sorry, my mistake. My amp is a Princeton 112+.

[KrS14]

A real simple "standby" or mute switch solution, if you don't want to rip your amp apart and add components: Get a 1/4 to 1/8 adapter, can be mono or stereo, and put it in the effects return jack. This will disconnect the power amp from the input/tone stack. This is a common trick that only works on series effects loops, to silence the output

I like the mods from Steve, they may be implemented in my 112+

I am going to be doing some pot switching to tame the steep reverse log of the volume pots. (exchanging the treble and bass pots in the clean channel with the clean and distortion channels vol pots.) They are the same value, just changing reverse log to log. Basically, the bass and treble controls will come on VERY fast, and the volumes will come on very strong at the end of the sweep, instead of between 0 and 2 I've got new filter caps coming as well for the power supply, i'll see if that helps with the POP some, it is quite annoying!

I would be interested in knowing why the dirty channel switches on when powering off the amp tho... Steve? I really wish I knew more about the electronics of amps!

[52 Bill]

I would be interested in knowing why the dirty channel switches on when powering off the amp tho...

All channel switching amps will have a default setting when the power supply is not powered up. Whether it uses mechanical relays or fet switches when the power supply turns off the relay will go to the normally closed setting or the fet will go low resistance.

If the switching power supply dies off before the audio power supply does, you will hear the signal switch to the default channel as the amp powers down.

[KrS14]

Which begs the question; why didn't they use the clean channel for the default lol. Kinda makes sense to me.

[Enzo]

It makes sense because you'd prefer it, but in the circuitry, there are things that are energized or not, and when power is removed, some things discharge faster than others. So the power to the channel switching relays drops out faster than the moment it takes for the amplifier portion to discharge. They did not chose to make one default for some marketing, it just happens to be the channel that is on when switching power disappears.

[KrS14]

I totally understand WHY the circuit acts as it does, i'm just questioning the unenergized state being the dirty channel. Most guitarists i know turn off amps on the clean channel. If this was the unenergized state, there would be no issue with what powers down first, since the switching is already in an idle state.

That's all i'm saying

[Jayz]

Thanks for all the helpful comments, MODS and suggestions. I bought a "project" P112+ recently and replaced broken parts including the reverb tank and REVERB DEPTH POT (according to the Fender schematic it's a B50K pot). Reverb works but I'm getting full reverb when the pot is only set to "1".
I assume that is not the way it was designed to work! Any suggestions would be appreciated.
Thanks.

[KrS14]

Did you replace the tank with the same type that originally came with the princeton? That may screw with levels if it's not the same spec wise.
Check R42 and R45 and make sure they aren't toasted at all (visually) I guess also make SURE that it's 50K and not 500K you put in there as well.

I'm sure Steve will pop in here with some other stuff to check