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.