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Old 17th May 2019, 9:15 am   #1
'LIVEWIRE?'
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Exclamation Quiescent Current of a Yacht Boy 300 Radio

Having just replaced a partially s/c transistor (T4) in one of the above, the radio now works fine, and, measuring the current when the set is powered from a stabilised PSU set to 7.5v, the minimum, with volume at zero is 20mA, and the maximum about 250mA with the o/p on speech peaking at around 1.4 watts into 4 ohms. Having mislaid my Yacht Boy 300 service sheets, I'm not sure if these are acceptable figures, so, can anyone tell me what the quiescent Current should be. The radio is only ever run from a 'wall-wart', but over the years the audio amplifier has failed twice. The last time I replaced all the transistors in the audio amp. with the types originally used, and, IIRC, one of the electrolytics, so I want to ensure, as far as possible that it's not likely to fail again.
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Old 17th May 2019, 10:11 am   #2
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Default Re: Quiescent Current of a Yacht Boy 300 Radio

Attached is the schematic which whilst it doesn't show current, the expected voltages are shown. Hope this helps.
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Old 17th May 2019, 10:26 am   #3
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Default Re: Quiescent Current of a Yacht Boy 300 Radio

20mA quiescent sounds plausible for this sort of design, though I don't have the actual figure.
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Old 17th May 2019, 10:59 am   #4
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Default Re: Quiescent Current of a Yacht Boy 300 Radio

Thanks, simpsons & Paul for your replies. There is a pre-set pot. on the PCB, but I may leave well alone as the radio works OK as it is. I don't recall from where I got the yacht Boy 200 & 300 diagrams. Those models aren't in the 'Instant Downloads' list.
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Old 23rd May 2019, 10:53 pm   #5
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Default Re: Quiescent Current of a Yacht Boy 300 Radio

I've now found the circuit diagran on http://radiomuseum.org, and have been listening to the Yacht Boy, which, to me, compared with my similar Brandt RS 902, sounds a little distorted, especially on FM Music Stations. Given that the audio amplifier in the YB has failed at least twice before, and, on the second occasion I replaced T12,13, 14, & 15, but on this occasion only T14, perhaps I should check for other faults &/or check and adjust R 80 in the Vbe multiplier circuit, the only adjustment provided, but no indication of Iq is given anywhere in the manual. (N.B. in post #1 I mistakenly referred to T14 as T4. As this is the Vbe Multiplier, then IMHO, the adjustment should be carried out, since the replacement transistor will have different gain to the original

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Old 24th May 2019, 1:47 pm   #6
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Default Re: Quiescent Current of a Yacht Boy 300 Radio

Having downloaded the circuit of the similar Yacht Boy 200, the Iq for that model is given as 25mA @ 7.5vDC, so the 20mA I measured seems a little on the low side, so a small adjustment is called for methinks.
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Old 25th May 2019, 1:56 am   #7
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Default Re: Quiescent Current of a Yacht Boy 300 Radio

If the idle current is set too high, then the battery life will be reduced. (The power dissipation in the output transistors will also be increased; but not by as much as it will increase anyway under signal, unless you set it outrageously high.) If it is set to low, crossover distortion will be audible. This will sound like the speaker voice coil is binding.

Think of the output transistors as a potential divider. If the "top" one is conducting, the output will be driven towards the positive rail; if the "bottom" one is conducting, the output will be driven towards the negative rail. Now, a transistor does not begin to conduct until it has a certain voltage across the B-E junction (about 0.7V for Silicon, 0.2V for Germanium). So the output transistors have to be biased so that the no-signal operating point is somewhere within the linear region of each transistor; if you superimpose the transfer curves of the output transistors, they cross over at the no-signal point. Both transistors will have to be conducting a little bit, meaning the current through the output stage with no signal will be greater than zero. The harder "on" the transistors are biased, the greater the current under no signal conditions; and this current can be measured as a useful indication of the biasing conditions.

If the biasing is insufficient and there is an input range where neither transistor is conducting, the speaker cone will not move until one or the other transistor begins conducting and the amplifier moves out of the dead zone. This is crossover distortion. It tends to manifest less at higher volume, since the signal is spending more of its time out of the dead zone. Of course, if you turn the volume up too high, the signal will become distorted again as clipping sets in .....

It's easy to see crossover distortion on an oscilloscope; but if you haven't got one, you can set the quiescent current approximately by ear. Find a good, strong station and turn the volume down until you have to start straining to hear it. Adjusting the current downwards until distortion is heard. Then tweak it up just a little, for luck. (You will need to tune away from your station and set the volume to minimum actually to read the current, of course, since it will increase when sound is being produced from the speaker.)

It is probably best to set the current using slightly-spent batteries, but then check it isn't too high with fresh batteries or a suitable PSU. (If you have an adjustable bench power supply, this can be turned down to simulate spent batteries.)
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Old 25th May 2019, 8:20 am   #8
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Default Re: Quiescent Current of a Yacht Boy 300 Radio

I'll bear your points in mind, Julie. I do have a variable bench supply and a 'scope. It's many years since i was taught the theory, but your post has reminded me of things I'd half-forgotten, so thanks for jogging my memory!
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