prosonic noise v3

[musicamex]

hi,

i have a fender prosonic on the bench that i hope for some tips on. the pcb amps really make me appreciate the old hard wired builds.

the amp is noisy. it has a crackle and hissing with nothing plugged into the inputs and with vols turned to zero. i pulled the PI and the noise stops, so i pulled v1 noise still there, pulled v2 noise still there pulled v3 noise stops. the noise also stops when i break the contact at the fx loop jack.

with all tubes replaced i tested the leads for v3 (only 1/2 of v3 is used) with a signal tracer. 1, 2 and 3 are all noisy. there are 2 related relays on the schematic. without pulling the amp apart to access the bottom of the pcb are there any suggestions? i usually suspect a bad contact/solder connection but it seems these amps have some other pcb related issues from what i've read.

in advance, i really appreciate any help. i know modern builds require pcbs for economy, but they sure have their own set of problems and mysteries. with this one it looks like i have to really do allot of disasembly to even access the solder side of the board.

[Enzo]

Frankly, circuit boards are more consistent an more reliable than hand wired circuits.


If you have already changed the tube, my first suspects will be a noisy resistor.

[musicamex]

Frankly, circuit boards are more consistent an more reliable than hand wired circuits.

thanks enzo, you are likely right on pcb reliability. but with many of today's tube circuits, it seems they come with additional modern complexity and are harder to work with, at least for an old guy who grew up around the classic old eyelet and turret board amps. it's probably just me, but i sure spend allot more time fixing a dependable pcb than i do the old style amps. to get to the solder on this prosonic to even see if there are cold solder joints, compared to an old leo era fender, is allot more bench time. maybe that's why, it is more cost effective for an entire board to be tossed and replaced than diagnosed and fixed.

If you have already changed the tube, my first suspects will be a noisy resistor.

the resistors appearance and the chopstick prodding don't show anything to be wrong so i guess i might have to bite the bullet and take apart the amp substantially more to get to the solder side of the board so i can desolder and test the resistors and caps in from v3 onward. i had read posts about the fx loop of this amp being very hot (+4 ? was mentioned in a post here) and thought there might be more info on this aspect of the prosonic. most info posted on this amp is from around the time of manufacture and a few years after. maybe production was fairly short and limited.

btw, the 4.7mf cap in the cap pan that connects to the output screens was ruptured along its side rather than blowing out the end. messed up the inside of the pan lid. but replacing it did nothing for the noise, so my "always check the filters first" was necessary but didn't solve the noise issue and i started pulling tubes and isolating with my homemade signal tracer.

[g1]

Some freeze spray on the resistors in the V3 circuit might help isolate the problem.

[Chuck H]

I agree with Enzo, but I'm interpreting that the amp sat for awhile since the caps dried up and failed on testing. I'd clean all the contacts (jacks. jack switches, pin holes, pins, etc.)

[Enzo]

It doesn't matter how they look, noisy parts don't look any different.


And there is not any way to test a resistor for noise. So unsoldering it to connect a meter won't help. What you need to do is troubleshoot to isolate the problem to a particular component.

[musicamex]

To be honest, freeze spray is something that i don't have and have never used. I don't think i could find in 100 miles from here. Is there a common non electronics substitute i might find in mexico? Drink glass freezer comes to mind but even that is something I have never seen before here. It sounds like a great diagnostic aid.

I think the amp was unused because of the noise. I am going to probe more mañana with my signal tracer, back through the tone stack since it feeds v3.

The caps are taiwanese so, even though the test ok on the meter and arent visibly leaking, 20 yrs mas o menos is a long time and imho they should be changed even if they aren't part of the noise problem. Looking back at previous posts here and elsewhere, noise is a common if not almost prevalent problem with this circuit. And i see im not alone in not looking forward to getting the solder side of the board wwhere i can get to it. I always appreciate am amp with the repair guy in mind. Leo's amps and pete traynor's always seem so straight forward and easy to work on. Traynors even have a schematic under the cover of many models.

Thanks allot for the suggestions. I'll post more info as i zero in on the source of the noise. Since it is on the grid if v3 the tone stack needs to be checked. And ill bite the bullet and do the disassembly necessary to flip over the pcb. Keep the idead coming please. Especially circuit freezing ideas.

[Chuck H]

Most often the noise will be from preamp tube plate load resistors.

Resist the urge to "fix" or otherwise mess with anything that hasn't been identified as the problem or, as is the case with the plate load resistors, very commonly the problem and untestable otherwise. Many amps end up with more problems when repairs are attempted without direction. I would clean all contacts, replace the plate resistors and perhaps replace any remaining original power supply and bias caps. I think the problem will go away and anything unnecessary will just be timely maintenance.

[Justin Thomas]

+1, ESPECIALLY since we dislike PCB amps! And to think this is one of the better ones, honestly. All the moving parts are chassis mounted. In mine, anyway...

Justin

[musicamex]

It doesn't matter how they look, noisy parts don't look any different.

With thanks and respect for your response, i'm not sure i understand this Enzo. The first and most productive thing i do to an amp i am to repair is to closely look at it. Leaking caps, burned or cracked resistors, corrosion (big problem on the coast), bad solder joints etc often ARE the problem. With 128vac here and hot summers obvious heat related issues show up visually.

I dont mean, on the prosonic, i would blindly begin unsoldering and testing components. But i have found perfectly visually good looking parts that are way out of spec and are the problem because the value is way off. I am going to continue to isolate the problem (after guitar classes today) with my signal tracer and o scope if necessary. My hunch as with allot of wave soldered boards, is bad solder. Since this says custom shop in the literature, maybe it was hand soldered?


And there is not any way to test a resistor for noise. So unsoldering it to connect a meter won't help. What you need to do is troubleshoot to isolate the problem to a particular component.

The freeze technique mentioned won't help test for noise? I never have used this and honestly don't have a clue, but it seems like something that might be useful. IF i can find a can of magic freeze juice here in the land of mañana. I agree, isolating saves allot of unnecessary work. So do the many comments from people who share experience and tips like here.

[musicamex]

Most often the noise will be from preamp tube plate load resistors.

Resist the urge to "fix" or otherwise mess with anything that hasn't been identified as the problem or, as is the case with the plate load resistors, very commonly the problem and untestable otherwise. Many amps end up with more problems when repairs are attempted without direction. I would clean all contacts, replace the plate resistors and perhaps replace any remaining original power supply and bias caps. I think the problem will go away and anything unnecessary will just be timely maintenance.

Since the signal is clean coming out of v2 and noisy on the grid of v3 and the tone stack is in between, that seem the best place to continue. But the electrolytics are on the replace list. I have 3 benches, one is luthier and my busiest and 2 electronics. This is a lower priority job since i just fixed the ownes laney . This time of the year i give priority to musicians needing to stay on stage. You're right, without narrowing down the problem, the shotgun approach isn't a productive method. All the best, and gracias.

[musicamex]

+1, ESPECIALLY since we dislike PCB amps! And to think this is one of the better ones, honestly. All the moving parts are chassis mounted. In mine, anyway...

Justin

You have a prosonic , justin? Also an aficionado of the older rugged builds?

[Justin Thomas]

Yes, a red combo from 98. I do like older builds only for the artwork and ease of modding. If a PCB amp is done right, I don't mind them at all. If the tube sockets and pots and switches and jacks are chassis mounted, and if it's not over-crowded, and is thick and sturdy with fairly wide traces and spacing between traces, it gets a pass in my book. The Prosonic is all of those things. The Fender Reissue series also falls into this camp, even thoug the pots have a dedicated PCB. Which is easily replaced... Old Ampegs, Marshalls, Sovteks are pretty good, old Peaveys... All good by me.

I made the "dislike PCB amps" comment a bit facetiously... I haven't played my Prosonic regularly for some time. It developed a cab rattle, and at the time I didn't know enough about how to address it. Sitting on it fixed it. I began to collect other amps in the meantime and stripped the tubes out of it. Also, the threads on the rectifier switch strippedd and I no longer felt comfortable gigging it. If/when I can get a new switch I'm probably going to sell it, maybe. But to be honest, the "Clean" channel alone is worth the price of admission. It still works perfectly fine, and I do like the amp. I just got distracted by others in my collection.

Justin

[Chuck H]

A little blathering about the Prosonic.

Most of the Fender Custom shop channel switchers of that era and for the next dozen years or so were more or less influenced by the Zinky Prosonic design. Many were refinements of the design done by Zinky himself. It's a stellar platform and holds up well to even modern stage requirements. IMHO the Prosonic is a stellar amp. I wish I hadn't sold mine (If you're listening, Justin ). I met Bruce Zinky at the 2009 Winter NAMM show. Much more of a nerdy, socially awkward genius type than I expected. He was never totally happy with the Prosonic. He posted here and there for years about small mods that he felt improved the amp. I was pretty happy with mine as it was when played into the right speakers. Being speaker specific (and not best with the common choices) was my only gripe. Some have complained about the switching noise. I can understand that too but it never bothered me that much. I've used the macro stage gain structure as a starting point for a couple of conversion mods in existing amps with excellent results too.

[g1]

As far as the freezing goes, if you are very cautious about the high voltage there are a few other options. Some have mentioned isopropyl or methyl alcohol on a swab will provide cooling effect (danger, flammable). But we are looking to create a thermal change so heat may be just as effective. A hair dryer may work but you want to isolate components so it may not be precise enough. Holding your solder iron close to the resistor may do the trick but you do not want to touch it. If you can get a reaction from heating or cooling you have probably found it. It doesn't matter if it makes the fault better or worse, just that it reacts.