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Fender princeton chorus issue-
Old 6 days ago
  #1
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Fender princeton chorus issue-

I just bought a fender princeton chorus, black knob for cheap because one speaker was "out". I got it home and began testing. Here is what I did:

First, switched the speaker leads to see if one of the speakers was blown. Both speakers work fine, but the left channel is super quiet regardless of what speaker is hooked up. Both speakers work great on right channel. With volume on 10, you can audibly hear signal coming out but it is barely audible. I know both speakers are good now, so I am thinking it is either the left channel amplifier ic chip or one of the big caps. It is a stereo amp, so each speaker is fed with it's own amp. (25watts per side)

Seeing as I do get signal through the circuit, it is just super weak, what is your opinion on the issue. Am I just hearing crosstalk from the other working side?

Other things I have done:

-bypassed both the stereo and mono fx loops.
-checked it with headphones-get signal for left and right just fine through headphones.
-tried inputs 1 and 2
-checked ohms on speakers, both were 8 ohms.
-visually inspected all solder joints and looked for any obvious burning, caps leaking, weird smells etc.

The amp looks virtually unused. My best guess is it is one of the 2 main caps or the left side ic amp chip. I am not an amp repair guy, but know how to fix a lot of things and am just trying to stumble my way through this using the limited knowledge I have. I have quite a bit of soldering experience, but it is mostly in PTP tube amps and analog stuff, guitar pots etc, so I have never soldered pcb stuff. I have a nice adjustable iron.

With how cheap these amps are I don't want to take it to a shop as just opening it up is almost the same cost as buying another amp. I have a lot of nice tube amps, this is my only solid state. I bought it as a cheaper backline amp for open mics and jams and such so I dont have a ton in it if it craps the bed (further)

On a side note-if I just ran the speakers in parallel dropping it to 4 ohm, do you think this would burn out the only good channel I have left? I could also run it in series and get it at 16ohm, but that would drop the output power and double my speaker surface area, which most likely would produce the same relative volume (or close enough anyway IME)

I have ordered another ic amp chip. As far as the caps, If I leave it sit for a few days unplugged, will the caps self drain enough to pull them?

Any help is greatly appreciated, thanks in advance.

Last edited by mintaka007; 5 days ago at 07:05 PM..
Old 6 days ago
  #2
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Sorry I don't have anything helpful to say but i do hope you get the amp running proper because I'm actually a big fan of the Princeton Chorus, especially with a quality clean tube pedal in front of it.
Old 6 days ago
  #3
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Quote:
Originally Posted by TurboJets View Post
Sorry I don't have anything helpful to say but i do hope you get the amp running proper because I'm actually a big fan of the Princeton Chorus, especially with a quality clean tube pedal in front of it.
That is why I bought it. For a cheap SS it is usable. I picked it up for $80, and even just running the one speaker it sounds good, just not loud enough for a band atmosphere. It looks brand new, so hopefully my $4.00 IC amp and $3.00 cap fixes it. I'm pretty sure it is the chip.
Old 6 days ago
  #4
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Quote:
Originally Posted by mintaka007 View Post
That is why I bought it. For a cheap SS it is usable. I picked it up for $80, and even just running the one speaker it sounds good, just not loud enough for a band atmosphere. It looks brand new, so hopefully my $4.00 IC amp and $3.00 cap fixes it. I'm pretty sure it is the chip.
Can't believe you scored that amp for $80. Usually they sell for $250+ which is what they went for brand new back in the 90's.

The stereo chorus is beautiful and the amp has that classic bottom end thump and spank. The Fender Blue Special speakers are perfect.

You sound a little more handy than me. Personally I take my amps to a tech when something goes wrong or if I need maintenance done.

I hope a few cheap parts fixes the problem for you.
Old 6 days ago
  #5
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A buddy of mine runs a pawn shop locally that specializes in music gear. He had 2 and let me mess with both for a while. The other one had the chorus rate pot snapped off, and no reverb. The one I picked was much perfect cosmetically, and I was hoping it was just a loose solder or wire.

The good thing is in my search to find the issue I have de-oxited all the pots and input jacks and inspected every solder connection, so If I can get it fixed I should have a nearly new amp that has been gone through.

I downloaded the schematic as I can read them ok for my tube amp stuff, but the one for this amp is above my pay grade. I can kindof trace signal path and see some values, but there is a lot going on in these pcb board amps. Now I know why techs don't like to mess with them.

We will see how it goes, never soldered those tiny pcb board connections before.

The chip will be here next tuesday, haven't ordered the cap yet.
Old 5 days ago
  #6
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I pulled the heatsink off and inspected the IC amp today. I don't see any visible signs of damage (cracks, craters, etc). Is it possible, or have you seen a bad IC amp with no visible signs of damage?

This question extends to the large caps as well. No visible signs of damage, odd smells, puffing or leaking etc. Could either one of these be bad and cosmetically not show it?

soldering and traces look perfect.

I ran it at 16 ohms today in series and does add a bit of volume but loses a touch of clarity maxed out. I think it will work for my purpose running this way, but would still like to fix the other channel down the road.

What do you think?
Old 5 days ago
  #7
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Personally I would drive it straight down to a local amp tech. Certainly worth a little scratch to have it properly repaired.
Old 3 days ago
  #8
Quote:
Originally Posted by mintaka007 View Post
I just bought a fender princeton chorus, black knob for cheap because one speaker was "out". I got it home and began testing. Here is what I did:

First, switched the speaker leads to see if one of the speakers was blown. Both speakers work fine, but the left channel is super quiet regardless of what speaker is hooked up. Both speakers work great on right channel. With volume on 10, you can audibly hear signal coming out but it is barely audible. I know both speakers are good now, so I am thinking it is either the left channel amplifier ic chip or one of the big caps. It is a stereo amp, so each speaker is fed with it's own amp. (25watts per side)

Seeing as I do get signal through the circuit, it is just super weak, what is your opinion on the issue. Am I just hearing crosstalk from the other working side?

Other things I have done:

-bypassed both the stereo and mono fx loops.
-checked it with headphones-get signal for left and right just fine through headphones.
-tried inputs 1 and 2
-checked ohms on speakers, both were 8 ohms.
-visually inspected all solder joints and looked for any obvious burning, caps leaking, weird smells etc.

The amp looks virtually unused. My best guess is it is one of the 2 main caps or the left side ic amp chip. I am not an amp repair guy, but know how to fix a lot of things and am just trying to stumble my way through this using the limited knowledge I have. I have quite a bit of soldering experience, but it is mostly in PTP tube amps and analog stuff, guitar pots etc, so I have never soldered pcb stuff. I have a nice adjustable iron.

With how cheap these amps are I don't want to take it to a shop as just opening it up is almost the same cost as buying another amp. I have a lot of nice tube amps, this is my only solid state. I bought it as a cheaper backline amp for open mics and jams and such so I dont have a ton in it if it craps the bed (further)

On a side note-if I just ran the speakers in parallel dropping it to 4 ohm, do you think this would burn out the only good channel I have left? I could also run it in series and get it at 16ohm, but that would drop the output power and double my speaker surface area, which most likely would produce the same relative volume (or close enough anyway IME)

I have ordered another ic amp chip. As far as the caps, If I leave it sit for a few days unplugged, will the caps self drain enough to pull them?

Any help is greatly appreciated, thanks in advance.
My opinion is this - if you bought it from a store, take it back. If you bought it elsewhere, chalk up the loss as a learning experience.

The amp bears no resemblance to a real Princeton other than in name.

Pull the speakers and chuck it.
Old 3 days ago
  #9
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Quote:
Originally Posted by John Eppstein View Post
My opinion is this - if you bought it from a store, take it back. If you bought it elsewhere, chalk up the loss as a learning experience.

The amp bears no resemblance to a real Princeton other than in name.

Pull the speakers and chuck it.
I bought it knowing it has a speaker issue, which is why I got it for cheap.

This is a solid state amp that is in almost everyway different than a princeton (I have one), so fully aware of that.

If you read the post you quoted, all of this information would be known to you.

Did you have any relevant information to my questions, or just wanted to attempt to cut this amp down?
Old 3 days ago
  #10
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To clarify John-

If you have technical experience on amp repair, if you look at the troubleshooting I have already done, it is not a speaker issue, it is extremely doubtful a preamp issue, it is not a wiring issue from the pcb to the speaker.

This amp has 2 separate power sections for each speaker, but all the tone shaping is shared, so that means that this issue is exclusive to the power section.

My testing has steered me towards the main cap or the ic amp. If either one went out, it could be partially caused by the other. I am going to replace both as it is a total of about $6.00 in parts.

My question, know my troubleshooting already done, is if there is anything else you think would be probable cause of this issue. This is a question for people with technical experience.

Your opinion of whether you like this amp or not is completely irrelevent.
Old 3 days ago
  #11
Quote:
Originally Posted by mintaka007 View Post
I bought it knowing it has a speaker issue, which is why I got it for cheap.

This is a solid state amp that is in almost everyway different than a princeton (I have one), so fully aware of that.

If you read the post you quoted, all of this information would be known to you.

Did you have any relevant information to my questions, or just wanted to attempt to cut this amp down?
My guess is that it's the amp hybrid. By my experience, I have a rather low opinion of these modules. Replacing it will probably get the channel functioning, but I wouldn't try pushing it very hard, either on it own or with pedals. Do not attempt to run it at lower than specified impedance. Do not stress the power amp.

At $80 half blown it's not what I would consider a good deal. There are listings on Reverb as low as $120 working.

Good luck.
Old 2 days ago
  #12
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Quote:
Originally Posted by John Eppstein View Post
My guess is that it's the amp hybrid. By my experience, I have a rather low opinion of these modules. Replacing it will probably get the channel functioning, but I wouldn't try pushing it very hard, either on it own or with pedals. Do not attempt to run it at lower than specified impedance. Do not stress the power amp.

At $80 half blown it's not what I would consider a good deal. There are listings on Reverb as low as $120 working.

Good luck.
When you say "amp hybrid", what are you referring to? Are you talking about the ten prong ic chip amp?

These sell constantly for 200, and the one on reverb looks like a great deal at 170 shipped, as all the rest are between 200 and 300 shipped. I went to every shop in my area and have been watching craigslist for weeks and this was the best cheap option available. I am in agreement with you about this amp not being the greatest compared to what else I have, or even compared to a cheap PCB tuber, but I don't want to use one of my nice amps for a jam backline atmoshpere. I wanted to find a well reviewed amp under $150, and this seemed to fit the bill.

I'm running it at 16ohms off of one side now and sounds fine, but would be borderline in a band atmoshere unless mic'd.

Fingers crossed my $6.00 invested in parts does the trick. Might sell it for a couple hundred and pick up a classic 30.
Old 2 days ago
  #13
Quote:
Originally Posted by mintaka007 View Post
When you say "amp hybrid", what are you referring to? Are you talking about the ten prong ic chip amp?

These sell constantly for 200, and the one on reverb looks like a great deal at 170 shipped, as all the rest are between 200 and 300 shipped. I went to every shop in my area and have been watching craigslist for weeks and this was the best cheap option available. I am in agreement with you about this amp not being the greatest compared to what else I have, or even compared to a cheap PCB tuber, but I don't want to use one of my nice amps for a jam backline atmoshpere. I wanted to find a well reviewed amp under $150, and this seemed to fit the bill.

I'm running it at 16ohms off of one side now and sounds fine, but would be borderline in a band atmoshere unless mic'd.

Fingers crossed my $6.00 invested in parts does the trick. Might sell it for a couple hundred and pick up a classic 30.
If you're talking about a Sanken type module (also made by Sanyo and a couple of other companies) it's not an "IC", it's a HYBRID. An IC is an Integrated Circuit - all the circuitry is on a single silicon chip that is no larger than the size of your thumbnail for a VLSI computer chip. The individual transistors are microscopic and not capable of handling any real amount of current - microamps at best. It is impossible to put a reasonable power amplifier on an IC chip. There are (or were) a few IC power amp chips but nothing above a couple watts or so, suitable for a weak headphone amp but nothing larger. I think there was a 5 watt battery powered amp that ran off a stereo 2.5 watt/channel chip operating in bridge mode, but I haven't seen one of those in decades. That's about max for an IC, otherwise the heat buildup is too great, even with a heatsink.

A Hybrid is a device consisting of one or more IC chips (for the low power circuitry) with external discrete power transistors incorporated into a sealed module. They're called hybrids because they're a hybrid of IC (chip) technology and discrete transistor technology. The 10 pin module you're talking about is a hybrid. Technologically ignorant people might call them "chips" but they're not.


Doing a quick look on the web, most of the hybrids I see are a 7 pin package - the 10 pin might actually be a stereo module being operated in bridge mode.

What is the part number on the module in your amp?

The module shouldn't cost you more than $25-$30 max.
Old 2 days ago
  #14
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Here it is. It isn't stereo. There are 2 of these, 1 for each channel. Price everywhere is about 4 bucks. Hadn't heard the term "hybrid" used before describing these, so thanks for the info.
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Old 2 days ago
  #15
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It arrives tomorrow, and the big cap gets here thurs or fri, so I'll get them in this weekend and know if my hypothesis is correct (or not) soon enough.
Old 16 hours ago
  #16
Quote:
Originally Posted by mintaka007 View Post
Here it is. It isn't stereo. There are 2 of these, 1 for each channel. Price everywhere is about 4 bucks. Hadn't heard the term "hybrid" used before describing these, so thanks for the info.
That's a bit different from the typical Sanken package (it looks a bit smaller), but still technically a hybrid, as there's no way to put the output transistors on the main circuit chip due to heat considerations. The price sounds about right. I'm actually a bit shocked that Fender would have chosen that particular package for a 25 watt guitar amp - that's REALLY cheaping out. Maybe they assumed that people wouldn't be pushing a stereo chorus amp very hard - I would not want to be pushing that chip to anywhere close to maximum output. Don't try to run it into a lower impedance that the load it was made for.
Old 14 hours ago
  #17
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Quote:
Originally Posted by John Eppstein View Post
That's a bit different from the typical Sanken package (it looks a bit smaller), but still technically a hybrid, as there's no way to put the output transistors on the main circuit chip due to heat considerations. The price sounds about right. I'm actually a bit shocked that Fender would have chosen that particular package for a 25 watt guitar amp - that's REALLY cheaping out. Maybe they assumed that people wouldn't be pushing a stereo chorus amp very hard - I would not want to be pushing that chip to anywhere close to maximum output. Don't try to run it into a lower impedance that the load it was made for.
25 watts x 2 for a total of 50 watts, its a reasonably loud amp and worthy of mic'ing for larger venues. Put a quality tube tone pedal in front of it and the amp really comes alive. Its an interesting amp with a beautiful and lush sounding stereo chorus. I really like the bottom end on the Princeton Chorus, but that's just me. A totally different animal from Fender's classic faves of course, but sonically has some classic Fender qualities nonetheless.
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