cyber twin no sound

[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