Vox AD120VT

[N_Cook]

Where's a hagiogolotre when you need one.
Do you really need to take the pa apart just to test the valves. Is there a
Jim Crow for this job, to avoid dismantling? plastic screwdriver to wedge
between valve and base ? long handled plastic pipe wrench to grab the
envelope, or some sort of suction cup to pull them out ?

In case its a safety issue. Owner does not know how it happened but the
plastic coating to the metal chassis around the IEC inlet is rucked from
partial melting? well it stretched back, as good as new, with low heat from
a hot air gun. Hopefully caused by such or an open fire. Unless the amp is
used with the speakers pointing to the floor then no convected heat from the
transformer, no sign of overheating of the Tx, and no sign of arcing or over
temp on leads,tags , IEC fuse , drawer etc- any known weird stock fault
leading to such internal localised heating.
No sign of any discolouring / pitting of the pins of the IEC , if it was a
bad IEC line plug arcing.

 

[N_Cook]

No, just undo the 20 or so screws needed to remove the amplifier chassis and
reverb tank from the cabinet and plonk the whole lot on the bench.

Done it quite a few times now, probably takes me 3 minutes max.
Certainly a lot less time than faffing around wedging screwdrivers and bits
of piping and then spending half an hour trying to get the valves back in
their sockets blind.

Time you bit the bullet, I reckon, and got out that powered screwdriver.


As for the melted IEC inlet, no doubt it was too close to the fire.



Cheers,


Gareth.

I had to take the other board apart anyway to replace a pot, but wondered
about more routine checking of valves. Replacement is not blind as there is
an access gap and maybe finger tips, in there , to guide into place.
With a radiant fire I expected some sort of discolouration of the Rexine,
presumably nearer to heat source. Low heat of my heat gun says 320 degree C,
whether higher or lower from numerous heater element joinings over the years
, with NiCad stainless steel "crimps", anyone's guess.

 

[N_Cook]

As far as I can tell (present owner not from new) I'm the first to go in
there from new.
TOP3 are 2x C5200 but one 2SA1943 and one KTA1943

 

[N_Cook]

Ah, this is the Valvetronix thing, I was talking about the re-issue AC30's.


Good luck.



Gareth.

Those are straightforward to work on.

Whats the chance of busting off a SMD device putting all those boards back
in place, like origami with spaghetti combined with that business of tapping
your head with one hand while the other hand moving round stomach, getting
all that stuff lined up again without breaking a valve top pip.
Just as well I have a little ratchet handle to take otherwise standard
crosshead hex driver bit for those 2 bright machine screws under the 7
segment LED section. Stubby screwdriver is not stubby enough.
Back in place sufficient to test properly, that for tomorrow, quite enough
cussing for today.

 

[N_Cook]

I've just downloaded the manual as although both PAs are live , only one is
getting a feed , something wrong with DG211 or the rear power selector or
something.
Manual does not include how to dismantle or mantle.
One tip is to cover the valves with some dense foam a bit longer than the
valve tip , supported by the too short standoffs , to protect the valves
when turning all this metalwork over for probing etc

 

[N_Cook]

Anyone happen to know of a designed-in function that would switch out 1
amplifier of the 2?
Analogue signal of right levels is getting to the inputs of both amps, which
both agree quiescent DC fashion but some control line is erroneous, I'm
assuming.
Anyone happen to know what switches the A/AB class function , via the 2x
DG211 analogue switches , I assume the user select, amp type, 16 way encoder
switch, but which for A and which for AB ? but either way seems to control
both amps simultaneously, not isolating one amp.
I thought it would be the 4 way power switch on the rear but that is
switched attenuators only to both amps, no control function. The phones
change over function via the MAX333 an. sw. should divert both amps by the
single ground switch in the phones socket .

 

[N_Cook]

Looking at my database, one of these had a mainboard failure (i.e. digital
stuff), uneconomic to repair, One I changed the MAX333, a zener and standard
diode, another was dirty switching contacts (power selector, headphones,
speaker sockets).

I would change the MAX if only to eliminate it. Could be an achilles heel.
I remember years ago I usd to regularly find blown analogue switches - a bit
like the old RAM chips in that they were obviously more prone to failure
than most other chips for some reason.




Gareth.

I think I now have a way to prop up a connected live PA board , to probe
stuff DC and AC, without it falling over or shorting anything. A couple of
plastic "ring " binder sleeves over lower front edge of the chassis so will
take the half rotated back edge of the pcb and the top of the heatsink,
safely over the top panel. Along with the foam protected valves

 

[N_Cook]

Looking at my database, one of these had a mainboard failure (i.e. digital
stuff), uneconomic to repair, One I changed the MAX333, a zener and standard
diode, another was dirty switching contacts (power selector, headphones,
speaker sockets).

I would change the MAX if only to eliminate it. Could be an achilles heel.
I remember years ago I usd to regularly find blown analogue switches - a bit
like the old RAM chips in that they were obviously more prone to failure
than most other chips for some reason.




Gareth.

Looks related to An Sw failures, FET failures
No signal on the grids of V1 but signal on Con2 with only a couple of
long-tail FETs in between.
Like a stereo amp comparing channels, signals on V2 grids and con1. DC
around the 2 An Sw are the same, but 0,0.4,220V dc sort of values on the bad
FETs and about 60,60,140V on the good. How long do you scratch your head
before you realise that the schematic is wrong. How could there be 0 V on
one end of a trace at an an sw and 60V on the other at a FET gate of the
working amp?
Supplies to these FET are from the valve anode rail
2002 version
http://www.valvetronix.net/docs/AD120VT_Service_Manual_Complete.pdf
about 1/3 down the pdf, left side
Now , with hindsight , it is bleeding obvious , a shorting connection
directly across both C51 and C48 is shown in the diagram but of course in
reality no such short.
Now to find some 240V,200mA rated FETs , dosn't seem right 240 V on a TO92
package

 

[N_Cook]

Much more mysterious failure mode , that yellow goo again?
Removed both FETs and they cold tested ok , changing conductance state, via
DVM "diode" test, so probably ok . There is that yellow goo in this area and
one resistor covered with it is 120K 1/3W dropper from the anode rail, so
difficult to check in situ as between 2 electros, sets the bias. Something
chemically has gone wrong with it. Small patch of body coating shows
perfectly normal cream and part of a red and brown band, again no thermal
discolouration. Under the R the board is perfectly normal green colour. But
the r is o/c , now replaced with 2x 240K 2/3W raised off the board , JIC a
heat problem but I doubt it. So as well as encouraging
deliquescence/hygroscopic or whatever the word is for taking moisture out of
the air and encouraging corrossion it looks as though high voltage has some
deliterious effect with the dreaded yellow peril.

 

[N_Cook]

I didn't think to mark the "amp type" knob before removing .
360 degree hex encoder , splined shaft, have I got this right

With my mark on one end of the shaft slot at 6 o'clock , (so could be 12
o/clock ) then A/AB control V at that contact is 5V. From the circuit 5V
probably relates to lack of negative FB so "class A",
Then going clockwise 000005555550050
Anunciators ,
at 6 o'clock , Boutique Cl then xlockwise
black 2x12
tweed 1x12
tweed 4x10
ac 15
15TB
ac30
30 TB
UK blues
UK 70s
uk 80s
uk 90s
uk modern
recto
US higain
Boutique OD

I've placed the knob so the block of 6x 5V cover Vox AC15 to UK 70s , right
or wrong?