Vox Cambridge 30 V9310/9320 Tremolo Tech Tip

[Robert M. Braught]

As some of you techs know by now, the tremolo 'assembly' (opto-coupler
p/n LT9914) has a reputation for failure... causing distortion in clean
channel operation, poor/no tremolo... one on the frizz can make the amp
sound like the output IC's blown...

Word from Korg USA last week, no 'recall' in the near future, no
substitute part recommended, etc. From my field experience it appears
the LDR portion fails, possibly linked to voltage/current (? customers
have reported after 'blasting' signal through their amp, it 'quit'.)
List price on the 'original' Vox(Korg USA) part $24 :(

I've successfully tested a 'regular' Vactrol, ala Fender p/n 16282, OEM
# VTL5C3/2 as a suitable replacement. Use both LDR elements in series
(leave the center wire unconnected / clipped.) The circuit will require
'recalibration', twiddle ref VR11 and VR12 (on the main board) 'til
tremolo sounds right ('normal' full depth) and signal pass is good with
tremolo 'off'. No harm will come from the adjustments being dimed or
turned back full CCW. This sub is less than half the part cost of the
orig Vox part, and my experience with the Vactrol units (as in Fenders,
etc) is that they are a very high reliability part, with long life
expectancy. FWIW *this will not change your tone*. :)

Hoping this helps someone,
-Robert
QTS
http://www.Braught.com

[Gilbert Bates]

Nice to know Robert, thanks. I think there was a large batch of them that
were bad initially or were damaged during the manufacturing process. This is
because we had around a half dozen of them that were bad in amps at the
store and about half of them were amps right out of the box.

I did buy one through Smart Parts for around $18 and it took over a month to
get. I initially said 'give me a handful, they can't be that expensive'
until they called back when they found out the price.

"Robert M. Braught" <elec...@grapevine.net> wrote in message
news:3D32BBF8...@grapevine.net...

[Robert M. Braught]

Glad I could help- we've seen quite a few too, the current replacement
part from Korg USA looks the same, and I felt I should do something for
the 'long term' (instead of just neutering the trem, like one customer
alluded to wanting after his amp crapped twice... FWIW 'full on' with a
working original unit ~5K ohms, a 4.7K resistor sub in a pinch.)

-Robert

[Gilbert Bates]

So you've had the replacement part crap out a second time? Do you think it
is indeed problems with the part and not due to problems with something like
the manufacturing process, like overheating the part during soldering? I
don't want to suggest putting in a new part to the store if they continue to
fail. I believe they have just started to send back any amp with the problem
but they have sold a few of them.

So 4.7k sub on the LDR side, right? What does this do? For the ones I
monkeyed with I didn't have a schematic and only knew it was the part while
not really considering where it was located within the circuit.

Gilbert

[Robert M. Braught]

Gilbert Bates wrote:
>
> So you've had the replacement part crap out a second time?

Evidently yes. I could tell by the workmanship it had been replaced
before (which is what the customer had reported, although they didn't
know precisely what had been done previously at another shop, only
'warranty work for same problem'.)

> Do you think it
> is indeed problems with the part and not due to problems with something like
> the manufacturing process, like overheating the part during soldering?

I kinda doubt overheating, although I suppose that's a possibility,
'weakening' the part for imminent failure. They fail after 3~6 months in
service too, not fresh out of the box. I've had customers report failure
after 'blasting' signal through their amp. I don't know the specific
mechanism of failure (thermal/mechanical/electrical), but I'm pretty
suspicious... ('no name' part, high failure rate)

> I don't want to suggest putting in a new part to the store if they continue to
> fail. I believe they have just started to send back any amp with the problem
> but they have sold a few of them.
>
> So 4.7k sub on the LDR side, right? What does this do? For the ones I
> monkeyed with I didn't have a schematic and only knew it was the part while
> not really considering where it was located within the circuit.

It's wired as a series attenuator, kinda like old Gibson designs, with
three resistors (and two caps) for a total of 12K resistance + LDR from
one op amp output to the input of another...
...I measured new parts to arrive at the ~5K 'on' (LED full on = tremolo
depth full CCW, calibration adjustments as set by factory) resistance
average. A 4.7K resistor effectively 'neuters' the tremolo, but prevents
the opto-isolator failure from shutting the amp down.

-Robert