Fender Cyber Twin Problem

[patlaw]

A fellow has asked me to take a look at a Fender Cyber Twin with the following problem description: "The negative is there are certain effects and amp models that sound like crap. Hisses, buzzes, etc. Clearly dysfunctional signals. I've mapped out most of the channels that don't work." Is the digital portion of the circuit board surface mount or through hole? If you have worked on one of these or one similar, what would you investigate first? It appears that Fender had problems with encoders, which are apparently no longer available from them. I'm trying to decide whether to take a look at it. My first inclination is no.

[Enzo]

DEpends on how you run your shop. I have a few exceptions (Marshall Mode 4, Fender Rhodes pianos) but I will LOOK at most anything. What if the only real issue is a 15v rail is failing?

And the thing is a computer. CERTAIN patches and effects sound bad? SO others sound fine? Perhaps those patches are corrupted, and a reset factory patches might be in order. None of that requires soldering surface mount.. Yes there is SM in there, but you may or may not encounter it.

Looking at it doesn't require you to accept the repair. If it were me, I'd look it over and THEN decide if I would repair it. I have repaired my share of them, mostly under warranty. Customers are notoriously poor judges of what is wrong. SO find out what is going on. I had a synth customer once who complained that all his patches sounded wrong, and he proceeded to reprogram each and every one. It still wasn't right. I held down the three buttons for reset and power off and on, et voila, all patches restored.

Encoders? I don't recall these being any less reliable than the encoders other companies use. If an encoder lets you call up a patch and the patch sounds bad, it isn't the encoder's fault, it did its job it got you there.

And he mapped out the bad ones? There are a couple 12AX7s in there, what if we find all the bad patches are ones that use the tubes? No soldering to change a tube. Bottom line is to find out what we are fighting. There is no shame is stating that this isn't something you feel confident to repair, on the other hand it is embarassing when he takes it elsewhere and reports back it was just a 12AX7, or a reinitialization.

[DrGonz78]

I had worked on one about 7 years ago. I don’t remember the symptoms but it was not working well. Opened it up and I immediately could tell there was a 12ax7 that had been shattered inside. Funny thing is that all the tubes were fine. So a shattered tube and they replaced the tube but left all the glass bits in the amp. Cleaned out the amp and it was working great! Thank god too since I was very new to working on amps and this thing way overly complexed. What with all the motorized pots when selecting different patch settings. I would look at the amp if one came to me but I would not give high hopes to fix it.

[tim]

Hey all...

It's been a while...I sold my home in NH and was in limbo for several years. My wife and I finally ended up in Southern Missouri last fall. I have set up a skeleton shop, enough to get my feet back on the ground. Wouldn't you know it...the first amp I get to work on out here is a Cyber Twin...

I think in the 40 years of working on amps, I've only dealt with one, maybe two of these beasts.

This one has a weird problem (don't they all?). This one, as best I can figure out has power out on certain presets but not others. I did the factory default reset but no change.

There seem to be a bunch of J113 fets in the preamp circuit. Are these used to switch gain stages in and out depending on what presets are being used? Both 12ax7s are good.

Any ideas?

[Enzo]

Probably. See if their gates get control voltages and whether the paths they control open and close. Pick one thing that goes missing and solve it. Ignore all else until layer. What you find for this may wind up being the issue for the other stuff as well.

DOubt it will make you feel batter, but if you think these things are whack, wait until you see a Fender Princeton Recording amp

[tim]

Probably. See if their gates get control voltages and whether the paths they control open and close. Pick one thing that goes missing and solve it. Ignore all else until layer. What you find for this may wind up being the issue for the other stuff as well.

DOubt it will make you feel batter, but if you think these things are whack, wait until you see a Fender Princeton Recording amp

I'll check those tomorrow. I believe the gate control voltages originate at the DSP IC's. I need to try and follow them back to the preamp.

[Enzo]

I'd just see if they are present at the JFETs

[tim]

I'd just see if they are present at the JFETs

OK Enzo, I had some time to figure out the paths of the sigs through the switching FETs and found that everything was going to where it was supposed to be going so the FETS are fine, as near as I can tell, but I found I was losing some signals through one of the 12AX7 tubes. I found an open 100K resistor that went to the plate of the tube. Now I need to dig through all my parts bins, still in storage boxes tomorrow. Pretty sure that will fix the problem. Thanks for your help.

It's been several years since I was elbows deep in amps so it feels good to be able to still figure this stuff out. One of my weaknesses is SS amps; always has been...I prefer tube amps. I know friends of mine that are amp techs and they would much rather work on SS amps, The high voltage in tube amps petrifies them .

[Enzo]

In my long career I have had to specialize in: tubes, RF, broadcasting, relay logic and control, digital logic (TTL, CMOS), electromechanical, finally transistors and op amps. (In essentially that order) And you know what? Troubleshooting is troubleshooting, I don't care what the technology is behind it. Tube? Op amp? just details. Look at the Peavey TransTube circuits. They essentially just built tube circuits with darlington transistors.

[tim]

In my long career I have had to specialize in: tubes, RF, broadcasting, relay logic and control, digital logic (TTL, CMOS), electromechanical, finally transistors and op amps. (In essentially that order) And you know what? Troubleshooting is troubleshooting, I don't care what the technology is behind it. Tube? Op amp? just details. Look at the Peavey TransTube circuits. They essentially just built tube circuits with darlington transistors.

I've spent more than my fair share of time troubleshooting transtube amps. They certainly are interesting but like just about everything else, if you are able to break it down into different circuits and figure out how they each work and interact with each other, it becomes much easier. One circuit that I once really struggled with was understanding a "long tailed pair"...I just couldn't make it work in my head until I once asked an electrical engineer where I was working about that circuit...he went through it with me and within minutes. it was like light dawned on marble head. Sometimes we make things too hard.

[Enzo]

One thing that has learned me more electronics than most anything else was teaching it. I really teach troubleshooting rather than electronics per se, but include the necessary electronics in th process. SO when I have to explain how something works, I often learn about it prepping up. And many times I discovered some notion I had held a long time was actually incorrect.

I think everyone should know how an op amp works. But you don't need to know HOW it works to determine that it DOESN'T work. Sure it helps,