Fender Princeton Chorus solid stater - no signal thru input

[Mr Johnny Birchwood]

I can check the headphone jack tomorrow. Also, see if I can get the input jacks gently disassembled to check for corroded contacts or internal breakage. Gotta get up early..ugh

I have a habit of hooking up a load to an amp under diagnostic because a tube amp can short/arc inside the output transformer without a load on the secondary. It's also handy for tracking down a problem in the signal path. I keep a few throwaway speakers on hand and throw a t-shirt on top of the cone to muffle the pops, hums, and oscillations caused by dmm probing. I also know that output transistors and mosfets without an isolating transformer can go up in smoke if you short the speaker leads. I'll get the hang of this SS stuff, nothing like gettin' your hands dirty to really understand how something works.

[DrGonz78]

Yup if not a dirty headphone jack or input jack. Then your gonna have to literally trace it down to a solder joint somewhere... Good luck man!

[Mr Johnny Birchwood]

Yup if not a dirty headphone jack or input jack. Then your gonna have to literally trace it down to a solder joint somewhere... Good luck man!

Alright, headphone jack works. Signal seems a bit weak. Running it into another amp the headphone signal at max volume on both the normal and high gain channels is quieter than a single coil pickup at same amp settings. After poking around a bit, It does pass signal thru both input jacks. When not running thru the headphone jack, I get a very very weak signal coming through the speaker. Almost inaudible. I believe the original hum I heard may have been coming from a faulty reverb tank. Do those small ceramic coupling caps in the front end ever go bad or drift out of spec? Also, the gain and volume knobs on the high gain channel have a scratchy, dc-on-the-pot sound to em. Not really a dirty pot sound, though the amp does have a bit of dust inside it. If it were a tube amp, I would be looking for a leaky coupling cap. That sound plausible for this '88 solid stater?

[52 Bill]

Yup if not a dirty headphone jack or input jack. Then your gonna have to literally trace it down to a solder joint somewhere... Good luck man!

You've already proven that the headphone jack was probably ok when you plugged a guitar into the FX return and got sound. If the signal works from the FX loop to the power amp, you have eliminated half of the circuitry. Checking all of the opamps for voltage on the outputs is a good idea, but I'd start by checking the input jacks for bad solder or a broken trace.

With the amp on and a guitar plugged in the input, does touching any of the components around the input jacks create a hum through the output?

The loud hum is probably a broken solder joint on one of the filter caps. Be sure to check both of them. In fact if you do pull the board, just resolder them both and be done with it.

[Mr Johnny Birchwood]

You've already proven that the headphone jack was probably ok when you plugged a guitar into the FX return and got sound. If the signal works from the FX loop to the power amp, you have eliminated half of the circuitry. Checking all of the opamps for voltage on the outputs is a good idea, but I'd start by checking the input jacks for bad solder or a broken trace.

With the amp on and a guitar plugged in the input, does touching any of the components around the input jacks create a hum through the output?

The loud hum is probably a broken solder joint on one of the filter caps. Be sure to check both of them. In fact if you do pull the board, just resolder them both and be done with it.

Input jacks pass a visual inspection. They are the clear top plastic pcb mount types. I wish it were as simple as a bad connection at the PS caps, but no. With the reverb tank unplugged the reverb pot produces a loud 60hz hum with the pot above the zero position. The first TL072 chip produces no hum when probing the pins with a dmm lead. U8 on the schematic above does produce some substantial hum and popping when I probe the leads. I'll get back to it this afternoon and focus on the front end of the amp. All the solder connections passed visual inspection and no broken traces evident. Gotta be a component in the front end circuit.

[52 Bill]

U8 takes you right back to the stereo FX loop, what about the mono FX loop, did you try a spare cable in there?

As for the reverb hum, try it with the tank connected before you assume that there is a problem with the circuit.

[Mr Johnny Birchwood]

U8 takes you right back to the stereo FX loop, what about the mono FX loop, did you try a spare cable in there?

As for the reverb hum, try it with the tank connected before you assume that there is a problem with the circuit.

Mono fx loop did no pass a signal to the output section when i tested it initially.. My plan was to get the amp stable before I plug in the reverb tank. Too many variables. Wont get back to it for an hour or so.

BTW the amp had a very loud hum when I first powered it up. Independent of any controls. Hum disappeared when I removed amp chassis. Best explanation I can come up with is that loose chassis nut, which is still MIA, was lodged under pcb shorting something.

Thanx, stay tuned...

[52 Bill]

Mono fx loop did no pass a signal to the output section when i tested it initially.. My plan was to get the amp stable before I plug in the reverb tank. Too many variables. Wont get back to it for an hour or so.

Something to note here is that this amp has mute chip that kills the signal unless there is a cable plugged into the input jacks. I believe that the mono FX loop is before one of these mutes, so you may need to try that test again.

BTW the amp had a very loud hum when I first powered it up. Independent of any controls. Hum disappeared when I removed amp chassis. Best explanation I can come up with is that loose chassis nut, which is still MIA, was lodged under pcb shorting something.

This is why I thought that you had a loose or broken filter cap.

[Mr Johnny Birchwood]

Tracked the problem down to faulty mono fx return jack. Broken internals. Inserting a dummy phono plug in the jack closes the switch and amp works just fine. Dummy plug out and the switch is open when it should be closed. I could jump the pins on the pcb(2 and 4 methinks) but then the amp would lose mono fx loop capability. Any fender techs out there know the part number/availability? Does fender stock replacement parts or do I have to order from a different supplier? Thanx for being so patient with a SS noob I appreciate the help immensely.

[DrGonz78]

Congrats!! Tracked the bugger down!

I am sure someone might know exactly which jack is used here, but you can reference these sites to get an idea what is out there online... How many legs? etc etc etc...

http://www.darrenriley.com/shop/cart...t_detail&p=426
Way too much to pay but it shows all the correct part numbers...

http://www.amprepairparts.com/jacks.htm
Look at Part # J-506

Edit:
I found that >>>Fender part# 0990912000 should do the job... (same part as old part#s 0037036000 & 0025929000)
That highlighted is the jack and the 0037036000 is nut hex that fits the jack

Also, might be able to find this on mouser, digikey, etc etc etc as well

[52 Bill]

Tracked the problem down to faulty mono fx return jack. Broken internals. Inserting a dummy phono plug in the jack closes the switch and amp works just fine. Dummy plug out and the switch is open when it should be closed.

Did you try cleaning the jack? I use Deoxit.

[Mr Johnny Birchwood]

Thanx guys. The jack passed initial visual inspection but after plugging in and out a few times the loose spring inside the switching jack showed itself to be the problem. Jack cannot be repaired. Tried to monkey the spring back into place with a micro screwdriver but had no luck. Got the part # from Fender USA and ordered it from NewOldSounds this morning. My buddy wants to use the fx loop so he opted to replace the part instead of going for the p2-p4 jumper.

On a side note, I don't usually use de-oxit for cleaning contacts. I found that the oily residue causes dust to stick to treated surfaces. What I found to work best for cleaning dirty contacts and tube sockets is 91% isopropyl alcohol and a set of mini rat tail files. Welding tip cleaner file sets can be had for about 25cents a set at a local junk shop, come in many different sizes, and are perfect for filing off any oxidation(alcohol cleans the dirt, not the oxidation). The alcohol leaves no residue. I've perfected this method for cleaning old tube sockets and it works pretty good for any hard to reach dirty contacts in jacks and such.

My eyes are trained to read tube amp schematics. Following a signal path through 10 opamp stages and logic gates and multiplexers and deplexifiers makes me want to pull my hair out and it's a skill I need to work on. Thank you guys so much for the help!

[DrGonz78]

Maybe that Deoxit is a mixture of contact cleaner and a lubricant, which is really good on cleaning pots. However, if that is what is in that version of deoxit then YES do not use it on jacks. I like the idea of using alcohol for cleaning but just basic contact cleaner is best for those really nasty jobs. No lubricants in basic contact cleaner.

[Enzo]

I have been using Deoxit professionally for at least 30 years and have had zero trouble with it. Spray some on a Qtip and swab out a jack, I say. Dust? Push a plug into the jack and any dust is pushed aside. The residue Deoxit leaves protects the surface from further oxidation, hence the name. That is not bad, that is good.

[DrGonz78]

I always buy the Radio Shack brand cleaner that has lubricant added for cleaning pots and use other cleaners for various stuff. I guess it comes down to cost that I never bought Deoxit ever before... Good to know why it is worth the little bit extra in costs.