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Fender Rock Pro 700 - MASSIVE channel problem! Need help

Started by monkeycabbage, August 29, 2009, 08:45:47 PM

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monkeycabbage


Hi everyone.... I'm new here so.... I'm Ronnie, I live in Kent, Uk. Nice to meet you all.

Funny little story:

I took my amp into a sparkie's a while ago for problems with the volume pot. The clean channel's volume was really unstable, raising and dropping randomly. He ordered a propper Fender pot from the states and had it shipped back to the UK, replaced it (and the worrying part) never charged me. When I went to pick it up, he seemed to have forgotten all about the volume pot and said "I've looked at your amp. There was a little bit of a loose solder connection so I just resoldered it for you."

He was a little on the old side, and I have a feeling he may have been missing 1 or 2 of his marbles. He didn't charge me as he said the solder wasn't worth the effort of opening the till and said see how the amp goes.

The problem.... I then moved 130 miles away a couple of weeks later and never had time to use the amp (I know I should have checked but I had a lot on with moving). This was 6 years ago!! (I've been using my old 30 watt practice amp in this time as it's small and more friendly for the neighbours.) I've just been asked to join a band and start gigging properly again and the amp is (for lack of a better word) BUGGERED!

When you turn the amp on, the overdrive channel light is on, even when the amp is on the clean channel. When you press the button for the overdrive channel, the light stays on.... and the amp stays on the clean channel. I would have worded that to say that nothing happens.... but something does. A very painful electric shock through the finger.

I've taken the amp apart and there's a massive wadd of solder underneath where it looks like the circuit on the board was cut and he's piled solder on to replace the board's paths.

I know he must have crossed a path here somewhere but although I can locate this on the diagram that came with the amp, this is a little over my head!!

PLEASE SOMEONE HELP!!!

Sorry this was so long.

STDog

Time to find a new tech that's nearby.

If you absolutely have to work on it, first, remove all the excess solder using a vacuum tool or solder wick.
Then you can see what's going on.

Maybe post pictures of the carnage so we can offer more advice.

J M Fahey

Hi monkeycab
Sorry to hear about that.
Please post, as asked above, a couple sharp pictures to see what really happened.
Don't want to rush to conclusions, maybe it can be repaired, but please start considering the possibility of rebuilding it, just salvaging hardware , chassis, power transformers, etc.
Why? Because "the PCB *is* the amplifier".
Anything else can be bought over the counter (well, sort of) or replaced by an equivalent, etc.
Electronic parts are "generic", what's unique is the PCB.
If somebody went trigger-happy with a soldering iron ....  :(
Anyway let's hope for the best.
Good luck.

monkeycabbage

Hi guys, thanks for coming back.

I've taken a couple of pics and had the chance over the last couple of days to have a closer look.

The first pic is the abbortion that he's made on the soldering job.... (Remeber, this guy is supposed to be a professional!!)



It looks like he's seen a break/soon to be break on the board and he's tried to patch it by re-routeing the paths with lines of solder. The pic looks like the board has burn out around that area, in fact... it's copius amounts of flux that he's used.

The next pic is a close up of where I believe the problem to be.



The arrow points to 2 "blobs" of solder that I don't think should be connected. The left part of the blob is the lower solder for component CR30 where the right blob looks like he slipt and didn't clean it up. This part of the circuit shouldn't join. To emphasise this, the next pic shows the opposite side of the board where the component can bee seen away from the path  of the adjasent curcuit.



I'm thinking (and hoping) that if I remove the excessive solder and break this connection that souldn't be there then I may be in luck.

I don't know if I'm thinking ahead here, but I read in another post on this forum that the rock pros send a current to the foot pedal and is intelligent to read the current coming back as to the selection of the pedal. Looking at the schematics for the pedal, I assume that using different resistors to limit the current for the reverb and OD channel gives the various combinations. As my amp seems to be "unable to decide" what channel to use, I am assuming that this is the part of the board that has been screwed up.

STDog

Those diodes, and the nearby capacitors and the resistors, look like part of a power supply circuit.
If the previous problem was in the PS, the diodes my have overheated and caused the traces to laminate for the board.

If so, he was trying to fix that, though not in the best manner.

Be careful when you remove the solder, and see if you can run wires from the good parts of the traces to the component leads.

That said, I think the connection your worried bout is correct, based on what I see from the top side.
A schematic or unmolested board would be the best way to tell.


I think your problem lies elsewhere, but without a schematic it's hard to say.

J M Fahey

So far I agree, we're talking about 2 different problems here.
The "big" one, at least physically, is that of the power supply.
Seems a diode died, the replacement was sloppy.
The channel switching one is elsewhere.
Just trusting my memory, those Fender guys love switching with CD4053, I'll dig my library.
If you can't find the RocPro700 schematic, search for the Performer650, it's practically the same.
A classical problem in all of that Fender line, is that they use boards with zillions of hair-thin tracks, which easily crack just where they join the solder pads, and are a pain in  the ___ to find.
In your second picture I think I see a thin track that´s cut or at least nicked, please check that, it´s on the lower right, the fourth diagonal thin track.
You´ll have to go all over the board, checking for pad-to-pad continuity when they are joined by a thin track. A good light and a jeweller's loupe are invaluable; a pair of cheap reading glasses, +4 power, also help a lot. You can soften flux deposits with an alcohol-wetted Q-Tip, scratching them must be avoided if possible or done very carefully to avoid damaging thin tracks.

teemuk

The schematics can be found from Fender's website under Support / Amplifier Schematics section.

STDog

The schematic and component diagram are helpful.

I also see several test points to help debug.

Form a quick glance, one does reverb work?
Then check TP30 and TP31 for the levels given on the schematic.
Since the LED is not working, I suspect them to be bad.

But maybe TP30 is good, and the problem is U8 or related components like CR39 & CR40, or R122 & R123.


Q7 (controls the LED) and appears to be conduction, but Q1 & Q3 appear off given the clean channel works.


monkeycabbage

ST Dog, you're right, the power supply goes straight through to that part of the board. I managed to strip the solder off and there definitely wasn't a path there before the "Boston Solderer Massacist" started his plight.

I cleaned it up to the degree of keeping the point and adjacent track seperate. I still need to chek that there is a circuit running through each of the "modified" tracks but apparently I was interupting the Mrs' TV. I'm not sure if the reverb works and I have to be honest.... after being shocked several times by the amp.... i'm not overly eager to plug something in to try it.

JM Fahey, I'm not sure that he has replaced a diode. The rest of the soldering on the whole board (except for the volume pot) seems to look very neet and untouched. Well spotted btw on the track in the pic though I'm afraid it is purely a very small drop of flux that didn't allow the light to reflect. I've checked the rest of the board and I'm pretty sure that there aren't any others.

Please forgive me for sounding "slightly simple" here (I've read some of your other posts!!) I am by nature a sound tech. I re-wire plugs, jack sockets, refit the odd speaker in a monitor here or there when a guitarist drops his pint on stage and generally sound check/mix all night long. The bigger in depth electronics is new to me and I'm learning as I go.

When you say:
Quote from: STDog on August 31, 2009, 01:06:05 PM
Then check TP30 and TP31 for the levels given on the schematic.
Since the LED is not working, I suspect them to be bad.

But maybe TP30 is good, and the problem is U8 or related components like CR39 & CR40, or R122 & R123.
Do you mean a continuity test to see that the circuit does continue through them or do you mean a voltage test to see what is going through them? Any extra tips on testing would be very helpfull and seriously appreciated.

Many thanks, Ronnie

STDog

Quote from: monkeycabbage on August 31, 2009, 07:49:57 PM
When you say:
Quote from: STDog on August 31, 2009, 01:06:05 PM
Then check TP30 and TP31 for the levels given on the schematic.
Since the LED is not working, I suspect them to be bad.

But maybe TP30 is good, and the problem is U8 or related components like CR39 & CR40, or R122 & R123.
Do you mean a continuity test to see that the circuit does continue through them or do you mean a voltage test to see what is going through them? Any extra tips on testing would be very helpfull and seriously appreciated.

Measure the voltage checking for the values called out in the schematic (you did download the schematic right?).


You may well be in over your head with this.
It's all basic stuff, but if you don't know how transistors (BJTs and FETs) are biased, or how an opamp works you're in deep.

I highly recommended finding a tech, or taking an electronics course.

93greenstrat

The problems you are having seem to be fairly common with these amps (I have a Performer 1000).  I don't have much that I can offer in way of a soloution, but it has been suggested that the tube in the drive channel may be at least part of the problem.  The heat that it generates may cause solder connections to melt and result in cold solder joints.  I actually need to open mine up and see what I can figure out.

monkeycabbage

93greenstrat,

If I get to the bottom of it i'll let you know.
Are you having the same problem with the selection switch?

I've managed to identify that the guy that last looked at the amp did botch the paths as the circle I identified shouln't have been connected at that point.

I haven't had the chance last week to do anything/look further as I've been swamped with work but the Mrs is now nagging me to clear the dining room table so I better get back on it!

Brymus

OK I agree with JM 100%
I too see a break on the same trace he mentioned.
And also that the switching is a seperate issue from the "solder job"
The voltages you talked about with the pedal are used to switch channels.
The Voltage that "SHOCKED" you several times greatly concerns me >:(
DO you have a DMM?
Can you check that the large filter caps are bled(at 0 volts) before proceeding?
If you have a DMM you can check the traces for continuity AFTER you make sure you dont have voltage left in the circuit.
Then with the schematic you can repair the traces,and hope you dont have a bad IC that you need to de-solder to fix the channel issue.
PLEASE BE CAREFUL
Also from the pic of the top of the PCB if you look under the diodes it looks like the trace was indeed an X before he soldered it.Unless thats just the solder I see,but it looks like copper in that pic.
Not saying thats a good solder job,cause it does lack but he may of actually used the right path.
Get your schematic,read up on draining or discharging capacitors and some other safety tips.
Wait till the missus is shopping and have at it again.