cyber twin no sound

[methodofcontrol]

i've got a cyber twin here that has no output. it's been a while since i've posted about it because i was waiting for parts. i ordered some new IC's as a sort of shotgun aproach to the repair. but only 3 of them arrived, and they are the least likely culprits. (figures)
anyway...for those of you who remember my story, skip this paragraph. for the others: got this amp in december to fix for a friend. he had it at a bar show and it was stolen, when it returned a few days later (apparently the theft was a drunken prank) it made some really terrible noises and died. "squeally noises" as they were described to me. i open the amp, find two fuses blown on main board...i replaced them and a faulty voltage regulator (7915, i believe). now i can turn the amp on, and everything appears to work, except i have no output sound.
i can change all of the parameters in the computer, i can use the onboard tuner, the input signal lights come on and interact w/ the "trim" pot accordingly. i'm just not sure where to go from here other than start replacing IC's. i'm a novice tech, but i know how to solder and i can follow directions. i'm looking for someone experienced and patient to guide me through some trouble shooting. thanks

[Enzo]

This is a very complex amp, and not just a tube circuit, there is DSP and such going on. Can you find documentation? It would be way larger than my dial up would want to handle.

[methodofcontrol]

i do have a schematic and layout. but it's divided among about 12 different pages, so it's difficult to sift through to find exactly what i'm looking for. which brings me to the next part....what am i looking for?

[drewl]

Start with the basics.
Does it have a pre amp out/power amp in?
see if either the preamp or power amp is working.
Then check power supplies.

[Enzo]

Sigh, OK look at drawing 55766 page 1.

If you plug into the input, the LEDs dance to the music? Or they are just ON? If they work, congratulations - the first stage works. That also means +/-15VDC is on.

Is there B+ on pins 1 and 6 of both tubes? DO they both light?

Do the headphones work?

Shunt the effects loop with a cord - any help?

Touch the right end of R301, 401 with a screwdriver (this is the input to the power amp). Does it make any hum out the speaker? Or you could inject a real signal here too for that matter.

Follow the signal through the amp as well as you can. What parts function?

[methodofcontrol]

in regards to "does it have a preamp out/poweramp in?":
it's got a line-out. but it's an XLR jack. pin1 is ground, pin 2 is +, pin 3 is -. i have a XLR to 1/4" adaptor, but the 1/4" side is mono. so that would make (-) and ground the same thing. not sure if that will work. if it will...jsut give me the word and i'll try plugging it into a power amp. would i be able to plug it into the the input of a regular amp? (ie:guitar ->cyber twin input -> CT line out-> other amp input. )
enzo...you mentioned drawing 55766, page 1. i don't have anything like that as far as i can tell. my schematic's page 1 is drawing 0064397000, revision A.
the input LED's dance based on how hard you strum the strings. although it seems like the input level knob (labled "trim") needs to be up to about 5 or more for the lights to start doing anything. i seem to recall it being more sensitive the last time i tried it. hopefully, i'm wrong and nothing else is breaking.
the B+ on pins 1 and 6 of the pre tubes are all between 356-360vdc.
the filaments glow just fine.
i don't get any sound out of the headphone jack.
shunting the effects loop (right or left channel) doesn't have any effect.
touching R301 or 401 with a screwdriver on the side nearest the front of the amp doesn't do anything. touching the side nearest the rear of the amp effects the hum from the speakers. i injected my guitar into this point of the circuit w/ a probe i made, and the guitar sound came out of the speakers perfectly. (the probe is just a 1/4" end for the guitar and the other end hacked off. the tip wire is attached to a .01 uf cap and the ring wire is attached to an aligator clip for grounding. the end that enters the amp is attached to a chopstick and insulated. i use the other lead off the .01 cap as the probe at the end of the chopstick.)

i assume this means the power amp is ok? thanks again for all of the help.

[Enzo]

MAking sound out the speakers verified the power amp worked, yes.

Those 300+ volts on the tubes sounded a little high, it that with the tubesd removed? Well a better question is what is on pins 3 and 8 of each. Those are the cathodes and ought to have a volt or two, as opposed to zero.

[methodofcontrol]

the B+ measurement was taken w/ the tubes removed, yes.
i measured the cathodes at pins 3 and 8 w/ tubes removed and all around was about 15mvdc.
i checked the other pins while i was there, and i found that V2 has +15vdc on pin 5, and V1 has -15vdc on pin 5. does that mean anything?

[Enzo]

Yes, it means the heaters are running on DC.


The voltage that appears on the cathode of a triode is the voltage dropped across the cathode resistor from the current flowing through it. Without a tube, there is no current flow, so the cathode pins of the sockets drop to zero - 15mv counts as zero - and the plate pins max to the B+ voltage. The normal plate voltage is likewise less than B+ because of the voltage drop across the plate load resistor from that same current. Stuff the tubes in there and see what you get.

[methodofcontrol]

i measured tube voltages again w/ tubes in place. it seems that the second triode of V1 is seeing some excessive voltage.

V1: pin 1- 206vdc
pin 6- 304vdc
pin 3-1.5vdc
pin 8- 95vdc
V2: pin 1- 210vdc
pin 6- 228vdc
pin 3- 1.5vdc
pin 8- 1vdc

[Enzo]

No, the tube is right, that stage is a cathode follower. Well the easy stuff is OK.

This will be extremely difficult to fix over the phone, so to speak.

The basic computer might work,but that is no guarantee the DSP circuits are OK.

Put your thumb onto every socketed chip on the host board - that is the smaller board with all the fancier chips on it behind the right end of the panel. Press each chip firmly into its socket for good contact.

At this poiunt I would have a steady signal at the input and I would look for it everywhere I see signal path. Digital signal path looks nothing like the real world path, but we can signal trace/signal inject all through the analog parts.

[methodofcontrol]

the two socketed chips on the host board seem to be in tight.
i'll set up a looping pedal at the input and probe the sound into a monitoring amp various points and see if we can't narrow it down a little better. thanks for the help.

[methodofcontrol]

update:
i injected a signal into the power amp at R301 and R401, and sound came out through the speakers. from anywhere before the power amp, i wasn't able to find another spot where the signal came through the speakers. (other than the ribbon cable that goes to the DSP board)
so i reversed my thinking and used a signal tracer i had made for pedal trouble shooting at the input. a 1/4" cable carrying steady signal goes into the input and a probe carries the signal from various points back to the speaker inside the signal tracer. using this method i found that all of the 8 pin chips in the pre-amp
seem to be good. on sheet 1 you can see that U4 pin 7 goes into U8 pin 5,7,9, and 11. i'm getting signalat the inputs of U8, but i'm not getting any signal coming out of U8 at pins 1,2,13, and 14. same situation seems to be applying to the other 14 pin chips. U14, U20, and U23. no signal at the outputs. could this be my problem?
or is it a bad DSP board? (in which case....am i screwed?)
thanks again for the help.

[Enzo]

If if it a DSP problem, I'd say take it to an authorized repair center.

OK, the power amps work. We suspected they would. Your preamp works up to U4-7. You get no signal from U8 for a reason. Step back and look what U8 does. It drives the level meter LEDs. It is a voltage comparator, not an amplifier. The outputs snap on and off depending upon the input voltage being higher or lower than a reference.

The other three chips mentioned are also voltage comparators, and they drive "control lines" per the schematic.

U3-7 is as far as it goes. That "PRE_DIST_LIN" and the right channel version is in the center at the right of the compressor. You will see those signal leaving the board center left on the ribbon to the HOST board. From there it returns to the tube section as "PRE_DIST_L" from the same ribbon.

If you have signal at the tube grid, then the first part of the host board is working. If not, the host is asleep or dead.

The host board has the DACs for the two output channels, and one op amp buffer for each.

[methodofcontrol]

hey enzo,
i applied my signal to the input and traced out from pins 2 and 7 off the 12ax7's. at pin 2 of V1 i get a good signal, and at pin 7 i get a very strong signal. on V2 i get little to no signal on the grids.
this would imply the host board is doing ok? or did i miss something?
thanks again for the help.

[Enzo]

It says PART of the host board works at least. Looking at the print, V1 is actually the second tube. The second half of that tube is a cathode follower, so there should be no signal on pin 6, just lots of B+. Strong signal on the pin a cathode though. Unless the board is lettered in reverse...

The output from the tubes flows up to U7-1. ANything there? And look just prior to that op amp input, there is a mute JFET. SIgnal on the left side of that?

[methodofcontrol]

on V1:
no signal on pin 6
when you said "...Strong signal on the pin a cathode...", i figured you mean
pin 3 off V1? there is a signal there, but not as strong as pin 8 or pin 7.
U7-1 has strong signal.
there are two muting JFETs. looking into the amp w/ the FET labels facing me, no signal on the left legs of either. low signal on the right sides. none in the middle. if you meant "left" in regards to the schematic, then i have weaker signal on one side of R84 and R94, and strong signal on the other. (i can't see the traces on the board to know which side of the resistor is on the FET. i'm assuming the strong signal is before the resistor and the weak signal is between the resistor and the FET)

[Enzo]

No, when I said no signal on pin 6 but strong signal on cathode, I meant the cathode associated with the pin 6 plate, so that would be pin 8 cathode.

Left and right on the drawing, yes. I have no idea which way anything faces on the circuit board.

Your problem is that nothing is getting to the power amp. If there is a strong signal at U7-1, then it doesn't matter how it gets there, it is there. SOmewhere between that point and the power amp input we are losing it. SO we can move on past the tubes, they seem to work enough to get something out U7-1.

We have not mentioned the mute ccircuit really. Look on the power amp page. Center left, right above the transformer is the mute circuit, it controls Q302,402 in the PA circuits. There are DC voltages all over, so check them all.

ANd make sure to have something plugged into the input jack, because that jack turns on the mute when it is empty too.

Meanwhile, the input signal at the two PAs is L_out and R_out. These come from the host board. I mentioned before there is a DAC and one op amp stage U27. Prrior to the DAC it is digital.

Got signal at the MONO FX send? If so, then can you inject signal at the L and R return jacks? Keep something plugged into the input jack up front still. Or just look for signal at U5 on the I/O board.

U7-1 is essentially the preamp out. From there back to the I/O board for thr FX loop jakcs and circuitry, then from there back to the host board where the signal goes through the other DAC. From that DAC we get analog again on to the PAs.

[methodofcontrol]

at C97 (+) the schematic shows 31.1 vdc. i'm reading a little over 29 vdc. and at the far sides of Q302 and 402 the scematic shows 31.7 vdc. i'm reading 30.1 vdc for both.
i do have signal at the mono effects send. if i inject signal at the L and R returns, i get signal at U5-1, 2, and 3 when injecting from the R return, and white noise off of U5-5, 6, and 7.
i get signal at U5-1, 2, 3, 5, 6, and 7 injecting into the L return.
i had a dummy plug at the input of the amp for all of this.
as always, thanks for the help.

[Enzo]

DOn't stop there, keep going. The loop returns pit signal out of U5, OK, and from there up to the ribbon connector as L_IN and R_IN. That goes to the main board where it simply transfers to the host board where those signals flow through U19,23. Pin 1 of each is the last step before it enters the DAC.

Whatever DSP goes on goes on and then the signal exits the same DAC, U20, flows through U27 and on to the power amps.

We did establish that the power amps respond when something injects at their input? Touch a small screwdriver or something to R301,401 to inject some hum. If it comes out the speakers, the PAs are OK. Of if surface mount is too tight for that, hold one meter lead tip in your fingers, and touch the other probe tip to the injection point.

There is not much real estate between the loop returns and the power amps other than the DAC and a few buffers. You ought to be able to check the buffers before the DAC by looking for signal at their outputs. From the DAC, you can test the outgoing buffers by injecting signal into pins 2 and 3 by touching. ANd if it is the DAC, then either there is a bad DAC, or the DSP host is nuts.

[methodofcontrol]

signal from effects returns pass all the way to U19-1 and U23-1 on the host board, respectively.
the power amps respond to signal injection at R301 and R401, respectively.

after that, you lost me a little with this part, "You ought to be able to check the buffers before the DAC by looking for signal at their outputs. From the DAC, you can test the outgoing buffers by injecting signal into pins 2 and 3 by touching. ANd if it is the DAC, then either there is a bad DAC, or the DSP host is nuts.".

DSP = "Digital Signal Processing"?
DAC = "Digital ..............."?

i don't understand where the buffers are that you mentioned. but if i'm following you...then at this point we know for a fact the main board is fine and the I/O board is fine. and that the problem is probably on the host board. the host board is all surface mount componants...is it possible fender would send me a whole new HOST board? would that likely cure what ails?
thanks again.

[Steve Conner]

ADC= Analog-to-Digital Converter
(turns friendly analog signal into a confusing mess of 0s and 1s)

DSP= Digital Signal Processor
(A special variety of microprocessor chip that does things to the 0s and 1s to produce effects like EQ, distortion, chorus and reverb)

DAC= Digital-to-Analog Converter
(turns the resulting mess back into an analog signal)

[methodofcontrol]

thanks steve! i learned that at one point, but i never used the knowledge so it kind of got lost in my brain.

[Enzo]

There are two DACs in this thing, and both operate as both DAC and ADC. I just call them all DACs because I can say "dack" when speaking. "Adck"... not so much. But I admit that is wrong half the time. These are either DACs ir ADCs depending upon which way the signal is flowing. There are other chips that are only one or the other.

Buffers just means the op amp stages. Coming on to the host board, the signal discussed go though an op amp or two on their way into the DAC. And coming out of the DAC, the signal goes through an op amp or two on its way back to the real world.

Look at the schemo for the host board. Two large DAC chips and op amps to their right.

Fender won't send YOU a host board unless you are a dealer. As to if they have themin stock, what the order number would be and cost, I don't know without calling myself

[methodofcontrol]

ok, that all makes sense.
well...i injected signal at U20 on the host board, pins 2,3,4, and 5. i get nothing out the speakers. when i inject signal into the effects loop i can retrieve the signal at U20-2,3,4, or 5 just fine.
so the problem is between U20 and the power amps. is my next step to track down a host board or a dealer to order one for me? or are there more diagnostics to check out?

[Enzo]

Not really, the factory diagnostic procedure just says look for signal at so and so pins on the connectors, with predicatable troubleshooting analysis. If the host board has the problem it says swap the host board.

However, U20 outputs feed through U27, then off board to the power amp. Inject at the inputs of U27. SHould come out the speaker. If not inject at the OUTPUT of U27. THAT ought to come out the speaker, since it is from there prety much a straight connection to R??? input to the power amp.

I bet you could change U27 if it were bad.

[methodofcontrol]

ok...here's U27 on the host board:
pin 1- if i inject signal here, i get no sound from speakers. if inject signal at the effects loop and try to receive it here, i get nothing.
pin 2- if i inject signal here, it comes out the speakers. if i inject signal at the effects loop and try to receive it here i get nothing but loud hum at the speakers.
pin 3- same as pin 2.
pin 5- same as pin 2.
pin 6- when i inject signal here, it comes out the speakers but alongside a loud hum. the hum being louder than the signal. if i inject signal at the effects loop and try to receive it here i get nothing but loud hum at the speakers.
pin 7- same as pin 1.

also...does it make sense that when i inject signal at R301 and R401 (power amp)....one one side of that resistor i get no sound out of the speakers, and on the other i get perfect sound out of the speakers?
thanks again.

[Enzo]

Maybe. WHich end of the resistors? Determine which end goes to the connector and which to the amp circuit. I assume the right end of the resistor is the good sound end, and maybe the output impedance of the op amp kills at the other end. SO my injecting at pin 1 was a bad idea - and pin 7.

Meanwhile, injecting at the INPUT pins of U27 comes out the speakers? That means whatever comes out the DAC would also go to the speakers. So nothing is gettin out that DAC

Hey, look left of U27, sorta in the middle - a few parts - R47, R50 etc. That takes +5 and divides in half for +5VA/2 - two and a half volts. That voltage is used to bias the inputs of U19,23 and U25,26. Is that voltage present?

Otherwise it looks like the problem is in the digital domain. Is there signal at the outputs of U19,23? That is injecting at the loop returns. They feed the DAC. If they have out put, then the DAC is getting hte signal, and since the inoput of U27 is hooked to the output if the same DAC, it seems surrounded.

Look at upper half of left side of each DAC. Scope it. ANy digital activity there?

[methodofcontrol]

i can't tell which end of those power amp resistors is which by looking at the board because the traces are small and it's a two sided board. but i can only assume the right side of the R301 and R401 is the side that i hear out of the speakers. since, the left side goes to that op amp output like you said and i don't get sound from there either.
at the spot left of U27, "sort of in the middle"...that 2.5vdc is present and accounted for.
and i DO have signal at the outputs of U19 and U23.
so is this conclusive that the DAC (U20, U21) is the problem? it's one thing to replace an 8pin surface mount IC, it's another to try to replace a 20 pin one.
if i had the board out, i'd be willing to give it a shot though. is that chip proprietary to fender? or is there a place i can order those? mouser doesn't seem to have any. any suggestions?
also...why would the DAC go bad? is there something that would cause that? or does it just happen sometimes?

[methodofcontrol]

i don't know if this is helpful...but this page seems to have all the info one would ever want on one of those chips.
http://www.asahi-kasei.co.jp/akm/en/...28/ak4528.html

[Enzo]

I don't know if it is the DAC itself or not, all we know at this point is that the problem is in the digital section. The DAC is merely the gateway between digital and analog. Could be the DAC, could be something behind it.

It is a DAC/ADC, but they refer to it as a CODEC (coder/decoder) It is not proprietary, it is an industry part. Fender gives it a part number, but that is no guarantee they stock it anywhere but where the boards are made.

Fender 0056861005 AK4528 CODEC $26.65

At this point a host board swap sounds attractive.

[Steve Conner]

The host board is probably the most expensive and hardest to get part in the whole amp, so by Murphy's law, it probably is what went bad. The AK4528 is indeed an industry standard codec, I have a few pieces of digital gear that use it as well as other chips in the same family.

Anyway...

Does the digital display light and display what it's supposed to, and do the digital controls work for changing patches and such like? (the large encoder knob etc) Do the motorized knobs spin when you change patches? If so, the DSP is probably good, and all the digital stuff is probably getting power. Therefore, the problem is either in the codec chips, or the power supplies to them, or in the analog part of the host board. (Maybe a shorted out op-amp, which would explain the original fuse blowing. The analog parts of the codec chips never run directly off the +/- 15V, so a bad codec would not blow a fuse.)

If the above digital stuff doesn't work at all, or seems cranky or corrupted, you should suspect the DSP or other digital component. The most likely cause is a bad logic power supply, that either is not giving enough voltage to run everything reliably, or gave too much when you were messing with the fuses, causing one of the fancy many-legged chips to blow out. (Murphy's law says it blew the one with the most and smallest pins, or even a ball grid array package.)

If you have a scope and X10 probe, you can check to see if any quartz crystals you find on the host board are oscillating at the right frequencies. That's one of the first things I do when debugging stuff with microprocessors in it. The X10 is mandatory, the piece of coax with alligator clips on the end that many newbies use with their scope has a lot of capacitance, and can load the crystal down and stop it.

[methodofcontrol]

i don't own a scope. so digital trouble shooting might be an issue.
if replacing the whole host board will definately fix the issue, i feel that so long as i could get one....i'd be best off doing it that way.
all of the digital display stuff seems to work just fine. the motorized knobs do change w/ different settings.
i need a fender dealer to get me the host board...is that correct? any suggestions there? what's my best bet to score one of those?

[Enzo]

You might try contacting Fender to ask just what part number to ask for so your dealer can order it and to find out if they are indeed still available, and what MSRP is on them. ANy Fender dealer who is willing to order parts for you can order the part, assuming availability. It is vetter if you do the research so they don't misinterpret what you want and order the little display board or something. Fender may require the dud back for example. In warranty service, the little digital board swaps are the only parts they ever want back. Although I do have a dead host board here.