Vox AC50 tech help!
John Stax
out of it gave me a clue..) but it all the fuses are intact... I've measured
the resistance of the choke, it's about 205 Ohms. I've measured this when
the choke was still in the circuit.
This is what I did:
I live in Europe and we have about 220-230V mains. The amps voltage selector
was set on 245V. I switched the voltage selector to 225V but the amp wouldnt
fire up, so normally I've played the amp on the 245V setting. I like the
sound of the amp, but I figured it could sound better if I could use the
correct 225V setting. I narrowed the problem down to the selector switch on
the 225V setting: it didnt make contact.
Since the 245V position worked OK, i thought I'd swap de 225V and 245V wires
on the switch. Did that, the amp lit up, but shortly thereafter made a
crackling sound and some smoke came out of it. I quickly shut it down and
opened it to spot the problem. I couldnt see any burnt components.
I've changed the voltage selector switch back to its orginal wiring. Then I
fired the amp up again and noticed after a few seconds the frying bacon
sound and smoke comming out of the choke.
Could this change have fried the choke? Before I put another choke in it,
should I check something else?
Thanks!
John Stax
fuses seem to be original (they have bulgin stickers on them) mains fuse =
3A, fuse inside the chassis is 1A.
jh
I suspect that the filter cap behind the choke (feeding the screengrids)
has shorted. This might have been an effect of the higher B+ voltages
due to the change of the tranny tap. (+10%) If the caps have never been
changed, this would be a realistic scenario. If you check the resistance
to ground and it is in the ohms or kOhms range, then the cap is the
culprit. Even if it ohms out correctly, then it might be a short, that
occurs only at operational voltages. If the caps are original, then they
should be changed anyway.
ducking and covering ??any boomerangs flying in from down under??
regards
Jochen
John Stax schrieb:
to...@nospamprepair.com
where. The choke presents a 200 ohm (ish) resistance in line with the B+
supply and it has smoked - so it has already pointed out a few things for
you. Forget the primary voltage 245 vs 225 for a minute, your problem is
elsewhere. I would troubleshoot by elimination: A shorted cap could likely
be the cause as already metioned here as it is the first component on the
other side of your smoking choke. I think you should determine what kind of
shape the rest of the amp is in before you go replacing caps though - only
to find other things wrong. Experience tells me that you most likely have an
EL34 with a screen grid short in it. There should be some 100 ohm resistors
on a turret board feeding the EL34's pins 4, check those - if you find one
open or way off value - there is your bad tube. BTW I would replace them
w/1K 5 W types. I would tack in a 200 ohm(ish) 5 or 10W resistor where the
choke used to be and pull both EL34's and see if your resistor heats up. If
it does, than you probably do have a shorted cap, or a short somewhere
further down the line. If it doesn't than you have likely narrowed it down
to power tubes, since you eliminated them from the circuit and your high
current problem dis-appeared. The 200 ohm resistor would be a much easier &
cheaper aid to discovery than buying another choke and smoking it. Once you
have the trouble fixed, replace the test resistor with a new choke.
A smarter method to go about this if you can:
Rig a light bulb in series with one side of the mains feeding this amp. You
could then more easily root out the problem by eliminating sections of the
circuit while evaluating the brightness of this bulb. A really bright bulb
would indicate that the short is still there. A dimly lit bulb would be
normal and indicate that the amp is pulling what it is supposed to. If you
had a similar healthy amp to give you a quick "brightness of bulb" reference
that could be helpful. You are working with 220 mains so I would recommend a
120W bulb. This would limit your mains current to about 1/2 Ampere and
enable you to work on the amp without going through a box of fuses or
burning up anything else.
In any case, all of this assumes that you are reasonably qualified and aware
of the dangers of electrocution while working on this kind of equipment so
BE CAREFUL! In case you don't know you must dis- charge all of the filter
caps even after power has been removed as they will hold high voltage and
could burn a hole in your finger if you touch them! Good luck
"John Stax" <as...@nospam.com> wrote in message
news:b3q76r$21$1...@reader08.wxs.nl...