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