Here's a bit of a puzzler.
This is the voltage regulator for the main +14V in a Kenwood KT-80 tuner:
Note compensation capacitor C87 (560p) below Q10.
This is the same in a 1-2 year newer KT-900. Almost identical topology and values, basically the same part types - 2SC945 or similar for the small transistors and 2SD330 for the big series transistor. (Current draw could be at least 50% higher though, and the 68 mA shown is decidedly not accurate, as are a number of things on this schematic. For one, R154 is actually a 5.6k, and voltage is 13.5 V and change.)
Note how C96 is now a .01 = 10n, as confirmed by the parts list.
This is the only model with this regulator topology that's taking such a drastic approach to compensation. No other model using it uses anything nearly as big (KT-615, 815, 80, 1000: 560p; KT-1100: 2200p or 0p). Feedback resistors and hence voltages tend to be of very similar values: 5k6/6k2, 4k7/5k1, 5k6/6k8, 4k7/3k9, 4k7/4k7.
Question of the day: Why would they have done this?
Most things are connected to this main rail via RC filtering usually using 100 ohms. The only thing with a lowish-impedance connection is the frontend, which sits after series silicon diode D5 alongside the IF strip and has LC filtering in the form of 1 µH and 100µF || 10n || 1n. (A bit of 10.7 MHz IF might be getting onto the rail via 100 ohm Wide bandwidth switch resistor R9, but I can't imagine it would be that much at this fairly early stage and preceded by a 10n to ground, plus I already swapped the poorly dimensioned R9 for a 470 ohm. The optimum value would be 560-750 ohms according to sim, but I only had 470R and 1k on hand.)
The PCB layout is quite different between KT-80 and KT-900, mind you.
I intend to port over my feedforward mod from the KT-80 (which incidentally works a treat for DC regulation)...
....and was wondering whether some attention to compensation may be warranted and how to check for potential abnormalities. (Have fancy multimeter and old 10 MHz analog scope.)
This is the voltage regulator for the main +14V in a Kenwood KT-80 tuner:
Note compensation capacitor C87 (560p) below Q10.
This is the same in a 1-2 year newer KT-900. Almost identical topology and values, basically the same part types - 2SC945 or similar for the small transistors and 2SD330 for the big series transistor. (Current draw could be at least 50% higher though, and the 68 mA shown is decidedly not accurate, as are a number of things on this schematic. For one, R154 is actually a 5.6k, and voltage is 13.5 V and change.)
Note how C96 is now a .01 = 10n, as confirmed by the parts list.
This is the only model with this regulator topology that's taking such a drastic approach to compensation. No other model using it uses anything nearly as big (KT-615, 815, 80, 1000: 560p; KT-1100: 2200p or 0p). Feedback resistors and hence voltages tend to be of very similar values: 5k6/6k2, 4k7/5k1, 5k6/6k8, 4k7/3k9, 4k7/4k7.
Question of the day: Why would they have done this?
Most things are connected to this main rail via RC filtering usually using 100 ohms. The only thing with a lowish-impedance connection is the frontend, which sits after series silicon diode D5 alongside the IF strip and has LC filtering in the form of 1 µH and 100µF || 10n || 1n. (A bit of 10.7 MHz IF might be getting onto the rail via 100 ohm Wide bandwidth switch resistor R9, but I can't imagine it would be that much at this fairly early stage and preceded by a 10n to ground, plus I already swapped the poorly dimensioned R9 for a 470 ohm. The optimum value would be 560-750 ohms according to sim, but I only had 470R and 1k on hand.)
The PCB layout is quite different between KT-80 and KT-900, mind you.
I intend to port over my feedforward mod from the KT-80 (which incidentally works a treat for DC regulation)...
....and was wondering whether some attention to compensation may be warranted and how to check for potential abnormalities. (Have fancy multimeter and old 10 MHz analog scope.)
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Rather than querying Kenwoods parts choice on its own , the answer you seek is in recognizing what the circuit provides. You should spot its a Darlington pair, arranged as a capacitance multiplier. The capacitance values at C86 and C87 they deemed appropriate, to decrease ripple sufficiently from the R/C network presented at the base, for supplying subsequent circuit stages,
The later 10nf value is better still for ripple reduction. Kenwood carefully looking at the Darlington pairs hfe. Substituting their transistor choice with others as your third diagram shows, has it appears not factored how much ripple reduction was firstly achieved by Kenwood's choices.
The later 10nf value is better still for ripple reduction. Kenwood carefully looking at the Darlington pairs hfe. Substituting their transistor choice with others as your third diagram shows, has it appears not factored how much ripple reduction was firstly achieved by Kenwood's choices.
The real oddity to me is L16 in series with the reference Zener diode in your second image.
I wouldn't worry to much over the whys and wherefores of what is really a classic text book implementation. It may that the design suffered from demodulation of high field strength RF signals (used in locations near powerful transmitters) and all this is just a belt and braces approach to force absolute stability under all possible conditions.
(Service manuals of this era and type often have lots of different variants shown as well, often with differences applicable to certain geographical regions)
I wouldn't worry to much over the whys and wherefores of what is really a classic text book implementation. It may that the design suffered from demodulation of high field strength RF signals (used in locations near powerful transmitters) and all this is just a belt and braces approach to force absolute stability under all possible conditions.
(Service manuals of this era and type often have lots of different variants shown as well, often with differences applicable to certain geographical regions)