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Old 17th Nov 2018, 6:09 pm   #1
kestrelmusic
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Default Calibrating a Taylor signal generator

I'm tidying up a Taylor Model 60 signal generator. I can't find a manual or circuit diagram for it (can anyone help there?) but the circuitry looks identical to the 65B except that it uses UX5 valves instead of Octals (might it have been made for an American market?) and the power supply is in the main case rather than behind the front panel. Incidentally it uses the contact between the front panel and the main case as both HT and LT returns - I have to remember to clip on an additional lead when removing it from the case for calibration! It has five air-cored coils, each one with its own trimmer capacitor.

The MW band functions nicely (I haven't started on the SW bands yet) but on the LW band I have the trimmer unscrewed to the point where the nut is on the point of dropping off, but it's still running too slow. I'm using BBC R4 at 198Kc/s as a reference point - I guess this is pretty accurate and stable, or am I being over-hopeful? I've checked the pointer position, and there is very little leeway there. Any thoughts on this gratefully received!
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Old 17th Nov 2018, 6:47 pm   #2
John_BS
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Default Re: Calibrating a Taylor signal generator

I'm not at all familiar with this instrument, but if it's producing too low a freqency on the LF range, you're only left with the inductor (assuming the anode capacitance of V3 is OK: if not it would affect other ranges as well). If you're testing with the unit partially dissembled, it may be that the LF coil, when fully cased, has a lower inductance due to one or more conducting surfaces contraining its magnetic fringes?

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Old 17th Nov 2018, 6:59 pm   #3
kestrelmusic
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Default Re: Calibrating a Taylor signal generator

Many thanks - I'll try replacing the valve. I doubt if it's a screening problem as each coil is individually screened and there's lots of space around the coil unit.
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Old 17th Nov 2018, 8:18 pm   #4
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Default Re: Calibrating a Taylor signal generator

Air cored coils sound a bit odd, especially for LF bands. There couldn't be any dust iron cores on brass screws missing perchance?

I'd expect capacitive trimmers to have a strong effect at the HF end of each band, and relatively little at the LF end. How far out is the LF end?

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Old 17th Nov 2018, 8:30 pm   #5
Bazz4CQJ
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Default Re: Calibrating a Taylor signal generator

I have an old Taylor 68, which as 'service-grade old sig gens' go, is really very good. Feeding it in to a Digital Frequency Meter invariably gives pleasing results. You can buy a basic DFM for little more than a fiver!

Could I respectfully suggest you proceed with caution with your unit? I'd really want to have the circuit and have considered any/all problems very carefully before touching the tuned circuits. You've heard of the "Phantom Twiddler" ?

The 68 had Hunt's black "mould seal" (?) caps in it, which are well-known offenders. In the 60?

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Old 18th Nov 2018, 9:59 pm   #6
kestrelmusic
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Default Re: Calibrating a Taylor signal generator

Circuit of the 65B attached. The 60 looks pretty similar except for UX instead of octal valves. I'd be surprised if the problem was outside the LW tuned circuit as the MW band is pretty well spot on. There's no sign of any cores in the coils - I've not dismantled the screening cans, but a dentist's mirror and a LED torch give me a reasonable view up the centre. Though why, in that case, the cans should have holes in their tops is another question...

Another oddity - the LT voltage is low, around 4.8V. The voltage selector is on the 220V setting. HT at the RF valve anode is around 138V. I haven't yet investigated the inside of the transformer screen. I'd suspect one or more of C1 to C4 of leakage but running it through a lamp limiter shows it taking very little current (60W lamp glowing dull red). And the box is staying cool and there's no nasty smell of burning dielectric.
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File Type: pdf Taylor 65B.pdf (98.6 KB, 78 views)
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