collapse

Author Topic: Complete newcomer in Amp Repairs.. Try to trace problem in Fender Princeton 112  (Read 6059 times)

blueboo

  • Chipper
  • *
  • Posts: 6
  • Chip Points: 0
    • View Profile
Hi,

i'm totally new to amp repairs and just did some googling lately to try and locate the problem I have with
 my Princeton Solid State Amp. (Fender Princeton 112, Tupe: PR226)
The amp still works but has a certain type of distortion on it as soon as you raise
the volume past 1 on the clean channel and as immediately on the drive channel.

The troubleshooting I did so far was to get the head of and have a quick look to see if I could see some obvious errors, I took some pictures while it was open:
http://imgur.com/a/zKp7T#0

After that I disconnected the speaker and reverb to see if that caused any problems.
Went with the headphone in the output only with the head and still had the same distortions but not as quick as on the speaker.

I recorded the distortion with my audio interface:
http://yourlisten.com/blueboo/amp-troubleshooting

Was wondering if there's any things that you think I should test / have a look at.
A friend of mine has a basic multimeter and knows how to work them so could measure some things up..

Also found a schematic for the amp online but the resoulution is quite low:
http://elektrotanya.com/fender_princeton-112-plus_sch.pdf/download.html

Hopefully this is the right place for this issue.

Thanks in advance,

Michiel

Roly

  • Legendary
  • ******
  • Posts: 2184
  • Chip Points: 288
    • View Profile
    • Australian Valve Amps
Hi blueboo, welcome.


Yep, you're in just the right place, and that information is a flying start.  :dbtu:  {and that circuit is not so bad when you zoom in.}

Measure and post the DC voltage at the speaker connection.  This should be no more than +/-0.25V.  Anything more than this, leave the speaker disconnected.

Wild early guess; it sounds like you might have AC ripple on one of your supplies (notice something is beating with your guitar notes), and a typical cause is a cracked solder joint where the big power supply capacitors fit to the board.

Another stock test if it has Fx Send and Fx Return, or Pre Out/Main In, is to jumper these with a known good lead and see if that makes any difference.


{techs with more experience of Princetons than I will be along shortly.  ;) }
If you say theory and practice don't agree you haven't applied enough theory.

Enzo

  • Legendary
  • ******
  • Posts: 1868
  • Chip Points: 195
    • View Profile
I am sure I have a clean factory schematic on my work computer.  I will post it as soon as I get over there.

PhredE

  • Regular
  • **
  • Posts: 19
  • Chip Points: 0
    • View Profile
Greetings all.

Hi guys.

FWIW, I had a Princeton 112 "Plus" model and happen to have the schematic sitting here on my computer (.pdf of course).  I *think* the only difference between the "Plus" and standard version of the SS Princeton 112 at that time was the power rating.  I believe, most or all the rest is very, very similar.

I had the Princeton 112 Plus (bought new in 1993) and loved it.  I never used the "Gain" / distortion channel, but the clean channel and the 2-way tone stack sounded quite good on it. Fender also did the old trick of using a linear taper pot for the volume control so it got loud really quick when turning it up, but after 4/5 it didn't seem to change much  :lmao:

Anyway, mine died about 1.5 yrs ago and I dismembered and re-used quite a lot of it.  As I recall, I researched the failures that amp series experienced and found some pretty common problem areas/components (these will probably sounds really darn familiar to subject matter experts here -- [ I am a mere newbie at all this...]):

a). bad / tired / burned filter caps 
b). burned power resistors  (mine was a nice tone of brown! - supposed to be ivory white)
c). failing Opamp or transistor in the preamp stage
d). toasted transformer
e). and, of course, the all too common broken solder joint (I found more than one on mine).
(etc)

Well, ok. That's all I've got. Good luck just the same.

PS. Oh, I see I have the old main board sitting here behind me.  I removed a good number of components, but most of the rest is still here.

Roly

  • Legendary
  • ******
  • Posts: 2184
  • Chip Points: 288
    • View Profile
    • Australian Valve Amps
Quote from: PhredE
b). burned power resistors  (mine was a nice tone of brown! - supposed to be ivory white)

And when they get really hot they turn snowy white again.  I've seen these glowing red hot, carbonised the laminate and burned a hole right through a PCB.  Pretty hard to kill these ceramic wirewounds.  ;)
If you say theory and practice don't agree you haven't applied enough theory.

PhredE

  • Regular
  • **
  • Posts: 19
  • Chip Points: 0
    • View Profile
Quote from: PhredE
b). burned power resistors  (mine was a nice tone of brown! - supposed to be ivory white)

And when they get really hot they turn snowy white again.  I've seen these glowing red hot, carbonised the laminate and burned a hole right through a PCB.  Pretty hard to kill these ceramic wirewounds.  ;)

I measured the resistance and mine went way, way off spec.  So, I swapped them out and later found the transformer was the culprit. Doh! (smacks forehead).  Oh well. Al least I didn't do it the other way around...

blueboo

  • Chipper
  • *
  • Posts: 6
  • Chip Points: 0
    • View Profile
Thanks for all the response so far!

Measure and post the DC voltage at the speaker connection.  This should be no more than +/-0.25V.  Anything more than this, leave the speaker disconnected.

Wild early guess; it sounds like you might have AC ripple on one of your supplies (notice something is beating with your guitar notes), and a typical cause is a cracked solder joint where the big power supply capacitors fit to the board.

Another stock test if it has Fx Send and Fx Return, or Pre Out/Main In, is to jumper these with a known good lead and see if that makes any difference.


I checked the pre-amp out with the speaker disconnected, put it in my mixer and your right, the distortion's gone on both on clean and gain channel through the pre-amp out.. So does this mean anything in particular?

Also going to check the voltage, my friend is on his way with a multimeter so will post within an hour, you think it is worth to get the circuit board out of the frame to have a look at the joints on it's bottomside?

Thanks

blueboo

  • Chipper
  • *
  • Posts: 6
  • Chip Points: 0
    • View Profile
Just checked the speaker connection with volume on 10 on guitar and amp I got minimum 0.00 V (with guitar on voume 0) and a maximum of 0.88 V (with guitar volume on 10) depending how hard I hit the strings it goes between 0.33 and 0.75 +/-

Roly

  • Legendary
  • ******
  • Posts: 2184
  • Chip Points: 288
    • View Profile
    • Australian Valve Amps
Quote from: blueboo
I checked the pre-amp out with the speaker disconnected, put it in my mixer and your right, the distortion's gone on both on clean and gain channel through the pre-amp out.. So does this mean anything in particular?

Yes, it means the preamp is in the clear and your fault is after the Fx point, i.e. in the power amp.

Did you try it with a known good lead between Fx Send and Fx Return?

Sorry, I should have been more specific, we are looking for the voltage across the speaker while the amp is idle, un-driven.
If you say theory and practice don't agree you haven't applied enough theory.

blueboo

  • Chipper
  • *
  • Posts: 6
  • Chip Points: 0
    • View Profile
With the device plugged in but not switched on : 0.3 mA
Or do you mean switched on but both volumes on 0?

It doesn't have a Send / Return only has Input, Footswitch, Pre-Amp Out, Power Amp In and Headphones

Roly

  • Legendary
  • ******
  • Posts: 2184
  • Chip Points: 288
    • View Profile
    • Australian Valve Amps
Quote from: blueboo
With the device plugged in but not switched on : 0.3 mA
Or do you mean switched on but both volumes on 0?

We want the voltage, not the current, and yes, turned on and turned right down so the amp is active but idle.


Quote from: blueboo
It doesn't have a Send / Return only has Input, Footswitch, Pre-Amp Out, Power Amp In and Headphones

Here we can treat Pre Out/Main In as the same as Fx Send/Fx Return, so plug a known good lead between Pre Out and Main In and see if that makes any difference to your symptoms.
If you say theory and practice don't agree you haven't applied enough theory.

blueboo

  • Chipper
  • *
  • Posts: 6
  • Chip Points: 0
    • View Profile
With the amp switched on and volume on 0 I read 0.004-0.005 V.

I tested the main power amp input / pre amp out and the distortion is gone so can use the speaker as a second amp.
It also works with the pre amp out to my mixer and just the normal input. Also the headphone output works with main power amp in (doesn't work with main input).

phatt

  • Legendary
  • ******
  • Posts: 2037
  • Chip Points: 242
    • View Profile
Roly simply means;

Using a normal working guitar cord, plug one end into preamp output and the other end into power amp input. (this bypasses the internal switch which is a very common fault)
 
If that solves the problem then you have 2 options;
1/ Buy a short lead and leave it alone,,, or 2/ replace the two sockets which means the whole pcb has to come out and unless you are teck savvy you may end up creating major issues and the cost can blow out big time. :-X

I've found a short cord to be far easier and much cheaper fix. hint 8|

Phil.

blueboo

  • Chipper
  • *
  • Posts: 6
  • Chip Points: 0
    • View Profile
Ok thanks sorry for the misunderstanding.
I just tried that but I still have the same distortion linked like that... :(

 

* User Controls

 
 
Welcome, Guest. Please login or register.
Did you miss your activation email?

* Recent Posts

Laney VC30 Output Transformer by ilyaa
[Today at 01:06:25 AM]


signal tracing by g1
[December 10, 2019, 02:26:30 PM]


Eight Audio Kit TDA7294 and TDA2050 from India by laikairawk
[December 10, 2019, 01:57:45 AM]


The Dangers of Speaker Impedance Mismatching by Enzo
[December 10, 2019, 01:15:39 AM]


Fender Mustang II v.2 by Enzo
[December 08, 2019, 04:46:37 PM]

* Sponsors