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One of the newer Vox AC30-6/TB (reissue) amps came in with 2 others. All the info I had from our Guitar Dept was 'not happy'....tubes? And, it having a higher asset number than the others, the Tolex covering hadn't been totally trashed, though I found I still had to spend a good half-hour with a staple gun and tack hammer patching up the 9 torn flaps of skin all over the amp. I often have to wonder if these go out with a sign on them asking "PLEASE TRASH ME". I know we have road cases for these, though I sometimes wonder if they bother to use them.
Anyway, I pulled the chassis out, moved it over to the test bench to sit it onto my lab jacks so I could begin with the power tubes. I had last looked at it in June of last year. Power tubes were still ok, though I got a better current balance by swapping one pair with each other. I found one of the vibrato tubes making a racket when tapping on it, and pulled it, cycling it in and out of it's tube socket and tried again, finding that seemed to cure it.
Had to exercise the volume pots to work out the gritty character, and now behaved without noise. I didn't find any issues until I moved it back into the cabinet and powered back up. Then the amp went into motorboating sounds with the three volume controls down.
Now, I had also removed the 3-jack PCB (high sensitivity jacks) to remove the fiber washers on them, as there wasn't enough exposed threaded bushing to properly tighten the plastic mtg nuts. To get the same washers off the other three jacks, I had to remove the knobs, control mtg nuts, jack nuts, then remove the six preamp tubes and release the PCB from those awful snap-lock standoffs enough to pry off the fiber washers from the other three jacks. Put all that back together, now having sufficient jack bushing thread to tighten the plastic nuts, and had found it was working ok on the bench outside the cabinet.
I stopped to ponder if I had put all the preamp tubes back in the order they were in, since I had rotated the chassis while extracting the PCB. No, I hadn't, so I swapped the tubes around and tried again. Stopped motorboating. Plugged signal back in, cranked it up, then it began motorboating again. It slowly subsided, and stopped. With signal applied, cranking it up, it was now adding that same rate of modulation on both the Brilliant and Normal Channels. WTF???!
So, pulled the chassis back out and went to fetch the schematic. Having too many versions of AC30 schematics on my folder, I wasn't finding the one that agreed with the PCB designation numbers, but finally found it....two pages, and it almost matched the PCB board number. My board in this case was AC30-60-00, while the drawings I have that DO match the PCB component numbers are AC30-60-02.
At this point, with the amp still motorboating, I went to the chopsticks and prodded components until I came to C36 power supply filter, a 33uF/350V cap that decouples the supply to the Brilliant Stage tubes V7 & V8. That cap could be moved, and doing so I could get the motorboating to start and stop.
Pulled the main PCB back up, found the (+) lead of that radial electrolytic cap had broken it's solder pad from the trace that led to R55 (plate resistor V7 pin 1) and R56 (decoupling resistor from the Choke/C46 supply cap). Replaced the cap with a 33uF/450V cap, and shaped the long (+) lead to fit over the solder pads of R55/R56, now tightly fitting onto the PCB.
I also noticed the power tubes didn't have their grid resistors at the tube sockets, so I added those, then jumpered over the other grid resistors on the PCB. That cured the motorboating.
Looking further at the schematics, I found connections numbers of sheet 1 didn't match those of sheet 2. Nor were there any pin numbers on all of the tubes, so I took the time to add that info, so I can pull these drawing into Adobe Illustrator or Adobe Acrobat and make the additions, as well as correct the numbers between the two pages. Those should be:
Sheet 1 To A1 Page 2 should be G6 (DC Power from C46)
Sheet 1 To A2 Page 2 should be F6 (input to Driver tube V6 pin 7)
Sheet 1 To A3 Page 2 should be E6 (Ground Buss)
Sheet 1 To A4 Page 2 should be D6 (input to Driver tube V6 pin 2)
Sheet 1 To A5 Page 2 should be C6 (Ground Buss)
I'll post the updated schematics when I get to that task.
AC3093Pre-Reissue.pdf
AC3093PA-Reissue.pdf
Anyway, I pulled the chassis out, moved it over to the test bench to sit it onto my lab jacks so I could begin with the power tubes. I had last looked at it in June of last year. Power tubes were still ok, though I got a better current balance by swapping one pair with each other. I found one of the vibrato tubes making a racket when tapping on it, and pulled it, cycling it in and out of it's tube socket and tried again, finding that seemed to cure it.
Had to exercise the volume pots to work out the gritty character, and now behaved without noise. I didn't find any issues until I moved it back into the cabinet and powered back up. Then the amp went into motorboating sounds with the three volume controls down.
Now, I had also removed the 3-jack PCB (high sensitivity jacks) to remove the fiber washers on them, as there wasn't enough exposed threaded bushing to properly tighten the plastic mtg nuts. To get the same washers off the other three jacks, I had to remove the knobs, control mtg nuts, jack nuts, then remove the six preamp tubes and release the PCB from those awful snap-lock standoffs enough to pry off the fiber washers from the other three jacks. Put all that back together, now having sufficient jack bushing thread to tighten the plastic nuts, and had found it was working ok on the bench outside the cabinet.
I stopped to ponder if I had put all the preamp tubes back in the order they were in, since I had rotated the chassis while extracting the PCB. No, I hadn't, so I swapped the tubes around and tried again. Stopped motorboating. Plugged signal back in, cranked it up, then it began motorboating again. It slowly subsided, and stopped. With signal applied, cranking it up, it was now adding that same rate of modulation on both the Brilliant and Normal Channels. WTF???!
So, pulled the chassis back out and went to fetch the schematic. Having too many versions of AC30 schematics on my folder, I wasn't finding the one that agreed with the PCB designation numbers, but finally found it....two pages, and it almost matched the PCB board number. My board in this case was AC30-60-00, while the drawings I have that DO match the PCB component numbers are AC30-60-02.
At this point, with the amp still motorboating, I went to the chopsticks and prodded components until I came to C36 power supply filter, a 33uF/350V cap that decouples the supply to the Brilliant Stage tubes V7 & V8. That cap could be moved, and doing so I could get the motorboating to start and stop.
Pulled the main PCB back up, found the (+) lead of that radial electrolytic cap had broken it's solder pad from the trace that led to R55 (plate resistor V7 pin 1) and R56 (decoupling resistor from the Choke/C46 supply cap). Replaced the cap with a 33uF/450V cap, and shaped the long (+) lead to fit over the solder pads of R55/R56, now tightly fitting onto the PCB.
I also noticed the power tubes didn't have their grid resistors at the tube sockets, so I added those, then jumpered over the other grid resistors on the PCB. That cured the motorboating.
Looking further at the schematics, I found connections numbers of sheet 1 didn't match those of sheet 2. Nor were there any pin numbers on all of the tubes, so I took the time to add that info, so I can pull these drawing into Adobe Illustrator or Adobe Acrobat and make the additions, as well as correct the numbers between the two pages. Those should be:
Sheet 1 To A1 Page 2 should be G6 (DC Power from C46)
Sheet 1 To A2 Page 2 should be F6 (input to Driver tube V6 pin 7)
Sheet 1 To A3 Page 2 should be E6 (Ground Buss)
Sheet 1 To A4 Page 2 should be D6 (input to Driver tube V6 pin 2)
Sheet 1 To A5 Page 2 should be C6 (Ground Buss)
I'll post the updated schematics when I get to that task.
AC3093Pre-Reissue.pdf
AC3093PA-Reissue.pdf
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