Fender Princeton 112 Plus power switch pop

[Kirby02]

I recently purchased a used and sightly abused Fender Princeton 112 Plus that had a few issues such as a reverb control with a loose shaft and missing knobs. I have it functional but when the power switch is turned there is a loud pop from the speaker. I am worried about damaging the speaker if this is not repaired. Any suggestions? Thanks in advance.

[DrGonz78]

The pop won't damage the speaker. Try the test of plugging the power cord in while the power switch is on and reverse that by turning it off by unplugging the power cord. This will tell us if the power switch is arcing which could make a popping noise at turn on/off. Then again many amps pop when turned on/off. Start with that test and report back to let us know the results.

[Kirby02]

Fender Princeton 112 Plus power switch pop

I recently purchased a used and sightly abused Fender Princeton 112 Plus that had a few issues such as a reverb control with a loose shaft and missing knobs. I have it functional but when the power switch is turned there is a loud pop from the speaker. I am worried about damaging the speaker if this is not repaired. Any suggestions? Thanks in advance.

The pop at turn on is not nearly as loud as at turn off. The pop is the same volume if the amp turned off with the switch or unplugged from the power.

[Jazz P Bass]

To see if it is a preamp or a power amp issue, insert a plug into the Return jack.

That plug will isolate the preamp from the power amp.

[Kirby02]

Fender Princeton 112 Plus power switch pop

To see if it is a preamp or a power amp issue, insert a plug into the Return jack.

That plug will isolate the preamp from the power amp.

When I insert a plug into the Power Amp In jack the amp is quiet while I turn it off.

[52 Bill]

Schematic?

Does that amp pop when there is no guitar cord plugged into the regular input?

[nevetslab]

Princeton 112 Plus Service Manual attached

Schematic?

Does that amp pop when there is no guitar cord plugged into the regular input?

Ah....solid state amp. That begins to suggest things.

Princeton_112_Plus_Service_Manual.pdf

So, breaking the signal between the Preamp Out (at the Send) by inserting plug (or guitar) into the Return jack eliminates the Turn-Off pop. Does it do the same at turn-on?

A brief look at the schematic shows a 22uF coupling cap at the preamp output (C32) ahead of the Send Jack, while following the Return jack there's just a 470nF cap (C34). Nothing large in stage-to-stage coupling upstream in the preamp circuits. And, in the power amp, there's the bootstrap cap C40 that can also produce a pop at turn-on, while everything is charging up all at the same time.

[Kirby02]

Fender Princeton 112 Plus power switch pop

Schematic?

Does that amp pop when there is no guitar cord plugged into the regular input?

Yes it does but not as loud as when the guitar is plugged in. If I can't find a fix this will be the shut off procedure I will be using.

[Kirby02]

Ah....solid state amp. That begins to suggest things.

[ATTACH]52382[/ATTACH]

So, breaking the signal between the Preamp Out (at the Send) by inserting plug (or guitar) into the Return jack eliminates the Turn-Off pop. Does it do the same at turn-on?

A brief look at the schematic shows a 22uF coupling cap at the preamp output (C32) ahead of the Send Jack, while following the Return jack there's just a 470nF cap (C34). Nothing large in stage-to-stage coupling upstream in the preamp circuits. And, in the power amp, there's the bootstrap cap C40 that can also produce a pop at turn-on, while everything is charging up all at the same time.

Wow, Thank you so much for the service manual with schematic. This will give me a starting point to trouble shoot this issue. I will check the caps you suggested. I contacted Fender about a schematic and received a vague answer about their system being down for upgrades.

[nevetslab]

Yes it does but not as loud as when the guitar is plugged in. If I can't find a fix this will be the shut off procedure I will be using.

Here's a simple solution. Plug in an open circuit TRS 1/4" plug into the headphone jack. That disconnects the speaker from the amp, and turn it on and off with the plug in place. There is room on the rear panel to put a Speaker On/Off switch, in lieu of having to mess with a dummy plug.

[52 Bill]

Some of these have mutes that turn off the signal into the power amp when there is nothing plugged into the amp. This one seems to have a mute that turns off the reverb only when there is nothing plugged in.

[Enzo]

SO the preamp is picking up the pop. Probably a spark in the power switch.

Try this, leave the power switch on, and just pull the power plug from the wall outlet. Does it still pop? You might need nothing more than a cap across the power switch.

In my shop we do not consider pops a problem, it will not hurt your speaker.

[glebert]

I think every Fender solid state amp I have had of this vintage has this same pop sound, worse at turn off. It is annoying, but something I just learn to live with.

[Enzo]

Most players do not turn it off during a performance.

[DrGonz78]

I only try to fix these solid state Fender on/off popping noises when they are really bad. Most times it is just little pops but only worth investigation when it seems excessive.