Fender Princeton Chorus solid stater - no signal thru input

[Mr Johnny Birchwood]

Hey guys, workin' on my buddy's Fender Princeton Stereo Chorus solid state guitar amp. Neither input jack will pass a signal. The stereo effects return jack will pass a quiet guitar signal(with guitar plugged directly into effects return) independent of volume control. Mono effects return does not pass guitar signal. Solid state stuff is not really my specialty, Anybody want to point me in the right direction of where to start tracking the problem down? Also, when I first powered it up, still assembled, there was a loud 60hz hum independent of volume control. Plugging a 1/4 inch jack into the stereo effects return killed the hum. Now that the chassis is out on the bench this problem has disappeared. I'm stumped. Help! Schematic from fender website included.

[Jazz P Bass]

Well, you got me confused.
Which problem 'disappeared'?
The 'no pass signal' or the '60 hz hum'?

[DrGonz78]

After reading this carefully I would have to assume that the input problem is still occurring, but the hum problem went away...? I guess that is confusing by itself too, but maybe look in the cab and make sure the shielding inside the cab is not torn or hanging down. Most times removing a chassis from a cabinet will make it hum more.

Also, what voltages are you getting at TP6, TP7 and TP8? Notice that TP8 will show different readings depending on whether you plug something into J2 input. Start there with the voltage readings to make sure we are getting the right stuff.

Edit: Sorry for the edit... Just forgot to say to look at bad solder on the input jacks or a bad jack too.

[Mr Johnny Birchwood]

After reading this carefully I would have to assume that the input problem is still occurring, but the hum problem went away...? I guess that is confusing by itself too, but maybe look in the cab and make sure the shielding inside the cab is not torn or hanging down. Most times removing a chassis from a cabinet will make it hum more.

Also, what voltages are you getting at TP6, TP7 and TP8? Notice that TP8 will show different readings depending on whether you plug something into J2 input. Start there with the voltage readings to make sure we are getting the right stuff.

Edit: Sorry for the edit... Just forgot to say to look at bad solder on the input jacks or a bad jack too.

First, yes: the hum disappeared. No shielding hanging down, visual inspection shows no obvious shorts caused by mechanical parts. One of the nuts for the side chassis bolt is missing from its bracket. Unable to locate. Nut may have been loose in chassis under the pcb shorting something but IDK at this point where the loud hum was coming from. Hum was constant and independent of any adjustment to controls. Weird. Thought maybe bad connection at main ps filter caps or maybe a bad cap, but plugging into the stereo effects return would not have solved that problem. Bad connection at input jack is next best bet. That's what I will check.

Second: just put my kid to bed and got back to work. The schematic I downloaded from Fender is the wrong one. This amp is the Princeton Chorus, red knobs, ca. late 80's? SN# LO-220319 The schematic is from a later Princeton Stereo Chorus Reverb DSP and though the schematic is quite similar to the circuit I'm looking at the pcb layout is different. Cant locate the right schematic on the Fender site, guess I will have to email them tomorrow. Sorry for the bad info.

Third: Input jacks are the plastic pcb mount types, and my next step is to remove pcb from chassis and do a thorough visual inspection. 90% of malfunctioning solid state amps from this era that end up on my bench have a loose connection somewhere caused by vibration, road wear, dropped the amp, etc.. Hope it's that simple. Still curious to track down the hum problem because it's embarrassing when you 'fix' someone's amp and it shows back up 2 weeks later with the same problem. Oy.

Thanx for taking the time to respond. I'm still learning the solid state ropes. Tube amps, no problem.

[DrGonz78]

Maybe this one will help??

Fender Princeton Stereo Chorus Schematic

[Mr Johnny Birchwood]

Maybe this one will help??

Fender Princeton Stereo Chorus Schematic

Thanx. That's the one. Tp5 reads 16.11 - Tp6 reads -16.44 - Tp7 reads 7.77 - Tp8 reads -8.0

Not sure if the test points you were asking for are still the same. Got the amp powered up on the bench, What readings do you want. I can post em within a few minutes.

[DrGonz78]

That is good news to start... I would make sure that IC's are getting their correct voltage 15-16volts from the B- rails. All the TL072 op amps should have 15-16volts on pins 4 and 8.

Edit: Also I would be looking to see if that nut that escaped from his cage had something to do with shorting something out? That sounded funny too but just had to say it. Also, it would be good to know that the speaker is disconnected and you have tested both sets of speaker terminals for any DC on the outputs.

[Mr Johnny Birchwood]

That is good news to start... I would make sure that IC's are getting their correct voltage 15-16volts from the B- rails. All the TL072 op amps should have 15-16volts on pins 4 and 8.

How do you determine pin numbers? Clockwise from ...? Which are the IC's? Remember I'm a SS noob..

[DrGonz78]

Just look at where there is a notch at the top of the chip. Then with the notch side facing you look at the right side of notch pin 4 (Typo-oops not pin 1) (last leg farthest from indent) and left side pin 8 (closest leg to the indent on the left side). Or in this pictorial of the top view of the chip that statement I just made is reversed. BTW those IC's are the TL072 8-legged critters used as gain stages on the preamp (much like you use a preamp tube 12ax7 or what not on a tube amp).

http://www.datasheetcatalog.org/data...ents/tl072.pdf

[Mr Johnny Birchwood]

u1 reads: p1 16.20 vdc 140mvac - p2 0.11vdc - p3 0vdc - p4 0vdc - p5 -16.7vdc - p6 0vdc 240mvac - p7 0vdc - p8 0.02 vdc. All pins read 18-20mvac ripple volts, unless specified otherwise. R149 reads -16.89vdc at each end, thought maybe it's open but no... Bad opamp?

[DrGonz78]

That U1 chip looks fine to me... But it was backwards. I probably confused us describing it funny Anyway Pin4 will have the -16vDC and Pin8 will have +16vDC. I think that is what we are getting right? Also, go through all the TL072 chips to make sure you are getting -16vDC on pin4 and -16vDC on pin8 respectively. If you are not getting those volts at some point we need to know which chip. Also, turn on the amp for a few minutes and then turn it off, and then feel if any chips are getting hot. Once we know these op amps are not suspects then time to proceed with look for other suspects.

P.S. Any DC on the output??

[Mr Johnny Birchwood]

The chips don't have a notch, but the pcb has a notched outline. I read the pins clockwise with the notch at 12 o clock. I will check the rest of the opamps, check for heat, and check for DC on outputs now. Gimme 15min stay tuned.

[DrGonz78]

Some chips are different where there is no center notch you will then just see a circular indent closest to what would be pin 1. Just like this pic in this link here...

http://ts3.mm.bing.net/th?id=H.47128...71294&pid=15.1

[Mr Johnny Birchwood]

Pretty sure I'm reading the pins right now. I'm going by the texas instruments datasheet. Here's a summary:

No DC on the spkr leads. I have a throwaway spkr hooked up to one set of leads so I can hear noise caused by DMM probe.

TL072CP chip readings:

U2 reads normal +/- Vdc at p4 and p8

u3 reads normal +/- vdc at p4 and p8. slight clicking noise when shorting pins 3 and 4 with dmm lead.

u4 reads normal...no noise from dmm probing.

u5 seems off to my amateur eyes: p8 15.78vdc p4 0vdc every other pin read 7.74vdc. no noise from dmm probing

u8 reads normal and get loud hum and popping when shorting pins with dmm lead. probe on pins 6 and 2 cause very slight hum in spkr

u13 readings seem odd: P1 +15.26vdc P2 +8.23vdc P3 +12.27vdc P4 0vdc P5 +8.23vdc P6 +8.7vdc P7+1.43 P8 +16.34

amp has been powered up for 15 minutes and no chips are warm, mains tfx barely warm to the touch. Still unable to locate the missing chassis nut...

[DrGonz78]

That hum on U8 pin 6 and 2 means you are just grounding out the signal path before it hits the chip. As far as everything listed and tested so far this amp seems to be doing just fine. Except lack of input and a hum that now has gone away.

So... Sorry I did not ask this right off the bat, because I really should have... Did you try the headphone jack to see if that works??

P.S. BTW you know you do not need to plug a load into the amp for any testing? Just checking, but yeah probably makes no difference in this case.

Edit: Just wanted to say that U5 and U13 have pin4 going to ground in the schematic so yes no voltage there... They must be using these two TL072's differently going in and out of the chorus circuit. Someone else will know way more about that than I would, but it seems normal. The 7.75vDC on every other pin U5 seems normal to schematic, I think. To me U13 seems funny but that might be something due to a chorus switch function or something else.