Vox V301H User manual

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Vox Continental models
V301H, 301J, 303J
TROUBLE-SHOOTING
COMBO-ORGANS

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TOPICS
1. Examine for cold-solder joints
2. Repair a lied track
3. Diagnose faulty transistors in a divider circuit (including use of a signal tracer).
4. Test a tuning coil
SAFETY
• Always power-down by disconnecng at the wall socket.
DO NOT power-down using the instrument on-o switch as that leaves AC supply
voltage sll present in the machine.
• Always power-down before performing physical work on any components.
• Know where AC supply voltage is present in the instrument. Likely suspect points are
at the terminaon of the AC supply cable, at on-o switches and power-on indicator
lamps.
• Before working on an instrument, put an insulang barrier around any exposed AC
supply connecons. The barrier could be temporary, or consider a permanent x
such as ng heat-shrink tubing.
• Look for any situaons where AC supply and signal wires are mixed together. Aim to
achieve reliable physical separaon. This may also help with any hum or clicks and
pops issues.
Exposed wiring at on-o switches is a common hazard

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1. EXAMINE FOR BAD SOLDER JOINTS
Good solder joints have a bright surface nish
and concave contour, with the lead wire
protruding from the joint. The concave shape is
the result of surface tension and is a posive
indicaon that the solder has bonded to the
component lead.
Be suspicious of any joints which appear as a
‘blob’ of solder with no lead protruding.
Cracked joints are not always this easy to spot.
Be suspicious of any joints which are
mechanically stressed or subject to vibraon.
Examples are connectors, wire terminaons or
heavier components. These will be the rst joints
to give problems.
Less extreme cracking in a joint may only be
visible under magnicaon with good lighng.
To repair a joint, rst remove the exisng solder.
Examine and, if necessary, mechanically clean
the surfaces before resoldering.
Photo credit, Wikimedia
Commons
Some helpful items when fault-nding; jeweller’s loupe, head torch and
illuminated magnifying glass
Photo credit, Wikimedia
Commons

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Use a temperature controlled soldering iron, set to around 450C.
While it may seem counter-intuive, the possibility of track or component damage is
reduced by having a hot iron, so geng the joint made quickly.
This is an 80 Wa unit which provides ample heang capacity for circuit card repairs.
The combinaon of good solder-sucker and uxed copper-braid will
greatly assist the removal of solder, especially where the card has copper
tracks on one side only.
A hobby-blade is useful to (gently) scrape surfaces clean, backed up by a
ux-pen to aid resoldering. The dried ux can be le in-place.

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2. HOW TO PREVENT A LIFTED TRACK
To avoid a lied track, use your soldering iron and de-soldering aids with care and avoid
any mechanical force which could push the copper track away from the card substrate.
Accidents can sll happen. This was the result of
simply inserng the leads of a new component.
To make a repair, if using a heavier gauge nned copper wire, form hooks to go around the
component lead ends. For longer wires, solder the wire to the original track at one or more points.
Alternavely, if the component lead is long
enough, it can be bent over to form a bridge to
a good secon of track.
A card of fuse-wire can be a handy source of
two smaller wire sizes. Wrapping the wire
around the component leads achieves a good
mechanical connecon prior to soldering.

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Both hand and machine soldered cards oen were not cleaned in manufacture. The ux residue is
safe to leave, but doesn’t look great. Isopropyl alcohol is the best solvent. As a scrubbing tool a
toothbrush works well, but should not be used where digital integrated circuits are involved.
Special an-stac brushes are available in that situaon.
Hold the card vercal to sluice the dirty uids away, but ensuring that the uids don’t get onto, or in,
any connectors.
A light spray of circuit board lacquer will leave your card looking beer than new.

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3. DIAGNOSE FAULTY TRANSISTORS IN A DIVIDER CIRCUIT
Many organ types use a series of divide-down circuits to derive the descending octaves of
a given note.
A typical divide-down stage comprises a pair of transistors plus peripheral resistors and
capacitors. Depending on how it’s been drawn, it’s usually possible to recognise the
crossed-connecon transistor pairs in an keyboard circuit diagram.
A divide-down stage is also known as a bistable ip-op, or latch. It has two stable output
states (o and on) and is triggered to change states by an input signal.
In a keyboard, the output of one stage is also the input to the next, each stage in turn
producing the next lowest octave.
Example of a divide-down stage from a VOX Connental 301
Output Input

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VOX divider cards
Card A is from a UK manufactured Connental, circa 1965. It uses PNP transistors
Card B is also from a UK manufactured double-manual Connental (with percussion).
It uses NPN transistors.
Both are built on the same printed circuit biscuit and have male pin connectors. However,
they are not interchangeable as they have opposite power supply voltage polaries.
Card C is from a US manufactured Connental, model V301H, circa 1966.
It uses PNP transistors and has female socket connectors.
The following pages will explore PNP and NPN transistors.
VOX DIVIDER CARDS

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PNP TRANSISTORS
Tesng a BC556 PNP transistor

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NPN TRANSISTORS
Tesng a BC337 NPN transistor

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With some experimentaon, it’s possible to determine whether an unknown device is PNP
or NPN, and which leg is the base.
The test will not dierenate between emier and collector, but with reference to the
circuit diagram it’s usually possible to idenfy these by observing what components are
connected to that leg. (Oen, the emier will be connected to circuit ground, but it’s not
guaranteed.)
Diodes and transistors are the two components which can be successfully tested in-circuit.
Aempts to test resistors and capacitors in-circuit will, at best, only give indicave
readings. For an accurate result, rst de-solder one leg of the component and li it away
from the circuit card track.
A Google search of the part number will usually nd the data and pin-out info for any
transistor.
Three methods of fault nding a divider card will be discussed in the following pages:
a) In the instrument, using the keyboard
b) In the instrument using an audio probe
c) On the bench using an audio probe or oscilloscope

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3a. DIAGNOSE A FAULTY CARD(S) USING THE KEYBOARD
• With only the 16’ drawbar out, play a descending chromac scale. Listen for any notes
which are missing, quiet, or the wrong octave
• Tag any faulty keys with masking tape, note the fault on the tape
• Repeat these two steps with only the 8’ drawbar out, then only the 4’ out
• At the end of this process, you will have a good picture of where any faults are located
If a note is missing in all octaves
and footages, it’s likely you have a
faulty oscillator.
This topic will be covered later in
the document.
If a note is missing in just one octave and one
footage, it’s likely to be the result of a broken
contact spring.
Replacement of a broken spring is not covered
here.
In all other cases, if you have a note (or
notes) not playing, or playing higher
than it should be, it’s likely to be due to
at least one faulty divider stage.
In this case, remove the card for bench
tesng.

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An audio signal tracer is a simple, but useful, test tool which will be referred to in following
pages. A quick Google search will nd several design approaches to making your own.
In the example below a 0.1mfd 400 volt capacitor is used to block any DC voltage and a
potenometer included to allow the sensivity to be readily controlled.
The components are housed in a small box complete with a guitar socket. In this way it can
be used plugged into a small pracce amp.
WARNING
A signal tracer must only be used to check low-level audio signals.
For your safety, DO NOT use an audio probe in circuit areas where mains AC voltage,
or any high voltages, may be present.
SIGNAL TRACER
Ground connector
Probe point

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3b. DIAGNOSE A FAULTY CARD(S) USING A SIGNAL TRACER
With the cards in the instrument:
• Touch the probe on each of the six audio outputs of each card, moving from the
highest to lowest octave
• Listen for an even progression of tones
• Tag any faulty cards nong the faulty octave (s)
UK and USA cards have the same pinout arrangement for the six octaves the cards
generate.

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3c. BENCH TESTING
• The following checks can be done with an audio probe or oscilloscope.
• The cards can be safely powered from a 9 volt baery
• A simple test jig can be helpful. A small press-buon switch in the baery circuit is
useful when checking for reliable oscillator start-up and avoids constant connecng and
disconnecng of the baery.
While not having the exact same pitch
spacing as the card, a six or eight-way
‘Molex’ connector allows easy connecon
to the male pins.
** The baery polarity shown here is for a
PNP card.
Reverse the polarity for an NPN card
Here, a length of medium gauge nned
copper wire with the end folded over
double makes an adequate male connector
pin for tesng purposes

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3c. BENCH TESTING cont.
Once it’s been established that you have at least one faulty divider stage, you will need to
locate the relevant components. For both UK and USA cards, the sequence of divider
stages one to six begins nearest the oscillator circuit.
Idenfy each pair of transistors which comprise a divider stage. In the UK cards above, the
output of each stage is on the collector of the right-hand-side transistor of each pair
(where it connects to the 2K2 resistor) as shown by the red arrows.
In the USA card above, several of the resistors and capacitors for each stage are located on
the vercal ‘couplates’. The yellow wires connect the output of one stage to the input of
the next. The stage output can also be found on the centre leg (collector) of the le-hand-
side transistor of each pair, as shown by the red arrows. Always start your tesng from the
highest frequency end, which in all cases is nearest to the oscillator.

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These oscilloscope traces illustrate a correctly working divider stage. Each rising edge (red
line) of the input signal triggers a change of state in the output signal, so halving the
frequency, which equates to lowering the note by one octave.
Two fault condions were seen in the
instrument I repaired. Either the divider stage
was not switching at all (atlining) or the
switching was not reliable (failure to latch) as
shown in the trace. Either fault will aect all
following stages.
OUTPUT
Each divider has a simple resistor-capacitor network which shapes the square-wave
divider output into a, more musical, rounded sawtooth paern, sll rich in harmonics.

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For all eight cards repaired from a USA
manufactured Connental, low-gain transistors
were the cause of every divider fault.
A transistor gain test funcon is included in
many digital mul-meters. Select the correct
test for the type of transistor you are working
with (PNP or NPN) and be sure to insert the
transistor leads into the correct sockets.
In technical literature, gain may be shown under the
term ‘hfe’. It is the rao of collector current divided by
the base current at some nominated test condion. The
design gain of a transistor will be presented in a range
(say 70-250).
Early type transistors as found in a Connental typically
have gains considerably lower than can be expected
from more modern devices. This example has a gain of
sixty.
This group represents a typical range of gain values (20 –92) in transistors from a USA
manufactured Connental. A gain of less than 30 will almost certainly cause faults in a
divider stage. It seems likely that, at least for this generaon of devices, the transistors
may loose gain over me.
Note also that the gain of these transistors was measured at room temperature.

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The gain of a transistor is temperature dependant. In the images above, the same
transistor is tested for gain at room temperature, then chilled with freeze spray.
This result suggests that chilling is not a useful test when looking for a marginal transistor,
as the gain of even a ‘good’ transistor will be reduced to the point that it would cease to
work in a divider stage.
However, to ush out any marginal transistors, it may be a useful to cool a divider card in a
refrigerator before tesng.

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REPLACEMENT OF FAULTY TRANSISTORS
There are two opons when replacing faulty transistors.
If you have access to transistors of the same part number, you may prefer to use those to
maintain authencity. Especially if your card has germanium PNP transistors, it would be
prudent to source replacements of the same part number, or a near equivalent. Gain test
the replacement parts before ng, and select the higher gain devices for use.
If your card has silicon NPN transistors, then any modern general purpose NPN transistor
should be ne as a replacement. Types BC337, BC546, BC547 have been used successfully.
Transistor pin orientaon for a UK manufactured card. The pin
orientaon is the same for both the PNP and NPN versions of the card.
B C E
Transistor pin orientaon for a USA manufactured card.
Here, BC337 transistors have been ed as replacements.
Note that the base and collector legs have been crossed to match the
transistor pin orientaon to the circuit card hole placement.
C B E
B C E