Fender Prosonic Noise on Clean Channel after 20 minutes operation

[caesparza]

Hi,

I got a Fender Prosonic amp early LOxxxx serial number. The amp works properly for the first 20 minutes - half an hour, after that time the clean channel starts to make sound like an airplane taking off, it's really loud and makes the amp completely
unusable.

I have read on some posts over here that it can be related to the C11 cap. I'm going to make some measurements for voltages and BIAS of the amp. Any help in any direction would be appreciated.

I've worked before on old Fender Amps (70's point to point wiring ones). So this one is a little new to me, even though the PCB seems pretty neat and organized.

All the best mates.
Carlos

[Chuck H]

Mt first guess would be a bad contact like a dirty tube pin socket, switching jack contact, compromised solder joint, etc. After that I would consider a bad tube. You can try gently wiggling the preamp tubes in their sockets when the problem manifests to see if that temporarily stops the issue or even worsens it. Resist the urge to replace any components until you have confirmed that they are bad and responsible for the problem.

[caesparza]

Mt first guess would be a bad contact like a dirty tube pin socket, switching jack contact, compromised solder joint, etc. After that I would consider a bad tube. You can try gently wiggling the preamp tubes in their sockets when the problem manifests to see if that temporarily stops the issue or even worsens it. Resist the urge to replace any components until you have confirmed that they are bad and responsible for the problem.

Thanks a lot Chuck H, I'll do that during the week and come back here with the results. Good day.

[Enzo]

This is also why they make freeze spray - circuit chiller. Once it warms up and gets noisy, you can chill small areas looking for what might be heat sensitive.

[Mick Bailey]

C11 is the 120pf cap in the tone stack. If this is the problem then a) turning the treble control right down will reduce the noise, and b) the turning the master volume down will eliminate it completely.

When troubleshooting any amp it's best not to get fixated on it being this fault or that, based on other people's experiences. There are lots of different faults that produce the same symptoms. The main objective is to identify the area where the fault is occurring and narrow it down from there. You have an FX loop, so when the amp is making the noise plug your guitar directly into the 'return'. Does it still make the noise? if it does then the problem lies after that, if not then you've eliminated the reverb, phase inverter and power amp sections.

Which controls affect the noise, and is it still the same with all controls zeroed?

EDIT; Just taken a second look at the circuit and most of the clean channel is shared with the drive channel, as is the tone stack. If the problem is confined to the clean channel and not the drive channel, this narrows it down a fair bit - to C3, P1, RLY1.

[caesparza]

Hi guys,

Thanks for the valuable information.

I did a whole maintenance on the amp today. Cleaned and tightened the tube sockets, clean the jacks, removed some kind of oxidation on the BIAS circuit, clean pots, etc...

I had the amp working for an hour with no problems of any kind. Everything seems fine by now.

I checked the BIAS for the Class A and Class AB and got the following data: Class A Inner Tube 28.24 Outer Tube 27.82 Plate Dissipation.
Class AB Inner Tube 16.87 Outer Tube 16.87.

Is this guy running kind of hot on Class A? Dissipation should be around 21, isn't it?

All the best, and thanks again.
CE

[caesparza]

During the inspection the only thing I found that could make me doubt of been in good shape was this resistor on the BIAS circuit. Is it normal the cracking on top?



Thanks a lot for your help.
CE

[Chuck H]

Ordinarily component fatigue is something to worry about, but with a block resistor like that I wouldn't even sweat it. If it was getting overly hot it would have melted it's contacts or caused damage to that electrolytic right next to it.So it's probably just a consequence of a bad filler mix or something. As long as it measures in spec

EDIT: As to the bias, keep in mind that it's the tube plate that is at 100% (ish) for class A so if you're measuring total tube dissipation you should subtract for screen current. Looks about right to me.

[caesparza]

Hi guys.

Plugged the amplifier today and again that horrible noise came. Clean channel only. It's funny that yesterday worked almost an hour with no noise at all. I guess will have to check all the chain on that channel.

Suggestions to begin?

Good day to all.
CE

[caesparza]

Connected guitar to effects return and no noise at all. So the problem has to be located on the preamp section of the clean channel.

Something funny happened, while checking the thing I mentioned before I saw that the return jack eliminated the noise when tapped with a wood stick, I cleaned again and the amp worked fine for a while, could this be the problem?

Should I require a scope to see in which section of the clean channel preamp the noise is been induced?

Thanks.
CE

[g1]

If the problem is on both channels, the return jack may be beyond cleaning and need replacement.
If the problem is only in one channel, then there is probably a bad connection that is just being jostled by vibration when you tap with the chopstick.

[Chuck H]

Yep. Chopstick poke around. It could also be one of the relays going hinky I suppose. Are we really sure it's isolated to the clean channel, or is it at all possible you just aren't hearing it as profoundly amid all the gain of the distortion channel. Maybe try running the distortion channel at lower gain settings when the problem is happening to be sure it's only affecting the clean channel.

[caesparza]

I'll check that again Chuck but I'm almost sure it's only on the clean channel also noticed that the noise is volume independent, I can put the clean channel volume control in 0 and still got the noise on the speakers.

I checked the schematic and almost everything is shared between both channels, so there wouldn't be a lot of components to check for that channel. Should I check the PCB on the underside? I poked around every component, the only one that cured the noise temporary was the effect loop return jack. When I lift the ground on that the noise disappears, it's the same when I connect the guitar on that jack straight to the output section, with the noise present on the normal input on the front panel.

I swapped all the preampo tubes to discard a bad tube, same behavior.

I'll keep digging around and tell you what I find.

Thanks for all the suggestions.
CE

[Chuck H]

Ok... Look at the diagram below. It's a simplified signal path for the Prosonic preamp. You can see that there are no gain stages exclusive to the clean channel. All are used in the dirty channel as well. So if the problem were with any of the gain stage operations you would hear it in both channels. The only things exclusive to the clean channel would be it's volume control and the specific relay switch positions. So I suspect you have a bad/dirty contact in the volume control or a relay. If jiggling and rotating the volume control doesn't make noise then I suspect dirty relay contacts. Those are sealed units so you can't clean them. They'll need to be replaced. Unless someone here chimes in to point out a flaw in my assessment that's what I would do.

[caesparza]

Ok... Look at the diagram below. It's a simplified signal path for the Prosonic preamp. You can see that there are no gain stages exclusive to the clean channel. All are used in the dirty channel as well. So if the problem were with any of the gain stage operations you would hear it in both channels. The only things exclusive to the clean channel would be it's volume control and the specific relay switch positions. So I suspect you have a bad/dirty contact in the volume control or a relay. If jiggling and rotating the volume control doesn't make noise then I suspect dirty relay contacts. Those are sealed units so you can't clean them. They'll need to be replaced. Unless someone here chimes in to point out a flaw in my assessment that's what I would do.

Yes Chuck!!, I narrowed down to the same components with the C3 cap included, volume control is not generating any type of noise, is dead clean. So I guess it has to be the relay. I wonder if I will be able to get a replacement where I live. Not even common on ebay.

I will check solder points under the pcb.

Thanks.
CE