No input sound to speaker on Fender Prosonic

[Skel]

Hi,

I have a Fender Prosonic, which is a 2 channel all tube dual rectifier combo with 2 6L6's. A few weeks ago it made a pretty loud screatchy noise for a few seconds, then blew the fuse.

I removed all the tubes except for the rectifier tube (not sure why I didn't remove this tube) and it would run on standby but would blow the fuse after switching it off standby. I noticed a slight hum when the amp was powered on. I tried new 6L6's to no avail, so I replaced the rectifier tube. Now it doesn't blow fuses but still nothing from the input signal comes out the speaker and that slight hum is present. No idea why changing the rectifier tube made it stop blowing the fuse (a 3 amp slo blo). I've tried both input jacks, check the obvious like making sure the speakers were connected to the right output, etc. I haven't replaced the 12AX7's - it has 5, plus a 12AT7 for the reverb driver. All the tubes light up.

Would looking for the problem in the 12AX7's be a logical place to start, or does this sound like a filter cap problem? Any ideas would be greatly appreciated.

Thanks.

[pdf64]

I think that your old tube rectifier has shorted out and blown the 2 diodes which are between the transformer winding and the rectifier switch, D101 and D102 on link below. You'll need to take it to a tech to get it fixed, unless you are competent yourself.
http://www.fender.com/support/amp_sc...ic_-_11x17.pdf
Hope that helps - Peter

[Skel]

I think that your old tube rectifier has shorted out and blown the 2 diodes which are between the transformer winding and the rectifier switch, D101 and D102 on link below. You'll need to take it to a tech to get it fixed, unless you are competent yourself.
http://www.fender.com/support/amp_sc...ic_-_11x17.pdf
Hope that helps - Peter

Wow - thanks so much for this insight. I'm not "competent" yet, but I understand how to drain the filter caps, then locate the diodes, desolder the old ones out and install new ones. I'll have to read up on whether there is a safe way to check the existing diodes to verify they are blown. I remember enough about electronics to know that a diode lets DC flow one direction only.

I will take it to a tech but I wish I could just check these diodes to verify this is the problem. Then the challenge is finding a tech who doesn't get offended by me explaining what needs to be fixed - sometimes techs would like to charge for a new P/T if you get my drift. I just need a tech I can trust, and I don't currently have one.

[Jag]

If you LEAVE THE AMP UNPLUGGED, in the standby mode, you should be able to check the diodes with a DMM. Set your DMM to the lowest resistance setting and read the resistance across each diode. It should read open in one direction and short in the other direction for each diode. Once again, there should be NO POWER to the amp while you test this.

Regarding the integrity of techs, don't go there here. Many of these guys are very trustworthy techs and take exception to blanket statements about unscrupulous techs.

[52 Bill]

Yes check the diodes, but I doubt that they are the cause of your output problem.

Replacing the bad rectifier tube fixed your fuse blowing problem, but now you have an audio problem. Check all of the obvious things first. Are the tubes lighting up? Do they get warm? Is the speaker ok? Have you tried testing the FX/pre/power loop jacks?

Is the amp still under warrantee? Then send it back to the manufacturer for repair. If you want, Fender will send you to an authorized repair station in your area. If not, talk to your friends to find out who they use to repair their equipment. Maybe there is an honest one in you area.

[Skel]

If you LEAVE THE AMP UNPLUGGED, in the standby mode, you should be able to check the diodes with a DMM. Set your DMM to the lowest resistance setting and read the resistance across each diode. It should read open in one direction and short in the other direction for each diode. Once again, there should be NO POWER to the amp while you test this.

Regarding the integrity of techs, don't go there here. Many of these guys are very trustworthy techs and take exception to blanket statements about unscrupulous techs.

The way you describe checking the diodes makes perfect sense. And I'm really sorry about inferring anything about the integrity of techs, I just had a bad experience with one recently, so I'm really just demonstrating ignorance.

My current goal is to buy all of Gerald Weber's books and CD's and along with consistenly doing what I'm doing now, eventually become a tech myself. Luckily every amp I own is broken -

Thanks for the help.

[Skel]

Yes check the diodes, but I doubt that they are the cause of your output problem.

Replacing the bad rectifier tube fixed your fuse blowing problem, but now you have an audio problem. Check all of the obvious things first. Are the tubes lighting up? Do they get warm? Is the speaker ok? Have you tried testing the FX/pre/power loop jacks?

Is the amp still under warrantee? Then send it back to the manufacturer for repair. If you want, Fender will send you to an authorized repair station in your area. If not, talk to your friends to find out who they use to repair their equipment. Maybe there is an honest one in you area.

Hi - yea all the tubes are lighting up and warm. it has 2 10"s for speakers, I'll connect them to a different amp just to make sure. I've never been a big effects loop user, but I guess it makes sense that I could just plug my guitar right into the return of the effects loop and see what happens (?) Hopefully this is what you meant.

Thanks for the help - much appreciated. I'm still gonna check the diodes just to see.

[52 Bill]

I've never been a big effects loop user, but I guess it makes sense that I could just plug my guitar right into the return of the effects loop and see what happens (?) Hopefully this is what you meant.

Often the FX loop jacks become oxidized and no longer make what is referred to as a "normal-ed" connection. This means that the jacks are normally connected to each other through a closed connection on the jack. When this happens, the signal line opens up and there is no sound getting through from the FX out to the FX return, thus killing the signal.

This can be an FX loop or it can be a pre-amp out/power amp-in, insert connection. In either case, you can check it by plugging in a regular guitar cable from the out jack to the in jack. If the signal comes back, then clean the jack connections. If the signal does not come back, then continue looking for the lost signal.

By plugging in your guitar to the power amp-in jack, you will be able to tell if the power amp section is working or not. If the power amp is working, you should be able to hear the guitar signal through the speakers. This means that the problem is in the pre-amp section. If you don't hear the guitar then the power amp is not working correctly.

[Enzo]

Since we had rectifier tube failures, I would be checking for B+ in the amp. The rectifier makes raw DC which is then filtered through several filter stages. A shorted filter cap or an open resistor would not be uncommon with a shorted rectifier tube. Without high voltage - your B+ - the amp won't operate.

[Skel]

If you LEAVE THE AMP UNPLUGGED, in the standby mode, you should be able to check the diodes with a DMM. Set your DMM to the lowest resistance setting and read the resistance across each diode. It should read open in one direction and short in the other direction for each diode. Once again, there should be NO POWER to the amp while you test this.

Regarding the integrity of techs, don't go there here. Many of these guys are very trustworthy techs and take exception to blanket statements about unscrupulous techs.

I tried this and on both diodes I'm getting "open" in one direction and about 2 Megaohms in the other direction. I tried this with the standby switch in both positions and got the same result either way.

Is this sounding like it actually is these diodes, or maybe my DMM isn't good enough? I can't set it for the "lowest setting", it only has one setting. For the heck of it I measured D103 and D104 and the same thing.

btw - for what I *think* are the filter cap(s) I'm seeing 3 100 V, 100 micro farad caps, and 1 50V, 2200 mfd "can" type caps. I don't know if all of these are "filter" caps, but I would guess the filter cap(s) are in this mix somewhere. I'll start reading up on a method for how to check these.

Thanks

[Skel]

Ok, nevermind about what I said about the filter caps in the previous post. I re-read the schematic and realized they are on top with a metal cover. I see three 500 volt 47 MicroFarad caps and (I think 3 other ones). Sorry.

[Skel]

Grrrr. I'll get this right sooner or later. There are actually 2 (the largest) 47 500 V, 47 Mfd caps (the smaller, 3rd one is a 4.7 mfd).

I read that if you measure the resistance across these 2 filter caps, the reading should be at least 15k ohms, usually much more. On mine, the first one, the reading just jumps all over the place constantly. The 2nd one is stable but it's still only about 14k ohms.

Any ideas? Thanks.

[Skel]

I read what Enzo said about the B plus (my son's bird pecked off my "plus" sign key). Since I wasn't sure about which pin was pin 3 on the power tubes (from trying to read upside down, etc) I read somewhere else where a guy said to just measure at each pin - it won't hurt anything. So I did that, and I'm not getting any voltage on any of the pins. Certainly not what I am assuming is B plus voltage, which is in the neighborhood of 600-plus volts?
FYI - I've got my DMM set to DC, it can measure up to 750 V.

So I'm thinking there is a B plus voltage problem. I just need to figure out where to take the first measurement in order to isolate the problem. Any input is greatly appreciated.

Thanks

[Enzo]

More like 465v but a lot more than nothing.

Make sure the standby switch works. With power removed, measure resistance to make sure both halves of the switch work.
And for that matter, when the amp is on, try it both ways in case the switch is upside down.

On the rear panel set the selector to solid state rectifier.

With power on, and the amp in operate mode, there should be 400-500v at pin 8 of the rectifier tube socket, or if you prefer, at the cathodes of diodes D103,104. If the voltage is missing there, then either the selector switch is bad, the standby switch is bad, diodes D101 AND 102 are open, the power transformer is shot, or the wiring between all these things is not right.

[Skel]

More like 465v but a lot more than nothing.

Make sure the standby switch works. With power removed, measure resistance to make sure both halves of the switch work.
And for that matter, when the amp is on, try it both ways in case the switch is upside down.

On the rear panel set the selector to solid state rectifier.

With power on, and the amp in operate mode, there should be 400-500v at pin 8 of the rectifier tube socket, or if you prefer, at the cathodes of diodes D103,104. If the voltage is missing there, then either the selector switch is bad, the standby switch is bad, diodes D101 AND 102 are open, the power transformer is shot, or the wiring between all these things is not right.

I'm almost laughing because you are so dead on - it was about 450 volts on pin 8 - no less.