Advertisement
Quick Links
Advertisement
Subscribe to Our Youtube Channel
Related Manuals for Vox V301H
Summary of Contents for Vox V301H
- Page 1 TROUBLE-SHOOTING COMBO-ORGANS Vox Continental models V301H, 301J, 303J...
- Page 2 TOPICS 1. Examine for cold-solder joints 2. Repair a lifted 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 disconnecting at the wall socket. • DO NOT power-down using the instrument on-off switch as that leaves AC supply voltage still present in the machine.
- Page 3 1. EXAMINE FOR BAD SOLDER JOINTS Good solder joints have a bright surface finish and concave contour, with the lead wire protruding from the joint. The concave shape is the result of surface tension and is a positive indication that the solder has bonded to the component lead.
- Page 4 Use a temperature controlled soldering iron, set to around 450C. While it may seem counter-intuitive, the possibility of track or component damage is reduced by having a hot iron, so getting the joint made quickly. This is an 80 Watt unit which provides ample heating capacity for circuit card repairs. The combination of good solder-sucker and fluxed copper-braid will greatly assist the removal of solder, especially where the card has copper tracks on one side only.
- Page 5 2. HOW TO PREVENT A LIFTED TRACK To avoid a lifted 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 still happen. This was the result of simply inserting the leads of a new component.
- Page 6 Both hand and machine soldered cards often were not cleaned in manufacture. The flux 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 anti-static brushes are available in that situation.
- Page 7 (off 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. Input Output Example of a divide-down stage from a VOX Continental 301...
- Page 8 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 polarities. Card C is from a US manufactured Continental, model V301H, circa 1966. It uses PNP transistors and has female socket connectors.
- Page 9 PNP TRANSISTORS Testing a BC556 PNP transistor...
- Page 10 NPN TRANSISTORS Testing a BC337 NPN transistor...
- Page 11 With some experimentation, it’s possible to determine whether an unknown device is PNP or NPN, and which leg is the base. The test will not differentiate between emitter and collector, but with reference to the circuit diagram it’s usually possible to identify these by observing what components are connected to that leg.
- Page 12 3a. DIAGNOSE A FAULTY CARD(S) USING THE KEYBOARD With only the 16’ drawbar out, play a descending chromatic 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 •...
- Page 13 SIGNAL TRACER An audio signal tracer is a simple, but useful, test tool which will be referred to in following pages. A quick Google search will find 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 potentiometer included to allow the sensitivity to be readily controlled.
- Page 14 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 •...
- Page 15 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 battery • A simple test jig can be helpful. A small press-button switch in the battery circuit is •...
- Page 16 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. Identify each pair of transistors which comprise a divider stage.
- Page 17 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. OUTPUT Two fault conditions were seen in the instrument I repaired.
- Page 18 For all eight cards repaired from a USA manufactured Continental, low-gain transistors were the cause of every divider fault. A transistor gain test function is included in many digital multi-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.
- Page 19 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’...
- Page 20 REPLACEMENT OF FAULTY TRANSISTORS There are two options when replacing faulty transistors. If you have access to transistors of the same part number, you may prefer to use those to maintain authenticity. Especially if your card has germanium PNP transistors, it would be prudent to source replacements of the same part number, or a near equivalent.
- Page 21 A CAVEAT. In some cases, cards which, after repairs, worked perfectly on the bench (but which still had some original transistors) did not work reliably in the instrument. With the card in the instrument, an audio probe was used to test the output pins to locate the faulty divider.
- Page 22 Although performing the same function, the UK and USA Continental cards have different oscillator circuits. The coil of a UK card has four terminals. VOX The coil of a USA card has three terminals. documentation suggests four coil variants across There are two coil variants across the twelve cards.
- Page 23 The initial pulse is present in the right-hand trace, but oscillation is not initiated. FAULT 1. C7 was identified as the ‘start’ capacitor. A chart in the VOX documentation gives the value of C7 for each of the twelve cards. C7 in a D# card should be 0.0027mfd (2.7nf) , however an incorrect value capacitor had been...
- Page 24 Fault 2. There were two other instances where the oscillator was unreliable. Changing the oscillator transistor Q1 did not help, but fitting new capacitors to replace the original C5 and C6 cured the problem. C5 and C6 are part of the resonant circuit together with coil L1. Curiously, the original parts tested to be within tolerance of their stated value.
Need help?
Do you have a question about the V301H and is the answer not in the manual?
Questions and answers