VOX AC100cp

[Phil Allison]

** Hi,

I have a VOX AC100cp ( Chines made) on the bench - it is about 6 years old.
It has all its original parts but is in a very bad way. The known faults
are:

1. One (Ruby) EL34 has lost vacuum.

2. Two scorched 5W screen resistors, one detached from the PCB.

3. One blown HT fuse, a 1 amp slow blow - which may well not be the only
one.

4. Output tranny OPEN on one side of the primary and exhibiting symptoms of
shorted turns.

5. Bridge rectifier only conducting on one half cycle - giving 50Hz
ripple.

The scorched 5W resistors are on the side with the open tranny winding.

The EL34 with no vacuum is on the other side.

Wanna have a go at the sequence of failures ?



.... Phil

 

[Phil Allison]

"Bitrex"

It looks like there are a few bridge rectifiers in the circuit, one for
the solid state preamp stuff, one for the output tube bias, and one for
the preamp heaters. Which one failed?

** Never said any bridges had failed.

There is 50Hz ripple on the HT, of course.

Since my post I discovered that R107 ( 22 ohms 7W) is open - this is is
series with the high voltage secondary.

All other supplies are OK.



.... Phil

 

[Phil Allison]

"Gareth Magennis"

Huge DC through the OPT primary partly via the scorched grid resistors on
the other side, and a nasty expensive smell.

** The HT fuse prevents huge DC through the output transformer primary.

I suggest you read the list of failures again.

The thing is to work out what part failed FIRST and how that led to each of
the others.




.... Phil

 

[Phil Allison]

"Bitrex"

http://www.ampix.org/albums/userpics/10003/Vox_AC100CPH.pdf

** Where did you find that link ?

Recent model VOX schems and service manuals are hard to find.


.... Phil

 

[Phil Allison]

"Gareth Magennis"

"Phil Allison"


I don't really know enough to achieve that, Phil.

** You had better sit and watch this one then.

I would guess again though that it started with the duff valve,

** Hint - how did one half of the OT primary go open ?



... Phil

 

[Phil Allison]

"Gareth Magennis"

Coca-cola spilt through the grille?!

** Not tricks here - I have detailed all the faults and there is no sign
of any such damage.




.... Phil

 

[Phil Allison]

"Gareth Magennis"

Well, one last try before I give up then.


The "lost vacuum" tube probably just went out of circuit, leaving one
valve vs. two.

** When a power tube loses vacuum, it usually conducts very heavily
regardless of grid bias setting. The *low pressure* gas inside ionises with
applied DC and it is lights up like a neon tube.

I think I'm right in saying that this will produce a net DC field in the
OPT primary that would push it towards saturation and overheating.

** This is a large transformer and it would normally have almost no temp
rise in operation - the primary resistance is 20 ohms per side. Even under
fault conditions, the 1 amp HT fuse protects it from damage.

So, the "two valve" primary winding got very hot, shorted some turns, then
went open circuit.

** With any large OT - this is a very unlikely scenario.

This meant that its Screen Grids became the Anodes, which got overloaded
until the resistors burnt or fell off the PCB.

** This is correct.

Half of the Bridge rectifier may well have burnt out

** Read the thread again. There are no faulty bridges.

(Or the customer could have replaced the blown HT fuse with a higher rated
one, then put the original back in before handing it in for repair).

** I figure more than one HT fuse has blown, but the one in the holder
looked like the original.

There might be other problems due to disconnection of speaker, or arcing
inside the OPT

** Now you are getting warm...

connecting secondary and primary

** No way is that the case here.



.... Phil

 

[Phil Allison]

"Gareth Magennis"

Well, are you going to tell us then?

** The OT failed first due to internal arcing in the primary winding. This
is induced by overdriving the output stage until "spiking" occurs - ie
spike voltages on each rising edge of the squared off wave. Spiking only
occurs with loudspeaker loads ( or simulations ) and is produced by the
energy stored in the inductance of the voice coils. The voltage spikes are
of the order of +/-4kV. The problem is mainly with EL34 valves and
Marshall type output stages.

Arcing damage will produce shorted turns and sometimes open windings - both
happened here.

Once half the primary went open, the two EL34 screen resistors on that side
began to severely overheat, the screens would have been glowing white.

The two EL34s on the other side began operating under great stress as the
tranny now had shorted turns and a much lower impedance than normal.
Dissipation goes well beyond the max 25 watts, the plates glow red and one
EL34 quickly became gassy and out of control of the grid.

Then the 1 amp fuse blew, another maybe larger fuse was tried and that
resulted in the 22 ohm resistor failing.


.... Phil